Asperity Failure Research Articles

Teleseismic finite-fault inversion of two Mw = 6.4 earthquakes along the East Anatolian Fault Zone in Turkey: the 1998 Adana and 2003 Bingöl earthquakes

The East Anatolian Fault Zone is a continental transform fault accommodating westward motion of the Anatolian fault. This study aims to investigate the source properties of two moderately large and damaging earthquakes which occurred along the transform fault in the last two decades using the teleseismic broadband P and SH body waveforms. The first earthquake, the 27 June 1998 Adana earthquake, occurred beneath the Adana basin, located close to the eastern extreme of Turkey’s Mediterranean coast. The faulting associated with the 1998 Adana earthquake is unilateral to the NE and confined to depths below 15 km with a length of 30 km along the strike (53°) and a dipping of 81° SE. The fixed-rake models fit the data less well than the variable-rake model. The main slip area centered at depth of about 27 km and to the NE of the hypocenter, covering a circular area of 10 km in diameter with a peak slip of about 60 cm. The slip model yields a seismic moment of 3.5 × 1018 N-m (Mw ≅ 6.4). The second earthquake, the 1 May 2003 Bingol earthquake, occurred along a dextral conjugate fault of the East Anatolian Fault Zone. The preferred slip model with a seismic moment of 4.1 × 1018 N-m (Mw ≅ 6.4) suggests that the rupture was unilateral toward SE and was controlled by a failure of large asperity roughly circular in shape and centered at a depth of 5 km with peak displacement of about 55 cm. Our results suggest that the 1998 Adana earthquake did not occur on the mapped Goksun Yakapinar Fault Zone but rather on a SE dipping unmapped fault that may be a split fault of it and buried under the thick (about 6 km) deposits of the Adana basin. For the 2003 Bingol earthquake, the final slip model requires a rupture plane having 15° different strike than the most possible mapped fault.

Smoothing of Fault Slip Surfaces by Scale‐Invariant Wear

AbstractPrevious field observations suggest that fault slip surface roughness may decrease with slip. However, measurements have yet to confidently isolate the effect of slip from other possible controls, such as lithology or tectonic setting. We describe the evolution of slip surfaces in normal faults in SE Utah that cut well‐sorted, high‐porosity sandstones and accommodated regional extension at 1‐ to 4‐km depth. Tight controls on fault offset and uniform tectonic history allowed us to isolate the effect of slip during early stages of faulting (0‐ to 55‐m slip). Slip surfaces progress from rough joints and deformation bands toward smooth, continuous, and mirror‐like surfaces with increasing slip. We collected 123 scans of pristine slip surfaces, which measure surface geometry from micrometers to meters in length scale. Results indicate that slip surface roughness is systematically smoother than joint surfaces and deformation band edges. Over the best resolved length scales (0.1–10 mm), we observe roughness decreases with slip according to a power law with exponent −1 in the slip direction. Slip‐perpendicular profiles, though rougher, exhibit the same smoothing trend. The Hurst scaling exponent does not change with slip. These observations require wear to be multiscale. Boundary element method models suggest that mechanical wear of completely mated surfaces occurs by asperity failure and that the wear rate depends on the aspect ratio of asperities. These results indicate that at large slip, asperity failure at all length scales can cause slip surfaces to smooth while maintaining the fractal geometry characteristic of faults.

‘Two go together’: Near-simultaneous moment release of two asperities during the 2016 Mw 6.6 Muji, China earthquake

