Strike-slip Earthquakes Research Articles

Seismic intensity assignments for the 2008 Andravida (NW Peloponnese, Greece) strike-slip event (June 8, Mw=6.4) based on the application of the Environmental Seismic Intensity scale (ESI 2007) and the European Macroseismic scale (EMS-98)

On June 8, 2008, a strike-slip earthquake (Mw=6.4) was generated NE of the Andravida town (NW Peloponnese, western Greece) due to the activation of the previously unknown western Achaia strike-slip fault zone (WAFZ). Extensive structural damage and earthquake environmental effects (EEE) were induced in the NW Peloponnese, offering the opportunity to test and compare the ESI 2007 and the EMS-98 intensity scales in a moderate strike-slip event. No primary EEE were induced, while secondary EEE including seismic fractures, liquefaction phenomena, slope movements and hydrological anomalies were widely observed covering an area of about 800 km<sup>2</sup>. The lack of primary effects and the relatively small surface deformation with respect to the earthquake magnitude is due to the thick Gavrovo flysch layer in the affected area that isolated and absorbed the subsurface deformation from the surface. According to the application of the EMS-98 scale, damage to masonry buildings ranged from grade 3 to 5, while damage in most of R/C buildings ranged from grade 1 to 3. A maximum ESI 2007 intensity VIII-IX is recorded, while the maximum EMS-98 intensity is IX. For all the sites where intensity VIII has been recorded the ESI 2007 and the EMS-98 agree, but for others the ESI 2007 intensities values are lower by one or two degrees than the corresponding EMS-98 ones, as it is clearly concluded from the comparison of the produced isoseismals. An exception to this rule is the Valmi village, where considerable structural damage occurs (IX<sub>EMS-98</sub>) along with the lack of significant EEE (V<sub>ESI 2007</sub>). This variability between the ESI 2007 and the EMS-98 intensity values is predominantly attributed to the vulnerability of old masonry buildings constructed with no seismic resistance design. Correlation of all existing data shows that the geological structure, the active tectonics, and the geotechnical characteristics of the alpine and post-alpine formations along with the construction type of buildings were of decisive importance in the damage and the EEE distribution.

Validation of Fault Rupture-Response Spectrum Analysis Method for Curved Bridges Crossing Strike-Slip Fault Rupture Zones

AbstractIn response to the bridge damages observed in past earthquakes and the absence of practical analysis methods, a simplified analysis procedure named the fault rupture-response spectrum analysis (FR-RSA) method was developed in this research as an alternative to the onerous nonlinear response history analysis (RHA) method for bridges crossing strike-slip earthquake fault rupture zones. For validation of the FR-RSA method, this research investigated seismic responses of two representative curved bridges in California, including one three-span bridge and one four-span bridge. The selected bridges were analyzed using both the FR-RSA method and the nonlinear RHA method. Ground motion, earthquake fault rupture location, and fault orientation were varied in the analyses to expand the validation work under a broad range of the parameters. Result comparisons show that the FR-RSA method provides estimates of peak bridge displacement response that are close enough to the exact nonlinear RHA results in all con...

Seismotectonic Implications of Strike-Slip Earthquakes in the Darjiling-Sikkim Himalaya

The Darjiling-Sikkim Himalaya (DSH) is located over the Dharan-Gorubathan salient-recess pair and moderate thrust and strike-slip earthquake occur here. The hypocentres cluster not only near the location of the Main Himalayan Thrust (MHT) or the basal decollement of the Himalayan wedge, but also well above and below it. The epicentres cluster over the mapped location of the Lesser Himalayan Duplex (LHD), suggesting that both MHT and LHD are active structures in DSH. The earthquakes below MHT can be related to transverse strike-slip faulting in DSH associated with salient-recess transition on both flanks of the Dharan salient. The 18 September 2011 (Mw 6.9) strike-slip event suggests that the western flank of the Dharan salient is also likely to contain an active transverse strike-slip fault like the Gish Transverse Fault (GTF) on its eastern flank. High-precision Global Positioning System measurements (1997-2006) indicate that a maximum of ∼4 mm/year convergence is being accommodated in the Tista Half-Window or LHD west of the surface trace of GTF and DSH is locked south of 27°N both east and west of GTF about 10 km north of the Himalayan mountain front. About 3-4 mm/year sinistral strike-slip is postulated on GTF north of 27°N. Dislocation based forward modelling using two thrust dislocations with oblique slip and a sinistral strike-slip dislocation generated velocities that were closest to the measured back-slip velocity field in DSH.

