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Bidirectional muographic measurements were conducted at the Imashirozuka burial mound, Japan. The mound was built in the beginning of the 6th century as a megalithic tomb and was later collapsed after a landslide caused by the 1596 Fushimi Earthquake, one of the largest earthquakes that have occurred in Japan over last few centuries. The measurements were conducted in order to find evidence of this past disaster recorded in this historical heritage sites. As a result, the vertical low-density regions were found at the top of the mound. These regions were interpreted as large-scale vertical cracks that caused the translational collapse process behind the rotational landslide that was already found in the prior trench-survey-based works. These results indicate that there was an intrinsic problem with the stability of the basic foundation of the Imashirozuka mound before the 1596 Fushimi Earthquake. [ABSTRACT FROM AUTHOR]

Natural fault networks are geometrically complex systems that evolve through time. The evolution of faults and their off-fault damage pattern are influenced by both dynamic earthquake ruptures and aseismic deformation in the interseismic period. To better understand each of their contributions to faulting we simulate both earthquake rupture dynamics and long-term deformation in a visco-elasto-plastic crust subjected to rate-and-state-dependent friction. The continuum mechanics-based numerical model presented here includes three new features. First, a 2.5-D approximation to incorporate effects of a viscoelastic lower crustal substrate below a finite depth. Second, we introduce a dynamically adaptive (slip-velocity-dependent) measure of fault width to ensure grid size convergence of fault angles for evolving faults. Third, fault localization is facilitated by plastic strain weakening of bulk rate-and-state friction parameters as inspired by laboratory experiments. This allows us to for the first time simulate sequences of episodic fault growth due to earthquakes and aseismic creep. Localized fault growth is simulated for four bulk rheologies ranging from persistent velocity-weakening to velocity-strengthening. Interestingly, in each of these bulk rheologies, faults predominantly localize and grow due to aseismic deformation. Yet, cyclic fault growth at more realistic growth rates is obtained for a bulk rheology that transitions from velocity-strengthening friction to velocity-weakening friction. Fault growth occurs under Riedel and conjugate angles and transitions towards wing cracks. Off-fault deformation, both distributed and localized, is typically formed during dynamic earthquake ruptures. Simulated off-fault deformation structures range from fan-shaped distributed deformation to localized Riedel splay faults. We observe that the fault-normal width of the outer damage zone saturates with increasing fault length due to the finite depth of the seismogenic zone. We also observe that dynamically and statically evolving stress fields from neighboring fault strands affect primary and secondary fault growth and thus that normal stress variations affect earthquake sequences. Finally, we find that the amount of off-fault deformation distinctly depends on the degree of optimality of a fault with respect to the prevailing but dynamically changing stress field. Typically, we simulate off-fault deformation on faults parallel to the loading direction. This produces a 6.5-fold higher off-fault energy dissipation than on an optimally oriented fault, which in turn has a 1.5-fold larger stress drop. The misalignment of the fault with respect to the static stress field thus facilitates off-fault deformation. These results imply that fault geometries bend, individual fault strands interact and that optimal orientations and off-fault deformation vary through space and time. With our work we establish the basis for simulations and analyses of complex evolving fault networks subject to both long-term and short-term dynamics. [ABSTRACT FROM AUTHOR]

A very high magnitude earthquake (9.1 MW) triggered a devastating Tsunami in the Indian Ocean on 26th December 2004. The epicentre was located at 3.3° N, 95.8° E with a focal depth of ~30 km. The impacts of Tsunami were felt as far away in Somalia, Tanzania and Kenya along the east coast of Africa. Considering the role of earthquake, in the present study the spatio-temporal analysis of long term (1901 to 2019) earthquake events was performed, which recorded by USGS to understand the genesis of Tsunami (2004) in the Indian Ocean. The study exhibited that the maximum frequency of earthquake was observed between the ranges of 4 MW to 6 MW on the Richter scale during 2001–2010. There was only one earthquake event > 8 MW on the Richter scale (26th December 2004 having depth 30 km) in the Indian Ocean recorded during 1901–2019. The study exhibited that the maximum earthquake was observed between 30–40 km below the surface, and primarily of moderate to low magnitudes. The proximity analysis along the major fault line indicates that the maximum earthquakes were in the buffer of 200 km from fault line in Bay of Bengal. The decadal variation of earthquake exhibits that the maximum number of earthquake events (8427 events) were triggered during the year 2001–2010, whereas during the year 2004, the total 902 earthquake events > 4 MW was recorded. The study indicates that the earthquakes > 7 MW (on Richter scale) and depth below 30 km (shallow earthquake) are primarily responsible to major Tsunami events in the Indian Ocean. The very high magnitude (> 9 MW on the Richter scale) and shallow depth (~30 km) are the major cause of 2004 Tsunami and its high level of damage. There were very low frequency (10–15 events) of earthquake occurred having magnitude > 7 and depth < 30 km. [ABSTRACT FROM AUTHOR]

A magnitude 7.5 earthquake occurred on 28 September 2018 at 10:02:43 UTC near the city of Palu on the Indonesian island of Sulawesi. It was a shallow, strike-slip earthquake with a rupture extending to a length of about 150 km and reaching the surface. Moreover, this earthquake was identified as one of very few events having a super-shear rupture speed. Clear and long-lasting infrasound signatures related to this event were observed by four infrasound arrays of the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization as well as by one national infrasound station in Singapore. Although these infrasound stations SING (Singapore), I39PW (Palau), I07AU (Australia), I40PG (Papua New Guinea) and I30JP (Japan) are located at large distances of between 1800 and 4500 km from the earthquake's epicentral region, the observed infrasound signals associated with this event were intense, including both seismic and acoustic arrivals. A detailed study of the event-related infrasound observations and the potential infrasound generation mechanisms is presented, covering range-dependent infrasound transmission loss and propagation modeling and characterization of the atmospheric background conditions, as well as identification of the regions of seismoacoustic activity by applying a back-projection method from the infrasound receivers to potential source regions. This back projection of infrasonic arrivals allows one to estimate that the main infrasound source region for the Sulawesi earthquake is related to the extended rupture zone and the nearby topography. This estimation and a comparison to other super-shear as well as large regional earthquakes identify no clear connection between the earthquake's super-shear nature and the strong infrasound emission. [ABSTRACT FROM AUTHOR]

A magnitude 7.5 earthquake occurred on September 28th 2018 at 10:02:43 UTC near the city of Palu on the Indonesian island of Sulawesi. It was a shallow, strike-slip earthquake with fractures up to the surface and a rupture length of about 150 km. Moreover, this earthquake was identified as one of very few events having a super shear rupture speed. Clear and long-lasting infrasound signatures related to this event were observed by four infrasound arrays of the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization as well as one national infrasound station in Singapore. Although these infrasound stations SING (Singapore), I39PW (Palau), I07AU (Australia), I40PG (Papua New Guinea) and I30JP (Japan) are located in large distances between 1800 km and 4500 km from the earthquake's epicentral region, the observed infrasound signals associated to this event were intense, including both seismic and acoustic arrivals. The seismic-to-acoustic coupling at nearby terrain features is supposed to generate distinct infrasonic signatures clearly recordable at remote infrasound arrays. A detailed study of the event-related observations and the potential infrasound generation mechanisms is presented covering range- and time-dependent infrasound attenuation and propagation modeling, characterization of the atmospheric background conditions as well as identification of the regions of seismoacoustic activity by applying a backtracking method from the infrasound receivers to potential source regions. The back-projection of infrasonic arrivals allows to estimate that the main infrasound source region for the Sulawesi earthquake is related to the extended rupture zone and the nearby topography. This estimation and the comparison to other super shear as well as large regional earthquakes identifies no clear connection between the earthquake's super shear nature and the strong infrasound emission. [ABSTRACT FROM AUTHOR]

