Showing papers on "Hypocenter published in 2015"

Detailed rupture imaging of the 25 April 2015 Nepal earthquake using teleseismic P waves

Abstract: We analyze the rupture process of the 25 April 2015 Nepal earthquake with globally recorded teleseismic P waves. The rupture propagated east-southeast from the hypocenter for about 160 km with a duration of ∼55 s. Backprojection of both high-frequency (HF, 0.2 to 3 Hz) and low-frequency (LF, 0.05 to 0.2 Hz) P waves suggest a multistage rupture process. From the low-frequency images, we resolve an initial slow downdip (northward) rupture near the nucleation area for the first 20 s (Stage 1), followed by two faster updip ruptures (20 to 40 s for Stage 2 and 40 to 55 s for Stage 3), which released most of the radiated energy northeast of Kathmandu. The centroid rupture power from LF backprojection agrees well with the Global Centroid Moment Tensor solution. The spatial resolution of the backprojection images is validated by applying similar analysis to nearby aftershocks. The overall rupture pattern agrees well with the aftershock distribution. A multiple-asperity model could explain the observed multistage rupture and aftershock distribution.

Ground‐Motion Prediction Equation for Small‐to‐Moderate Events at Short Hypocentral Distances, with Application to Induced‐Seismicity Hazards

Abstract: The evaluation of seismic hazard from induced seismicity requires the development of ground‐motion prediction equations (GMPEs) that are tuned to the key magnitude–distance range for such applications. I use events of M 3–6 at hypocentral distances less than 40 km, drawn from the Next Generation Attenuation‐West 2 (NGA‐West 2) database, to develop a GMPE that accounts correctly for point‐source scaling in both magnitude and distance space for such events. The developed GMPE is in demonstrable agreement with the NGA‐West 2 database and with the predictions of a stochastic point‐source simulation model. The database is sparse at close distances, implying epistemic uncertainty of as much as a factor of 2 in ground‐motion amplitudes within 10 km of the hypocenter. An important conclusion from this study is that the ground‐motion amplitudes for moderate induced events could be much larger near the epicenter than predicted by most of the NGA‐West 2 GMPEs. The potential for large motions is a consequence of the shallow depth of induced events, which places the earthquake fault only a short distance beneath the epicenter.

Integrated seismic source model of the 2015 Gorkha, Nepal, earthquake

Abstract: We compared spatiotemporal slip-rate and high-frequency (around 1 Hz) radiation distributions from teleseismic P wave data to infer the seismic rupture process of the 2015 Gorkha, Nepal, earthquake. For these estimates, we applied a novel waveform inversion formulation that mitigates the effect of Green's functions uncertainty and a hybrid backprojection method that mitigates contamination by depth phases. Our model showed that the dynamic rupture front propagated eastward from the hypocenter at 3.0 km/s and triggered a large-slip event centered about 50 km to the east. It also showed that the large-slip event included a rapid rupture acceleration event and an irregular deceleration of rupture propagation before the rupture termination. Heterogeneity of the stress drop or fracture energy in the eastern part of the rupture area, where aftershock activity was high, inhibited rupture growth. High-frequency radiation sources tended to be in the deeper part of the large-slip area, which suggests that heterogeneity of the stress drop or fracture energy there may have contributed to the damage in and around Kathmandu.

Fluid-faulting interactions: Fracture-mesh and fault-valve behavior in the February 2014 Mammoth Mountain, California, earthquake swarm

Abstract: Faulting and fluid transport in the subsurface are highly coupled processes, which may manifest seismically as earthquake swarms. A swarm in February 2014 beneath densely monitored Mammoth Mountain, California, provides an opportunity to witness these interactions in high resolution. Toward this goal, we employ massive waveform-correlation-based event detection and relative relocation, which quadruples the swarm catalog to more than 6000 earthquakes and produces high-precision locations even for very small events. The swarm's main seismic zone forms a distributed fracture mesh, with individual faults activated in short earthquake bursts. The largest event of the sequence, M 3.1, apparently acted as a fault valve and was followed by a distinct wave of earthquakes propagating ~1 km westward from the updip edge of rupture, 1–2 h later. Late in the swarm, multiple small, shallower subsidiary faults activated with pronounced hypocenter migration, suggesting that a broader fluid pressure pulse propagated through the subsurface.