Statistical analysis of landslides caused by the Mw 6.9 Yushu, China, earthquake of April 14, 2010

The April 14, 2010 Yushu, China, earthquake (Mw 6.9) triggered a great number of landslides. At least 2,036 co-seismic landslides, with a total coverage area of 1.194 km2, were delineated by visual interpretation of aerial photographs and satellite images taken following the earthquake, and verified by field inspection. Based on the mapping results, a statistical analysis of the spatial distribution of these landslides is performed using the landslide area percentage (LAP), defined as the percentage of the area affected by the landslides, and landslide number density (LND), defined as the number of landslides per square kilometer. The purpose is to clarify how the landslides correlate the control factors, which are the elevation, slope angle, slope aspect, slope position, distance from drainages, lithology, distance from the surface rupture, and peak ground acceleration (PGA). The results show that both LAP and LND have strongly positive correlations with slope angle and negative correlations with distance from the surface rupture and distance from drainages. The highest LAP and LPD values are in places of elevations from 3,800 to 4,000 m. The slopes producing landslides are mostly facing toward NE, E, and SE. The geological units of Q4 al-pl, N, and T3 kn 1 have the highest concentrations of co-seismic landslides. No apparent correlations are present between LAP and LND values and PGA. On both sides of the surface rupture, the landslide distributions are almost similar except a few exceptions, likely associated with the nature of the strike-slip seismogenic fault for this event. The bivariate statistical analysis shows that, in descending order, the earthquake-triggered landslide impact factors are distance from surface rupture > slope angle > distance from drainages > lithology > PGA. Besides, as the detailed co-seismic landslides inventories related to strike-slip earthquakes are still few compared with that of thrusting-fault earthquakes, this case study would shed new light on the subject. For instance, the landslide spatial distribution on both sides of the strike-slip seismogenic fault is rather different from that of thrusting-fault earthquakes. It reminds us to take different strategies of measures for prevention and mitigation of landslides induced by earthquakes with different mechanisms.

How well do surface slip measurements track slip at depth in large strike-slip earthquakes? The importance of fault structural maturity in controlling on-fault slip versus off-fault surface deformation

Comparisons of observed surface displacements with geodetically inferred slip at depth in six large (Mw⩾7.1) strike-slip earthquakes reveal a correlation between the structural maturity of the fault and the ratio of deep slip to surface slip that occurs on localized zones of surface rupture. Specifically, structurally immature faults (⩽25 km of total displacement) manifest only ∼50–60% of total slip on narrow fault surface traces versus ∼85–95% for structurally mature faults (⩾85 km total slip). The same pattern holds when structurally simple parts of a surface rupture are analyzed separately from parts that exhibit obvious structural complexity (e.g., discontinuous or multiple traces or significant dip slip). These results imply that geologic measurements of surface slip along structurally immature faults are likely to significantly underestimate the true slip at depth in large earthquakes. This observation has implications for a number of important problems, including determination of fault slip rates, which are based on surface offsets; earthquake probability assessments, which are based on geologic fault slip rates; comparisons of geologic and geodetic rate data in the search for strain transients; the structural evolution of fault zones; estimation of paleo-earthquake magnitudes based on geomorphic offsets; analyses of the relative importance of faulting vs. distributed deformation in accommodating relative plate motions in continental crust; the design and construction of infrastructure built near active faults; and possibly for the prediction of strong ground motions, which may at least partially depend on the degree of slip localization to the surface.

