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Noviembre de 2017
Geophysics From Terrestrial Time-Variable Gravity Measurements
Authors: Michel Van Camp, Olivier de Viron et al
Link: Click here

In a context of global change and increasing anthropic pressure on the environment, monitoring the Earth system and its evolution has become one of the key missions of geosciences. Geodesy is the geoscience that measures the geometric shape of the Earth, its orientation in space, and gravity field. Time-variable gravity, because of its high accuracy, can be used to build an enhanced picture and understanding of the changing Earth. Ground-based gravimetry can

determine the change in gravity related to the Earth rotation fluctuation, to celestial body and Earth attractions, to the mass in the direct vicinity of the instruments, and to vertical displacement of the instrument itself on the ground. In this paper, we review the geophysical questions that can be addressed by ground gravimeters used to monitor time-variable gravity. This is done in relation to the instrumental characteristics, noise sources, and good practices. We also discuss the next challenges to be met by ground gravimetry, the place that terrestrial gravimetry should hold in the Earth observation system, and perspectives and recommendations about the future of ground gravity instrumentation.

Noviembre de 2017
Quake warnings, seismic culture
Authors: Richard M. Allen
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Since 1990, nearly one million people have died from the impacts of earthquakes. Reducing those impacts requires building a local seismic culture in which residents are aware of earthquake risks and value efforts to mitigate harm. Such efforts

include earthquake early warning (EEW) systems that provide seconds to minutes notice of pending shaking. Recent events in Mexico provide an opportunity to assess performance and perception of an EEW system and highlight areas for further improvement. We have learned that EEW systems, even imperfect ones, can help people prepare for earthquakes and build local seismic culture, both beneficial in reducing earthquake-related losses.

Noviembre de 2017
Observations and modeling of the elastogravity signals preceding direct seismic waves
Authors: Martin Vallée, Jean Paul Ampuero et al
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After an earthquake, the earliest deformation signals are not expected to be carried by the fastest (P) elastic waves but by the speed-of-light changes of the gravitational field. However, these perturbations are weak and, so far, their detection has not been accurate enough to fully understand

their origins and to use them for a highly valuable rapid estimate of the earthquake magnitude. We show that gravity perturbations are particularly well observed with broadband seismometers at distances between 1000 and 2000 kilometers from the source of the 2011, moment magnitude 9.1, Tohoku earthquake. We can accurately model them by a new formalism, taking into account both the gravity changes and the gravity-induced motion. These prompt elastogravity signals open the window for minute time-scale magnitude determination for great earthquakes.

Noviembre de 2017
A Comparison of Geodetic and Geologic Rates Prior to Large Strike-Slip Earthquakes: A Diversity of Earthquake Cycle Behaviors?
Authors:James F. Dolan and Brendan J. Meade
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Comparison of pre-event geodetic and geologic rates in three large-magnitude (Mw=7.6-7.9) strike-slip earthquakes reveals a wide range of behaviors. Specifically, geodetic rates of 26-28 mm/yr for the North Anatolian fault along the 1999 MW=7.6 Izmit rupture are ∼40% faster than Holocene geologic rates. In contrast, geodetic rates of 6-8 mm/yr along the Denali fault prior to the 2002 MW=7.9 Denali earthquake are only half as fast as the latest Pleistocene-Holocene geologic rate of 12 mm/yr. In the third example where a sufficiently long pre-earthquake geodetic

time series exists, the geodetic and geologic rates along the 2001 MW=7.8 Kokoxili rupture on the Kunlun fault are approximately equal at 11 mm/yr. These results are not readily explicable with extant earthquake cycle modeling, suggesting that they may instead be due to some combination of regional kinematic fault interactions, temporal variations in the strength of lithospheric-scale shear zones, and/or variations in local relative plate motion rate. Whatever the exact causes of these variable behaviors, these observations indicate that either the ratio of geodetic to geologic rates before an earthquake may not be diagnostic of the time to the next earthquake, as predicted by many rheologically based geodynamic models of earthquake cycle behavior, or that different behaviors characterize different fault systems in a manner that is not yet understood or predictable.

Noviembre de 2017
Compact Single Layer Travelling-Wave Antenna Design Using Metamaterial Transmission-Lines
Author: M. Alibakhshikenari, B. Singh Virdee et al
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This paper presents a single-layer travelling-wave antenna (TWA) that is based on composite right/left-handed (CRLH)-metamaterial (MTM) transmission-line (TL) structure, which is implemented by using a combination of inter-digital capacitors and dual-spiral inductive slots. By embedding dual-spiral inductive slots inside

the CRLH MTM-TL results in a compact TWA. Dimensions of the proposed CRLH MTM-TL TWA is 21.5×30.0 mm2 or 0.372λ0×0.520λ0 at 5.2 GHz (center frequency). The fabricated TWA operates over 1.8–8.6 GHz with a fractional bandwidth greater than 120%, and it exhibits a peak gain and radiation efficiency of 4.2 dBi and 81%, respectively, at 5 GHz. By avoiding the use of lumped components, via-holes or defected ground structures (DGS), the proposed TWA design is economic for mass production as well as easy to integrate with wireless communication systems.

Noviembre de 2017
Nucleation Phase and Dynamic Inversion of the Mw 6.9 Valparaíso 2017 Earthquake in Central Chile
Author: S. Ruiz, F. Aden-Antoniow et al
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The Valparaiso 2017 sequence occurred in the Central Chile megathrust, an active zone where the last mega-earthquake occurred in 1730. Intense seismicity started 2 days before the Mw 6.9 mainshock, a slow trenchward movement was observed in the coastal GPS antennas and was accompanied by foreshocks and repeater-

type seismicity. To characterize the rupture process of the mainshock, we perform a dynamic inversion using the strong-motion records and an elliptical patch approach. We suggest that a slow slip event preceded and triggered the Mw 6.9 earthquake, which ruptured an elliptical asperity (semiaxis of 10 km and 5 km, with a subshear rupture, stress drop of 11.71 MPa, yield stress of 17.21 MPa, slip weakening of 0.65 m, and kappa value of 1.98). This earthquake could be the beginning of a long-term nucleation phase to a major rupture, within the highly coupled Central Chile zone where a megathrust earthquake like 1730 is expected.