Numerical Shake Prediction for Earthquake Early Warning: Data Assimilation, Real‐Time Shake Mapping, and Simulation of Wave Propagation

Abstract: Many of the present earthquake early warning (EEW) systems quickly determine an event’s hypocenter and magnitude and then predict strengths of ground motions. The M w 9.0 Tohoku earthquake, however, revealed some technical issues with such methods: (1) underprediction at large distances due to the large extent of the fault rupture and (2) overprediction because the system was confused by multiple aftershocks that occurred simultaneously. To address these issues, we propose a new concept for EEW, in which the distribution of the present wavefield is estimated precisely in real time (real‐time shake mapping) by applying a data assimilation technique, and then the future wavefield is predicted time evolutionally by simulation of seismic‐wave propagation. Information on the hypocenter location and magnitude is not necessarily required. We call this method, in which physical processes are simulated from the precisely estimated present condition, numerical shake prediction because of its analogy to numerical weather prediction in meteorology. By applying the proposed method to the 2011 Tohoku earthquake and the 2004 Mid‐Niigata Prefecture earthquake ( M w 6.7), we show that numerical shake prediction can precisely and rapidly predict ground motion in real time. Online Material: Animations as examples of numerical shake prediction.

Hypocenter migration and crustal seismic velocity distribution observed for the inland earthquake swarms induced by the 2011 Tohoku-Oki earthquake in NE Japan: Implications for crustal fluid distribution and crustal permeability

Abstract: After the occurrence of the 2011 magnitude 9 Tohoku earthquake, the seismicity in the overriding plate changed. The seismicity appears to form distinct belts. From the spatiotemporal distribution of hypocenters, we can quantify the evolution of seismicity after the 2011 Tohoku earthquake. In some earthquake swarms near Sendai (Nagamachi-Rifu fault), Moriyoshi-zan volcano, Senya fault, and the Yamagata–Fukushima border (Aizu-Kitakata area, west of Azuma volcano), we can observe temporal expansion of the focal area. This temporal expansion is attributed to fluid diffusion. Observed diffusivity would correspond to the permeability of about 10−15 (m2). We can detect the area from which fluid migrates as a seismic low-velocity area. In the lower crust, we found seismic low-velocity areas, which appear to be elongated along N–S or NE–SW, the strike of the island arc. These seismic low-velocity areas are located not only beneath the volcanic front but also beneath the fore-arc region. Seismic activity in the upper crust tends to be high above these low-velocity areas in the lower crust. Most of the shallow earthquakes after the 2011 Tohoku earthquake are located above the seismic low-velocity areas. We thus suggest fluid pressure changes are responsible for the belts of seismicity.

Effects of three-dimensional crustal structure and smoothing constraint on earthquake slip inversions: Case study of the Mw6.3 2009 L'Aquila earthquake

Abstract: Earthquake slip inversions aiming to retrieve kinematic rupture characteristics typically assume 1-D velocity models and a flat Earth surface. However, heterogeneous nature of the crust and presence of rough topography lead to seismic scattering and other wave propagation phenomena, introducing complex 3-D effects on ground motions. Here we investigate how the use of imprecise Green's functions—achieved by including 3-D velocity perturbations and topography—affect slip-inversion results. We create sets of synthetic seismograms, including 3-D heterogeneous Earth structure and topography, and then invert these synthetics using Green's functions computed for a horizontally layered 1-D Earth model. We apply a linear inversion, regularized by smoothing and positivity constraint, and examine in detail how smoothing effects perturb the solution. Among others, our tests and resolution analyses demonstrate how imprecise Green's functions introduce artificial slip rate multiples especially at shallow depths and that the timing of the peak slip rate is hardly affected by the chosen smoothing. The investigation is extended to recordings of the 2009 Mw6.3 L'Aquila earthquake, considering both strong motion and high-rate GPS stations. We interpret the inversion results taking into account the lessons learned from the synthetic tests. The retrieved slip model resembles previously published solutions using geodetic data, showing a large-slip asperity southeast of the hypocenter. In agreement with other studies, we find evidence for fast but subshear rupture propagation in updip direction, followed by a delayed propagation along strike. We conjecture that rupture was partially inhibited by a deep localized velocity-strengthening patch that subsequently experienced afterslip.