Full Moment Tensor Variations and Isotropic Characteristics of Earthquakes in the Gulf of California Transform Fault System

The full moment tensor is a mathematical expression of six independent variables; however, on a routine basis, it is a common practice to reduce them to five assuming that the isotropic component is zero. This constraint is valid in most tectonic regimes where slip occurs entirely at the fault surface (e.g. subduction zones); however, we found that full moment tensors are best represented in transform fault systems. Here we present a method to analyze source complexity of earthquakes of different sizes using a simple formulation that relates the elastic constants obtained from independent studies with the angle between the slip and the fault normal vector, referred to as angle $$ \theta $$ ; this angle is obtained from the full moment tensors. The angle $$ \theta $$ , the proportion of volume change $$ \left( k \right) $$ and the constant volume (shear) component $$ \left( T \right) $$ are numerical indicators of complexity of the source; earthquakes are more complex as $$ \theta $$ deviates from $$ \pi /2 $$ or as T and k deviate from zero as well. These parameters are obtained from the eigensolution of the full moment tensor. We analyzed earthquakes in the Gulf of California that exhibit a clear isotropic component and we observed that the constant volume parameter T is independent of scalar moments, suggesting that big and small earthquakes are equally complex. In addition, simple models of one single fault are not sufficient to describe physically all the combinations of $$ \theta $$ in a source type plot. We also found that the principal direction of the strike of the Transform Fault System in the Gulf of California is following the first order approximation of the normal surface of the full moment tensor solution, whereas for deviatoric moment tensors the principal direction does not coincide with the strike of the Transform Fault System. Our observations that small and large earthquakes are equally complex are in agreement with recent studies of strike-slip earthquakes.

Tectonic setting of the recent damaging seismic series in the Southeastern Betic Cordillera, Spain

In this work we analyze the tectonic setting of the recent damaging seismic series occurred in the Internal Zones of the eastern Betic Cordillera (SE Spain) and surrounding areas, the tectonic region where took place the 11th May 2011 Mw 5.2 Lorca earthquake. We revisit and make a synthesis of the seven largest and damaging seismic series occurred from 1984 to 2011. We analyze their seismotectonic setting, and their geological sources under the light of recent advances in the knowledge on active faults, neotectonics, seismotectonics and stress regime, with special attention focused on the Lorca Earthquake. These seismic series are characterized by two types of focal mechanisms, produced mainly by two sets of active faults, NNW–SSE to NNE–SSW small (no larger than 20–30 km) extensional faults with some strike slip component, and E–W to NE–SW large strike slip faults (more than 50 km long) with some compressional component (oblique slip faults). The normal fault earthquakes related to the smaller faults are dominant in the interior of large crustal tectonic blocks that are bounded by the large E–W to NE–SW strike-slip faults. The strike slip earthquakes are associated to the reactivation of segments or intersegment regions of the large E–W to NE–SW faults bounding those crustal tectonic blocks. Most of the seismic series studied in this work can be interpreted as part of the background seismicity that occurs within the crustal blocks that are strained under a transpressional regime driven by the major strike slip shear corridors bounding the blocks. The seismotectonic analysis and the phenomenology of the studied series indicate that it is usual the occurrence of damaging compound earthquakes of M $$\sim $$ 5.0 associated with triggering processes driven by coseismic stress transfer. These processes mainly occur in the seismic series generated by NNW–SSE to NNE–SSW faults. These mechanical interaction processes may induce a higher frequency of occurrence of this kind of earthquakes than considered in traditional probabilistic seismic hazard assessments and it should be taken into account in future seismic hazard assessments.

Strike-slip intraplate earthquakes in the Western Philippine Sea Plate

Abstract On 26 April 2010, a strike-slip earthquake (Mw 6.5) occurred in the Western Philippine Sea Plate. We deployed 14 ocean-bottom seismometers to record the corresponding aftershocks to acquire information regarding these intraplate events. Our results show that the aftershocks were located along two linear features that intersect with an angle of approximately 120° and are considered a conjugate fault set. The P axis of the mainshock focal mechanism is consistent with the compressive stress direction induced by the arc–continent collision occurring in eastern Taiwan. The pre-existing oceanic fracture zones and tectonic fabrics do not appear to be reactivated based on the distinct rupture directions determined from the relocated aftershocks. However, the abrupt halt of the aftershocks at the border of the fracture zone suggests that pre-existing weak zones could act as a barrier to rupture propagation. Moreover, most large earthquakes have occurred near fracture zones, indicating that the pre-existing weakness may favor the generation of earthquakes compared to the other portion of the oceanic plate due to the relatively low rock strength of this zone.