Noviembre de 2017
Sloshing of Earth's core may spike big quakes
Author: Paul Voosen
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For decades, scientists have charted tiny fluctuations in the length of Earth's day: Gain a millisecond here, lose a millisecond there. Last week at the annual meeting of the Geological Society of America in Seattle, Washington, two geophysicists argued that these minute changes

could be correlated with the timing of major earthquakes—and potentially help forecast them. During the past 100 years, Earth's slowdowns have matched surprisingly well with periods of global increases in the frequency of magnitude-7 and larger earthquakes. Each spike happens well after the slowdown, offering a 5-year heads up on future quakes. So far, researchers have only fuzzy ideas about how changes in Earth's molten iron core might cause this pattern, but they say the finding is too provocative to ignore.

Octubre de 2017
Utilización de Análisis de Componentes Principales para mejorar el desempeño de la predicción de la magnitud de los terremotos en Japón
Autores: G Asencio-Cortés, F Martínez-Álvarez, A Morales-Esteban, J Reyes (NT2 Labs) and A Troncoso
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Durante la última decada se ha incrementado la atención en la investigación de la predicción de terremotos por medio de técnicas de Data Mining. Muchos trabajos han presentado diversos indicadores sísmicos como inputs para Clasificadores Supervisados. Sin embargo estos indicadores han sido utilizados sin una previa transformación. En este trabajo utilizamos Análisis de Componentes Principales (ACP) para reducir la dimensionalidad de los vectores input para así generar nuevos datsets y colocar este procedimiento como un paso intermedio en una exitosa metodología que hoy se utiliza para predecir terremotos. Nuestra investigación la aplicamos a Tokyo, una de las ciudades japonesas más amenazadas por grandes terremotos. Utilizamos diversosy muy bien conocidos Clasificadores junto con ACP y reportamos notorias mejoras en la predicción.

Octubre de 2017
Using principal component analysis to improve earthquake magnitude prediction in Japan
Authors: G Asencio-Cortés, F Martínez-Álvarez, A Morales-Esteban, J Reyes (NT2 Labs) and A Troncoso
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Increasing attention has been paid to the prediction of earthquakes with data mining techniques during the last decade. Several works have already proposed the use of certain features serving as inputs for supervised classifiers. However, they have been successfully used without any further transformation so far. In this work, the use of principal component analysis (PCA) to reduce data dimensionality and generate new datasets is proposed. In particular, this step is inserted in a successfully already used methodology to predict earthquakes. Tokyo, one of the cities mostly threatened by large earthquakes occurrence in Japan, is studied. Several well-known classifiers combined with PCA have been used. Noticeable improvement in the results is reported.

earthquake japan

Octubre de 2017
Rupture-Depth-Varying Seismicity Patterns for Major and Great (Mw ≥ 7.0) Megathrust Earthquakes
Authors: Nadav Wetzler, Thorne Lay et al
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Large earthquakes on subduction zone plate boundary megathrusts result from intervals of strain accumulation and release. The mechanism diversity and spatial distribution of moderate-size aftershocks is influenced by the mainshock rupture depth extent. Mainshocks that rupture across the shallow megathrust to near the trench have greater intraplate aftershock faulting diversity

than events with rupture confined to deeper portions of the megathrust. Diversity of intraplate aftershock faulting also increases as the size of the mainshock approaches the largest size event to have ruptured that region of the megathrust. Based on these tendencies, we identify “breakthrough” ruptures as those involving shallow rupture of the megathrust with volumetrically extensive elastic strain drop around the plate boundary that allows activation of diverse intraplate faulting influenced by long-term ambient deformation stresses. In contrast, homogeneity of the aftershock faulting mechanisms indicates only partial release of elastic strain energy and remaining potential for another large rupture.

Octubre de 2017
Constraining the date of the Martian dynamo shutdown by means of crater magnetization signatures
Authors: Foteini Vervelidou, Vincent Lesur et al
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Mars is believed to have possessed a dynamo that ceased operating approximately 4 Ga ago, although the exact time is still under debate. The scope of this study is to constrain the possible timing of its cessation by studying the magnetization signatures of craters. The study uses the latest available model of the lithospheric magnetic field of Mars, which is based on Mars Global Surveyor data. We tackle the problem of non-uniqueness that characterises the inversion of magnetic field data for the magnetization by inferring only the visible part of the magnetization, i.e., the part of the magnetization that gives rise to

the observed magnetic field. Further on, we demonstrate that a zero visible magnetization is a valid proxy for the entire magnetization being zero under the assumption of a magnetization distribution of induced geometry. This assumption holds for craters whose thermoremanent magnetization has not been significantly altered since its acquisition. Our results show that the dynamo shut off after the impacts that created the Acidalia and SE Elysium basins and before the crust within the Utopia basin cooled below its magnetic blocking temperature. Accounting for the age uncertainties in the dating of these craters, we estimate that the dynamo shut off at an N(300) crater retention age of 2.5-3.2 or an absolute model age of 4.12 - 4.14 Ga. Moreover, the Martian dynamo may have been weaker in its early stage, which if true implies that the driving mechanism of the Martian dynamo was not the same throughout its history.