Strong-Motion Observations of the M 7.8 Gorkha, Nepal, Earthquake Sequence and Development of the N-SHAKE Strong-Motion Network

Abstract: We present and describe strong-motion data observations from the 2015 M 7.8 Gorkha, Nepal, earthquake sequence collected using existing and new Quake-Catcher Network (QCN) and U.S. Geological Survey NetQuakes sensors located in the Kathmandu Valley. A comparison of QCN data with waveforms recorded by a conventional strong-motion (NetQuakes) instrument validates the QCN data. We present preliminary analysis of spectral accelerations, and peak ground acceleration and velocity for earthquakes up to M 7.3 from the QCN stations, as well as preliminary analysis of the mainshock recording from the NetQuakes station. We show that mainshock peak accelerations were lower than expected and conclude the Kathmandu Valley experienced a pervasively nonlinear response during the mainshock. Phase picks from the QCN and NetQuakes data are also used to improve aftershock locations. This study confirms the utility of QCN instruments to contribute to ground-motion investigations and aftershock response in regions where conventional instrumentation and open-access seismic data are limited. Initial pilot installations of QCN instruments in 2014 are now being expanded to create the Nepal–Shaking Hazard Assessment for Kathmandu and its Environment (N-SHAKE) network. Online Material: Figures of Pg arrivals, earthquake locations, epicenter change vectors, and travel-time misfit vector residuals, and tables of QCN and NetQuake stations and relocated hypocenter timing, location, and magnitude.

Seismogeodesy of the 2014 Mw6.1 Napa earthquake, California: Rapid response and modeling of fast rupture on a dipping strike‐slip fault

Abstract: Real-time high-rate geodetic data have been shown to be useful for rapid earthquake response systems during medium to large events. The 2014 Mw6.1 Napa, California earthquake is important because it provides an opportunity to study an event at the lower threshold of what can be detected with GPS. We show the results of GPS-only earthquake source products such as peak ground displacement magnitude scaling, centroid moment tensor (CMT) solution, and static slip inversion. We also highlight the retrospective real-time combination of GPS and strong motion data to produce seismogeodetic waveforms that have higher precision and longer period information than GPS-only or seismic-only measurements of ground motion. We show their utility for rapid kinematic slip inversion and conclude that it would have been possible, with current real-time infrastructure, to determine the basic features of the earthquake source. We supplement the analysis with strong motion data collected close to the source to obtain an improved postevent image of the source process. The model reveals unilateral fast propagation of slip to the north of the hypocenter with a delayed onset of shallow slip. The source model suggests that the multiple strands of observed surface rupture are controlled by the shallow soft sediments of Napa Valley and do not necessarily represent the intersection of the main faulting surface and the free surface. We conclude that the main dislocation plane is westward dipping and should intersect the surface to the east, either where the easternmost strand of surface rupture is observed or at the location where the West Napa fault has been mapped in the past.

Preceding seismic activity and slow slip events in the source area of the 2011 Mw 9.0 Tohoku-Oki earthquake: a review

Abstract: The 2011 Tohoku-Oki earthquake ruptured a large area of the megathrust east of NE Japan. The earthquake’s magnitude was 9.0, substantially larger than predicted. It is important to know what occurred in the source area prior to this great megathrust earthquake to improve understanding of the nucleation processes of large earthquakes and risk assessments in subduction zones. Seafloor observation data revealed the existence of two extremely large slip patches: one just updip of the mainshock hypocenter and the other 80–100 km to the north near the trench axis. For 70–90 years before 2003, M > 6 events and slips of M > c. 7 events on the megathrust occurred in the areas surrounding these two large slip patches. Seismic activity had increased since at least 2003 in the downdip portion of the source area of the Tohoku-Oki earthquake. In addition, long-term accelerated slow slip occurred in this downdip portion of the source area in the decades before the Tohoku-Oki earthquake. About 1 month before the earthquake, a slow slip event (SSE) took place at relatively shallow depths between the two large slip patches, accompanied by foreshock activity. Both the slow slip and foreshocks propagated from north to south toward the southern large slip patch. Two days before the earthquake, an M 7.3 foreshock and an associated postseismic slip began at relatively deep depths in the megathrust between the two large slip patches. In addition, a slow slip type event seems to have occurred approximately half a day after the M 7.3 foreshock near the mainshock hypocenter. This slow slip event and the foreshock activity again propagated from north to south toward the mainshock hypocenter. These long- and short-term preceding seismic and aseismic slip gradually reduced the interplate coupling, increased shear stresses at the two large slip patches (i.e., two strong asperity patches), and finally led to the rupture of the great Tohoku-Oki earthquake.