The 2011 MW 6.8 Burma earthquake: fault constraints provided by multiple SAR techniques

We used two tracks of ALOS PALSAR images to investigate the focal mechanism and slip distribution of the 2011 March 24, M W 6.8 Burma strike-slip earthquake. Three different SAR techniques, namely conventional interferometry, SAR pixel offsets (SPO) and multiple-aperture InSAR (MAI), were employed to obtain the coseismic surface deformation fields along the ∼30 km length of the fault rupture. Along-track measurements from SPO and MAI techniques show a high correlation, and were subsequently used to precisely determine the location and extent of the surface fault trace. The best-fitting fault model geometry derived from an iterative inversion technique suggests that the rupture occurred on a near-vertical sinistral strike-slip fault west of the Nam Ma fault with a strike of 70°. A maximum slip of 4.2 m occurs at a depth of 2.5 km, with significant slip constrained only to the upper 10 km of the crust.

Parameterization of 18th January 2011 earthquake in Dalbadin Region, Southwest Pakistan

An earthquake of magnitude 7.3Mw occurred on 18th January 2011 in Southwestern Pakistan, Baluchistan province (Dalbadin Region). The area has complex tectonics due to interaction of Indian, Eurasian and Arabian plates. Both thrust and strike slip earthquakes are dominant in this region with minor, localized normal faulting events. This earthquake under consideration (Dalbadin Earthquake) posed constraints in depth and focal parameters due to lack of data for evaluation of parameters from Pakistan, Iran or Afghanistan region. Normal faulting mechanism has been proposed by many researchers for this earthquake.In the present study the earthquake was relocated using the technique of travel time residuals. Relocated coordinates and depth were utilized to calculate the focal mechanism solution with outcome of a dominant strike slip mechanism, which is contrary to normal faulting. Relocated coordinates and resulting mechanism are more reliable than many reporting agencies as evaluation in this study is augmented by data from local seismic monitoring network of Pakistan. The tectonics in the area is governed by active subduction along the Makran Subduction Zone. This particular earthquake has strike slip mechanism due to breaking of subducting oceanic plate. This earthquake is located where oceanic lithosphere is subducting along with relative movements between Lut and Helmand blocks. Magnitude of this event i.e. Mw=7.3, re evaluated depth and a previous study of mechanism of earthquake in same region (Shafiq et al., 2011) also supports the strike slip movement.

The October 28, 2012 Mw 7.8 Haida Gwaii underthrusting earthquake and tsunami: Slip partitioning along the Queen Charlotte Fault transpressional plate boundary

