Strike-slip Earthquakes Research Articles

On- and off-fault deformation associated with the September 2013 Mw 7.7 Balochistan earthquake: Implications for geologic slip rate measurements

The 24 September 2013 Mw7.7 Balochistan, Pakistan earthquake ruptured a ~200km-long stretch of the Hoshab fault in southern Pakistan and produced the second-largest lateral surface displacement observed for a continental strike-slip earthquake. We remotely measured surface deformation associated with this event using high-resolution (0.5m) pre- and post-event satellite optical imagery. We document left lateral, near-field, on-fault offsets (10m from fault) using 309 laterally offset piercing points, such as streams, terrace risers, and roads. Peak near-field displacement is 13.6 +2.5/−3.4m. We characterize off-fault deformation by measuring medium- (<350m from fault) and far-field (>350m from fault) displacement using manual (259 measurements) and automated image cross-correlation methods, respectively. Off-fault peak lateral displacement values are ~15m and exceed on-fault displacement magnitudes for ~85% of the rupture length. Our observations suggest that for this rupture, coseismic surface displacement typically increases with distance away from the surface trace of the fault; however, nearly 100% of total surface displacement occurs within a few hundred meters of the primary fault trace. Furthermore, off-fault displacement accounts for, on average, 28% of the total displacement but exhibits a highly heterogeneous along-strike pattern. The best agreement between near-field and far-field displacements generally corresponds to the narrowest fault zone widths. Our analysis demonstrates significant and heterogeneous mismatches between on- and off-fault coseismic deformation, and we conclude that this phenomenon should be considered in hazard models based on geologically determined on-fault slip rates.

A Laboratory Earthquake‐Based Stochastic Seismic Source Generation Algorithm for Strike‐Slip Faults and its Application to the Southern San Andreas Fault

There is a sparse number of credible source models available from large-magnitude past earthquakes. A stochastic source-model-generation algorithm thus becomes necessary for robust risk quantification using scenario earthquakes. We present an algorithm that combines the physics of fault ruptures as imaged in laboratory earthquakes with stress estimates on the fault constrained by field observations to generate stochastic source models for large-magnitude (M w 6.0–8.0) strike-slip earthquakes. The algorithm is validated through a statistical comparison of synthetic ground-motion histories from a stochastically generated source model for a magnitude 7.90 earthquake and a kinematic finite-source inversion of an equivalent magnitude past earthquake on a geometrically similar fault. The synthetic dataset comprises three-component ground-motion waveforms, computed at 636 sites in southern California, for 10 hypothetical rupture scenarios (five hypocenters, each with two rupture directions) on the southern San Andreas fault. A similar validation exercise is conducted for a magnitude 6.0 earthquake, the lower magnitude limit for the algorithm. Additionally, ground motions from the M w 7.9 earthquake simulations are compared against predictions by the Campbell–Bozorgnia Next Generation Attenuation relation, as well as the ShakeOut scenario earthquake. The algorithm is then applied to generate 50 source models for a hypothetical magnitude 7.9 earthquake originating at Parkfield, California, with rupture propagating from north to south (toward Wrightwood), similar to the 1857 Fort Tejon earthquake. Using the spectral element method, three-component ground-motion waveforms are computed in the Los Angeles basin for each scenario earthquake and the sensitivity of ground-shaking intensity to seismic source parameters (such as the percentage of asperity area relative to the fault area, rupture speed, and rise time) is studied. Online Material: Figures of source and simulated peak ground motions for an M w 6.05 scenario on the southern San Andreas fault, and table of V S30 and basin depth at stations.