Continued Geothermal Reservoir Stimulation Experiments in the Cooper Basin (Australia)

Abstract: Two fluid injection experiments were conducted in the Cooper Basin, Australia, in 2010 and 2012. Following previous stimulations in the Habanero reservoir, approximately 34,000 m3 of water were injected into the granitic crust at 4160 m depth through the Habanero 4 well. More than 29,000 induced seismic events were recorded by a local 24‐station network. Event magnitudes are in the range between M L−1.6 and 3.0. Hypocenter locations could be determined for 21,720 events. The spatial hypocenter distribution indicates that the seismicity occurred on the same subhorizontal layer structure identified previously. Fault‐plane solutions determined for 525 of the strongest events are mostly consistent with the regional stress field acting on a larger scale fracture zone with an orientation as outlined by the hypocenter distribution. The fracture zone is interpreted to be of tectonic origin. Large stress accumulations near a reservoir boundary indicate that the hypocenter distribution is bounded by a structural limit of the fracture zone. High‐pressure stimulation at similar depth in a nearby well caused only minor seismic activity. During an eight‐day stimulation period, less than 80 induced seismic events were recorded followed by another 139 events over the next six months. The maximum event magnitude was M L 1.6. Hypocenters are located in close vicinity of the injection well and align along steeply dipping fractures consistent with their fault mechanisms. These fractures are not well oriented for shearing in the regional stress field. They were seismically activated at extremely large overpressures and accepted only small amounts of fluid. Our findings indicate that natural flowing fractures play a key role for hydraulic stimulations in the crystalline crust.

Upper plate reverse fault reactivation and the unclamping of the megathrust during the 2014 northern Chile earthquake sequence

Abstract: After 137 years without a great earthquake, the Mw 8.1 Pisagua event of 1 April 2014 occurred in the central portion of the southern Peru–northern Chile subduction zone. This megathrust earthquake was preceded by more than 2 weeks of foreshock activity migrating ∼3.5 km/day toward the mainshock hypocenter. This foreshock sequence was triggered by an Mw 6.7 earthquake on a reverse fault in the upper plate that strikes at a high angle to the trench, similar to well-documented reverse faults onshore. These margin-oblique reverse faults accommodate north-south shortening resulting from subduction across a plate boundary that is curved in map view. Reverse slip on the crustal fault unclamped the subduction interface, precipitating the subsequent megathrust foreshock activity that culminated in the great Pisagua earthquake. The combination of crustal reverse faults and a curved subduction margin also occurs in Cascadia and northeastern Japan, indicating that there are two additional localities where great megathrust earthquakes may be triggered by upper plate fault activity.

Seismicity and structure of Akutan and Makushin Volcanoes, Alaska, using joint body and surface wave tomography

Abstract: Joint inversions of seismic data recover models that simultaneously fit multiple constraints while playing upon the strengths of each data type. Here we jointly invert 14 years of local earthquake body wave arrival times from the Alaska Volcano Observatory catalog and Rayleigh wave dispersion curves based upon ambient noise measurements for local Vp, Vs, and hypocentral locations at Akutan and Makushin Volcanoes using a new joint inversion algorithm. The velocity structure and relocated seismicity of both volcanoes are significantly more complex than many other volcanoes studied using similar techniques. Seismicity is distributed among several areas beneath or beyond the flanks of both volcanoes, illuminating a variety of volcanic and tectonic features. The velocity structures of the two volcanoes are exemplified by the presence of narrow high-Vp features in the near surface, indicating likely current or remnant pathways of magma to the surface. A single broad low-Vp region beneath each volcano is slightly offset from each summit and centered at approximately 7 km depth, indicating a potential magma chamber, where magma is stored over longer time periods. Differing recovery capabilities of the Vp and Vs data sets indicate that the results of these types of joint inversions must be interpreted carefully.

The 2013 Mw 6.2 Khaki-Shonbe (Iran) Earthquake: insights into seismic and aseismic shortening of the Zagros sedimentary cover†

Abstract: Determining the relationship between folding and faulting in fold and thrust belts is important for understanding the growth of geological structures, the depth extent of seismic slip, and consequently, the potential earthquake hazard. The 2013 Mw 6.2 Khaki-Shonbe earthquake occurred in the Simply Folded Belt of the Zagros Mountains, Iran. We combine seismological solutions, aftershock relocations, satellite interferometry, and field observations to determine fault geometry and its relationship with the structure, stratigraphy, and tectonics of the central Zagros. We find reverse slip on two along-strike, southwest dipping fault segments. The main shock rupture initiated at the lower northern end of the larger northwest segment. Based upon the hypocenter and rupture duration, slip on the smaller southern segment is likely aseismic. Both faults verge away from the foreland, toward the high-range interior, contrary to the fault geometries depicted in many structural cross sections of the Zagros. The modeled slip occurred over two mutually exclusive depth ranges above 10 km, resulting in long (∼16 km), narrow rupture segments (∼7 km). Aftershocks cluster in the depth range 3–14 km. This indicates reverse slip and coseismic shortening occurred mostly or exclusively in the sedimentary cover, with slip distributions likely to be lithologically controlled in depth by the Hormuz salt at the base of the sedimentary cover (∼10–12 km), and the Kazhdumi Formation mudrocks at upper levels (∼4–5 km). Our findings suggest lithology plays a significant role in the depth extent of slip found in reverse faults in folded belts, providing an important control on the potential size of earthquakes.