The Pacific/North American plate boundary is undergoing predominantly right-lateral strike–slip motion along the Queen Charlotte and Fairweather transform faults. The Queen Charlotte Fault (QCF) hosted the largest historical earthquake in Canada, the 1949 MS 8.1 strike–slip earthquake, which ruptured from offshore northern Haida Gwaii several hundred kilometers northwestward. On January 5, 2013 an Mw 7.5 strike–slip faulting event occurred near the northern end of the 1949 rupture zone. Along central and southern Haida Gwaii the relative plate motion has ∼20% oblique convergence across the left-stepping plate boundary. There had been uncertainty in how the compressional component of plate motion is accommodated. The October 28, 2012 Mw 7.8 Haida Gwaii earthquake involved slightly (∼20°) oblique thrust faulting on a shallow (∼18.5°) northeast-dipping fault plane with strike (∼320°) parallel to the QCF, consistent with prior inferences of Pacific Plate underthrusting beneath Haida Gwaii. The rupture extended to shallow depth offshore of Moresby Island beneath a 25–30km wide terrace of sediments that has accumulated in a wedge seaward of the QCF. The shallow thrusting caused seafloor uplift that generated substantial localized tsunami run-up and a modest far-field tsunami that spread across the northern Pacific, prompting a tsunami warning, beach closure, and coastal evacuation in Hawaii, although ultimately tide gauges showed less than 0.8m of water level increase. The mainshock rupture appears to have spread with a ∼2.3km/s rupture velocity over a length of ∼150km, with slip averaging 3.3m concentrated beneath the sedimentary wedge. The event was followed by a substantial aftershock sequence, in which almost all of the larger events involve distributed intraplate normal faulting extending ∼50km oceanward from the QCF. The highly oblique slip partitioning in southern Haida Gwaii is distinctive in that the local plate boundary-parallel motion on the QCF may be accommodated either by infrequent large strike–slip ruptures or by aseismic creep, as seems to be the case for deeper oblique relative plate motion beneath Haida Gwaii, while the sedimentary terrace accumulates plate boundary-perpendicular compressional strain that releases in almost pure thrust faulting earthquakes, seaward of the QCF.

Evaluation of seismo-electric anomalies using magnetic data in Taiwan

Abstract. The Parkinson vectors derived from 3-component geomagnetic data via the magnetic transfer function are discussed with respect to epicentre locations and hypocentre depths of 16 earthquakes (M ≥ 5.5) in Taiwan during a period of 2002–2005. To find out whether electric conductivity changes would happen particularly in the seismoactive depth ranges, i.e. in the vicinity of the earthquake foci, the frequency dependent penetration depth of the electromagnetic waves (skin effect) is taken into account. The background distributions involving the general conductivity structure and the coast effect at 20 particular depths are constructed using the Parkinson vectors during the entire study period. The background distributions are subtracted from the time-varying monitor distributions, which are computed using the Parkinson vectors within the 15-day moving window, to remove responses of the coast effect and underlying conductivity structure. Anomalous depth sections are identified by deviating distributions and agree with the hypocentre depths of 15 thrust and/or strike-slip earthquakes with only one exception of a normal fault event.

Long-term versus short-term deformation of the meizoseismal area of the 2008 Achaia–Elia (MW 6.4) earthquake in NW Peloponnese, Greece: Evidence from historical triangulation and morphotectonic data

The deformation of the meizoseismal area of the 2008 Achaia–Elia (MW 6.4) earthquake in NW Peloponnese, of the first significant strike slip earthquake in continental Greece, was examined in two time scales; of 102years, based on the analysis of high-accuracy historical triangulation data describing shear, and of 105–106years, based on the analysis of the hydrographic network of the area for signs of streams offset by faulting. Our study revealed pre-seismic accumulation of shear strain of the order of 0.2μrad/year in the study area, consistent with recent GPS evidence, but no signs of significant strike slip-induced offsets in the hydrographic network. These results confirm the hypothesis that the 2008 fault, which did not reached the surface and was not associated with significant seismic ground deformation, probably because of a surface flysch layer filtering high-strain events, was associated with an immature or a dormant, recently activated fault. This fault, about 150km long and discordant to the morphotectonic trends of the area, seems first, to contain segments which have progressively reactivated in a specific direction in the last 20years, reminiscent of the North Anatolian Fault, and second, to limit an 150km wide (recent?) shear zone in the internal part of the arc, in a region mostly dominated by thrust faulting and strong destructive earthquakes.

Earthquake nucleation and triggering on an optimally oriented fault

Seismic surface waves from large, distant earthquakes commonly trigger smaller earthquakes. However, delay times of hours to days between the surface waves and the triggered earthquakes weaken the causal connection. Furthermore, when there is no delay, the triggered earthquakes are typically too small or too obscured to obtain reliable source mechanisms. We present observations of instantaneous triggering of a strike-slip earthquake in central Alaska. Shear strain from the optimally aligned teleseismic Love wave induced a 24s exponential foreshock signal leading to the triggered earthquake. This nucleation phase, and the alignment of the triggered earthquake source mechanism with the teleseismic stress field, reveal the behavior of an existing fault under well-calibrated strain conditions. The Alaska earthquake provides the first observation of combined nucleation and triggering, and it suggests that transient stresses during nucleation may influence the subsequent earthquake rupture. Laboratory and theoretical studies of nucleation and triggering may help discriminate between different interpretations for the Alaska earthquake.

