Rupture Plane Research Articles

Breakdown of Earthquake Self‐Similar Scaling and Source Rupture Directivity in the 2016–2017 Central Italy Seismic Sequence

AbstractTo analyze the source characteristics of the 2016–2017 central Italy seismic sequence, source spectra of 78 earthquakes of ML = 3.5–6.1 were separated from the S wave Fourier spectra using the two‐step nonparametric generalized inversion technique. Source parameters (e.g., stress drop) were estimated from the source spectra following the ω‐square model. Stress drops were found mainly in the range 0.113–12.190 MPa. The significant dependence of stress drop on magnitude indicates the breakdown of earthquake self‐similar scaling in this sequence. The low stress drops for small events following the release of high stress by the mainshock might have led to stress accumulation on the unruptured fault, which could explain the subsequent occurrence of multiple major events. We investigated the source rupture directivity for 36 events through the azimuthal variation of apparent source spectra. Significant variation was observed at specific frequency bands (generally, over corner frequencies) in 10 events caused by rupture directivity, which was verified by the stable estimation of rupture plane. The rupture parameters confirmed unilateral ruptures predominantly on the NNW–SSE striking fault with fast rupture velocity (2.52–2.84 km/s) for the 10 events. Rupture parameters for an additional four events with stable estimated rupture plane were also analyzed. These were characterized by slow rupture velocity causing weak directivity effects. According to the rupture parameters for the 14 events, prevailing SSE or SEE (NNW or NNE) rupture directivity was a common feature for events to the north (south) of the mainshock in this earthquake sequence.

Intraslab Deformation in the 30 November 2018 Anchorage, Alaska, MW 7.1 Earthquake

AbstractAnchorage, Alaska, was strongly shaken on 30 November 2018 by an MW 7.1 earthquake that ruptured within the underthrust Pacific plate at depths of from 45 to 65 km. Ground failures occurred in saturated lowlands filled with sediments, producing notable road damage, but there was limited structural damage in Anchorage, only ~12 km south of the epicenter. The earthquake has a normal faulting geometry with a shallowly dipping east‐west tension axis indicating intraslab deformation, likely between the underthrust Yakutat terrane and adjacent Pacific seafloor. Separate and joint inversions of teleseismic P and SH waves, regional strong ground motions, and GPS static displacements provide a weak preference for a westward steeply dipping rupture plane with up to 2 m of slip distributed over a single slip patch with dimensions of 20 × 20 km. The ~12 s long rupture expanded northward. Aftershocks occur at shallower depths than the mainshock slip zone.

Multi-stage structural and kinematic analysis of a retrogressive rock slope instability complex (Preonzo, Switzerland)

Large catastrophic rock slope failures are difficult to predict because the underlying mechanisms causing slope accelerations are difficult to study under in-situ conditions. For slope failures with compound basal rupture planes, not only fracture propagation and slip in the basal rupture surface, but also within the landslide body control the displacement evolution and time-to-failure. The Preonzo instability complex in southern Switzerland failed several times since 2002 and offers a unique opportunity to study these mechanisms under in-situ conditions. The largest failure at Preonzo occurred in May 2012 and was conditioned by large slope parallel (NNE-SSW) fracture zones, which are intersected by NE and SW and E-W striking faults creating diverging lateral boundaries. Whereas in the head scarp region oblique and flexural toppling along pre-existing fracture sets is the prevailing mechanism, sliding of either planar or wedge type is the preferred kinematic mode facilitated by the new discontinuities developed within the basal rupture surfaces. These dominating mechanisms, leading to a classical rock slope collapse, could only be revealed in retrospect. We compare fracture patterns in stable ground (tectonic and unloading fractures) with new fractures related to rock slope failure observed on basal rupture surfaces and in the head scarp. Simplified mechanical and kinematic analyses show that old pre-existing tectonic fractures cannot accommodate substantial deformations under the in-situ stress conditions and propagate/connect through curved wing cracks forming at high angles. This situation favors the development of new nearly slope parallel fractures, which are interpreted as synthetic P-shears along localized shear zones. Fractographic markings on these fractures indicate a continuous and relatively fast formation, which presumably has taken place during a period of about 60 years prior to catastrophic failure. This detailed description of the multi-stage evolution of slope failure at Preonzo represents a unique data set for future numerical studies of progressive failure in crystalline rocks.