Primary Surface Ruptures of the Ludian Mw 6.2 Earthquake, Southeastern Tibetan Plateau, China

Abstract: Online Material: Google Earth images. A M w 6.2 earthquake struck Ludian County at 08:30:11 UTC (16:30:10.2 local time) on 3 August 2014 and is one of the most destructive earthquakes that has occurred on an unmapped active fault east of the Xianshuihe–Xiaojiang fault system between the Chuandian rhomb block (the Sichuan–Yunnan block), southeastern Tibetan plateau, and the South China block, a stable tectonic terrain (Fig. 1; Wen et al. , 2013; X. Xu, Jiang, et al. , 2014; Cheng et al. , 2015). Its hypocenter is located at 27.11° N, 103.35° E, at a depth of ∼12 km, adjacent to Longtoushan Township, ∼29 km southwest of Zhaotong City (Liu et al. , 2014; L. Xu et al. , 2014; Y. Zhang et al. , 2014). The relocation of the aftershocks and a little of the reported fieldwork results may demonstrate the seismogenic fault of the Ludian earthquake is the Baogunao–Xiaohe fault, a northwest-trending left-lateral strike-slip fault, which cut through the Zhaotong–Lianfeng fault zone (http://www.eq-igl.ac.cn/contents/41/867.html, last accessed August 2014; Fang et al. , 2014; Liu et al. , 2014; W. Wang, et al. , 2014; X. Xu, Cheng, et al. , 2014; X. Xu, Jiang, et al. , 2014). We know nothing at all about the former, whereas the latter is a highly developed northeast-trending reverse fault with a right-slip component (Wen et al. , 2013). The finite-source inversion using regional seismic broadband data suggests the hypocenter of the Ludian earthquake was initiated from ∼12 km in the upper crust, then propagated unilaterally to the southeast and upward to the shallow with a peak left slip of ∼0.7 m; it likely offset the ground to form a surface rupture zone 10 km in length (Liu et al. , 2014), but there is no any reliable evidence to support or oppose this view. The significant directivity of the rupture propagation during …

Impoundment of the Zipingpu reservoir and triggering of the 2008 Mw 7.9 Wenchuan earthquake, China

Abstract: Impoundment of the Zipingpu reservoir (ZR), China, began in September 2005 and was followed 2.7 years later by the 2008 Mw 7.9 Wenchuan earthquake (WE) rupturing the Longmen Shan Fault (LSF), with its epicenter ~12 km away from the ZR. Based on the poroelastic theory, we employ three-dimensional finite element models to simulate the evolution of stress and pore pressure due to reservoir impoundment, and its effect on the Coulomb failure stress on the LSF. The results indicate that the reservoir impoundment formed a pore pressure front that slowly propagated through the crust with fluid diffusion. The reservoir loading induced either moderate or no increase of the Coulomb failure stress at the hypocenter prior to the WE. The Coulomb failure stress, however, grew ~9.3–69.1 kPa in the depth range of 1–8 km on the LSF, which may have advanced tectonic loading of the fault system by ~60–450 years. Due to uncertainties of fault geometry and hypocenter location of the WE, it is inconclusive whether impoundment of the ZR directly triggered the WE. However, a small event at the hypocenter could have triggered large rupture elsewhere on fault, where the asperities were weakened by the ZR. The microseismicity around the ZR also showed an expanding pattern from the ZR since its impoundment, likely associated with diffusion of a positive pore pressure pulse. These results suggest a poroelastic triggering effect (even if indirectly) of the WE due to the impoundment of the ZR.

The Aftershock Sequence of the 2011 Mineral, Virginia, Earthquake: Temporal and Spatial Distribution, Focal Mechanisms, Regional Stress, and the Role of Coulomb Stress Transfer