Apply of Explicit Finite Element in Seismic Ground Motion Computation

In this paper, we will use the explicit finite element to compute ground motion due to Tangshan earthquake. The explicit finite-element method uses one integration point and an hourglass control scheme. We implement the coarse-grain method in a structured finite-element mesh straightforwardly. At the same time, we also apply the coarse-grain method to a widely used, slightly unstructured finite-element mesh, where unstructured finite elements are only used in the vertical velocity transition zones. By the finite-element methods, we can compute the ground velocity with some distance to the seismogenic fault in Tangshan earthquake. Through the computation, we can find the main character of ground motion for the strike slip earthquake events and the high frequency vibration motion of ground motion.

Aftershocks due to property variations in the fault zone: A mechanical model

Studies of many large strike-slip earthquakes have shown that the seismicity rate increases in quadrants in which the Coulomb stress is increased by the earthquake rupture. This mechanism explains the concentration of aftershocks near the tips of the fault rupture, but cannot explain the aftershocks on the sides of the rupture zone, where the aftershocks are commonly abundant. We suggest that these aftershocks may be induced by variations of fault-zone properties not included in the previous models. In a finite-element model that treats the rupture plane as a zone with heterogeneous physical properties, as suggested by field observations and laboratory experiments, the Coulomb-stress changes triggered by the mainshock no longer uniformly decrease on the sides of the rupture zone, but show complex patterns of negative and positive changes. These results may explain some of the aftershocks on the sides of the mainshock rupture.

Earthquake Fault Scaling: Self-Consistent Relating of Rupture Length, Width, Average Displacement, and Moment Release

In this paper, I propose the scaling relation W = C 1 L β (where β ≈2/3) to describe the scaling of rupture width with rupture length. I also propose a new displacement relation ![Graphic][1] , where A is the area ( LW ). By substituting these equations into the definition of seismic moment (![Graphic][2] ), I have developed a series of self-consistent equations that describe the scaling between seismic moment, rupture area, length, width, and average displacement. In addition to β , the equations have only two variables, C 1 and C 2, which have been estimated empirically for different tectonic settings. The relations predict linear log–log relationships, the slope of which depends only on β . These new scaling relations, unlike previous relations, are self-consistent, in that they enable moment, rupture length, width, area, and displacement to be estimated from each other and with these estimates all being consistent with the definition of seismic moment. I interpret C 1 as depending on the size at which a rupture transitions from having a constant aspect ratio to following a power law and C 2 as depending on the displacement per unit area of fault rupture and so static stress drop. It is likely that these variables differ between tectonic environments; this might explain much of the scatter in the empirical data. I suggest that these relations apply to all faults. For small earthquakes ( M ∼5) earthquakes β =2/3, so L 2.5 applies. For dip-slip earthquakes this scaling applies up to the largest events. For very large ( M >∼7.2) strike-slip earthquakes, which are fault width-limited, β =0 and assuming ![Graphic][3] , then L 1.5 scaling applies. In all cases, M ∝ A 1.5 fault scaling applies. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif

Mechanics of 3-D shear cracks between Rayleigh and shear wave rupture speeds

Though mode II shear fractures (primarily strike-slip earthquakes) can not only exceed the shear wave speed of the medium, but can even reach the compressional wave speed, steady-state calculations showed that speeds between the Rayleigh and shear wave speeds were not possible, thus defining a forbidden zone. For more than 30 years it was believed that this result in which the rupture jumps over the forbidden zone, also holds for 3-D ruptures, in which mode II and mode III (mainly dip-slip faulting) are mixed. Using unprecedentedly fine spatial and temporal grids, we show that even in the simple configuration of homogeneous fault properties and linear slip-weakening friction law, a realistic 3-D rupture which starts from rest and accelerates to some higher velocity, actually does pass smoothly through this forbidden zone, but very fast. The energy flux from the rupture tip is always positive, even within the so-called forbidden zone, contrary to the 2-D case. Finally, our results show that the width of the cohesive zone initially decreases, then increases as the rupture exceeds the shear wave speed and finally again decreases as the rupture accelerates to a speed of ∼90% of the compressional wave speed.

