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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]

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]

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]

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]

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]

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]