Abstract: The aftershocks of the 23 August 2011 M w 5.7 Mineral, Virginia, earthquake were recorded by 36 temporary stations installed by several institutions. We located 3960 aftershocks from 25 August 2011 through 31 December 2011. A subset of 1666 aftershocks resolves details of the hypocenter distribution. We determined 393 focal mechanism solutions. Aftershocks near the mainshock define a previously recognized tabular cluster with orientation similar to a mainshock nodal plane; other aftershocks occurred 10–20 km to the northeast. A large percentage of the aftershocks occurred in regions of positive Coulomb static stress change, and ∼80% of the focal mechanism nodal planes were brought closer to failure. However, the aftershock distribution near the mainshock appears to have been influenced strongly by rupture directivity. Aftershocks at depths less than 4 km exhibit reverse mechanisms with north‐northwest‐trending nodal planes. Most focal mechanisms at depths greater than 6 km are similar to the mainshock, with north‐northeast‐trending nodal planes. A concentration of aftershocks in the 4–6 km depth range near the mainshock are mostly of reverse type but display a 90° range of nodal‐plane trend. Those events appear to outline the periphery of mainshock rupture, where positive Coulomb stress transfer is largest. The focal mechanisms of aftershocks at depths less than 4 km and those greater than 6 km, along with the mainshock, point to the possibility of a depth‐dependent stress field prior to the occurrence of the mainshock. Analysis of earthquake occurrence using a new magnitude scale () indicates a Gutenberg–Richer law b ‐value of 0.864 and an Omori law p ‐value of 1.085, indicative of a typical aftershock sequence. Online Material: Catalogs of aftershock location, magnitude, and focal mechanisms.

Chapter 10: Earthquake, Tomographic, Seismic Reflection, and Gravity Evidence for a Shallowly Dipping Subduction Zone beneath the Caribbean Margin of Northwestern Colombia

Abstract: Earthquake hypocenter relocations, earthquake focal mechanisms, P-wave velocity anomaly tomography, interpretation of deep-penetration seismic reflection lines, and gravity modeling are integrated to define an ESE, 110°-dipping zone of shallow subduction beneath northwestern Colombia. These data define a 15- to 16-km-thick (9.3–9.9 mi), late Cretaceous oceanic plateau (Caribbean plate) that is actively subducting with anomalous low Benioff zone seismicity at a dip of 3–8° over a down-dip distance of 200 km (124 mi) beneath a deformed sedimentary wedge (South Caribbean deformed belt). At a down-dip distance of 450 km (280 mi) from the frontal thrust of the accretionary wedge and at a depth of 130 km (81 mi), tomographic data show that the largely aseismic, subducting Caribbean plate bends and steepens to a dip of 28°–50°. In the depth range of 80–130 km (50–81 mi), tomographic data show that the subducted Caribbean slab exhibits a low-velocity anomaly that we interpret as evidence for slab delamination and enhanced dehydration by rising asthenosphere. Tomographic data beneath the middle Magdalena Basin of northern Colombia show a thick, cold continental lithosphere (ca 60–100 km [37–62 mi]) while gravity data beneath the lower Magdalena Basin show a thin continental crust (24–27 km [15–17 mi] thick) beneath which the Caribbean slab dips in the range of 40–50°. Minor subduction-related volcanism is present in the eastern Cordillera likely as a result of shallow subduction limiting the size of the mantle wedge that is needed for slab melting and arc-related volcanism. Understanding the subduction setting of northern Colombia is fundamental for understanding its heat flow, tectonic history, controls on subsidence, and other parameters needed for petroleum exploration both onshore and offshore.

Imaging supershear rupture for the 2014 Mw 6.9 Northern Aegean earthquake by backprojection of strong motion waveforms

Abstract: The seismic source spatiotemporal evolution of the Mw 6.9 event on 24 May 2014 in Northern Aegean is imaged by backprojection of strong motion envelopes. The results indicate that the event ruptured on two different fault segments. In the first one, rupture propagated from the hypocenter westward for ∼20 km. In the second delayed segment to the east, rupture propagated eastward for ∼65 km with a supershear velocity (∼5.5 km/s). At the end of this rupture the largest stacking amplitudes are imaged, associated with possible stopping phases from the abrupt cessation of a fast slip. Low-aftershock seismicity on the supershear segment implies a simple and linear fault geometry there. This is the third large event in the Northern Aegean Trough-Northern Anatolian Fault zones that has ruptured with supershear speed. This characteristic should be taken into account in studying past events and estimating seismic hazard in this area.