The 2016 Mw 6.0 Hutubi earthquake: A blind thrust event along the northern Tian Shan front

The Tian Shan Range, which trends E-W along the southern margin of the Junggar Basin, is one of the longest and most active intracontinental orogenic belts in central Asia. On 8 December 2016 (05:15:04 UTC), a Mw 6.0 earthquake ruptured the northern Tian Shan front. Here, we use Sentinel-1 radar imagery to investigate the deformation and source parameters related to this event. The co-seismic surface deformation was predominated by uplift without surface rupture. Ascending and descending interferograms indicate that the event triggered small co-seismic deformations with maximum line-of-sight displacements of 22 mm and 24 mm, respectively. Although the north-dipping and south-dipping plane solutions can both fit the observations well, the north-dipping solution with a dip of 58° is preferred in consideration of the relocated aftershocks and regional geological structure. Significant slip is located between depths of 12 km and 17 km, suggesting that the event was caused by a completely blind thrust fault. This blind rupture is characterized largely by a compact thrusting patch with a peak slip of 56 cm at a depth of 13 km. The source model generates a geodetic moment of 6.678 × 1017 N m corresponding to a Mw 5.85 event. Both the interferometric synthetic aperture radar modeling and the aftershock locations indicate that the rupture plane is linked to the Huoerguosi-Manas-Tugulu fault at a depth of ∼16 km, a typical locking depth in the Tian Shan. We suggest that the 2016 Hutubi earthquake more likely occurred on a back-thrust of the Huoerguosi-Manas-Tugulu fault, and the back-thrust is interpreted to represent a preexisting normal fault beneath the Qigu anticline belt that was tectonically inverted during the Cenozoic.

New insights into the 2010 Yushu Mw6.9 mainshock and Mw5.8 aftershock, China, from InSAR observations and inversion

The Mw6.9 Yushu earthquake occurred on 14 April 2010, in Qinghai, China; the largest aftershock, a Mw5.8 event, occurred west of the mainshock on 29 May 2010 (˜40 days later). The aftershock had a different focal mechanism from the mainshock. Furthermore, seismicity after 29 May showed different spatial characteristics in terms of focal depth and distribution direction. To better understand the faulting and the relationship between these two events, we derived the whole displacement field caused by the Yushu mainshock and the Mw5.8 aftershock based on multi-perspective Interferometric Synthetic Aperture Radar (InSAR) data. We then conducted a robust inversion of the slip distribution jointly constrained by InSAR and GPS data. The results indicate that the Mw5.8 aftershock produced a separated deformation field with significant displacement changes of up to ˜4–6 cm, which indicates another intersecting ruptured fault at the west end of the Yushu fault. The slip distribution shows a 75-km NW rupture with a maximum slip of ˜2.1 m at a depth of ˜0–10 km on the main Yushu fault, and a 20 km NE rupture with peak slip of ˜0.4 m at a depth of ˜5–15 km on a vertical hidden fault. Both events showed a dominant left-lateral component. The total rupture length associated with the 2010 Yushu earthquake sequence reached ˜95 km. By calculating Coulomb stress changes, we confirmed that the mainshock triggered the Mw5.8 aftershock. Our results imply that the increased stress at the western end of the Yushu fault caused by the mainshock rupture may have played an important role in transferring the rupture plane from the Yushu fault to the NE hidden fault.

Geomechanical Assessment of the Effect of Inhomogeneities on the Propagation of Hydrofractures in Coal Seams

The article gives a theoretical propagation of the development of hydrofractures in coal seams from the position of geomechanics, including the possibilities and conditions for the fracture to intersect a natural crack. A technique is proposed for carrying out experimental studies of the effect of inhomogeneities on the extension of hydraulic fractures by physical modeling on samples of equivalent materials and the facility for their conduct. The results of experiments based on the proposed technique on samples made on the basis of cement, sand, fly ash, and water are presented, with modeling of inhomogeneities of various types. The results made it possible to conclude that when the closed cavities intersect in the form of gaping cracks of limited prevalence, their rupture fluid envelops with the preservation of the rupture plane. It is noted that when crossing unbounded gaping cracks, the established enveloping phenomenon will obviously occur along the “bridge” at the points of contacts of the opposite banks of cracks that take place in a real massif, and the very intersection of the fracture of the natural crack, mainly, depends on its deformation properties, the magnitude of the angle between the planes of the fracture and the crack, and also the fluid pressure.