On 25 November 2016, a Mw 6.6 earthquake ruptured the Muji fault in western Xinjiang, China. We investigate the earthquake rupture independently using geodetic observations from Interferometric Synthetic Aperture Radar (InSAR) and regional seismic recordings. To constrain the fault geometry and slip distribution, we test different combinations of fault dip and slip direction to reproduce InSAR observations. Both InSAR observations and optimal distributed slip model suggest buried rupture of two asperities separated by a gap of greater than 5 km. Additional seismic gaps exist at the end of both asperities that failed in the 2016 earthquake. To reveal the dynamic history of asperity failure, we inverted regional seismic waveforms for multiple centroid moment tensors and construct a moment rate function. The results show a small centroid time gap of 2.6 s between the two sub-events. Considering the >5 km gap between the two asperities and short time interval, we propose that the two asperities failed near-simultaneously, rather than in a cascading rupture propagation style. The second sub-event locates ∼39 km to the east of the epicenter and the centroid time is at 10.7 s. It leads to an estimate of average velocity of 3.7 km/s as an upper bound, consistent with upper crust shear wave velocity in this region. We interpret that the rupture front is propagating at sub-shear wave velocities, but that the second sub-event has a reduced or asymmetric rupture time, leading to the apparent near-simultaneous moment release of the two asperities.

Characteristics of shear-induced asperity degradation of rock fractures and implications for solute retardation

Direct shear tests on a series of fractured Beishan granite samples were performed under different normal stresses, to investigate the characteristics of asperity degradation during shear and to quantify their effects on solute retardation in fractures. A three-dimensional laser scanner and a laser diffraction particle size analyzer were used to examine the shear-induced asperity volume loss and the size distribution of sheared-off fragments. The results identified two main modes of asperity degradation during shear, i.e., instantaneous failure of asperities occurring at the peak shear stress, and crushing of the generated fragments with further shear displacement. Based on the volume of asperity degradation, a new joint damage coefficient was proposed. The size of the sheared-off fragments under different normal stresses was found to follow a Weibull distribution. Combining the proposed joint damage coefficient and the Weibull size distribution of the sheared-off fragments can approximately predict the potential effects of sheared-off fragments on solute retardation coefficient in rock fractures. The results showed that the shear-induced asperity degradation significantly increases the values of the solute retardation coefficient, by offering more sorption surfaces in fracture voids.

Dynamics of a fault model with two mechanically different regions

We consider a fault containing two regions with different mechanical behaviours: a strong, velocity-weakening region (asperity) and a weak, velocity-strengthening region. The fault is embedded in a shear zone subject to a constant strain rate by the motions of adjacent tectonic plates. The fault is modelled as a discrete dynamical system whose state is described by two variables expressing the slip deficits of the two regions. Because of plate motion, the asperity accumulates stress and eventually releases it, producing an earthquake, when a frictional threshold is exceeded. The weak region is subject to a very slow creep during interseismic intervals and may slip at a higher rate (afterslip) as a consequence of coseismic stress imposed by the asperity failure. The evolution equations of the system are solved analytically for the interseismic intervals, the asperity slip and the afterslip in the weak region. It is found that the amount of afterslip is proportional to the seismic slip of the asperity, in agreement with observations. The model shows that afterslip is a natural consequence of seismic slip in a fault containing a velocity-strengthening region. Afterslip may have any duration, according to the intensity of velocity strengthening, thus accounting for the wide range of observed durations. The model is applied to the fault of the 2011 Tohoku-Oki earthquake. The results suggest that the first four months after the event were dominated by afterslip, while the subsequent postseismic deformation was probably due to viscoelastic relaxation in the asthenosphere.

Control of asperities size and spacing on seismic behavior of subduction megathrusts

AbstractThe majority of the largest subduction megathrust earthquakes share the common characteristic of rupturing more than one asperity along strike of the margin. Understanding the factors that control coseismic failure of multiple asperities, and thus maximum magnitude, is central for seismic hazard assessment. To investigate the role of asperities size and spacing on maximum magnitude, seismicity rate, and percentage of synchronized ruptures, we use analog models simulating along‐strike rupture behavior of megathrust earthquakes. We found negative correlations between the barrier‐to‐asperity length ratio Db/Da and maximum magnitude and seismicity rate. Db/Da also controls the process of asperities synchronization along the megathrust. A permanent barrier behavior is observed for Db/Da > 0.5. Comparing our experimental results to the Nankai Trough historical seismicity, we propose that the distribution of megathrust frictional heterogeneities likely explains the diversity of earthquakes which occurred there.