Structural heterogeneity of the midcrust adjacent to the central Alpine Fault, New Zealand: Inferences from seismic tomography and seismicity between Harihari and Ross

Abstract: Determining the rates and distributions of microseismicity near major faults at different points in the seismic cycle is a crucial step toward understanding plate boundary seismogenesis. We analyze data from temporary seismic arrays spanning the central section of the Alpine Fault, New Zealand, using double-difference seismic tomography. This portion of the fault last ruptured in a large earthquake in 1717 AD and is now late in its typical 330 year cycle of Mw∼8 earthquakes. Seismicity varies systematically with distance from the Alpine Fault: (1) directly beneath the fault trace, earthquakes are sparse and largely confined to the footwall at depths of 4–11 km; (2) at distances of 0–9 km southeast of the trace, seismicity is similarly sparse and shallower than 8 km; (3) at distances of 9–20 km southeast of the fault trace, earthquakes are much more prevalent and shallower than 7 km. Hypocenter lineations here are subparallel to faults mapped near the Main Divide of the Southern Alps, confirming that those faults are active. The region of enhanced seismicity is associated with the highest topography and a high-velocity tongue doming at 3–5 km depth. The low-seismicity zone adjacent to the Alpine Fault trace is associated with Vp and Vs values at midcrustal depths about 8 and 6% lower than further southeast. We interpret lateral variations in seismicity rate to reflect patterns of horizontal strain rate superimposed on heterogeneous crustal structure, and the variations in seismicity cutoff depth to be controlled by temperature and permeability structure variations.

Scenario Source Models and Strong Ground Motion for Future Mega‐earthquakes: Application to Lima, Central Peru

Abstract: The 2011 moment magnitude ( M w) 9.0 Tohoku‐Oki Japan earthquake occurred in a region where giant megathrust earthquakes were not expected. This earthquake proved the difficulty in assessing seismic hazard by relying mainly on information from historical and instrumental seismicity. To help improve the seismic‐hazard assessment for such rare events, we propose a methodology to estimate the slip distribution of future megathrust earthquakes based on a model of interseismic coupling distribution in subduction margins, as well as information of historical earthquakes, and apply the method to the central Peru region, Lima. The slip model obtained from geodetic data represents the large scale features of asperities within the megathrust, which is appropriate for simulation of long‐period waves and tsunami modeling. For the simulation of a broadband strong ground motion, we add small scale heterogeneities to the source slip to be able to simulate high frequencies. To achieve this purpose, we propose broadband source models constructed by adding short‐wavelength slip distributions obtained from a Von Karman power spectral density function, to the slip model inferred from interseismic geodetic data. Using these slip models and assuming several hypocenter locations, we calculate a set of strong ground motions for Lima and incorporate site effects obtained from microtremors surveys and geotechnical data. Our simulated average pseudospectral accelerations (period 0.3 s) are above ![Graphic][1] for wide areas in Lima, which may be critical in terms of damage of low‐ to midrise masonry and reinforced concrete buildings, which characterize the majority of buildings in Lima. [1]: /embed/inline-graphic-1.gif

A more accurate relocation of the 2013 M s 7.0 Lushan, Sichuan, China, earthquake sequence, and the seismogenic structure analysis

Abstract: We use a combined earthquake location technique to relocate the M s7.0 Lushan, Sichuan, China, earthquake sequence of April 20, 2013. A stepwise approach, employing three existing location methods (the HYPOINVERSE method, the Minimum 1-D model, and the Double Difference method), is used to improve location precision by iteratively revising the velocity model station corrections, and hypocenter relocations throughout the process. Our stepwise approach has significantly improved the location precision of the Lushan earthquake sequence, yielding hypocenter locations with final errors of 359, 309, and 605 m in the E-W, N-S, and vertical directions, respectively, with average travel time residuals of 0.12 s. Furthermore, we analyzed the seismogenic structure surrounding the Lushan earthquake sequence by combining the results of the relocated hypocenter distribution with new focal mechanism solutions and information from regional geological and geophysical investigations. From our analysis, we conclude that the vast majority of the aftershocks of the Lushan earthquake sequence occurred at depths of 6–9 km, near the front of the southwestern segment of the NE-trending Longmenshan fault zone. Densely aligned hypocenters clearly suggest that the seismogenic structure of the mainshock consists of a set of basal thrust faults dipping to the NW at 40–50°, at a ramp of the deep basal decollement-thrust system at depths of 7–18 km. Focal mechanism solutions suggest that the seismogenic faults have produced almost pure thrusting. At least one SE-dipping back-thrust is also observed within the basement, as indicated by the hypocenter relocations, which points to either a secondary rupture plane during the mainshock or a plane of aftershock slips. A small number of minor events in the Lushan sequence are located at depths of 0–6 km, with a distribution suggesting that the three NE-trending faults with surface traces running through or passing close to the aftershock area are confined to the upper Mesozoic sedimentary cover, making them independent of the deeper thrust faults that ruptured during the mainshock. Therefore, the 2013 M s7.0 Lushan earthquake was a blind thrust fault generated on active thrust faults within the basement of the southwestern Longmenshan fault zone, with an upper limit estimation of the rupture length, average down-dip width, and rupture area of 40, 16, and 640 km2, respectively.