Seismotectonics and crustal stress field in the Kumaon–Garhwal Himalaya

We present fault plane solutions of 94 well located small-to-moderate sized (1.5≤ML≤5.4) earthquakes, which occurred in the Kumaon–Garhwal Himalaya during 2005–2008, using P-wave polarity and body wave amplitudes. These earthquakes show a mixture of thrust, normal and strike-slip type mechanism, with a majority of thrust type. Most of the thrust earthquakes occur at a depth of 8–22km in the Main Central Thrust (MCT) zone and the Lower Himalaya. The spatial distribution of these earthquakes suggest that the strain resulting from the ongoing collision of the Indian plate with the Eurasian plate is being consumed by thrust fault movement mainly on the north dipping Munsiari Thrust and south dipping Tons Thrust. The strike-slip earthquakes are mainly observed in the Lower Himalaya as well as around the Munsiari region in the MCT zone. The normal earthquakes are also observed in different parts of the Kumaon–Garhwal Himalaya and the Gangetic plain. Their occurrence is attributed to the local structure(s) as well as the flexure of the Indian plate. Stress tensor inversion of the calculated fault plane solutions indicates that the maximum compressive stress in the Gangetic plain is N–S directed and near vertical; whereas in the Kumaon–Garhwal Himalaya, it is near horizontal and NNE–SSW directed, and correlating with the prevailing stress condition due to northward movement of Indian plate.

Source parameters of the 2014 Mw 6.1 South Napa earthquake estimated from the Sentinel 1A, COSMO-SkyMed and GPS data

Using the combination of two InSAR and one GPS data sets, we present the detailed source model of the 2014 Mw 6.1 South Napa earthquake, the biggest tremor to hit the San Francisco Bay Area since the 1989 Mw 6.9 Loma Prieta earthquake. The InSAR data are from the Sentinel-1A (S1A) and COSMO-SkyMed (CS) satellites, and GPS data are provided by Nevada Geodetic Laboratory. We firstly obtain the complete coseismic deformation fields of this event and estimate the InSAR data errors, then using the S1A data to construct the fault geometry, one main and two short parallel sub-faults which haven't been identified by field investigation. As expected the geometry is in good agreement with the aftershock distribution. By inverting the InSAR and GPS data, we derive a three segment slip and rake models. Our model indicates that this event was a right-lateral strike-slip earthquake with a slight reverse component in the West Napa Fault as we estimated. The fault is ~30km long and more than 80% of the seismic moment was released at the center of the fault segment, where the slip reached its maximum (up to 1m). We also find that our geodetic moment magnitude is 2.07×1018Nm, corresponding to Mw 6.18, larger than that of USGS (Mw 6.0) and GCMT (Mw 6.1). This difference may partly be explained by our InSAR data including about one week's postseismic deformation and aftershocks. The results also demonstrate high SNR and great ability of the newly launched Sentinel-1A in earthquake study. Furthermore, this study suggests that this earthquake has potential to trigger nearby faults, especially the Green Valley fault where the coulomb stress was imparted by the 2014 South Napa earthquake.

Reassessment of the Maximum Fault Rupture Length of Strike-Slip Earthquakes and Inference on Mmax in the Anatolian Peninsula, Turkey