Micromechanics of sea ice frictional slip from test basin scale experiments.

We have conducted a series of high-resolution friction experiments on large floating saline ice floes in an environmental test basin. In these experiments, a central ice floe was pushed between two other floes, sliding along two interfacial faults. The frictional motion was predominantly stick-slip. Shear stresses, normal stresses, local strains and slip displacement were measured along the sliding faults, and acoustic emissions were monitored. High-resolution measurements during a single stick-slip cycle at several positions along the fault allowed us to identify two phases of frictional slip: a nucleation phase, where a nucleation zone begins to slip before the rest of the fault, and a propagation phase when the entire fault is slipping. This is slip-weakening behaviour. We have therefore characterized what we consider to be a key deformation mechanism in Arctic Ocean dynamics. In order to understand the micromechanics of sea ice friction, we have employed a theoretical constitutive relation (i.e. an equation for shear stress in terms of temperature, normal load, acceleration, velocity and slip displacement) derived from the physics of asperity-asperity contact and sliding (Hatton et al. 2009 Phil. Mag. 89, 2771-2799 (doi:10.1080/14786430903113769)). We find that our experimental data conform reasonably with this frictional law once slip weakening is introduced. We find that the constitutive relation follows Archard's law rather than Amontons' law, with [Formula: see text] (where τ is the shear stress and σn is the normal stress) and n = 26/27, with a fractal asperity distribution, where the frictional shear stress, τ = ffractal Tmlws, where ffractal is the fractal asperity height distribution, Tml is the shear strength for frictional melting and lubrication and ws is the slip weakening. We can therefore deduce that the interfacial faults failed in shear for these experimental conditions through processes of brittle failure of asperities in shear, and, at higher velocities, through frictional heating, localized surface melting and hydrodynamic lubrication.This article is part of the themed issue 'Microdynamics of ice'.

Asperity generation and its relationship to seismicity on a planar fault: a laboratory simulation

Author(s): Selvadurai, PA; Glaser, SD | Abstract: Earthquake faults, and all frictional surfaces, establish contact through asperities. A detailed knowledge of how asperities form will enable a better understanding of the manner in which they communicate during foreshock failure sequences that are observed, leading to the larger main shock. We present results of experiments where a pressure sensitive film was used to map, size and measure the magnitudes of the normal stresses at asperities along a seismogenic section of a laboratory simulated fault. We measured seismicity acoustically and foreshocks were found to be the result of localized asperity failure during the nucleation phase of gross fault rupture. Since surface roughness plays an important role in how asperities are formed, two Hurst exponents were measured to characterize a highly worn interface using roughness profiles: (i) long wavelength estimates (H ~ 0.45) and (ii) short wavelength estimates (H ~ 0.8-1.2). The short wavelength roughness estimates were computed at the scale of single asperity junction points. Macroscopically, the number of asperities and real contact area increased with additional application of normal force while the mean normal stress remained constant. The ratio of real to nominal contact area was low - ranging from 0.02 l Ar/A0 l 0.05-predicting that the asperities should be elastically independent of each other. Results from the pressure sensitive film showed that asperities were closely spaced and could not be treated as mechanically independent. Larger asperities carried both higher levels of average normal stress and higher levels of normal stress heterogeneity than smaller ones. Using linear stability theorem, the critical slip distance on foreshocking asperities was estimated to be d0 ~ 0.65-3 μm. The critical slip distance d0 was ~1.8-11.5 per cent of the premonitory slip needed to initiate gross fault rupture of the interface (20-40 μm) and the overall slip necessary to initiate gross fault rupture was on the order of the average asperity diameter (52 μm). Foreshocks may be due to a change in the critical slip distance, at localized sections of the fault, caused by the two distinct roughness profiles measured at short and long length scales.