The analysis of earthquake time series in a new time domain termed natural time enables the uncovering of hidden properties in time series of complex systems and has been recently employed as the basis of a method to estimate seismic risk. Natural time also enables the determination of the order parameter of seismicity, which is a quantity by means of which one can identify when the system approaches the critical point (the mainshock occurrence is considered the new phase). Applying this analysis, as an example, to the Japanese seismic data from 1 January 1984 until the super-giant M 9 Tōhoku earthquake on 11 March 2011, we find that almost 3 months before its occurrence the entropy change of seismicity under time reversal is minimized on 22 December 2010, which signals an impending major earthquake. On this date the order parameter fluctuations of seismicity exhibit an abrupt increase. This increase is accompanied by various phenomena; e.g., from this date the horizontal GPS azimuths start to become gradually oriented toward the southern direction, while they had random orientation during the preceding period. Two weeks later, a minimum of the order parameter fluctuations of seismicity appears accompanied by anomalous Earth magnetic field variations and by full alignment of the orientations of GPS azimuths southwards leading to the most intense crust uplift. These phenomena are discussed and found to be in accordance with a physical model which seems to explain on a unified basis anomalous precursory changes observed either in ground-based measurements or in satellite data. [ABSTRACT FROM AUTHOR]

The Great East Japan Earthquake (magnitude 9.0; occurred on 11th March 2011) and subsequent huge tsunami caused widespread damage along the Pacific Ocean coast of eastern Honshu, Japan. This research utilizes multi-resolution remote sensing images to clarify the impact on landscapes caused by this disaster, and also to monitor the subsequent survival and recovery process in the Sendai Bay region. The coastal landscape in the target area features a narrow strip of coastal sand barrier, historically stabilized by planted pine groves; backed by a low-lying plain that has traditionally been diked and converted to irrigated rice paddies. Farmsteads on the flat alluvial plain are surrounded by groves called "Igune", consisting primarily of conifers. MODIS data (250 m resolution) were employed to map the overall extent of inundation and damage on the regional landscape scale. The major damage caused by the tsunami, destruction of coastal pine forests and inundation or rice paddies on the plain, was identified at this level. Progressively finer scale analysis were then implemented using SPOT/HRG-2 (10 m resolution) data; GeoEye-1 fine resolution data (0.5 m) and very fine resolution aerial photographs (10 cm) and LiDAR. These results demonstrated the minute details of the damage and recovery process. Some patches of pine forest, for example, were seen to have survived, and some coastal plant communities were already recovering only a year after the disaster. Continuous monitoring using field work and remote sensing is required for balanced regional strategies that provide for economic and social recovery and as well as restoration of vegetation, biodiversity and vital ecosystem services. [ABSTRACT FROM AUTHOR]

The modelling or prediction of complex geospatial phenomena (like formation of geo-hazards) is one of the most important tasks for geoscientists. But in practice it faces various difficulties, caused mainly by the complexity of relationships between the phenomena itself and the controlling parameters, as well by limitations of our knowledge about the nature of physical/ mathematical relationships and by restrictions regarding accuracy and availability of data. In this situation methods of artificial intelligence, like artificial neural networks (ANN) offer a meaningful alternative modelling approach compared to the exact mathematical modelling. In the past, the application of ANN technologies in geosciences was primarily limited due to difficulties to integrate it into geo-data processing algorithms. In consideration of this background, the software advangeo® was developed to provide a normal GIS user with a powerful tool to use ANNs for prediction mapping and data preparation within his standard ESRI ArcGIS environment. In many case studies, such as land use planning, geo-hazards analysis and prevention, mineral potential mapping, agriculture & forestry advangeo® has shown its capabilities and strengths. The approach is able to add considerable value to existing data. [ABSTRACT FROM AUTHOR]

Due to the 11 March 2011 Tohoku earthquake, massive radioactive materials were released from the Fukushima Daiichi Nuclear Power Plant (Fukushima NPP). It is crucial to predict the regional distribution and magnitude of the effects on wildlife by radioactive materials. However, during the post-accident early stage in 2011, limited information on large-scale pollution and prediction maps was open to public. Hence, this paper aimed to provide (1) the pollution map covering areas within 300 km from the Fukushima NPP where the radiation intensity exceeded 0.5 µSv/h, (2) pollution maps which predicted air dose for the next 30 years after the accident, and (3) maps of areas where wildlife might be affected by radioactive isotopes by the Fukushima nuclear accident. First, the relative contributions of 131I, 134Cs, and 137Cs were estimated from time series observation data. Second, a 30-year prediction of the pollution was calculated based on the isotope half-lives. Third, the chronic radiation effects on vertebrates were estimated using the threshold dose rate proposed by Sazykina et al. (2009). We examined the chronic radiation effects on morbidity, reproduction, and longevity. The results indicated that radioactive materials could have affected vertebrate morbidity within a 350 km2 area in early April 2011; the threshold level was the median result of Sazykina et al. (2009) with bootstrapping. Based on the prediction, a 15.5 km² region will remain affected after 30 years. These areas should be monitored to confirm the effects of radioactivity on wildlife. [ABSTRACT FROM AUTHOR]

Eruptive rates in volcanic arcs increase significantly after subduction mega-thrust earthquakes. Over short to intermediate time periods the link between mega-thrust earthquakes and arc response can be attributed to dynamic triggering processes or static stress changes, but a fundamental mechanism that controls long-term pulses of volcanic activity after mega-thrust earthquakes has not been proposed yet. Using geomechanical, geological, and geophysical arguments, we propose that increased eruption rates over longer timescales are due to the relaxation of the compressional regime that accompanies mega-thrust subduction zone earthquakes. More specifically, the reduction of the horizontal stress σh promotes the occurrence of short-lived strike-slip kinematics rather than reverse faulting in the volcanic arc. The relaxation of the pre-earthquake compressional regime facilitates magma mobilisation by providing a short-circuit pathway to shallow depths by significantly increasing the hydraulic properties of the system. The timescale for the onset of strike-slip faulting depends on the degree of shear stress accumulated in the arc during inter-seismic periods, which in turn is connected to the degree of strain-partitioning at convergent margins. We performed Coulomb stress transfer analysis to determine the order of magnitude of the stress perturbations in present-day volcanic arcs in response to five recent megathrust earthquakes; the 2005 M8.6, 2007 M8.5, and 2007 M7.9 Sumatra earthquakes; the 2010 M8.8 Maule, Chile earthquake; and the 2011 M9.0 Tohoku, Japan earthquake. We find that all but one the shallow earthquakes that occurred in the arcs of Sumatra, Chile and Japan show a marked lateral component. We suggests that the long-term response of volcanic arcs to subduction zone mega-thrust earthquakes will be manifested as predominantly strike-slip seismic events, and that these future earthquakes may be followed closely by indications of rising magma to shallower depths, e.g. surface inflation and seismic swarms. [ABSTRACT FROM AUTHOR]