Research Article| April 15, 2015 Reassessment of the Maximum Fault Rupture Length of Strike‐Slip Earthquakes and Inference on Mmax in the Anatolian Peninsula, Turkey Arnaud Mignan; Arnaud Mignan aInstitute of Geophysics, Swiss Federal Institute of Technology (ETH) Zürich, Sonneggstrasse 5, CH‐8092 Zürich, Switzerlandarnaud.mignan@sed.ethz.ch Search for other works by this author on: GSW Google Scholar Laurentiu Danciu; Laurentiu Danciu bSwiss Seismological Service, Swiss Federal Institute of Technology (ETH) Zürich, Sonneggstrasse 5, CH‐8092 Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Domenico Giardini Domenico Giardini aInstitute of Geophysics, Swiss Federal Institute of Technology (ETH) Zürich, Sonneggstrasse 5, CH‐8092 Zürich, Switzerlandarnaud.mignan@sed.ethz.ch Search for other works by this author on: GSW Google Scholar Author and Article Information Arnaud Mignan aInstitute of Geophysics, Swiss Federal Institute of Technology (ETH) Zürich, Sonneggstrasse 5, CH‐8092 Zürich, Switzerlandarnaud.mignan@sed.ethz.ch Laurentiu Danciu bSwiss Seismological Service, Swiss Federal Institute of Technology (ETH) Zürich, Sonneggstrasse 5, CH‐8092 Zürich, Switzerland Domenico Giardini aInstitute of Geophysics, Swiss Federal Institute of Technology (ETH) Zürich, Sonneggstrasse 5, CH‐8092 Zürich, Switzerlandarnaud.mignan@sed.ethz.ch Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2015 by the Seismological Society of America Seismological Research Letters (2015) 86 (3): 890–900. https://doi.org/10.1785/0220140252 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Arnaud Mignan, Laurentiu Danciu, Domenico Giardini; Reassessment of the Maximum Fault Rupture Length of Strike‐Slip Earthquakes and Inference on Mmax in the Anatolian Peninsula, Turkey. Seismological Research Letters 2015;; 86 (3): 890–900. doi: https://doi.org/10.1785/0220140252 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search Seismic‐hazard analyses and stress tests for critical infrastructures show limitations in the treatment of extreme events. These extreme events can be great earthquakes and/or their cascading effects, generally not foreseen in risk analysis and management (e.g., Komendantova et al., 2014). For instance, earthquake ruptures are known to potentially propagate over several segments (e.g., Eberhart‐Phillips et al., 2003; Fliss et al., 2005), yet fault segments are still modeled as individual faults in most regional seismic‐hazard models based on expert opinion and on limited paleoseismic data. Rate anomalies (known as the bulge) in the Uniform California... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

No surface breaking on the Ecemiş Fault, central Turkey, since Late Pleistocene (~ 64.5 ka); new geomorphic and geochronologic data from cosmogenic dating of offset alluvial fans

The Ecemiş Fault Zone (EF) has been recognized as a major left lateral strike-slip fault in the Central Anatolian Fault Zone (CAFZ) of Turkey. However, its Quaternary slip-rate has been challenging to determine due to the difficulty of dating offset markers. Using high-precision offset measurements and 36Cl cosmogenic nuclide dating, we present the first geochronologically determined Late Quaternary slip-rate for the EF. Our study focuses on the excellent exposures of offset alluvial fan surfaces, originating from the Aladağlar, a Late Quaternary glaciated mountain. Analysis of airborne orthophotogrametry and GNSS (Global Navigation Satellite System) surveys indicates 168±2m left lateral and 31±1m vertical displacements. In-situ terrestrial cosmogenic 36Cl geochronology obtained from eleven surface boulders provides a minimum abandonment/incision age of 104.2±16.5ka for the oldest offset alluvial fan surface. Our geomorphic observations together with Self-potential geophysical surveys revealed the presence of an unfaulted alluvial fan terrace, which allows us to constrain the timing of deformation. The abandonment/incision age of this fan is 64.5±5.6ka based on thirteen 36Cl depth profile samples. Accordingly, we obtained a geologic fault slip-rate of 4.2±1.9mma−1 horizontally and 0.8±0.3mma−1 vertically for the time frame between 104.2±16.5ka and 64.5±5.6ka. Our analysis indicates that the EF has not been producing a major surface breaking earthquake on the main strand at least since 64.5±5.6ka (mid-Late Pleistocene). This could be the result of abandonment of the main strand and accommodation of deformation by other faults within the EF. Nevertheless, a recently occurred (30 September 2011) low magnitude (ML: 4.3) left lateral strike-slip earthquake indicates recent seismic activity of the EF. Comparison of the recent GPS velocity field with the longer slip history along the CAFZ indicates a constant but low strain release without surface breaking and very long large earthquake recurrence intervals.