Anomalous ULF Emissions and Their Possible Association with the Strong Earthquakes in Sumatra, Indonesia, during 2007-2012

Abstract: Eleven strong Sumatran earthquakes, with their epicenter less than 550 km away from the Kototabang (KTB) geomagnetic station (2007-2012), were studied to examine the occurrence of anomalous ultra-low frequency emissions (ULF-EM). Anomalous ULF signals, possibly associated with the earthquake’s precursors, were determined by the Welch ratio SZ/SH at 0.06 Hz at the KTB station. These ULF anomalies were then compared with geomagnetic data observed from two reference stations in Darwin and Davao, to prevent misinterpretation of global geomagnetic disturbances as precursors. This study aims to analyze the relationship between earthquake magnitude and hypocenter radius, and seismic index against lead time during ULF-EM anomalies. We used the polarization ratio Welch method in terms of power spectrum density to evaluate the geomagnetic data by overlapping windows and applying fast Fourier transform (FFT). The results showed anomalous variations in onset and lead time, determined using the standard deviation controlling the SZ/SH power pattern. Our positive correlation between lead time of ULF emission and earthquake magnitude as well as between lead time and seismic index. It shows a negative correlation between hypocenter distances to KTB station against lead time.

Structural interpretation and geo-hazard assessment of a locking line: 2005 Kashmir earthquake, western Himalayas

Abstract: The 08 October 2005, magnitude (Mw) 06 Kashmir earthquake occurred along the Balakot–Bagh fault (BBF) with about 30° dip toward NE in the internal part of the western Himalayas in north Pakistan. It was accompanied by a ground rupture of about 75 km with an average slip of about 5 m along the causative fault. The epicenter of the thrust was located at about 19 km to the NE of its surface trace in Muzaffarabad with about 11 km depth of the hypocenter. The geometry of the fault based on a structural cross section has allowed us to interpret it as a thrust restricted to a roof sequence along a triangle zone across the Hazara–Kashmir syntaxis (HKS). The triangle zone is occupied at depth by a wedge of the Higher Himalayan Sequence (HHS) in the core zone of the HKS. The core–wedge is bounded between the NE-dipping BBF and SE- to SW-dipping thrust stack of the Lesser Himalayan Sequence (LHS) along the northeastern and southwestern limb of the HKS, respectively. Based on surface geology, the overlapping BBF and MBT are interpreted to merge at depth in a roof thrust of Pre-Cambrian (Late Proterozoic) rocks above a duplex which is inferred to have a floor thrust in Early Proterozoic/Archean rocks. The core–wedge is located over a ramp which is connected to the floor thrust in the basement. The BBF is inferred to be active, at least since 1–0.5 Ma, with recurrence interval of about 625 ± 125 years. This out-of-sequence deformation is represented by linear seismicity, both along emergent and blind thrusts in the system, with likelihood of major events as a result of strain buildup due to slow convergence rates (~7 mm/year) in the region. Many towns located along the active fault trace were destroyed or largely damaged due to the earthquake. Major destruction of human dwellings and infrastructure occurred as a consequence of earthquake-triggered landslides, mostly along fault, high river terraces, and road cuts. To minimize future damages in earthquake-prone areas, several mitigation measures are suggested including: (1) avoiding new settlements near the fault trace and landslide susceptible areas, (2) establishment of new township schemes in relatively safer areas with earthquake-sustainable structural designs, and (3) extensive forestation for slope stability, erosion control, and provision of wood for flexible earthquake-resistant structures. The measures are needed for the sustainable development of the region.

Rupture directivity of the August 3rd, 2014 Ludian earthquake (Yunan, China)

Abstract: An M6.5 earthquake occurred on August 3rd, 2014 in Ludian of Yunnan Province in China, causing severe casualty and economic loss. Local broadband waveform inversion with the CAP method demonstrates that the earthquake is a strike-slip event, with the strike along 70° and 160° for the two nodal planes respectively. However, the geological structure in the epicentral region is complicated with abundant active faults, and it is challenging to identify the seismogenic fault with the focal plane solutions due to nodal-plane ambiguity. We resolved the rupture directivity by measuring the difference between centroid location and hypocenter of the Ludian earthquake with the time shift from CAP inversion, and found that the nodal plane with the strike of 160° is the ruptured fault plane. Moreover, the rupture is found to propagate from northwest to southeast. rupture directivity, focal mechanism, Ludian earthquake, strong ground motion