The 2015 Fillmore Earthquake Swarm and Possible Crustal Deformation Mechanisms near the Bottom of the Eastern Ventura Basin, California

The 2015 Fillmore swarm occurred about 6 km west of the city of Fillmore in Ventura, California, and was located beneath the eastern part of the actively subsiding Ventura basin at depths from 11.8 to 13.8 km, similar to two previous swarms in the area. Template‐matching event detection showed that it started on 5 July 2015 at 2:21 UTC with an M ∼1.0 earthquake. The swarm exhibited unusual episodic spatial and temporal migrations and unusual diversity in the nodal planes of the focal mechanisms as compared to the simple hypocenter‐defined plane. It was also noteworthy because it consisted of >1400 events of M ≥ 0.0, with M 2.8 being the largest event. We suggest that fluids released by metamorphic dehydration processes, migration of fluids along a detachment zone, and cascading asperity failures caused this prolific earthquake swarm, but other mechanisms (such as simple mainshock–aftershock stress triggering or a regional aseismic creep event) are less likely. Dilatant strengthening may be a mechanism that causes the temporal decay of the swarm as pore‐pressure drop increased the effective normal stress, and counteracted the instability driving the swarm.

Fracture closure and conductivity decline modeling – Application in unpropped and acid etched fractures

Natural fractures, hydraulically generated fractures, and acid etched fractures have some degree of fracture face roughness that generates flow conductivity. While it has been proven both theoretically and experimentally that fracture conductivity depends on fracture face roughness, there are limited models that can predict fracture conductivity at different closure stresses for these various fracture roughness patterns. In addition, some of the models require detailed statistical and topographical surface profile parameters, which limit their field application.A numerical model is developed to study the closure of rough surfaces in contact. Both asperities and semi-infinite half-spaces are assumed to be deformable. The mechanical interaction among asperities is accounted for and its effect on the fracture closure is investigated. Asperity failure is also considered in the model and the results are compared to that of perfectly elastic contact. Aperture profiles that are the output of the closure model are used to solve the fluid flow problem and study the effect of closure stress on fracture conductivity.It is evident in our results that the closure behavior depends on the etching pattern as well as the elastic properties of the surface. The performance of a rough fracture depends on its initial aperture, asperity height distribution, roughness pattern, and the closure stress range. Certain fracture roughness patterns were able to withstand the closure stress while undergoing lower amounts of closure. Our model tends to predict fracture closure and conductivity behavior better than widely used correlations.This paper discusses the closure of fractures and attempts to shed more light on the performance of such a stimulation technique by comparing the closure behavior of some particular surface patterns. Our model can be used to determine the most optimum fracture system for a given reservoir condition.

High‐resolution images of tremor migrations beneath the Olympic Peninsula from stacked array of arrays seismic data

AbstractEpisodic tremor and slip (ETS) in subduction zones is generally interpreted as the manifestation of shear slip near the base of earthquake‐generating portion of the plate interface. Here we devise a new method of cross‐correlating stacked Array of Arrays seismic data that provides greatly improved tremor locations, a proxy for the underlying slow slip, beneath the Olympic Peninsula. This increased resolution allows us to image many features of tremor that were not visible previously. We resolve the spatial transition between the rupture zones of the inter‐ETS and major ETS episodes in 2010, suggesting stress redistribution by the former. Most tremor migrations propagated along the slowly advancing main tremor front during both the inter‐ETS and the major ETS episodes, even though the main front of the former deviated strongly from its usual (along‐dip) orientation. We find a distinct contrast between along‐dip rupture extent of large‐scale rapid tremor reversals (RTRs) to the south and that to the north in our study region that anticorrelates with the locations of inter‐ETS events. These RTRs originate from the main front, similar to smaller‐scale RTRs previously observed at high‐resolution, and many start by propagating along the main front. This could be consistent with RTRs being triggered by a cascading failure of brittle asperities. After initiation, the RTRs repeatedly occupy the same source region, and the early repetitions appear not to be tidally driven. Their stress drop may come from continuing fault weakening processes within the tremor zone, or loading by aseismic slip in surrounding regions.