Postseismic Deformation Following the 2010 $$M = 7.2$$ M = 7.2 El Mayor-Cucapah Earthquake: Observations, Kinematic Inversions, and Dynamic Models

Due to its location on a transtensional section of the Pacific-North American plate boundary, the Salton Trough is a region featuring large strike-slip earthquakes within a regime of shallow asthenosphere, high heat flow, and complex faulting, and so postseismic deformation there may feature enhanced viscoelastic relaxation and afterslip that is particularly detectable at the surface. The 2010 M=7.2 El Mayor-Cucapah earthquake was the largest shock in the Salton Trough since 1892 and occurred close to the US-Mexico border, and so the postseismic deformation recorded by the continuous GPS network of southern California provides an opportunity to study the rheology of this region. Three-year postseismic transients extracted from GPS displacement time-series show four key features: (1) 1–2 cm of cumulative uplift in the Imperial Valley and ∼ 1 cm of subsidence in the Peninsular Ranges, (2) relatively large cumulative horizontal displacements > 150 km from the rupture in the Peninsular Ranges, (3) rapidly decaying horizontal displacement rates in the first few months after the earthquake in the Imperial Valley, and (4) sustained horizontal velocities, following the rapid early motions, that were still visibly ongoing 3 years after the earthquake. Kinematic inversions show that the cumulative 3-year postseismic displacement field can be well fit by afterslip on and below the coseismic rupture, though these solutions require afterslip with a total moment equivalent to at least a M=7.2 earthquake and higher slip magnitudes than those predicted by coseismic stress changes. Forward modeling shows that stress-driven afterslip and viscoelastic relaxation in various configurations within the lithosphere can reproduce the early and later horizontal velocities in the Imperial Valley, while Newtonian viscoelastic relaxation in the asthenosphere can reproduce the uplift in the Imperial Valley and the subsidence and large westward displacements in the Peninsular Ranges. We present two forward models of dynamically coupled deformation mechanisms that fit the postseismic transient well: a model combining afterslip in the lower crust, Newtonian viscoelastic relaxation in a localized zone in the lower crust beneath areas of high heat flow and geothermal activity, and Newtonian viscoelastic relaxation in the asthenosphere; and a second model that replaces the afterslip in the first model with viscoelastic relaxation with a stress-dependent viscosity in the mantle. The rheology of this high-heat-flow, high-strain-rate region may incorporate elements of both these models and may well be more complex than either of them.

The M w 6.9 Sikkim–Nepal earthquake of September 2011: a perspective for wrench faulting in the Himalayan thrust zone

In the present study, we analyze the strike-slip-type earthquakes along the Himalayan arc that are transverse to the collision front, with a special emphasis on the Sikkim region. The study examines seismicity and focal mechanism data in conjunction with crustal structure derived from receiver functions and gravity gradients, supported by results from GPS data. We present a relation between the north–south depth distribution of the strike-slip earthquakes in Sikkim, with the northward dipping trend of the Indian crust beneath Eurasia deciphered from teleseismic receiver function analysis of data recorded by our seismic network. Results suggest that transverse faults caused by thrust partitioning along the Himalayan arc manifest as vertical strike-slip faults cutting across the crust of the descending Indian plate down to 60 km. Further, we argue in favor of a consistent right-lateral sense of slip among all the strike-slip-type earthquakes cutting across the Himalayan arc from Kumaon in the west through Sikkim and Bhutan Himalaya, to the Burmese arc and the Sagaing fault on the easternmost end. We infer a wrench fault tectonic model for the Indian plate described by a series of right-lateral strike-slip faults from west to east in the Himalaya, in conjunction with under-thrusting of the Indian plate beneath Eurasia.