Octubre de 2017
Presentation and Analysis of a Worldwide Database of Earthquake-Induced Landslide Inventories
Authors: Hakan Tanyas, Cees J. van Weste et al
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Earthquake-induced landslide (EQIL) inventories are essential tools to extend our knowledge of the relationship between earthquakes and the landslides they can trigger. Regrettably, such inventories are difficult to generate and therefore scarce, and the available ones differ in terms of their quality and level of completeness. Moreover, access to existing EQIL inventories is currently difficult because there is no centralized database. To address these issues, we compiled EQIL inventories from around the globe based on an extensive literature study. The database contains information on 363 landslide-triggering earthquakes and includes 66 digital landslide inventories. To make these data openly available, we created a repository to host the digital

inventories that we have permission to redistribute through the U.S. Geological Survey ScienceBase platform. It can grow over time as more authors contribute their inventories. We analyze the distribution of EQIL events by time period and location, more specifically breaking down the distribution by continent, country and mountain region. Additionally, we analyze frequency distributions of EQIL characteristics, such as the approximate area affected by landslides, total number of landslides, maximum distance from fault rupture zone, and distance from epicenter when the fault plane location is unknown. For the available digital EQIL inventories, we examine the underlying characteristics of landslide size, topographic slope, roughness, local relief, distance to streams, peak ground acceleration, peak ground velocity, and Modified Mercalli Intensity. Also, we present an evaluation system to help users assess the suitability of the available inventories for different types of EQIL studies and model development.

Septiembre de 2017
The hidden simplicity of subduction megathrust earthquakes
Authors: M.-A. Meier, J. P. Ampuero et al
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The largest observed earthquakes occur on subduction interfaces and frequently cause widespread damage and loss of life. Understanding the rupture behavior of megathrust events is crucial for earthquake rupture physics, as well as for earthquake early-warning systems. However, the large variability in behavior between

individual events seemingly defies a description with a simple unifying model. Here we use three source time function (STF) data sets for subduction zone earthquakes, with moment magnitude Mw = 7, and show that such large ruptures share a typical universal behavior. The median STF is scalable between events with different sizes, grows linearly, and is nearly triangular. The deviations from the median behavior are multiplicative and Gaussian—that is, they are proportionally larger for larger events. Our observations suggest that earthquake magnitudes cannot be predicted from the characteristics of rupture onsets.

Septiembre de 2017
Uplift, rupture, and rollback of the Farallon slab reflected in volcanic perturbations along the Yellowstone adakite hot spot track
Authors:Bertrand Rouet-Leduc, Claudia Hulbert et al
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We apply machine learning to data sets from shear laboratory experiments, with the goal of identifying hidden signals that precede earthquakes. Here we show that by listening to the acoustic signal emitted by a laboratory fault, machine learning can predict the time remaining

before it fails with great accuracy. These predictions are based solely on the instantaneous physical characteristics of the acoustical signal and do not make use of its history. Surprisingly, machine learning identifies a signal emitted from the fault zone previously thought to be low-amplitude noise that enables failure forecasting throughout the laboratory quake cycle. We infer that this signal originates from continuous grain motions of the fault gouge as the fault blocks displace. We posit that applying this approach to continuous seismic data may lead to significant advances in identifying currently unknown signals, in providing new insights into fault physics, and in placing bounds on fault failure times.

Septiembre de 2017
Post-injection normal closure of fractures as a mechanism for induced seismicity
Authors: E. Ucar, I. Berre et al
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Understanding the controlling mechanisms underlying injection-induced seismicity is important for optimizing reservoir productivity and addressing seismicity-related concerns related to hydraulic stimulation in Enhanced Geothermal Systems. Hydraulic stimulation enhances permeability through elevated pressures, which cause normal deformations and the shear slip of pre-existing fractures. Previous experiments

indicate that fracture deformation in the normal direction reverses as the pressure decreases, e.g., at the end of stimulation. We hypothesize that this normal closure of fractures enhances pressure propagation away from the injection region and significantly increases the potential for post-injection seismicity. To test this hypothesis, hydraulic stimulation is modeled by numerically coupling flow in the fractures and matrix, fracture deformation and matrix deformation for a synthetic reservoir in which the flow and mechanics are strongly affected by a complex three-dimensional fracture network. The role of the normal closure of fractures is verified by comparing simulations conducted with and without the normal closure effect.

Septiembre de 2017
Ionospheric anomalies immediately before Mw7.0–8.0 earthquakes
Authors: Liming He, Kosuke Heki et al
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Recent observations suggested that ionospheric anomalies appear immediately before large earthquakes with moment magnitudes (Mw) of 8.2 or more. Do similar phenomena precede smaller earthquakes? Here we answer this question by analyzing vertical total electron contents (VTEC) observed near the epicenters before and after 32 earthquakes with Mw7.0–8.0 using data from nearby Global Navigation Satellite Systems stations. To detect anomalies, we

defined the reference curves to fit the observed VTEC and considered the departure from the curves as anomalies. In estimating the reference curves, we excluded time windows, prescribed for individual earthquakes considering Mw, possibly affected by earthquakes. We validated the method using synthetic VTEC data assuming both preseismic, coseismic, and postseismic anomalies. Out of the 32 Mw7.0–8.0 earthquakes, eight earthquakes showed possible preseismic anomalies starting 10–20 min before earthquakes. For earthquakes of this Mw range, we can observe preseismic ionospheric changes probably when the background VTEC is large, say 50 TECU (total electron content unit, 1 TECU = 1016 el m-2) or more.

Septiembre de 2017
Geophysics From Terrestrial Time-Variable Gravity Measurements
Authors:Michel Van Camp, Olivier de Viron et al
Link: Click here

In a context of global change and increasing anthropic pressure on the environment, monitoring the Earth system and its evolution has become one of the key missions of geosciences. Geodesy is the geoscience that measures the geometric shape of the Earth, its orientation in space, and gravity field.Time-variable gravity, because of its high accuracy, can be used to build an enhanced picture and understanding of the changing Earth. Ground-based gravimetry can

determine the change in gravity related to the Earth rotation fluctuation, to celestial-body and Earth attractions, to the mass in the direct vicinity of the instruments, and vertical displacement of the instrument itself on the ground.
In this paper, we review the geophysical questions that can be addressed byground gravimeters used to monitor time-variable gravity. This is done in relation to the instrumental characteristics, noise sources and good practices. We also discuss the next challenges to be met by ground gravimetry,the place that terrestrial gravimetry should hold in the Earth observation system, and perspectives and recommendations about the future of ground gravity instrumentation.