In recent years many devastating earthquakes with Mw ≥ 7.5 occurred during 2017-2018 asa result of active plate interactions near the major mega-thrust subduction zones. Theyreveal tectonic complexities and astonishing deformation styles associated with theplate movements. In this study, we determined source mechanism parameters andfinite-fault slip distribution models of four destructive earthquakes (Mw ≥ 7.5) byperforming point-source and finite-fault slip inversions. The shapes and amplitudesof long-period and broad-band P- and SH-waveforms recorded by the Federationof Digital Seismograph Networks (FDSN) and the Global Digital SeismographNetwork (GDSN) stations in the distance range of 30˚ - 90˚ are compared withsynthetic waveforms. Finite-fault slip models of earthquakes are estimated by applyinga hybrid back-projection method that uses teleseismic P-waveforms to integratethe direct P-phase with reflected phases from structural discontinuities near thesource. Overall results indicate various faulting mechanisms at relatively shallowfocal depths (h&#x003C;70 km). For example, the February 25, 2018 Papua New Guinea(Mw: 7.5) earthquake indicates a thrust faulting mechanism associated with theconvergence of Australia and Pacific Plates. The September 28, 2018 Sulawesi (Indonesia)earthquake (Mw: 7.5), occurred within the Molucca Sea micro-plate at the eastern part ofIndonesia shows a strike-slip faulting mechanism reflecting the complex deformationsrelated to the interactions between Australia, Sunda, Pacific and Philippine Seaplates. Regardless strike-slip mechanism involved, this large earthquake producedunexpected tsunami waves along the coastal planes of Sulawesi. Finite-fault slipmodels reveal several heterogeneous rupture propagations and slip distributions onfault planes of those earthquakes. In addition, distributions of P- wave first motionpolarities recorded at near-field and regional seismic stations are consistent withthe best-fitting minimum misfit source mechanism solutions of earthquakes. Thisstudy is partially supported by the Turkish Academy of Sciences (TÜBA-GEBIP). [ABSTRACT FROM AUTHOR]

Along the Japan trench, quasi-periodic slow slip was found from repeating earthquakes (repeaters) and on-land GPS data before the 2011 Tohoku-oki earthquake. After the earthquake, the occurrence of Very Low Frequency Earthquakes (VLFEs) was recognized and new GPS-Acoustic (GPS-A) stations were deployed along the shallow plate boundary. In this study, we examine the postseismic activity of the slow slip from repeater, GPS-A and VLFE data. The repeaters are selected form their inter-event waveform similarity and the VLFEs are selected by waveform similarity with template matching at low-frequency range (0.02 – 0.1 Hz). Both data analysis periods were extended from previous studies to until the end of 2016. The GPS-A data from 2012 to 2016 was also re-analyzed to obtain better positions by incorporating the effect of horizontal variations in the sound speed of the ocean. The repeater analysis shows that the plate-boundary slip rate in the area close to the trench off Sanriku (to the north of the coseismic slip area) has large temporal variations with overall decay after the Tohoku-oki earthquake. On the other hand, the slip rate decreased monotonically with time in the area close to the trench off Fukushima (to the south of the coseismic slip area). In the off-Sanriku area, a slip-rate peak in 2015 correlated well with an active period of the VLFEs and a period of trenchward movement of the GPS-A stations. These observations suggest the occurrence of episodic slow slip caused the surface trenchward movement and repeater and VLFE activities. The amount of the inferred fault slip from the repeater activity was ~27 cm for the 2015 event, consistent with the seafloor GPS-A station movements (7~16 cm). The largest 5 earthquakes in the region during the 2015 episode were M5.5 to 6.9, but the total surface deformation expected from these earthquakes is too small (~2 cm) to explain the GPS-A station movements. This suggests the event was mostly aseismic. We also found the distribution of the VLFEs and repeaters are spatially complementary, suggesting that variations in local fault material properties near the plate boundary control the differences in slip mode. [ABSTRACT FROM AUTHOR]

Due to a serious disaster caused by the 2011 Tohoku-oki tsunami, improvement of the tsunami forecast has been an urgent issue in Japan. National Institute of Disaster Prevention installed a cable network system of earthquake and tsunami observation (S-NET) at the ocean bottom along the Japan and Kurile trench. This cable system includes 150 pressure sensors (tsunami meters) which are separated by 30 km. The system is the densest tsunami observation network in the world. Real-time tsunami forecast has depended on estimation of earthquake parameters, such as epicenter, depth, and magnitude of earthquakes. Recently, we developed a tsunami simulation method assimilating ocean bottom pressure data near tsunami areas for tsunami forecast (Tanioka 2018, Tanioka and Gusman 2018). The method does not need any tsunami source information nor earthquake source information. The method uses only initial parts of observed waveforms at ocean bottom pressure sensors. Therefore, the under-estimation of forecast tsunami heights due to the under-estimation of earthquake source or tsunami source should be prevented. However, it was tested only for an equally distributed ocean bottom sensor network (with a 30 km separation). This is not realistic because we do not have such an equally distributed network along the Japan and Kurile trench. In this paper, the 150 ocean bottom pressure sensors were located at exact positions of the S-NET sensors. We developed an interpolation method of observed heights at the S-NET sensors to create data at equally distributed stations with a 10 minute (about 18 km) separation. Then, we applied the tsunami simulation method assimilating those data for the 1968 Tokachi-oki earthquake case. First, the tsunami waveforms at the S-NET stations were numerically computed from a simple rectangle source model of the 1968 Tokachi-oki earthquake (Mw8.2). The fault parameters, strike=156, dip=20, rake=38, were obtained from the previous studies. The fault length of 200 km, the fault width of 100 km, and the slip amount of 4 m, are assumed. By using those computed tsunami waveforms as observation data at the S-NET stations, the tsunami numerical simulation with the assimilation was carried out using the assimilation method in this study. Those computed tsunami wave fields from the method were compared with those computed directly from the earthquake source model. The tsunami heights are slightly overestimated by the assimilation. The wavelength of tsunami from the assimilation method were slightly larger than those directly from the earthquake source model. The result suggests that the tsunami wave fields from the assimilation were overall similar to those from the earthquake source model. However, it also suggests that we may need more stations to improve an accuracy of the tsunami forecast. [ABSTRACT FROM AUTHOR]

Although interseismic locking distributions have been used in qualitatively evaluating the future earthquake potential, quantitatively estimating how an earthquake may rupture through the locked interface is a more useful tool for quantifying both seismic and tsunami hazards. Here, we investigate rupture scenarios from interseismic locking models along the megathrust interface below Nicoya peninsula, Costa Rica using spontaneous rupture simulations. We first estimate initial stress from locking, then initiate spontaneous ruptures at different nucleation points and observe the eventual earthquake magnitudes and slip distribution. We find that ~40% of nucleations tested develop into large earthquakes of Mw>7.2 based on present interseismic locking models. Of these events, those nucleated from deeper depths have a tendency for larger-amplitude shallow slip, suggesting increased tsunami potential. Furthermore, irrespective of the input locking models we do not observe rupture scenarios of earthquakes with intermediate magnitudes between 6 and 7, a result consistent with observations outside of the aftershock period in Nicoya. The results of hypocentre-dependent earthquake magnitudes and tsunamigenic potential not only pose challenges in estimating rupture extents from locking models, but also underscore the significance of quantitatively evaluating seismic and tsunami hazard in subduction zones. [ABSTRACT FROM AUTHOR]