The surge of great earthquakes from 2004 to 2014

During the decade from mid-2004 to mid-2014 18 great (Mw≥8.0) earthquakes occurred globally (∼1.8 per year), compared to 71 from 1900 to mid-2004 (∼0.68 per year), yielding a short-term rate increase of 265%. Six events had Mw≥8.5, larger than any prior event since the 1965 Rat Islands earthquake. The December 26, 2004 Mw 9.2 Sumatra earthquake had the longest recorded rupture length of 1300+ km and a rupture duration exceeding 450 s. The largest recorded strike-slip earthquake (Mw 8.7) occurred in the Indo–Australian plate on April 11, 2012. The largest recorded deep focus earthquake (Mw 8.3) occurred under the Sea of Okhotsk on May 24, 2013. While this overall surge of activity has not been demonstrated to be causally linked, regional spatio-temporal clustering is clearly evident for great events along the Sumatra, Kuril and Tonga subduction zones, and longer-range interactions have been established for global seismicity and seismic tremor at lower magnitudes following some of the events. This recent decade of intense great earthquake activity coincided with vastly expanded global networks of seismometers, GPS stations, tsunami gauges, and new satellite imaging capabilities such as InSAR and LandSAT interferometry and gravity measurements by GRACE and GOCE, enabling unprecedented analyses of precursory, co-seismic and post-seismic processes around the subduction zone environments where most of the events occurred. Individual events such as the March 11, 2011, Tohoku, Japan Mw 9.0 earthquake produced more ground motion and tsunami recordings than available for all great earthquakes of the last century collectively. Joint inversion and modeling of the diverse data sets exploit complementary sensitivity of the signals to different aspects of the earthquake processes. Major advances have been achieved in quantifying frictional locking and strain accumulation prior to some great events and in relating it to co-seismic slip heterogeneity. Many surprising aspects of these well-quantified great earthquakes have been manifested, associated with their rupture dimensions, tectonic location, compound faulting, triggering interactions, slow slip and foreshock migration precursors, aftershock complexity, and depth-varying seismic radiation characteristics. Regions with potential for near-future great ruptures include mature seismic gaps along the Mentawai Islands and northern Chile, as well as western North America and the Himalayan front, so more great earthquake activity can certainly be anticipated.

Coseismic ionospheric disturbance of the large strike-slip earthquakes in North Sumatra in 2012: Mw dependence of the disturbance amplitudes

We studied ionospheric responses to the 2012 April 11 Mw 8.6 North Sumatra earthquake using total electron content (TEC) measurements with the regional Global Navigation Satellite System network. This earthquake ruptured the oceanic lithosphere off the Indian Ocean coast of North Sumatra, and is known as the largest strike-slip earthquake ever recorded. Coseismic ionospheric disturbances (CIDs) with rapid TEC enhancement of a few TEC units propagated northward with a speed of acoustic waves (∼1 km s–1). Resonant atmospheric oscillation with a frequency ∼4 mHz have been found as monochromatic oscillation of TEC lasting for an hour after the main shock and the largest aftershock. We compared CID amplitudes of 21 earthquakes world-wide with moment magnitudes (Mw) 6.6–9.2. They roughly obeyed a law such that CID amplitude increases by two orders of magnitude for the Mw increase of three. The 2012 North Sumatra earthquakes slightly deviated negatively from the trend possibly reflecting their strike-slip mechanisms, that is small vertical crustal movements for their magnitudes.

Rupture velocity inferred from near-field shear strain analysis

AbstractWe propose a new technique to determine the rupture velocity of large strike slip earthquakes. By means of simple numerical ground motion simulations, we show that when the rupture penetrates a shallow layer of sediment or fractured rock, shock waves propagate along the surface fault trace in the forward rupture direction. Such shock waves, which are insensitive to the complexity of slip over the fault plane, propagate at a phase velocity equal to the rupture speed. We show that those shock waves can be easily isolated in the frequency domain, and that phase velocity can then be simply obtained from shear strain.

Kinematic earthquake rupture inversion in the frequency domain

We develop a frequency-based approach to earthquake slip inversion that requires no prior information on the rupture velocity or slip-rate functions. Because the inversion is linear and is performed separately at each frequency, it is computationally efficient and suited to imaging the finest resolvable spatial details of rupture. We demonstrate the approach on synthetic seismograms based on the Source Inversion Validation Exercise 1 (SIV1) of a crustal Mw 6.6 strike-slip earthquake recorded locally. A robust inversion approach is obtained by applying a combination of damping, smoothing and forcing zero slip at the edge of the fault model. This approach achieves reasonable data fits, overall agreement to the SIV1 model, including slip-rate functions of each subfault, from which its total slip, slip time history and rupture velocity can be extracted. We demonstrate the method's robustness by exploring the effects of noise, random timing errors, and fault geometry errors. The worst effects on the inversion are seen from errors in the assumed fault geometry.