DISJUNCTIVE DISLOCATION AND THEIR ROLE IN STRUCTURE AND TRAP FORMATION

The results of data generalization оf tectonic disturbances value in formation of hydrocarbon structures and traps are presented. The double role of disjunctives in the processes of oil and gas accumulation, known and possible screening factors is considered. Disjunctive dislocations in dependence of their nature and geological conditions of existence in the process of oil and gas accumulation can play a structuring role, be a main component in the formation of disjunctly screened traps, or act as fluids migration ways. Genetic and morphologic characteristics of disjunctives in many cases control the possibility of forming and conservation of deposits and the nature of oil and gas territories of their development. Numerous characteristics of disjunctive tectonic disturbances that are differentiated by a set of different features are considered. Predominantly structural forming are deep faults and Earth crust rupture. Among intra-sedimentary-cover ruptures, which are predominantly structure complicating forse, the function of forming structures is most commonly inherent to lystric disjunktives. Deep faults function as main ways of heat and mass transferring from mantle sources, including hydrocarbon compounds. The role of an oil anaerobic oxidation on contact with aquifers across rupture plane accentuated as a possible factor forming disjunctive screened traps.

Recent seismicity rate forecast for North East India: An approach based on rate state friction law

In this study, we have investigated the association of stress-field disparities through earthquake production rate in both spatially and temporally for Northeast region of India. Here, we have implemented the rate and state dependent friction law for forecasting the seismicity rate over MW ≥ 5.0 earthquake events during the period 2013–2015. The most valuable input model parameter of forecasting seismicity rate is Coulomb stresses changes (ΔCFF), which promote the seismicity rate. The ΔCFF distributions that exhibit significant stress increase in the close proximity of the mainshock sources are found to be diminished with the inverse of the distance from the fault rupture plane. Basically, background seismicity depends on the Coulomb stress changes. These Coulomb stress changes amplify the background seismicity, so a little change in stress can produce very low or high seismicity rate in areas of high ambient seismicity. In several zones the high seismicity rate is revealed by the dominancy of the low b-value. The observed background seismicity rate lies between the ranges of 0–4.0. The significant b-value is achieved in the range of 0.57–1.54 along with average value of 0.93. We have taken the value of constant consecutive parameters (i.e. Aσ = 0.05 MPa) and constant effective friction coefficient of μ'=0.4 for all the faults. However, in the forecast model we have obtained the stress perturbations and heterogeneous b-value to infer the reliable result in comparison to reference forecast model for the period of 1963–2012. In this study, we have also adopted the RELM experiment in CSEP testing Centre to verify the consistency of the result related to forecast for the duration of 2013–2015. The statistical test provided the satisfactory result for this study region.

Subsurface fault investigation in Chiang Rai province, northern Thailand by integrated geophysical surveys

A magnitude 6.3 earthquake, the biggest instrumentally recorded earthquake in Thailand, occurred in Chiang Rai Province on 5 May 2014 and caused large damages in the affected area. The earthquake generated numerous aftershocks that could portray the location of the fault plane beneath the ground. In this work, we conducted integrated geophysical surveys consisting of 2D seismic reflection and 2D resistivity imaging surveys to explore for active faults that could have caused this earthquake. A seismic reflection survey line with a total length of 3,750 metres and resistivity survey with a total length of 1,975 metres were conducted along the Chiang Rai earthquake’s aftershock locations. The subsurface fault geometry was imaged from this integrated geophysical survey. Numerous subsurface discontinuities detected from both seismic reflection and 2D resistivity imaging survey were interpreted as potential faults along the survey line with depths from a few meters to around 500 metres. These subsurface discontinuities correspond well with the aftershock locations which could suggest a fault rupture plane.