Slow Slip Events (SSEs) are episodic slip events that play a significant role in the moment budget along subduction megathrust. They share many similarities with regular earthquakes, and have been observed in major subduction regions like, for example, Cascadia, Japan, Mexico, New Zealand. They show striking regularity, suggesting that it might be possible to forecast their size and timing, but the prediction of their extension and exact timing is still yet to come. They certainly are a great natural system to study how friction works at scale of the order of hundreds or thousands of km, and their recurrence time being much shorter than that of regular earthquakes, they give us the possibility to study multiple cycles and test their predictability. Here we focus on the Cascadia region, where SSEs recur every about 1 or 2 years, depending on the latitude. We use a catalog containing dozens of SSEs derived from the study of GPS position time series during the time span ranging from 2007 to 2017. The data show a clear segmentation with a few major patches interacting with one another, a behavior that recalls that of a discrete body system. We use both classical embedding theory and Extreme Value Theory applied to the study of dynamical systems to show that, where the Signal to Noise Ratio is sufficiently high, a low-dimensional (< 5) non-linear chaotic system is more appropriate to describe the dynamics than a stochastic system. We calculate major properties of the strange attractor like its correlation and instantaneous dimension, its instantaneous extremal index and a possible range for the metric entropy of the system. This knowledge is important for the determination of the predictability of the system, since it is related to the rate at which two trajectories in the phase space diverge. We further validate our results with a fuzzy inference system model to check the predictability of the slip and slip rate. In conclusion, SSEs in Cascadia can be described as a deterministic, albeit chaotic, system rather than as a random process. As SSEs might be regarded as earthquakes in slow motion, regular earthquakes might be similarly chaotic and predictable. [ABSTRACT FROM AUTHOR]

The earthquake nucleation process is inherently complex, due to an involvement of several deformation mechanisms with multiple spatial and time scales. We here focus on a foreshock sequence which took place prior to a Mw 5.8 shallow crustal earthquake in 2013, at NE Japan. After relocating the foreshocks using travel time differential data constrained by waveform cross-correlation, we searched for events with similar waveforms to those of each relocated template event, applying a matched filter technique. Furthermore, we extracted one pair of repeating earthquakes from the reconstructed catalog (M>1.8), based on waveform similarity and nearly overlapping locations. The cumulative number of events showed an episodic increase toward the mainshock failure time. The initial crisis characterized as a tiny seismic cluster started from three weeks before the mainshock rupture. During about three days prior to the mainshock, a planer seismic cluster with ~0.5 km length had developed. Less than one hour just before the mainshock break, the foreshock activity abruptly became intensive and being concentrated along a eastward-steeply dipping fault plane (~0.5 km length) very close to the mainshock hypocenter. We recognize a distinct sequence of earthquake migration at an average speed of ~10 km/day over the fault plane along fault-dip and fault-strike, just after the largest foreshock (M3.6). Its migration front, slowing down with time, is well approximated by a parabolic curve and moved toward the rupture initiation point, which is analogous to the foreshock migration observed before the 2011 M9.0 Tohoku earthquake despite quite different spatial scales. Combining the foreshock migration with repeating earthquake, we suggest that slow slip transient propagated towards the nucleation point of the mainshock rupture. The partial unlocking of fault might take place episodically through interplay between fast and slow slip modes, prompting the subsequent unstable dynamic rupture. [ABSTRACT FROM AUTHOR]

Seismic hazard assessment rests on the distance from the rupture zone to population centers, and on the magnitude estimation. The latter is controlled by the area ruptured along the fault plane and by the amount of slip accommodated during the earthquake. While progress has been made to estimate the ruptured area, the determination of how much slip occurs during an earthquake remains a major issue in improving hazard assessment. This is particularly true for thrust events, where traditional scaling relationships between seismic moment, area and slip fail, due to lack of understanding of the fault mechanical behavior close to the surface. Thrust faults are commonly known to produce larger slip, damage and ground acceleration as the rupture approaches the free surface. Preliminary work from Gabuchian et al. [2017] supports a novel solution which simply built on the natural asymmetry of the geometry of reverse faults. Combining numerical and experimental simulations, they showed that, when propagating up dip toward the free surface, the earthquake rupture induces large deformation, especially rotation of the hanging wall, the region above the fault.In this work, we take advantage of new numerical algorithms for dynamic modeling of earthquake rupture to describe more carefully the physical mechanisms behind the free surface effect. We use enhanced numerical algorithms for earthquake rupture recently developed by the Los Alamos National Laboratory. The numerical method behind these algorithms is called combined finite-discrete element method (FDEM) and can effectively reproduce earthquake rupture nucleation and propagation. It also captures the rich, and complex, deformation of the surrounding media, as it reproduces the dynamically activated off-fault fracture network. We first use this new numerical tool to better characterize rupture propagation on a dipping fault with dynamically-generated coseismic damage. Through a systematic analysis of case studies, we investigate the effect of the fault geometry, friction parameters and damage properties in the rupture evolution. We then use this numerical model to predict surface displacements and ground motions. These predictions will next serve as synthetic data when comparing with recorded surface deformation from past earthquakes. [ABSTRACT FROM AUTHOR]

On 14 November 2016, the Northern part of New Zealand's South Island was struck by the Mw7.8 Kaikōura earthquake, an event of unprecedented geometric complexity rupturing at least 21 segments of the contractional North Canterbury and of the Marlborough fault systems. Geological, geodetic, tsunami and seismic data reveal puzzling features as well as observational difficulties of the event leading to competing views on its key features, such as the role played by the Hikurangi subduction interface, the exact rupture sequence, and the higher than expected local tsunami. We present a dynamic rupture model of the 2016 Mw 7.8 Kaikōura earthquake to unravel the event's riddles in a physics-based manner and provide insights on the mechanical viability of competing hypotheses proposed to explain them. The event is modeled using the open-source software SeisSol (www.seissol.org), based on a arbitrary high-order accurate DERivative Discontinuous Galerkin method (ADER-DG). We verify our model with strong ground-motions, high rate GPS, long-period teleseismic and tsunami data. Our model reproduces key characteristics of the event and constraints puzzling features inferred from high-quality observations including a large gap separating surface rupture traces, the possibility of significant slip on the subduction interface, the non-rupture of the Hope fault, and slow apparent rupture speed. We show that a rupture cascade sequentially breaking the Papatea and Kekerengu faults is dynamically consistent with regional stress estimates and a crustal fault network geometry inferred from seismic and geodetic data. A coseismically triggered tsunami caused by the rare occurrence of an earthquake starting on-land and crossing to two faults off-shore matches local tide gauge data.We find, that such complex rupture cascade links the crustal fault system when operating at low apparent friction thanks to the combined effects of overpressurized fluids, low dynamic friction and stress concentrations induced by deep fault creep. Understanding the physical conditions that allow rupture cascades on complex fault systems advances our ability to quantify earthquake hazard, especially to evaluate the possibility of extreme events on real fault networks. [ABSTRACT FROM AUTHOR]