Septiembre de 2017
Uplift, rupture, and rollback of the Farallon slab reflected in volcanic perturbations along the Yellowstone adakite hot spot track
Authors: Victor E. Camp, Martin E. Ross et al
Link: Click here

Field, geochemical, and geochronological data show that the southern segment of the ancestral Cascades arc advanced into the Oregon back-arc region from 30 to 20 Ma. We attribute this event to thermal uplift of the Farallon slab by the

Yellowstone mantle plume, with heat diffusion, decompression, and the release of volatiles promoting high-K calc-alkaline volcanism throughout the back-arc region. The greatest degree of heating is expressed at the surface by a broad ENE-trending zone of adakites and related rocks generated by melting of oceanic crust from the Farallon slab. A hiatus in eruptive activity began at ca. 22–20 Ma but ended abruptly at 16.7 Ma with renewed volcanism from slab rupture occurring in two separate regions. The eastern rupture resulted in the extrusion of Steens Basalt

Septiembre de 2017
Seismoelectric responses to an explosive source in a fluid above a fluid-saturated porous medium
Authors: Yongxin Gao, Mengqiang Wang et al
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We numerically compute seismoelectric wavefields generated at a fluid/porous medium interface by an explosive source in the fluid. Our numerical experiments show that electromagnetic (EM) signals accompanying the P, S, and interface waves can be observed at receivers located in the fluid regions near the interface. Such accompanying EM signals are produced by the inhomogeneous EM waves that are generated by the seismic waves at the interface and their amplitudes decrease with the distance from interface. Under the excitation of an explosive

source whose strength is within the capability of industry air guns, electric and magnetic fields that accompany the Scholte wave are on the order of 1 μV/m and 0.01 nT, respectively. This means that the EM signals arising from the electrokinetic effect at an ocean bottom are detectable and suggest that it is possible to measure the EM signals during marine seismic explorations to study the properties of the seafloor material. EM signals that accompany the P, S, and interface waves are also observed in the porous medium region near the interface. Component analysis shows that they contain contributions from multiple modes of waves, among which the slow compressional wave contributes significantly to the vertical electric field, leading to a much stronger vertical electric field than the horizontal electric field during the passage of a seismic wave along the interface.

Septiembre de 2017
Triggering of volcanic eruptions by large earthquakes
Authors: Finn Løvholt, Stein Bondevik et al
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Landslides are the second most important cause of tsunamis after earthquakes, and their potential for generating large tsunamis depend on the slide process. Among the world's largest submarine landslides is the Storegga Slide that generated a large tsunami over an ocean-wide scale, while no traces of a tsunami generated from the similar and nearby Trænadjupet Slide have been found. Previous models for such landslide tsunamis have not been able to capture

the complexity of the landslide processes and are at odds with geotechnical and geomorphological data that reveal retrogressive landslide development. The tsunami generation from these massive events are here modeled with new methods that incorporate complex retrogressive slide motion. We show that the tsunamigenic strength is closely related to the retrogressive development and explain, for the first time, why similar giant landslides can produce very different tsunamis, sometimes smaller than anticipated. Because these slide mechanisms are common for submarine landslides, modeling procedures for dealing with their associated tsunamis should be revised.

Septiembre de 2017
Investigation of back-projection uncertainties with M6 earthquakes
Authors: Wenyuan Fan and Peter M. Shearer
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We investigate possible biasing effects of inaccurate timing corrections on teleseismic P-wave back-projection imaging of large earthquake ruptures. These errors occur because empirically-estimated time shifts based on aligning P-wave first arrivals are exact only at the hypocenter and provide approximate corrections for other parts of the rupture. Using the Japan subduction zone as a test region, we analyze 46 M6–7 earthquakes over a ten-year period, including many aftershocks of the 2011 M9 Tohoku earthquake, performing waveform cross-correlation of their initial P-wave arrivals to obtain hypocenter timing

corrections to global seismic stations. We then compare back-projection images for each earthquake using its own timing corrections with those obtained using the time corrections from other earthquakes. This provides a measure of how well sub-events can be resolved with back-projection of a large rupture as a function of distance from the hypocenter. Our results show that back-projection is generally very robust and that the median sub-event location error is about 25 km across the entire study region (∼700 km). The back-projection coherence loss and location errors do not noticeably converge to zero even when the event pairs are very close (<20 km). This indicates that most of the timing differences are due to 3D structure close to each of the hypocenter regions, which limits the effectiveness of attempts to refine back-projection images using aftershock calibration, at least in this region.

Septiembre de 2017
Tsunami hazard assessments with consideration of uncertain earthquake slip distribution and location
Authors:Ignacio Sepúlveda, Philip L.-F. Liu et al
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This paper proposes a stochastic approach to model the earthquake uncertainties in terms of the rupture location and the slip distribution for a future event, with an expected earthquake magnitude. Once the statistical properties of earthquake uncertainties are described, they are then propagated into the tsunami response and the inundation at assessed coastal areas. The slip distribution is modeled as a random field within a nonrectangular rupture area. The Karhunen-Lòeve (K-L) expansion method is used to generate samples of the random slip, and a translation model is employed to obtain target probability properties. A strategy is developed to specify the accuracy of the random samples in

terms of numbers of subfaults of the rupture area and the truncation of the K-L expansion. The propagation of uncertainty into the tsunami response is performed by means of a Stochastic Reduced Order Model. To illustrate the methodology, we investigated a study case in north Chile. We first demonstrate that the stochastic approach generates consistent earthquake samples with respect to the target probability properties. We also show that the results obtained from SROM are more accurate than those obtained with classic Monte Carlo simulations. To validate the methodology, we compared the simulated tsunamis and the tsunami records for the 2014 Chilean earthquake. Results show that leading wave measurements fall within the tsunami sample space. At later times, however, there are mismatches between measured data and the simulated results, suggesting that other sources of uncertainties are as relevant as the uncertainty of earthquakes.