Source characteristics of the 2017 Mw6.4 Moijabana, Botswana earthquake, a rare lower-crustal event within an ancient zone of weakness

The 2017 Moijabana earthquake in central Botswana (Mw6.4) was a large and deep event for a continental interior and occurred in a region with little historical seismicity. Based on InSAR measurements of surface deformation spanning the event and teleseismic observations, we determine the ruptured fault plane and finite rupture model of the earthquake. Although this oblique normal-faulting earthquake is too deep to uniquely determine the rupture plane geometry from InSAR alone, the best-fitting fault plane constrained by the joint inversion of teleseismic waveforms and InSAR data has a southwest dip and a strike of 126°, roughly consistent with the geologically mapped strike of the Kaapvaal craton's northern edge. Our results indicate that the earthquake had a total duration of ∼10 s, characterized by two major asperities. The first asperity nucleated in the lower crust and then the rupture propagated up-dip. The lower crustal asperity shows a much shorter rise time compared with the shallower asperity, indicating that contrasts in stress or material properties may have played an important role in the rupture process. The earthquake appears to have occurred in the Limpopo belt, a Proterozoic orogenic belt that represents an ancient zone of weakness between the Archean Zimbabwe and Kaapvaal cratons. In the present day, this zone of weakness may be responding to the stress field imposed by the East African Rift System.

Double wedge model for computing seismic sliding displacements of cantilever retaining walls

A double wedge model has been proposed to compute seismic sliding displacements of cantilever retaining walls. Experimental observations indicate formation of rupture planes in the form of an inverted triangular wedge in the backfill of retaining walls. Computation of critical yield acceleration at each time instant considering the v-shaped weakest rupture planes which evolve during the ground motion is the preliminary aim of the double wedge model. The model computes translational displacements considering the relative movement of soil wedge along these rupture planes. It considers velocity compatibility along with the acceleration compatibility between the wall and soil wedge. It also computes approximate depth of subsidence of the backfill soil during ground motion. A simplified double wedge model has been also considered wherein the yield acceleration at all time instants is computed with respect to a fixed wedge. The double wedge model and its simplified version have been compared with different cases studies, which show the seismic sliding displacements matching well with the real measurements.

A High‐Frequency Distance Metric in Ground‐Motion Prediction Equations Based on Seismic Array Backprojections

AbstractTypical ground‐motion prediction equations (GMPEs) measure source‐to‐site distances relative to the closest point on the rupture plane (Rrup). However, for megathrust earthquakes (Mw > 8), the oversimplification of the earthquake source characteristics in distance metrics results in significant bias. Recent studies suggest that the high‐frequency (HF) and low‐frequency (LF) energy tend to emanate from different portions of the megathrusts. This phenomenon motivates an alternative distance metric based on the array backprojection imaging technique that effectively captures regions releasing HF energy. Herein, we define an HF distance metric (Rhf) as the distance from the site to the high‐frequency radiation zone. We study five Mw > 7.2 megathrust earthquakes in Japan and Chile and find that Rhf outperforms Rrup in predicting the ground shaking intensity between 0.5 and 4 Hz. We consider Rhf as a complementary measure to conventional GMPE distance metrics and a more accurate ground‐motion predictor in many cases.

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

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

Coulomb stress changes due to main earthquakes in Southeast Iran during 1981 to 2011