Effects of the Earth's rotation on tsunami wave dynamics has repeatedly been discussed in the literature. Usually it was concluded that these effects are minor. Such a conclusion is easily explained. It is known from geophysical hydrodynamics that the Earth's rotation effects are significant for processes for which the spatial scale is comparable with the barotropic Rossby deformation radius Ro (~2000 km) or exceeds it. Tsunami spatial scale Rts (~100 km) is determined by the size of co-seismic deformations area which is normally much inferior to the Rossby radius. There are exceptions to this rule: mega-earthquakes with moment magnitude Mw>9.0 and strong deep-focus earthquakes. In both cases co-seismic deformations cover extensive area which size can be comparable with the Rossby radius. The aim of this study is to reveal effects of the Earth's rotation on waves generated by two deep-focus seismic events: the Okhotsk Sea earthquake on May 24, 2013 (Mw=8.3, h=611 km) and the Fiji earthquake on August 19, 2018 (Mw=8.2, h=580 km). Being completely hazard-less due to a few centimeter amplitude, these unusually long tsunami-like waves are of great interest from the viewpoint of geophysical hydrodynamics. Our method is based on comparison of results of numerical experiments performed with and without the Coriolis force. It was shown that in case of the 2013 Okhotsk Sea event the Earth's rotation has a large effect on the spatial distribution of the maximum amplitudes and wave forms (Rts/Ro=1.4). In particular, taking into account the Coriolis force we obtain nearly twofold increase of wave amplitude in some bays of Okhotsk Sea. The observed differences are mostly caused by Kelvin waves, which exist only in a rotating ocean. For the 2018 Fiji earthquake the Earth's rotation effect, because of the proximity of the equator, turned out to be smaller (Rts/Ro=0.22). However, even in this case the Coriolis force noticeably alters the leading wave amplitude and tsunami wave-forms.AcknowledgementsThis work was supported by the Russian Foundation for Basic Research, projects 16-05-00053, 19-05-00351. [ABSTRACT FROM AUTHOR]

On April 2014 the Iquique Mw 8.1 earthquake ruptured the interpolate contact between the oceanic Nazca and continental South American plates offshore northern Chile between 19.5°S to 21°S in April 2014. This earthquake did not fully release the strain accumulated since the last great megathrust (Mw 8.8) in 1877 and had left an unbroken segment in the south. From December 2014 to November 2016, we deployed an offshore network of 15 Ocean Bottom Seismometers (OBS) that covered the rupture area and the unbroken southern segment using the Chilean Navy ship OPV Toro and R/V Sonne. That data set is supplemented by five weeks of data from 67 OBS installed for a controlled source seismic experiment during cruise MGL1610 of the R/V Marcus Langseth in late 2016. Data acquired onshore by stations from of the IPOC (Integrated Plate Boundary Observatory Chile) and CSN (Chilean Seismological Service) networks are also included.We present first results of this ongoing project, which include double-difference hypocenter relocations based on waveform cross-correlation. Most of the seismicity occurs between 19.5 and 21°S up-dip of the patch of maximum coseismic slip during the 2014 earthquake, while the seismicity in seismogenic depths is highly concentrated forming well-defined clusters. The observed seismicity provides constraints on the structure of the marine forearc and enables us to relate the seismicity distribution to the background seismicity, seafloor morphology, and regional tectonics. [ABSTRACT FROM AUTHOR]

Northeast India is sandwiched between the seismically active plate boundaries of the Eastern Himalaya to the north and the Indo-Burman subduction zone to the east. This plate boundary system is different from the central Nepal Himalaya, due to oblique convergence across two orthogonal plate boundaries, resulting in a zone of distributed deformation both within and away from the plate boundary. Previous studies of spatial distribution and source mechanism of earthquakes have shown that the N20E convergence between India and Tibet is partitioned into N-S underthrusting and E-W subduction of the Indian plate beneath the Himalaya and Burma micro-plate, respectively. However, south-west of the plate boundaries, the middle-to-lower crust deforms by strike-slip faulting in the Kopilli Fault Zone (KFZ); upper-to-mid crustal strike-slip faulting along the western Brahmaputra Valley; and lower crustal strike-slip faulting beneath the Bengal Basin. Strike-slip faulting in the KFZ also extends northwards beneath the Eastern Himalaya and southeastward beneath the Naga fold-thrust belt. In order to understand the role of these strike-slip faults in accommodating the GPS derived convergence across northeast India, we study the source mechanism of recent earthquakes using teleseismic (for earthquakes with Mw>5) and local (4.0

A better characterization of seismic and aseismic slip behaviors of active faults is a crucial challenge for the seismic hazard estimation because it can influence the size and timing of large earthquakes. Geodetic and terrestrial measurements can provide a better picture of the fault slip budged and spatio-temporal evolution of creep mechanism. A number of studies based on Interferometric Synthetic-Aperture Radar (InSAR) and Global Positioning System (GPS) satellite observations until 2012 have found that the central segment of the August 17, 1999 Mw 7.4 Izmit earthquake on the North Anatolian Fault started slipping aseismically following the earthquake. In this study, we used new InSAR time series, obtained from TerraSAR-X and Sentinel 1A/B radar images acquired over the period 2011-2017, with near field GPS measurement campaigns performed every 6 months from 2014 to 2016. The mean velocity fields reveal that creep on the central segment of the 1999 Izmit fault rupture continues to decay, more than 19 years after the earthquake, in overall agreement with models of postseismic afterslip decaying logarithmically with time for a long period of time. Along the fault section that experienced supershear velocity rupture during the Izmit earthquake creep continues with a rate up to ~ 8 mm/yr. A significant transient accelerating creep is detected in December 2016 on the Sentinel-1 time series, near the maximum creep rate location, associated with a total surface slip of 10 mm released in one month only. Additional analyses of the vertical velocity fields show a persistent subsidence on the hanging wall block of the Golcuk normal fault that also ruptured during the Izmit earthquake. Our results demonstrate that afterslip processes along the North Anatolian Fault, east-southeast of Istanbul are more complex than previously proposed as they vary spatio-temporally along the fault. [ABSTRACT FROM AUTHOR]

Studying the mechanical response of the crust to large earthquakes provides unique insight into the processes of deformation in preparation for future earthquakes. Noise-based seismic velocity monitoring can directly probe the mechanical state of the crust at depth continuously in time. In this work, we study the deformation of the crust concerning the Mw 9.0, 2011 Tohoku-oki earthquake. In addition to the Hi-net short period sensors, we employ here for the first time for the noise-based monitoring the particular dense network of Hi-net tiltmeters as long period (8 – 50 s) seismometers to sample the crust below 5 km depth. Spatial distribution of strong velocity decreases at short periods can be limited to strong ground shaking induced by the 2011 Tohoku-oki earthquake, while the long period velocity changes correlate well with modeled static strain induced by viscoelastic relaxation and afterslip at depth. This observation indicates that the variations at depth are not a spurious effect of changes in the shallow layers. The amplitudes of coseismic velocity changes diminish with increasing depth. The temporal evolution of velocity changes at different depth shows that the maximum drops of velocity at greater depth are delayed in time with respect the date of the earthquake. This delay could find its origin in both the visco-elastic response of the crust to substantial strain changes or a complex response of crustal seismic velocities to transient deformation. [ABSTRACT FROM AUTHOR]