Septiembre de 2017

La secuencia sísmica de Valparaíso de 2017 y la fragmentación del megaterremoto de Chile Central
Authors: Jennifer L. Nealy , Matthew W. Herman et al
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En abril de 2017 una secuencia de sismos ocurrieron mar adentro en Valparaíso, Chile, elevando la probabildiad de un megaterremoto en el futuro cercano. El evento más grande en la secuencia de 2017 fue un M6.9, ocurrido el 24 de abril y aparentemente ubicado cerca del último gran terremoto gatillado en la región (un M8.0 en marzo de 1985). La historia de los grandes terremotos en esta región muestra una variación significativa en tamaño de rotura y extensión y se destaca típicamente por una yuxtaposición de grandes rupturas intercaladas con secuencias de menor magnitud. En esta investigación demostramos que la secuencia de 2017 rompió un área entre los dos remiendos principales del terremoto de 1985,
fracturando nuevamente un fragmento que había resbalado previamente durante la secuencia del terremoto M6.5 de octubre de 1973. Al día de hoy y en caso de que persista bloqueado, existe una importante brecha en las rupturas históricas, directamente al sur de la secuencia de 2017, con
sufiente momento para gatillar un terremoto de gran tamaño en el futuro próximo.

2017 Valparaíso earthquake sequence and the megathrust
patchwork of central Chile
Authors: Jennifer L. Nealy , Matthew W. Herman et al
Link: Click here

In April 2017, a sequence of earthquakes offshore Valparaíso, Chile, raised concerns of a
potential megathrust earthquake in the near future. The largest event in the 2017 sequence was a M6.9 on
24 April, seemingly colocated with the last great-sized earthquake in the region—a M8.0 in March 1985. The
history of large earthquakes in this region shows significant variation in rupture size and extent, typically
highlighted by a juxtaposition of large ruptures interspersed with smaller magnitude sequences. We show
that the 2017 sequence ruptured an area between the two main slip patches during the 1985 earthquake,
rerupturing a patch that had previously slipped during the October 1973 M6.5 earthquake sequence. A
significant gap in historic ruptures exists directly to the south of the 2017 sequence, with large enough
moment deficit to host a great-sized earthquake in the near future, if it is locked.

Agosto de 2017
Triggering of volcanic eruptions by large earthquakes
Author: Takeshi Nishimura
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When a large earthquake occurs near an active volcano, there is often concern that volcanic eruptions may be triggered by the earthquake. In this study, recently accumulated, reliable data were analyzed to quantitatively evaluate the probability of the occurrence of new eruptions of volcanoes located near the epicenters of large earthquakes. For volcanoes located within 200 km of large earthquakes of magnitude 7.5 or

greater, the eruption occurrence probability increases by approximately 50% for 5 years after the earthquake origin time. However, no significant increase in the occurrence probability of new eruptions was observed at distant volcanoes or for smaller earthquakes. The present results strongly suggest that new eruptions are likely triggered by static stress changes and/or strong ground motions caused by nearby large earthquakes. This is not similar to the previously presented evidence that volcanic earthquakes at distant volcanoes are remotely triggered by surface waves generated by large earthquakes.

Agosto de 2017
Physics-based forecasting of induced seismicity at Groningen gas field, the Netherlands
Authors: David Dempsey and Jenny Suckale
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Earthquakes induced by natural gas extraction from the Groningen reservoir, the Netherlands, put local communities at risk. Responsible operation of a reservoir whose gas reserves are of strategic importance to the country requires understanding of the link between extraction and earthquakes. We synthesize observations and a model for Groningen seismicity to produce forecasts for felt seismicity (M > 2.5) in the period

February 2017 to 2024. Our model accounts for poroelastic earthquake triggering and rupture on the 325 largest reservoir faults, using an ensemble approach to model unknown heterogeneity and replicate earthquake statistics. We calculate probability distributions for key model parameters using a Bayesian method that incorporates the earthquake observations with a nonhomogeneous Poisson process. Our analysis indicates that the Groningen reservoir was not critically stressed prior to the start of production. Epistemic uncertainty and aleatoric uncertainty are incorporated into forecasts for three different future extraction scenarios. The largest expected eart hquake was similar for all scenarios, with a 5% likelihood of exceeding M 4.0.

Agosto de 2017
The influence of sulfur on the electrical resistivity of hcp iron: Implications for the core conductivity of Mars and Earth
Authors: Sho Suehiro, Kenji Ohta et al.
Link: Click here

Cosmochemical and geochemical studies suggest sulfur (S) as a light alloying element in the iron-rich cores of telluric planets, but there is no report of sulfur's alloying effect on the electrical and thermal transport properties of iron (Fe); a subject that is closely related to the dynamo

action and thermal evolution of planetary cores. We measured the electrical resistivity of hexagonal-closed-packed (hcp) structured Fe alloy containing 3 wt. % silicon (Si) and 3 wt. % S up to 110 GPa at 300 K. Combined with the reported resistivities of hcp Fe and hcp Fe-Si alloy, we determined the impurity resistivity of S in a hcp Fe matrix at high pressures. The obtained impurity resistivity of S is found to be smaller than that of Si. Therefore, S is a weaker influence on the conductivity of Fe alloy, even if S is a major light element in the planetary cores.

Agosto de 2017
The Value of Real-Time GNSS to Earthquake Early Warning
Authors: C. J. Ruhl, D. Melgar et al.
Link: Click here

GNSS-based earthquake early warning (EEW) algorithms estimate fault-finiteness and unsaturated moment magnitude for the largest, most damaging earthquakes. Because large events are infrequent, algorithms are not regularly exercised and insufficiently tested on few available datasets. We use 1300 realistic, time-dependent, synthetic earthquakes on the Cascadia megathrust to rigorously test the

Geodetic Alarm System. Solutions are reliable once six GNSS stations report static offsets, which we require for a “first alert.” Median magnitude and length errors are -0.15±0.24 units and -31±40% for the first alert, and -0.04±0.11 units and +7±31% for the final solution. We perform a coupled test of a seismic-geodetic EEW system using synthetic waveforms for a Mw8.7 scenario. Seismic point-source solutions result in severely underestimated PGA. Geodetic finite-fault solutions provide more accurate predictions at larger distances, thus increasing warning times. Hence, GNSS observations are essential in EEW to accurately characterize large (out-of-network) events and correctly predict ground motion.