Inferring fault mechanical conditions from the source parameters of a complex microseismic multiplet in the Corinth rift, Greece

AbstractWe develop a mechanical model of tight clusters of coplanar seismic asperities, to investigate a particular microearthquake swarm located at 8 km depth in the Corinth rift in Greece, which was active between 2001 and 2007. Although it is classified as a multiplet based on waveform similarity, this seismic sequence is much more complex than a repeating earthquake sequence and cannot be interpreted as the regular failure of a single asperity forced by surrounding aseismic creep. Here we suggest that such complex sequences could be generated by the failure of a set of coplanar asperities interacting in a small region of a fault segment. We show that in order to reproduce the dynamics of the observed sequence and the characteristics of the events, the cluster of asperities has to be located very close to an aseismically slipping fault segment, which could be an updip extension of the deep detachment zone in the rift, creeping at 1.5 cm/yr. For more general cases of coplanar clustered asperities, we show that the shape of the cumulative coseismic displacement pattern associated with the repeated failures of the asperities is strongly controlled by the behavior of the fault area surrounding the asperity cluster. In particular, if the cluster is part of a locked fault area, the resulting long‐term cumulative displacement is maximum at the center of the cluster. In contrast, an asperity cluster surrounded by aseismic creep leads to a uniform cumulative coseismic slip pattern. The ratio between cumulative slip at the center of the seismogenic patch and cumulative slip at its periphery could therefore be an indicator of the mechanical conditions prevailing on the fault. A systematic study of the source parameters of complex microseismic sequences could therefore provide insights into the mechanical state of active faults continuously generating microseismicity.

Observation of the Degradation Characteristics and Scale of Unevenness on Three-dimensional Artificial Rock Joint Surfaces Subjected to Shear

The present study explores the degradation characteristics and scale of unevenness (small-scale roughness) on sheared rock joint surfaces at a low-stress regime. While the degradation characteristics of unevenness and the normal stress are mutually interrelated, an understanding of the degradation patterns of the three-dimensional roughness of rock joints is one of the important components needed to identify asperity failure characteristics and to quantify the role of damaged unevenness in establishing a shear strength model. A series of direct shear tests was performed on three-dimensional artificial rock joint surfaces at different normal stress levels. After shearing, the spatial distributions and statistical parameters of degraded roughness were analysed for the different normal stress levels. The length and area of the degraded zones showed bell-shaped distributions in a logarithmic scale, and the dominant scale (or the most frequently occurring scale) of the damaged asperities (i.e., unevenness) ranged from approximately, 0.5 to 5.0 mm in length and 0.1–10 mm2 in area. This scale of the damaged unevenness was consistent regardless of the level of normal stress. It was also found that the relative area of damaged unevenness on a given joint area, and thus the contribution of the mechanical asperity failure component to shear strength increased as normal stress increased.

Dual megathrust slip behaviors of the 2014 Iquique earthquake sequence

The transition between seismic rupture and aseismic creep is of central interest to better understand the mechanics of subduction processes. A Mw 8.2 earthquake occurred on April 1st, 2014 in the Iquique seismic gap of northern Chile. This event was preceded by a long foreshock sequence including a 2-week-long migration of seismicity initiated by a Mw 6.7 earthquake. Repeating earthquakes were found among the foreshock sequence that migrated towards the mainshock hypocenter, suggesting a large-scale slow-slip event on the megathrust preceding the mainshock. The variations of the recurrence times of the repeating earthquakes highlight the diverse seismic and aseismic slip behaviors on different megathrust segments. The repeaters that were active only before the mainshock recurred more often and were distributed in areas of substantial coseismic slip, while repeaters that occurred both before and after the mainshock were in the area complementary to the mainshock rupture. The spatiotemporal distribution of the repeating earthquakes illustrates the essential role of propagating aseismic slip leading up to the mainshock and illuminates the distribution of postseismic afterslip. Various finite fault models indicate that the largest coseismic slip generally occurred down-dip from the foreshock activity and the mainshock hypocenter. Source imaging by teleseismic back-projection indicates an initial down-dip propagation stage followed by a rupture-expansion stage. In the first stage, the finite fault models show an emergent onset of moment rate at low frequency (<0.1 Hz), while back-projection shows a steady increase of high frequency power (>0.5 Hz). This indicates frequency-dependent manifestations of seismic radiation in the low-stress foreshock region. In the second stage, the rupture expands in rich bursts along the rim of a semi-elliptical region with episodes of re-ruptures, suggesting delayed failure of asperities. The high-frequency rupture remains within an area of local high trench-parallel gravity anomaly (TPGA), suggesting the presence of subducting seamounts that promote high-frequency generation. Our results highlight the complexity of the interactions between large-scale aseismic slow-slip and dynamic ruptures of megathrust earthquakes.