We observed a shallow (H<10 km) seismic activity containing both low frequency events (LFE) and regular earthquakes using a dense local seismic observation network installed recently at northeastern Japan. Significant recurring activities of deep (H>20 km) LFE have been monitored carefully around several subduction zones including Japan. Additionally, activities of shallow and deep LFE are also reported worldwide occurring beneath several potentially damaging active volcanoes. Deep (H>15 km) LFEs have also been observed to occur in inland areas not associated with active volcanoes or volcanism.The shallow seismic activity we observed was located on the Tsugaru Channel, between the Hokkaido and Honshu Islands of Japan. We installed a local seismic network consisting of 36 stations, called AS-net, in 2013 and 2014 and are monitoring shallow and deep seismic activities in the region. AS-net improved both the earthquake detection capability and the accuracy of hypocenter determination around this region significantly. The shallow activity of earthquakes and LFEs was observed at just ~2 km south of an AS-net station. The seismic record at the station revealed that the depth of this seismic activity was shallower than 10 km. From 2014 to 2017, 23 earthquakes and 19 or more LFEs were detected. Both earthquakes and LFEs swarmed individually and intermittently. Magnitudes of all events were less than two. Both types of events seemed to occur in the vicinity of one and other however this is not clear because of the limit of the accuracy of the hypocenter location. At ~10 km eastward of this activity, we observed another activity of deep (15~35 km) LFEs in an area without volcanos. Above this curious shallow seismic activity, Yamagata et al. (1989) pointed out the existence of a volcano based on their research about tephra layers over Hokkaido. They estimated that a tephra layer on tens of thousands of years ago could be from Zenigame Caldera, a depression (1~2 km in diameter) at seafloor of Tsugaru Channel. Because it is under the sea, it is not easy to survey, and no other studies about this potential volcano have been reported.In Japan, opposite to deep LFEs, shallow LFEs are only monitored around potentially damaging volcanoes. Outside of such regions, the presence of shallow LFE is not clear. Shallow LFE activity and its mechanism as well as its relation to volcanoes should be clarified. By doing so, we will be able to locate potential volcanic activity by monitoring shallow LFEs; even underwater. Additionally, we have recently started to install several temporal seismic observation stations in the area surrounding this seismic activity to investigate it in detail. AcknowledgementsThe AS-net has been constructed and is being managed thanks to financial contributions by Tohoku Electric Power Co., Inc., Electric Power Development Co., Ltd., Japan Nuclear Fuel Limited and Recyclable-Fuel Storage Company. [ABSTRACT FROM AUTHOR]

Numerical simulations are conducted to examine the effect of heterogeneity of frictional property on earthquake cycles. Rate- and state-dependent friction is assumed on a planar fault in an infinite uniform elastic medium. One to four circular patches of steady-state velocity-weakening frictional property is assumed on the model fault plane, and velocity-strengthening friction is assumed outside the circular patches. Friction parameters are uniform within each patch, and the values of characteristic slip distance L in the patches are varied so that the effect of interaction between the patches with different frictional property may be examined. Simulation results indicate that simple periodic (period-1), multi-periodic (period-n, n > 1), and aperiodic earthquake cycles occur in the models. Here, a period-n cycle indicates that n different slip events are included in a cycle. In the one-patch model, simple periodic earthquake cycles occur for smaller L, where the critical nucleation radius h*, which is proportional to L, is smaller than the radius of the velocity-weakening patch, and multi-periodic and aperiodic sequences of slip events occur for larger L, where the patch radius is close to h* (Kato, 2014, Geophys. J. Int., 198, 727-736). In the two- to four-patch models, a patch is ruptured in some cases and some or all patches are simultaneously ruptured in the other cases. This causes complex earthquake cycles. Slip events of different combinations of patches occur. The possible number of combinations is larger for larger number of patches on a fault. Moreover, aseismic slip events occur within velocity-weakening patches with larger L, which also complicates sequences of slip events. Even in a velocity-weakening patch whose radius is smaller than h*, seismic slip may occur because it is triggered by seismic slip nucleated in another patch. In a parameter study where the characteristics of earthquake cycles are examined with a change in L in a patch, we find that earthquake cycles can be classified into several patterns according to the combination of ruptured patches in earthquakes. Complex aperiodic earthquake cycles tend to occur in a parameter range between parameter ranges of different patterns. Frequencies of multi-periodic and aperiodic earthquake cycles increase with the number of velocity-weakening patches, suggesting that more complex cycles may be generated in a fault with more heterogeneous frictional property. Frequency distribution of recurrence intervals of simulated earthquakes is discrete in the single-patch model and it becomes more continuous and realistic with an increase in the number of patches. These simulation results suggest that realistic complex earthquake cycles are caused by interaction among many patches of different frictional property. [ABSTRACT FROM AUTHOR]

The promotion of earthquake research in Japan covers comprehensive basic policies for thepromotion of seismic research through the observation, measurement and survey ofearthquakes. HERP (Headquarter for Earthquake Research Promotion) proposed a frameworkof survey and observation plan of earthquakes in Japan. At the first stage of this framework, itconsists of observation of crustal activities and surveying of underground structures. Nextstep is to improve the methods of predicting strong ground motion and increase accuracy,and thus the improvement of seismic hazard maps. In order to fully capture theseismic hazard of a region, we must understand the effects of the regional 2D/3Dunderground structure on wave propagation. Selection and modification of groundmotion time series to represent a specific hazard at a site has a large impact on theresults of nonlinear response history analyses using these input motions, indicatingthe need to establish rational procedures for ground motion selection, scaling andmodification. Validated seismological methods may be used to generate ground motiontime series that incorporate near-fault rupture directivity effects and basin effects,and appropriately represent the duration and long period energy content of theselarge design events. These effects cannot be fully predicted by simply adoptingGMPEs that are developed for other regions. The main objective of this project is todetermine time-histories for distant and near-field earthquakes through ground motionsimulations. This joint project proposes to investigate the validation of the modeling procedure ofdetermination of geological models proposed by HERP for seismic hazard map in Japan, andoutput of this validation will be applied to the Eskisehir basin in Turkey. We canunderstand the performance of the procedure through the application to the Kumamotoarea by the Japanese side (using HERP procedure), and will be compared with theresults from Eskisehir by Turkish side. We also incorporate the methods used in theJapanese procedure into estimation of geological models in the Eskisehir basin. Inthis frame, we attempt to calculate broadband period ground motion time seriesfrom near-field and far-field earthquakes by using source modeling and groundmotion techniques. The effects of ground motions having a broadband period rangecan be studied in two parts; first one is the rupture directivity effects and secondlybasin edge effects. The prediction and simulation of such ground motions needsource modeling, determination of 2D/3D basin geometry and S-wave velocitystructure. We performed both array- and single-station microtremor measurements at a number oflocations so as to characterize the area to determine 1-D S-wave velocity structures down tothe bedrock depth beneath the strong motion sites for the broadband simulation. Finalgeological models obtained will be verified by seismic interferometry method andmicrogravity measurements. As next step, we make ground-motion simulations to get timehistories to analyze the effects of ground motions to buildings. In spite of increasing numberof buildings having different heights and characteristics in urbanized areas in Turkey, a fewstudies exist on the interaction of such buildings with ground motions having broadbandperiods. [ABSTRACT FROM AUTHOR]