Agosto de 2017
A unified continental thickness from seismology and diamonds suggests a melt-defined plate
Authors: Saikiran Tharimena, Catherine Rychert et al.
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Thick, rigid continents move over the weaker underlying mantle, although geophysical and geochemical constraints on the exact thickness and defining mechanism of the continental plates are widely discrepant. Xenoliths suggest a chemical continental lithosphere ~175 kilometers thick, whereas seismic tomography supports a

much thicker root (>250 kilometers) and a gradual lithosphere-asthenosphere transition, consistent with a thermal definition. We modeled SS precursor waveforms from continental interiors and found a 7 to 9% velocity drop at depths of 130 to 190 kilometers. The discontinuity depth is well correlated with the origin depths of diamond-bearing xenoliths and corresponds to the transition from coarse to deformed xenoliths. At this depth, the xenolith-derived geotherm also intersects the carbonate-silicate solidus, suggesting that partial melt defines the plate boundaries beneath the continental interior.

Agosto de 2017
How “good” are real-time ground motion predictions from Earthquake Early Warning systems?
Author: Men-Andrin Meier
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Real-time ground motion alerts, as can be provided by Earthquake Early Warning (EEW) systems, need to be both timely and sufficiently accurate to be useful. Yet how timely and how accurate the alerts of existing EEW algorithms are is often poorly understood. In part, this is because EEW algorithm performance is usually evaluated not in terms of ground motion prediction accuracy and timeliness but in terms of other metrics (e.g., magnitude and location estimation errors), which do not directly reflect the usefulness of the alerts from an end user perspective. Here we attempt to

identify a suite of metrics for EEW algorithm performance evaluation that directly quantify an algorithm's ability to identify target sites that will experience ground motion above a critical (user-defined) ground motion threshold. We process 15,553 recordings from 238 earthquakes with M > 5 (mostly from Japan and southern California) in a pseudo-real-time environment and investigate two end-member EEW methods. We use the metrics to highlight both the potential and limitations of the two algorithms and to show under which circumstances useful alerts can be provided. Such metrics could be used by EEW algorithm developers to convincingly demonstrate the added value of new algorithms or algorithm components. They can complement existing performance metrics that quantify other relevant aspects of EEW algorithms (e.g., false event detection rates) for a comprehensive and meaningful EEW performance analysis.

Agosto de 2017
How variable is the number of triggered aftershocks?
Authors: D. Marsan and A. Helmstetter
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Aftershock activity depends at first order on the main shock magnitude but also shows important fluctuations between shocks of equal magnitude. We here investigate these fluctuations, by quantifying them and by relating them to the main shock stress drop and other variables, for

southern California earthquakes. A method is proposed in order to only count directly triggered aftershocks, rather than secondary aftershocks (i.e., triggered by previous aftershocks), and to only quantify fluctuations going beyond the natural Poisson variability. Testing of the method subjected to various model errors allows to quantify its robustness. It is found that these fluctuations follow a distribution that is well fitted by a lognormal distribution, with a coefficient of variation of about 1.0 to 1.1. A simple model is proposed to relate this observed dependence to main shock stress drop variability.

Agosto de 2017
A seismic shift in continental tectonic plates
Author: Brian Savage
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Continents have tolerated billions of years of tectonic stresses and disfigurement, yet they continue to survive. Compared with their oceanic counterpart, where a sinking demise is an almost certainty, continents and their internal cores, or cratons, are much thicker (>175 km), older (>2 billion years), colder, and more buoyant. However, their basic attributes, such as size and shape, are

a still a matter of debate because of large uncertainties in deceivingly straightforward, but entirely complicated, measurements. Continental cratons are rigid bodies composed of both crust and mantle, and their thickness was thought to be related to temperature and extend to depths of 250 to 350 km. On page 580 of this issue, Tharimena et al. (1) use reflections of seismic waves within the cratons to constrain their thickness globally. The strength of the reflections suggests that the base of the cratonic plate is defined by a partial melt of carbon-laced silicate mantle, not temperature.

Julio de 2017
Efficiency of coseismic ionospheric perturbations in identifying crustal deformation pattern: Case study based on Mw 7.3 May Nepal 2015 earthquake
Authors: Mala S. Bagiya, A. S. Sunil et al.
Link: Click here

The amplitude asymmetry and initial polarity of seismic induced ionospheric perturbations around the epicenter are considered to be important in providing information about the rupture propagation and related vertical surface deformation. To comprehend this, we study ionospheric perturbations related to the 12 May 2015, Mw 7.3 Nepal earthquake. We model the

coseismic slip associated with the event using the interferometric synthetic aperture radar derived surface deformation data. The ionospheric perturbations associated with the obtained surface deformation are explained in terms of rupture propagation, favorable geomagnetic field-wave coupling, and satellite geometry effects. We discuss the effects of phase cancelation on the perturbation evolution for various receiver satellite line-of-sight configurations invoking an elementary version of satellite geometry factor. The present study thus elucidates further the role of nontectonic forcing mechanisms while identifying ground source pattern using the associated ionospheric perturbations.

Julio de 2017
Seismic evidence for partial melting at the root of major hot spot plumes
Authors: Kaiqing Yuan and Barbara Romanowicz l et al.
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Ultralow-velocity zones are localized regions of extreme material properties detected seismologically at the base of Earth's mantle. Their nature and role in mantle dynamics are poorly understood. We used shear waves diffracted at the core-mantle boundary to

illuminate the root of the Iceland plume from different directions. Through waveform modeling, we detected a large ultralow-velocity zone and constrained its shape to be axisymmetric to a very good first order. We thus attribute it to partial melting of a locally thickened, denser- and hotter-than-average layer, reflecting dynamics and elevated temperatures within the plume root. Such structures are few and far apart, and they may be characteristic of the roots of some of the broad mantle plumes tomographically imaged within the large low-shear-velocity provinces in the lower mantle.