New criterion for rock joints based on three-dimensional roughness parameters

The shear behavior of rock joints is important in solving practical problems of rock mechanics. Three group rock joints with different morphologies are made by cement mortar material and a series of CNL (constant normal loading) shear tests are performed. The influences of the applied normal stress and joint morphology to its shear strength are analyzed. According to the experimental results, the peak dilatancy angle of rock joint decreases with increasing normal stress, but increases with increasing roughness. The shear strength increases with the increasing normal stress and the roughness of rock joint. It is observed that the modes of failure of asperities are tensile, pure shear, or a combination of both. It is suggested that the three-dimensional roughness parameters and the tensile strength are the appropriate parameter for describing the shear strength criterion. A new peak shear criterion is proposed which can be used to predict peak shear strength of rock joints. All the used parameters can be easily obtained by performing tests.

Physical modeling of seismic source generation in failure of fault asperities

A series of full-scale experiments was performed to study the influence of impact loads on the parameters of seismic vibrations initiated in variable friction. The study was conducted on a test setup Tribo which is a movable concrete slab modeling an allochthon on a rough plane of the Angara fault segment in Baikal region. Contact interactions of asperities in the slip zone were recorded using strain and load measuring equipment and four seismic stations Baykal-7HR widely used for recording earthquakes. The proposed physical modeling method and obtained results can be of interest for the development of new physical models of differently scaled sources of seismic energy dissipation in tectonic faults and can be useful for seismological studies, related data interpretation, and improvement of extended forecast of rock bursts and earthquakes.

Evolution of Wear and Friction Along Experimental Faults

We investigate the evolution of wear and friction along experimental faults composed of solid rock blocks. This evolution is analyzed through shear experiments along five rock types, and the experiments were conducted in a rotary apparatus at slip velocities of 0.002–0.97 m/s, slip distances from a few millimeters to tens of meters, and normal stress of 0.25–6.9 MPa. The wear and friction measurements and fault surface observations revealed three evolution phases: A) An initial stage (slip distances <50 mm) of wear by failure of isolated asperities associated with roughening of the fault surface; B) a running-in stage of slip distances of 1–3 m with intense wear-rate, failure of many asperities, and simultaneous reduction of the friction coefficient and wear-rate; and C) a steady-state stage that initiates when the fault surface is covered by a gouge layer, and during which both wear-rate and friction coefficient maintain quasi-constant, low levels. While these evolution stages are clearly recognizable for experimental faults made from bare rock blocks, our analysis suggests that natural faults “bypass” the first two stages and slip at gouge-controlled steady-state conditions.

Doubly Stochastic Earthquake Source Model: “Omega-Square” Spectrum and Low High-Frequency Directivity Revealed by Numerical Experiments