The rupture-zone averaged static stress drop in the 2011 M=9 Tohoku-oki earthquake wasless than 5 MPa according to all published slip models, but it caused a reversal of the state ofstress in most of the offshore forearc, from predominantly compressive to predominantlyextensional. In this work, we demonstrate that this stress reversal has the followingimportant geodynamic implications. (1) The reversal unequivocally indicates avery weak subduction megathrust. In order to reproduce this reversal in a finiteelement model of force balance that quantifies the effects of gravity and megathrustfriction, the effective coefficient of friction of the megathrust has to be about 0.032. Alower value would make the forearc too extensional before the earthquake, and ahigher value would make it too compressive after the earthquake. (2) The very weakmegathrust results in very low differential stresses in the upper plate, consistent withpreviously reported sensitivity of the state of stress to small perturbations. Applying thedynamic Coulomb wedge model, we demonstrate that the inner wedge, and byinference the nearshore-onshore forearc, is in a stable state throughout subductionearthquake cycles, far from failure. (3) The outer wedge is normally in a stable state butmay reach an extensionally or compressively critical state during an earthquake,depending on the behavior of the shallow megathrust. Gravitational collapse of theouter wedge is prevented by a finite strength of the underlying shallow megathrust.Therefore, complete stress drop (i.e., effective coefficient of friction decreases tozero) is unlikely to have happened over the main part of the shallow megathrustexcept for limited local areas. (4) The presence of permanent deformation such asearthquakes and active faulting in the overall stable forearc can be explained bystructural and stress heterogeneities. We propose that for the most part, the upper platenear a subduction zone is much below its yield stress and is an elastic body, butpermanent deformation can locally occur in areas of low strength, high stress, orhigh fluid pressure. (5) The concept of dynamic Coulomb wedge (Wang and Hu,2006, JGR) needs to be expanded to address timescales much beyond subductionearthquake cycles. The strength of the megathrust varies over geological timescales,affecting the stability of the outer wedge and its response to earthquake rupture. [ABSTRACT FROM AUTHOR]

In the latest decade new and more complex physics-based simulators have been developed and have acquired a growing interest as a tool for the comprehension and testing of seismic process models.Here we introduce a new generation earthquake simulation algorithm and its potentiality for modelling both short- and long-term spatiotemporal process of strong earthquakes preparation. A specific seismogenic structure is modelled by quadrilateral faults constituted by thousands of cells of few kilometres size each. The physical model on which the latest version of our simulation algorithm is based includes, besides tectonic stress loading and static stress transfer as in the previous versions, also the Rate & State constitutive law. The simulator code can be run on a relatively modest computer and is capable of simulating thousands years of seismic activity producing catalogs of tens of thousands events in a wide range of magnitudes. The resulting synthetic catalogs exhibit typical magnitude, space and time features, which are comparable with those of real observations.In this study we applied the simulator code to a physical model of the Nankai mega trust, a well-known seismic structure 650 km long, aligned with the Pacific Ocean coast of Southern Japan, which generated several earthquakes of magnitude larger than 8.0 in the last few centuries. This structure is typically modelled as subdivided in five main segments characterized by different slip-rates, which can rupture separately from or simultaneously with each other.The results of this simulation provide interesting inferences on the spatiotemporal properties of seismic activity in the study area. In particular, the recurrence time of large events and their spatial relation, can be studied. As the simulator algorithm allows displaying the stress pattern on all the cells constituting the seismic structure, in this study we have focused our attention on the time evolution of such stress before, during and after large earthquakes. In particular, we have quantitatively recognized that the ratio between the average stress and its standard deviation on the cells constituting a specific fault segment always increases with accelerating rate before a large rupture on a part or all of such segment, or even several joint segments. [ABSTRACT FROM AUTHOR]

Dams are extremely important components of public and private infrastructure for flood prevention as well as water storage and often energy production for governments and their citizens globally. A failure of a dam or potential failure of a dam can cause major evacuations downstream as well as has the possibility of many deaths and major economic losses.Earthquakes provide a large risk for dams as a result of damaging ground motions causing cyclic loading of the dam structure as well as associated components to cause progressive damage. As many dams are built on faults, the fault movement and fault rupture also must be explored.Global dam databases such as ICOLD and GRaND provide a view into the location of dams globally and many can be shown to be in locations of high risk. In addition, the age of many dams calls into question the methods used to determine the design earthquake loads and construction methodologies used.Using the CATDAT Damaging Earthquakes Database and other research studies, a database of historical dam losses has been developed globally as a result of earthquake shaking and secondary effects such as landslides and liquefaction, and shows the compound effects of earthquake loading. This includes dam collapse events such as the Fujinuma Dam in Tohoku 2011 and the 1925 Santa Barbara earthquake. Events in which earthquake damage was recorded such as 1957 Montana, 1962 Guangdong, 1967 India, 1971 San Fernando, 1983 Japan, 1990 Manjil-Rudbar, 2001 Gujarat etc., have been collected and documented with various parameters such as socioeconomic damage, settlement, damage level etc. Over 100+ historic dam damage reports have been collected, not including tailings dams.In addition, a database of concurrent earthquake and rainfall events are examined to also check the relationship of previous damage to dams, with potentially influenced effects from earthquakes even if not the main cause of failure, and the importance of dam break studies for downstream elements of infrastructure.This work hopes to provide a first-order view into the compounding effects of damaging earthquake events on dams and a first-step to providing vulnerability functions for the assessment of global risk to dams. [ABSTRACT FROM AUTHOR]

There are many reports on Ionospheric electron distribution anomalies possibly related tolarge earthquakes. In this paper, Total Electron Content (TEC), Ionosonde, and tomographyanalyses have been applied to investigate the spatial and temporal distributions of ionosphericelectron density prior to the 2011 Off the Pacific Coast of Tohoku earthquake (Mw9.0).Results show significant TEC enhancements and an interesting three dimensional structureprior to the main shock. As for temporal TEC changes, the TEC value increases 3-4 daysbefore the earthquake remarkably, when the geomagnetic condition was relatively quiet. Inaddition, the abnormal TEC enhancement area in space was stalled above Japan during theperiod. Tomographic results show that three dimensional distribution of electron densitydecreases around 250 km altitude above the epicenter (peak is located just the east-region ofthe epicenter) and increases the mostly entire region between 300 and 400 km. TheIonospheric electron distribution caused by solar-terrestrial events have been alsoanalyzed. Detail characteristics and differences will be shown in the presentation. [ABSTRACT FROM AUTHOR]