Julio de 2017
Searching for patterns in caldera unrest
Authors: Laura Sandri and Valerio Acocella.
Link: Click here

The ultimate goal of volcanology is forecasting eruptions. This task is particularly challenging at calderas, where unrest is frequent, affects wider areas and its evidence is often masked by the activity of hydrothermal systems. A recent study has compiled a database on caldera unrest, derived from seismicity, geodetic, gravity, and geochemical monitoring data at calderas worldwide, from 1988 to 2014. Here we exploit this database, searching for the most recurring features of unrest and, in turn, its possible dynamics. In particular, we focus on (a) the duration of unrest at calderas; (b) recurring patterns in unrest; (c) unrest episodes culminating in eruptions, including time-predictability or size-predictability and a

multivariate regression analysis. Our analysis indicates that preeruptive unrest is shorter than noneruptive unrest, particularly with open or semiplugged calderas, calderas with mafic or mixed composition of past eruptive products, or unrest driven by mafic magma; conversely, lack of data on preeruptive unrest driven by felsic magma and/or at felsic or plugged calderas prevents an analysis of these specific subsets. In addition, 72% of preeruptive unrest lasts <10 months and shows high seismicity and degassing. The remaining 28% (a) is essentially aseismic in calderas with open-conduit (17%), or (b) lasts between 10 and 18 months, with seismicity and degassing, constituting a longer-duration tail of the preeruptive unrest with seismicity and degassing (11%). Surface deformation is not always reliable to characterize preeruptive unrest. Our analysis suggests that magma may withstand only a limited period of “eruptability,” before becoming stored in the upper crust.

Julio de 2017
Source model for the Copahue volcano magma plumbing system constrained by InSAR surface deformation observations
Authors: Paul Lundgren, Mehdi Nikkhoo et al.
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Copahue volcano straddling the edge of the Agrio-Caviahue caldera along the Chile-Argentina border in the southern Andes has been in unrest since inflation began in late 2011. We constrain Copahue's source models with satellite and airborne interferometric synthetic aperture radar (InSAR) deformation observations. InSAR time series from descending track RADARSAT-2 and COSMO-SkyMed data span the entire inflation period from 2011 to 2016, with their initially high rates of 12 and 15 cm/yr, respectively, slowing only slightly despite ongoing small eruptions through 2016. InSAR ascending and descending track time series for the 2013–2016 time period

constrain a two-source compound dislocation model, with a rate of volume increase of 13 × 106 m3/yr. They consist of a shallow, near-vertical, elongated source centered at 2.5 km beneath the summit and a deeper, shallowly plunging source centered at 7 km depth connecting the shallow source to the deeper caldera. The deeper source is located directly beneath the volcano tectonic seismicity with the lower bounds of the seismicity parallel to the plunge of the deep source. InSAR time series also show normal fault offsets on the NE flank Copahue faults. Coulomb stress change calculations for right-lateral strike slip (RLSS), thrust, and normal receiver faults show positive values in the north caldera for both RLSS and normal faults, suggesting that northward trending seismicity and Copahue fault motion within the caldera are caused by the modeled sources. Together, the InSAR-constrained source model and the seismicity suggest a deep conduit or transfer zone where magma moves from the central caldera to Copahue's upper edifice.

Julio de 2017
Reawakening of large earthquakes in south central Chile: The 2016 Mw 7.6 Chiloé event
Authors: S. Ruiz, M. Moreno et al.
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On 25 December 2016, the Mw 7.6 Chiloé earthquake broke a plate boundary asperity in south central Chile near the center of the rupture zone of the Mw 9.5 Valdivia earthquake of 1960. To gain insight on decadal-scale deformation trends and their relation with the Chiloé earthquake, we combine geodetic, teleseismic, and regional seismological data. GPS velocities increased at continental scale after the 2010

Maule earthquake, probably due to a readjustment in the mantle flow and an apparently abrupt end of the viscoelastic mantle relaxation following the 1960 Valdivia earthquake. It also produced an increase in the degree of plate locking. The Chiloé earthquake occurred within the region of increased locking, breaking a circular patch of ~15 km radius at ~30 km depth, located near the bottom of the seismogenic zone. We propose that the Chiloé earthquake is a first sign of the seismic reawakening of the Valdivia segment, in response to the interaction between postseismic viscoelastic relaxation and changes of interseismic locking between Nazca and South America.

Julio de 2017
Efficiency of coseismic ionospheric perturbations in identifying crustal deformation pattern: Case study based on Mw 7.3 May Nepal 2015 earthquake
Authors: Masashi Kawamura and Kate Huihsuan Chen
Link: Click here

To explore where earthquakes tend to recur, we statistically investigated repeating earthquake catalogs and background seismicity from different regions (Parkfield, Hayward, Calaveras, and Chihshang Faults). We show that the location of repeating earthquakes can be mapped using the spatial distribution of the seismic a and b values

obtained from the background seismicity. Molchan's error diagram statistically confirmed that repeating earthquakes occur within areas with high a values (2.8–3.8) and high b values (0.9–1.1) on both strike-slip and thrust fault segments. However, no significant association held true for fault segments with more complicated geometry or for wider areas with a complex fault network. The productivity of small earthquakes responsible for high a and b values may thus be the most important factor controlling the location of repeating earthquakes. We inferred that the location of high creep rate in planar/listric fault structures might be indicated by a values of ~3 and b values of ~1.