Since its formulation in 1967–1970, the classical ω −2 model of earthquake source spectrum awaits a consistent theoretical foundation. To obtain one, stochastic elements are incorporated both into the final structure of the fault and into the mode of rupture propagation. The main components of the proposed “doubly stochastic” model are: (1) the Andrews’s concept, that local stress drop over a fault is a random self-similar field; (2) the concept of rupture with running slip pulse, after Heaton; (3) the hypothesis that a rupture front is a tortuous, multiply connected (“lacy”) fractal polyline that occupies a strip of finite width close to the slip-pulse width; and (4) the assumption that the propagation distance of fault-guided, mostly Rayleigh waves from a failing spot on a fault is determined by the slip-pulse width. Waveforms produced by this model are determined based on the fault asperity failure model after Das and Kostrov. Properties of the model are studied by numerical experiments. At high frequency, simulated source spectra behave as ω −2, and acceleration spectra are flat. Their level, at a given seismic moment and rms stress drop, is inversely related to the relative width of the slip pulse. When this width is relatively low, a well-defined second corner frequency (lower cutoff of acceleration spectrum) is seen. The model shows clear dependence of propagation-related directivity on frequency. Between the first and the second corner frequency, amplitude spectra are strongly enhanced for the forward direction; whereas, above the second corner frequency, directivity is significantly reduced. Still, it is not inhibited totally, suggesting incomplete incoherence of the simulated radiator at high frequencies.

Implications of recent asperity failures and aseismic creep for time-dependent earthquake hazard on the Hayward fault

The probability of large seismic events on a particular fault segment may vary due to external stress changes imparted by nearby deformation events, including other earthquakes and aseismic processes, such as fault creep and postseismic relaxation. The Hayward fault (HF), undergoing both seismic and aseismic fault slip, provides a unique opportunity to study the mutual relation of seismic and aseismic processes on a fault system. We use surface deformation data obtained from InSAR (interferometric synthetic aperture radar), creepmeters and alinement arrays, together with constraints provided by repeating earthquakes to investigate the kinematics of fault creep on the northern HF and its relation to two seismic clusters (Mw≤4.1) in October 2011 and March 2012, and an Mw 4.2 event in July 2007. Recurrences of nearby repeating earthquakes show that these episodes involved both seismic and aseismic slip. We model the stress changes due to fault creep and the recent seismic activity on the locked central asperity of the HF, which is believed to be the rupture zone of past and future M~7 earthquakes. The results show that the shallow fault creep stresses the major locked central patch at an average rate of 0.001–0.003MPa/yr, in addition to background stressing at 0.01–0.015MPa/yr. Given the time-dependent nature of the creep, occasional deviations from this stressing rate occur. We find that the 2011 seismic cluster occurred in areas on the fault that are stressed up to 0.01MPa/yr due to aseismic slip on the surrounding segments, suggesting that the occurrence of these events was encouraged by the fault creep. Changes in the probability of major earthquakes can be estimated from the imparted stress from the recent earthquakes and associated fault creep transients. We estimate that the 1-day probability of a large event on the HF only increased by up to 0.18% and 0.05% due to the static stress increase and stressing rate change by the 2011 and 2012 clusters. For the July 2007 south Oakland event (Mw 4.2) the estimated increase of short-term probabilities is 50%, highlighting the importance of short-term probability changes due to transient stress changes.

23 October 2011 Van, Eastern Anatolia, earthquake (M W 7.1) and seismotectonics of Lake Van area

The 23 October 2011 Van earthquake took place in the NE part of Lake Van area, surprisingly on a fault (the Van fault) that is not present in the current active fault map of Turkey. However, occurrence of such a large magnitude earthquake in the area is not surprising regarding the historical seismicity of the region. The comparison of the damage patterns suggests that the earthquake is much likely a recurrence of the 1715 Van earthquake. The finite fault modelling of the earthquake using teleseismic broadband body waveforms has shown that the earthquake rupture was unilateral toward SW, was mostly reverse faulting, confined to below the depth of 5 km, did not propagate offshore, and was dominated by a failure of a single asperity with a peak slip of about 5.5 m. The total seismic moment calculated for the model is 4.6 × 1019 Nm (M W ≈ 7.1). The finite fault model coincides with the field observations indicating blind faulting and the vertical displacements over the free surface estimated from it correlate well with the maximum reported uplift along the coast of Lake Van above the hanging wall. The possible offshore continuations of the Van fault and some other faults lying its south are also discussed by assessing a previous offshore seismic reflection study and the earthquake epicentres and focal mechanisms.