The 2004 Mw 9.1-9.3 Great Sumatra-Andaman Earthquake is one of the strongest and most devastating earthquakes in recent history. It generated a powerful tsunami, causing more than 220,000 fatalities. Despite its huge spatial extent, the analysis of the earthquake is impeded by a lack of near-source observations. Static ground-deformation data, polluted by significant after-slip in the near-source region, and teleseismic data do not fully constrain the source, leading to significant differences in its characterization. Specifically, the tsunamigenic aspects of the earthquake, such as the large but mostly horizontal seafloor displacements and potential splay fault activation, remain puzzling.In this study, we investigate the earthquake using high-resolution 3D dynamic rupture simulations. We use the open-source software SeisSol (www.seissol.org) which is based on an arbitrary high-order accurate DERivative Discontinuous Galerkin method (ADER-DG). We account for complex megathrust-splay fault geometries, high resolution topography and bathymetry, 3D subsurface structure and off-fault plasticity. We resolve the full frictional sliding process as well as the seismic wave field in the to-date longest (500 s) and largest (1500 km) dynamic rupture earthquake scenario.We discuss macroscopic earthquake source characteristics inferred from a smoothly initialized model, in which we prescribe regional, smoothly varying fault pre-stresses constrained by earthquake focal mechanisms, interacting with frictional fault conditions changing only depth-dependently. The interplay of complex fault geometry and simple pre-stress state yields good agreement of ground-deformations in the near field and very long-period teleseismic data. We are specifically interested in the effects of off-fault plasticity: overall reducing rupture speed but increasing or decreasing fault slip depending on regional stress orientation in conjunction with the fault geometry. We then refine the regional stress state based on static inversion and mechanical viability arguments to fully capture dynamic features such as lower slip release in the Southern megathrust and slow rupture velocity at around 8° latitude. Our models are verified by generating synthetic geodetic, teleseismic and tsunami records, employing off-line coupling of SeisSol to established global seismic wave propagation and tsunami simulation tools. [ABSTRACT FROM AUTHOR]

Recent seismological and geodetic observations, as well as sophisticated regional models, indicate that similar physical processes are active during the earthquake cycle at different subduction margins. Part of the observed complexity at these margins is controlled by the fact that they are in different stages of the earthquake cycle. The observations capture critical physical processes like (partial) locking of the plate interface, the detailed co-seismic slip, and mantle relaxation and afterslip.We analyze geodetic observations along sections of the South America, Japan, and Sunda trenches during the interseismic part of the megathrust earthquake cycle. The results show that overriding plates shorten from the trench to a "backstop", where interseismic velocities become close to zero. In most, but not all, regions, the backstop location from trench-perpendicular GPS velocities agrees with that from parallel velocities. The distance of this interseismic backstop to the trench varies along major subduction margins. We explore what controls this distance by correlating backstop location with locking area on the plate contact, with boundaries of tectonic blocks in the overriding plate (mechanical contrasts), and with estimates of the recurrence time of the largest events.An enigmatic observation is that co-seismic geodetic displacements of great earthquakes, including the 2004 Sumatra earthquake, the 2010 Maule earthquake, and the 2011 Tohoku earthquake extended into the overriding plates well beyond what we infer to be the backstops. We use cyclic 3D geodynamic models to further understand and quantify these correlations. Co-seismic and subsequent slip on the subduction interface are dynamically (and consistently) driven. This setup allows us to access the sensitivity of the geodetic signals to mechanical components of the overriding plate and the slab system. We conclude that gradients in the effective elastic thickness of the overriding plate explain the observations. [ABSTRACT FROM AUTHOR]

Mongolia was affected in the 20th century by four major strike-slip earthquakes (M8) with spectacular surface ruptures still fresh in the landscape. These ruptures happened along fault sections that presently help accommodate the extrusion of tectonic blocks North of the India Eurasia collision. These structures, however, were preexisting to those earthquakes and have a prior long standing polyphased tectonic history. In addition to these major earthquakes, a large event, the M7 1967 Mogod earthquake, happened off of what was considered the major tectonic structures at the margin of a tectonic setting usually considered as a stable continental region. This earthquake ruptured a 40 km-long stretch of faults between the Hangay dome and eastern Mongolia. The rupture is divided in 3 main fault strands, which show either evidence of left-lateral strike-slip or thrust motion. Despite the spectacular surface ruptures associated to the 1967 event, in most places no accumulated seismic scarp could be found, raising questions about the past history of this active fault, its seismogenic potential and associated seismic hazard. We documented a paleoseismological trench through one of the 1967 rupture sections to unravel the past history of this active fault system. The excavation through the local reverse fault scarplet revealed at least 3 earthquakes that deformed and offset the superficial units by recumbent folding, drag folding and backthrusting. The chronology of the sequence is constrained by 7 OSL dates of sands sampled within the main units. The last surface rupture in 1967 CE was preceded by a similar event that happened between 20 and 40 ky and a third one around 50 ky. These result suggests that this slow slipping intraplate fault has a seismic cycle generating M7 events with an inter- event time of about 25 ka, an activity rate consistent with a slip on the fault around 0.1 mm/yr. [ABSTRACT FROM AUTHOR]

After the 2011 Mw 9.0 Tohoku earthquake ruptured the subduction megathrust offshore Japan, many normal faulting earthquakes were observed in regions that previously hosted dominantly thrust faulting events. The rate of normal faulting seismicity has decreased since 2011, but is still significantly higher than before the Tohoku earthquake. Similar normal faulting earthquake sequences have been observed after other Mw 8.5+ megathrust events and in some cases appear to last for decades. In this study, we investigate whether the spatial distribution and temporal evolution of normal faulting seismicity in subduction zones can be explained by earthquake cycle deformation processes, particularly co-seismic and post-seismic stress changes. We develop a three-dimensional finite element model of the subduction zone offshore Honshu with an appropriate plate interface geometry and a rheology consisting of an elastic upper plate above a linear visco-elastic mantle wedge. We spin up the model using internally consistent earthquake cycles to generate pre-stresses; stresses build across the locked plate interface during the inter-seismic stage, driving co-seismic slip. Afterwards, the interface is re-locked as afterslip and bulk viscous relaxation occur, leading back into the subsequent inter-seismic stage. We incorporate the co-seismic slip from known megathrust earthquakes occurring before 2011 to better represent the slip deficit and state of stress at the time of the Tohoku earthquake. The model results show that the immediate co-seismic effect of a great megathrust earthquake is to cause approximately trench-normal extension of the upper plate (relative to the pre-earthquake stress field). This extension occurs above and landward of the rupture zone in the regions that currently host significant normal faulting earthquakes. Early afterslip occurring on the subduction interface down-dip of the rupture results in further extension of both the upper and subducting plates, promoting normal faulting earthquakes in these regions. Although the upper plate continues to move trenchward as a consequence of the bulk viscous relaxation of the deeper visco-elastic regions, this does not have a significant effect on the stress field in the shallower, elastic regions. The dominant process responsible for reestablishing the pre-earthquake state of stress is the plate convergence after the re-locking of the subduction megathrust. We find good agreement between the predicted spatial and temporal pattern of tensile stresses and the post-seismic normal faulting earthquakes in Japan. [ABSTRACT FROM AUTHOR]