Strike-slip earthquakes can also be detected in the ionosphere

It is generally assumed that co-seismic ionospheric disturbances are generated by large vertical static displacements of the ground during an earthquake. Consequently, it is expected that co-seismic ionospheric disturbances are only observable after earthquakes with a significant dip–slip component. Therefore, earthquakes dominated by strike-slip motion, i.e. with very little vertical co-seismic component, are not expected to generate ionospheric perturbations. In this work, we use total electron content (TEC) measurements from ground-based GNSS-receivers to study ionospheric response to six recent largest strike-slip earthquakes: the Mw7.8 Kunlun earthquake of 14 November 2001, the Mw8.1 Macquarie earthquake of 23 December 2004, the Sumatra earthquake doublet, Mw8.6 and Mw8.2, of 11 April 2012, the Mw7.7 Balochistan earthquake of 24 September 2013 and the Mw 7.7 Scotia Sea earthquake of 17 November 2013. We show that large strike-slip earthquakes generate large ionospheric perturbations of amplitude comparable with those induced by dip–slip earthquakes of equivalent magnitude. We consider that in the absence of significant vertical static co-seismic displacements of the ground, other seismological parameters (primarily the magnitude of co-seismic horizontal displacements, seismic fault dimensions, seismic slip) may contribute in generation of large-amplitude ionospheric perturbations.

A Bayesian approach to determine the average shape of the slip-rate function and the rupture velocity using near-field phases: the 2003 Mw6.6 Bam earthquake example

SUMMARY Slip-rate function and the rupture velocity are two important parameters that are critical in understanding the physics of earthquakes. When conventional objective functions are used, the slip-rate function is not well resolved from seismic data. Here, we propose a new method to obtain the slip-rate function by utilizing the near-field phases recorded near the fault rupture. First we illustrate the sensitivity of near-field phases to the moment accumulation and modify the objective function in order to take advantage of this sensitivity. By utilizing near-field P waves along with S pulses on the near-source records and using a Bayesian approach, we show that we can constrain the average slip-rate function as well as the average rupture velocity for a strike-slip earthquake. As a case example, we apply this technique to the record of the 2003, Mw6.6 Bam Earthquake. Our results indicate an asymmetric slip-rate function, with acceleration duration of about 0.4s, and deceleration duration of 1.4s. The slip-rate function obtained from kinematic modelling of the 2003 Bam earthquake is consistent with those predicted by dynamic rupture simulations. The rupture velocity is about 82–90 per cent of the shear wave velocity, implying a sub-Rayleigh rupture velocity close to the Rayleigh wave speed. In future cases where abundant near-source strong-motion data exist and slip is well constrained, the method described in this study can be applied to obtain the variation of the slip-rate function along the fault which would improve our understanding of earthquake rupture physics.

Using regional moment tensors to constrain the kinematics and stress evolution of the 2010–2013 Canterbury earthquake sequence, South Island, New Zealand

On September 3, 2010, a MW 7.0 (U.S. Geological Survey moment magnitude) earthquake ruptured across the Canterbury Plains in South Island, New Zealand. Since then, New Zealand GNS Science has recorded over 10,000 aftershocks ML 2.0 and larger, including three destructive ~MW 6.0 earthquakes near Christchurch. We treat the Canterbury earthquake sequence as an intraplate earthquake sequence, and compare its kinematics to an Andersonian model for fault slip in a uniform stress field. We determined moment magnitudes and double couple solutions for 150 earthquakes having MW 3.7 and larger through the use of a waveform inversion technique using data from broadband seismic stations on South Island, New Zealand. The majority (126) of these double couple solutions have strike-slip focal mechanisms, with right-lateral slip on ENE fault planes or equivalently left-lateral slip on SSE fault planes. The remaining focal mechanisms indicate reverse faulting, except for two normal faulting events. The strike-slip segments have compatible orientations for slip in a stress field with a horizontal σ1 oriented ~N115°E, and horizontal σ3. The preference for right lateral strike-slip earthquakes suggests that these structures are inherited from previous stages of deformation. Reverse slip is interpreted to have occurred on previously existing structures in regions with an absence of existing structures optimally oriented for strike-slip deformation. Despite the variations in slip direction and faulting style, most aftershocks had nearly the same P-axis orientation, consistent with the regional σ1. There is no evidence for significant changes in these stress orientations throughout the Canterbury earthquake sequence.