When an oceanic tectonic plate cracks

Analyses of two recent earthquakes of great magnitude show how complex the breaking of the oceanic lithosphere can be, how it is linked to earlier great events and how it triggers seismicity worldwide. See Letters p.240, p.245 & p.250 On 11 April 2012, two of the largest strike-slip earthquakes ever recorded — at magnitudes of 8.7 and 8.2 — occurred in the northeastern Indian Ocean, a few hundred kilometres off the coast of Sumatra. Three groups now report the analysis of seismic data from the days and months before and after these events, as well as the events themselves. Matthias Delescluse and co-authors show that these earthquakes are part of the ongoing boost of intraplate deformation between India and Australia that followed the Aceh 2004 and Nias 2005 megathrust earthquakes. They conclude that the Australian plate, driven by slab-pull forces at the Sunda trench, is gradually detaching from the Indian plate. Han Yue and colleagues show that the 11 April event involved a complex four-fault rupture lasting several minutes, followed two hours later by a magnitude-8.2 aftershock. These great ruptures on a lattice of strike-slip faults that extends through the crust and into the upper mantle represent large lithospheric deformation that may eventually create a localized boundary between the Indian and Australian plates. Fred Pollitz and colleagues show that, in the six days following 11 April, the global rate of remote earthquakes with magnitudes greater than 5.5 increased nearly fivefold, and events up to magnitude 7 seem to have been triggered. The unprecedented delayed triggering power of this earthquake may arise from its strike-slip source geometry, or because it struck at a time of an unusually low global earthquake rate and increased the number of nucleation sites that were very close to failure.

En échelon and orthogonal fault ruptures of the 11 April 2012 great intraplate earthquakes

The Indo-Australian plate is undergoing distributed internal deformation caused by the lateral transition along its northern boundary--from an environment of continental collision to an island arc subduction zone. On 11 April 2012, one of the largest strike-slip earthquakes ever recorded (seismic moment magnitude M(w) 8.7) occurred about 100-200 kilometres southwest of the Sumatra subduction zone. Occurrence of great intraplate strike-slip faulting located seaward of a subduction zone is unusual. It results from northwest-southeast compression within the plate caused by the India-Eurasia continental collision to the northwest, together with northeast-southwest extension associated with slab pull stresses as the plate underthrusts Sumatra to the northeast. Here we use seismic wave analyses to reveal that the 11 April 2012 event had an extraordinarily complex four-fault rupture lasting about 160 seconds, and was followed approximately two hours later by a great (M(w) 8.2) aftershock. The mainshock rupture initially expanded bilaterally with large slip (20-30 metres) on a right-lateral strike-slip fault trending west-northwest to east-southeast (WNW-ESE), and then bilateral rupture was triggered on an orthogonal left-lateral strike-slip fault trending north-northeast to south-southwest (NNE-SSW) that crosses the first fault. This was followed by westward rupture on a second WNW-ESE strike-slip fault offset about 150 kilometres towards the southwest from the first fault. Finally, rupture was triggered on another en échelon WNW-ESE fault about 330 kilometres west of the epicentre crossing the Ninetyeast ridge. The great aftershock, with an epicentre located 185 kilometres to the SSW of the mainshock epicentre, ruptured bilaterally on a NNE-SSW fault. The complex faulting limits our resolution of the slip distribution. These great ruptures on a lattice of strike-slip faults that extend through the crust and a further 30-40 kilometres into the upper mantle represent large lithospheric deformation that may eventually lead to a localized boundary between the Indian and Australian plates.