Julio de 2017
Ionospheric anomalies immediately before Mw 7.0-8.0 earthquakes
Authors: Liming He and Kosuke Heki
Link: Click here

Recent observations suggested that ionospheric anomalies appear immediately before large earthquakes with moment magnitudes (Mw) of 8.2 or more. Do similar phenomena precede smaller earthquakes? Here we answer this question by analyzing vertical total electron contents (VTEC) observed near the epicenters before and after 32 earthquakes with Mw7.0-8.0 using data from nearby Global Navigation Satellite

System (GNSS) stations. To detect anomalies, we defined the reference curves to fit the observed VTEC, and considered the departure from the curves as anomalies. In estimating the reference curves, we excluded time windows, prescribed for individual earthquakes considering Mw, possibly affected by earthquakes. We validated the method using synthetic VTEC data assuming both pre-, co- and postseismic anomalies. Out of the 32 Mw7.0-8.0 earthquakes, 8 earthquakes showed possible preseismic anomalies starting 10-20 minutes before earthquakes. For earthquakes of this Mw range, we can observe preseismic ionospheric changes probably when the background VTEC is large, say 50 TECU or more.

Julio de 2017
Rapid transport of ash and sulfate from the 2011 Puyehue-Cordón Caulle (Chile) eruption to West Antarctica
Authors: Bess G. Koffman, Eleanor G. Dowd et al.
Link: Click here

The Volcanic Explosivity Index (VEI) 5 eruption of the Puyehue-Cordón Caulle volcanic complex (PCC) in central Chile, which began 4 June 2011, provides a rare opportunity to assess the rapid transport and deposition of sulfate and ash from a mid-latitude volcano to the Antarctic ice sheet. We present sulfate, microparticle concentrations of fine-grained (~5 μm diameter) tephra, and major oxide geochemistry, which document the depositional sequence of volcanic products from the PCC eruption in West Antarctic snow and shallow firn. From the depositional phasing and duration of ash and sulfate peaks, we infer that transport occurred primarily through the troposphere but that ash and sulfate transport

were decoupled. We use Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) back-trajectory modeling to assess atmospheric circulation conditions in the weeks following the eruption, and find that conditions favored southward air parcel transport during 6-14 June and 4-18 July, 2011. We suggest that two discrete pulses of cryptotephra deposition relate to these intervals, and as such, constrain the sulfate transport and deposition lifespan to the ~2-3 weeks following the eruption. Finally, we compare PCC depositional patterns to those of prominent low- and high-latitude eruptions in order to improve multiparameter-based efforts to identify “unknown source” eruptions in the ice core record. Our observations suggest that mid-latitude eruptions such as PCC can be distinguished from explosive tropical eruptions by differences in ash/sulfate phasing and in the duration of sulfate deposition, and from high-latitude eruptions by differences in particle size distribution and in cryptotephra geochemical composition.

Junio de 2017
Integrated geophysical characteristics of the 2015 Illapel, Chile, earthquake
Authors: Matthew W. Herman, Jennifer L. Nealy et al.
Link: Click here

On 16 September 2015, a Mw 8.3 earthquake ruptured the subduction zone offshore of Illapel, Chile, generating an aftershock sequence with 14 Mw 6.0–7.0 events. A double source W phase moment tensor inversion consists of a Mw 7.2 subevent and the main Mw 8.2 phase. We determine two slip models for the mainshock, one using teleseismic broadband waveforms and the other using static GPS and InSAR surface displacements, which indicate high slip north of the epicenter and west-northwest of the epicenter near the oceanic trench. These models and slip distributions published in other studies suggest spatial slip uncertainties of ~25 km and have

peak slip values that vary by a factor of 2. We relocate aftershock hypocenters using a Bayesian multiple-event relocation algorithm, revealing a cluster of aftershocks under the Chilean coast associated with deep (20–45 km depth) mainshock slip. Less vigorous aftershock activity also occurred near the trench and along strike of the main aftershock region. Most aftershocks are thrust-faulting events, except for normal-faulting events near the trench. Coulomb failure stress change amplitudes and signs are uncertain for aftershocks collocated with deeper mainshock slip; other aftershocks are more clearly associated with loading from the mainshock. These observations reveal a frictionally heterogeneous interface that ruptured in patches at seismogenic depths (associated with many aftershocks) and with homogeneous slip (and few aftershocks) up to the trench. This event likely triggered seismicity separate from the main slip region, including along-strike events on the megathrust and intraplate extensional events.

Junio de 2017
Influences on the location of repeating earthquakes determined from a and b value imaging
Authors: Masashi Kawamura and Kate Huihsuan Chen et al.
Link: Click here

To explore where earthquakes tend to recur, we statistically investigated repeating earthquake catalogs and background seismicity from different regions (Parkfield, Hayward, Calaveras, and Chihshang Faults). We show that the location of repeating earthquakes can be mapped using the spatial distribution of the seismic a and b values

obtained from the background seismicity. Molchan's error diagram statistically confirmed that repeating earthquakes occur within areas with high a values (2.8–3.8) and high b values (0.9–1.1) on both strike-slip and thrust fault segments. However, no significant association held true for fault segments with more complicated geometry or for wider areas with a complex fault network. The productivity of small earthquakes responsible for high a and b values may thus be the most important factor controlling the location of repeating earthquakes. We inferred that the location of high creep rate in planar/listric fault structures might be indicated by a values of ~3 and b values of ~1.

Junio de 2017
A colossal impact enriched Mars' mantle with noble metals
Authors: R. Brasser and S. J. Mojzsis
Link: Click here

Once the terrestrial planets had mostly completed their assembly, bombardment continued by planetesimals left over from accretion. Highly siderophile element (HSE) abundances in Mars' mantle imply that its late accretion supplement was 0.8 wt %; Earth and the Moon obtained an additional 0.7 wt % and 0.02 wt %, respectively.

The disproportionately high Earth/Moon accretion ratio is explicable by stochastic addition of a few remaining Ceres-sized bodies that preferentially targeted Earth. Here we show that Mars' late accretion budget also requires a colossal impact, a plausible visible remnant of which is the emispheric dichotomy. The addition of sufficient HSEs to the Martian mantle entails an impactor of at least 1200 km in diameter to have struck Mars before ~4430 Ma, by which time crust formation was well underway. Thus, the dichotomy could be one of the oldest geophysical features of the Martian crust. Ejected debris could be the source material for its satellites.

 

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