Complementary slip distributions of the August 4, 2003 Mw 7.6 and November 17, 2013 Mw 7.8 South Scotia Ridge earthquakes

The South Scotia Ridge Transform (SSRT) plate boundary between the Scotia and Antarctic plates experienced large strike-slip earthquakes on August 4, 2003 (Mw 7.6) and November 17, 2013 (Mw 7.8). These events have overlapping aftershock zones, which is unusual. A 36°–45° southward dipping fault zone ruptured with left-lateral displacements in each event along the northern margin of the South Orkney micro-continent near 60°S. Slip distributions for the two events are determined using teleseismic body and surface wave recordings along with constraints from GPS ground motion recordings at station BORC on Laurie Island (South Orkney Islands), just south of the SSRT. The aftershock distributions, high-frequency back-projections, and unconstrained body wave finite-fault inversions permit significant overlap of the 2003 and 2013 slip zones; however, the GPS static displacements resolve differences in the large-slip regions of the two ruptures. The 2013 earthquake sequence along the SSRT initiated with Mw 6.1 (November 13) and Mw 6.8 (November 16) foreshocks located ∼50 km west of the mainshock hypocenter, and had aftershocks extending ∼250 km eastward. The rupture spread primarily eastward at ∼2.5 km/s with a total rupture duration of about 120 s, with two distinct patches of large-slip located northwest and northeast of the South Orkney Islands. The rupture swept past BORC, with high-rate GPS (HRGPS) ground motion recordings capturing the time-varying slip history of the faulting. Traditional GPS data require that the largest-slip region of the shorter rupture in 2003 is located in the gap NNE of BORC between the two patches that ruptured in 2013. There appears to be some overlap of lower slip regions. The complementary slip distributions comprise a relatively uniform offset along this portion of the SSRT, which is one of the most seismically active regions of the entire Antarctic plate boundary.

Slow Slip Acceleration beneath Andaman Islands Triggered by the 11 April 2012 Indian Ocean Earthquakes

The M-w 8.6 and 8.2 strike-slip earthquakes that struck the northeast Indian Ocean on 11 April 2012 resulted in coseismic deformation both at near and distant sites. The slip distribution, deduced using seismic-wave analysis for the orthogonal faults that ruptured during these earthquakes, is sufficient to predict the coseismic displacements at the Global Positioning System (GPS) sites, such as NTUS, PALK, and CUSV, but fall short at four continuous sites in the Andaman Islands region. Slip modeling, for times prior to the events, suggests that the lower portion of the thrust fault beneath the Andaman Islands has been slipping at least at the rate of 40 cm/yr, in response to the 2004 Sumatra-Andaman coseismic stress change. Modeling of GPS displacements suggests that the en echelon and orthogonal fault ruptures of the 2012 intraplate oceanic earthquakes could have possibly accelerated the ongoing slow slip, along the lower portion of the thrust fault beneath the islands with a month-long slip of 4-10 cm. The misfit to the coseismic GPS displacements along the Andaman Islands could be improved with a better source model, assuming that no local process contributed to this anomaly.

Source study of the Jan Mayen transform fault strike-slip earthquakes

Seismic source parameters of oceanic transform zone earthquakes have been relatively poorly studied. Previous studies showed that this type of earthquakes has unique characteristics such as not only the relatively common occurrence of slow events with weak seismic radiation at high frequencies but also the occurrence of some events that have high apparent stress indicating strong high frequency radiation. We studied 5 strike-slip earthquakes in the Jan Mayen fracture zone with magnitudes in the range of 5.9<Mw<6.7. We determined finite-fault slip distributions with the iterative deconvolution method from teleseismic data. The slip models were characterized by determining source parameters on the asperities and background areas. We found that the areas of the asperities represent about 23–28% of the total rupture area with two different criteria based on average slip and maximum displacement. Sensitivity tests were carried out to estimate the variability of slip patterns and source parameters. We observed robust solutions and reliable source parameter estimates from the models. We also determined the radiated seismic energy and apparent stress in order to identify possible slow events. Our results suggest that the analyzed events in Jan Mayen have a regular nature in terms of apparent stress, seismic energy and centroid time delay compared to other oceanic transform fault earthquakes.

Localized slip and distributed deformation in oblique settings: the example of the Denali fault system, Alaska

In most fault systems the direction of the relative plate motion is oblique to the azimuth of the existing faults. Hence, during earthquakes the displacement may be partitioned between several faults that accommodate different components of the total motion. Here, we quantify the effect of the obliquity of the fault system relatively to the plate-motion direction on the distribution of the deformation in the fault system, during distinct periods of the seismic cycle. The 2002 November, M w 7.9, Denali strike-slip earthquake ruptured 341 km of the Denali fault. The azimuth of the fault varies by more than 50 • over the total rupture length, making the Denali fault an ideal system to test the effect of obliquity. From west to east, thrust dominates the first part of the rupture while strike-slip dominates the central and eastern sections. Using a kinematic model that considers the obliquity of the plate-motion direction relative to the local fault azimuth, we explored how much of the far-field tectonic loading is accommodated on the main strike-slip fault during the earthquake, and how much is accommodated by distributed deformation off the main fault, on secondary structures. Using a dataset of 735 focal mechanisms, we represent the deformation using strain rosettes and we compare seismological data with model results using the areal strain. Then we developed the parameter Ca, the coefficient of accommodation, which allows a direct quantification of the efficiency of a fault to accommodate oblique motion. Using these indicators, we show that in oblique setting, such as in the Denali case, the aftershocks and the background seismicity are organized to accommodate a significant part of the deformation that is not taken on the Denali strike-slip fault during the main earthquakes. The westward increase of the obliquity actually increases the amount of such deformation accommodated through distributed thrust faults, leading to the westward widening of the Alaska Range, located north of the Denali fault. Therefore, the strain partitioning between localized slip on pre-established major faults and distributed deformation accommodated through aftershocks and background seismicity on smaller faults (highlighted here by the longer-term topography) seems to be needed during the seismic cycle to accommodate the boundary conditions in such oblique settings.

Radiated Energy of Great Earthquakes from Teleseismic Empirical Green’s Function Deconvolution

We expand on the empirical Green’s function deconvolution method of Ide et al. (2011) to estimate radiated energy for the six largest earthquakes worldwide over the last 10 years: 2011 M w 9.0 Tohoku-Oki, 2004 M w 9.1 Sumatra, 2010 M w 8.8 Maule, 2005 M w 8.7 Nias, 2007 M w 8.5 Bengkulu, and 2012 M w 8.6 off-Sumatra. Deconvolution of P, SV and SH components gives consistent energy results that are comparable to estimates found independently by other researchers. Apparent stress for the five great thrust earthquakes is between 0.4 and 0.8 MPa, while the 2012 off-Sumatra strike-slip earthquake has a higher apparent stress of 3 MPa, which is consistent with other studies that find a tendency for strike-slip events to be more energetic. Our results are within the spread of apparent stress from the wider global earthquake population over a large magnitude range. The azimuthal distribution of energy in each case shows signs of directivity, and in some cases, shows less energy radiated in the trench-ward direction, which may suggest enhanced tsunami potential. We find that eGfs as small as ~M 6.5 can be used for teleseismic deconvolution, and that an eGf-mainshock magnitude difference of 1.5 units yields stable results. This implies that M 8 is the minimum mainshock size for which teleseismic eGf deconvolution will work well. We propose that a database of eGf events could be used to calculate radiated energy and apparent stress of great, hazardous events in near real time, i.e., promptly enough that it could contribute to rapid response measures.