Southeast of Iran experienced eight destructive earthquakes during 30 years from 1981 to 2011. Six of these events with M > 6.5 were fatal and caused great human and financial losses in the region. The 1981 July 28 (Mw 7.2) Sirch earthquake with 65 km surface rupture was the largest event in this region since 1877 and with other three earthquakes occurred in Golbaf-Sirch region during 17 years. The 26 December 2003 (Mw 6.6) Bam earthquake was one of the most destructive events in the recorded history of Iran. There were more than 26,000 killed, 30,000 to 50,000 injured people, and more than 100,000 were homeless. We calculated the static coulomb stress changes due to this earthquake sequence (four earthquakes) between 1981 and 1998 on the Golbaf-Sirch right-lateral fault and the Shahdad reverse fault and a slow slip on the Shahdad fault. Our calculations showed positive stress changes due to previous events on the ruptured plane of next earthquake. For example, the rupture plane of the 14 March 1998 (Mw 6.6) Fandoqa earthquake received a maximum positive stress change about 2.3 MPa. Also, some parts of the surrounding faults received positive stress changes due to these events. Stress changes on the planes of other four events until 2011 were calculated in this study. The 26 December 2003 (Mw 6.6) Bam earthquake and the 20 December 2010 (Mw 6.5) first Rigan earthquake received negligible (about thousandth (0.001)) negative stress changes in this sequence. The last event in our study area, the 27 January 2011 (Mw 6.2) second Rigan earthquake, experienced more than 0.5 MPa coseismic coulomb stress changes especially in its hypocenter and according to our calculations, it is mostly due to the first Rigan event. By using well-located aftershocks of the Rigan earthquake, we investigated the correlation between coulomb stress changes and aftershocks distribution. Calculated coulomb stress changes due to these two events on the optimally oriented strike-slip faults for the first event showed that most of the well-located seismicity occurred in regions of stress increase and majority of them concentrated near the ruptured plane where the stress changes are in the highest value. Based on our computation for the second event, it would be concluded that most of the aftershocks located in the places that imposed stress are positive and some of them are in places where the imposed stress changes are zero or very small. So, there is a good correlation between coulomb stress changes and aftershocks distribution for both Rigan events. Calculating imparted coulomb stress changes that resolved on the nodal planes of the Rigan first event aftershocks has also been considered to examine whether they were brought closer to failure or not by using different fault friction. Various values of effective coefficient of friction (0.2, 0.4, and 0.8) were used to find the best value of fault friction that produces the highest gain in positively stressed aftershocks. Based on these calculations, majority of aftershocks received positive stress changes by increasing the effective coefficient of friction.

Further studies on the 1988MW5.9 Saguenay, Quebec, earthquake sequence

Many small earthquakes occur annually in Eastern Canada, but moderate to strong earthquakes are infrequent. The 25 November 1988 MW5.9 Saguenay mainshock remains the largest earthquake in the last 80 years in eastern North America. In this article, some aspects of that earthquake sequence were re-analyzed using several modern methods. The regional depth-phase modeling procedure was used to refine the focal depths for the foreshock, the aftershocks, and other MN≥ 2.5 regional earthquakes. The hypocenters of 10 earthquakes were relocated using hypoDD. The spatial distribution of eight relocated hypocenters defines the rupture plane of the mainshock. The moment tensor for the mainshock was retrieved using three-component long-period surface wave records at station HRV (Harvard seismograph station) with additional constraints from P-wave polarities. One nodal plane is conclusively identified to be close to the rupture plane, and its strike is similar to the trend of the south wall of the Saguenay Graben. Based on the consistency between the strike of the nodal plane and the trend of the Graben, as well as the deep focal depth distribution, we suggest that the Saguenay earthquake sequence is related to the reactivation of one of the faults of the Saguenay Graben.

Source model of 2016 Mw6.6 Aketao earthquake, Xinjiang derived from Sentinel-1 InSAR observation

On 25 November 2016 (14:24:30 UTC), an Mw6.6 earthquake occurred in Aketao county of Xinjiang. We derived the coseismic deformation field of the earthquake from ESA's Sentinel-1B images and inverted the fault geometry and slip distribution by using a homogenous half-space elastic model and the Steepest Decent Method (SDM) program. The slip model shows that the rupture plane is about 68 km in length and 40 km in width and has an average strike of 105.2°SE and an average dip of 76.0°S. The average amount of slip is ∼0.16 m and the maximum amount of slip is 0.59 m. The estimated seismic moment is 1.31 × 1019 N·m and the corresponding moment magnitude is Mw6.68. The dislocation is mainly distributed in the depth of 5–22 km, where the rupture center is located at 39.17°E, 74.38°N, and a depth of ∼8.8 km, so this earthquake is a shallow earthquake. In terms of the tectonic settings, the earthquake was triggered by the release of residual stresses on the seismogenic Muji fault. The ruptures were stimulated simultaneously by the main shock, which made the slip distribution pattern of a strike-slip fault with a double fracture. Due to the tectonic setting and the seismic gap between the ruptures, the Muji fault would have a risk of rupture to some extent and trigger an earthquake in future.

Source Complexity of the 2015 Mw 7.9 Bonin Earthquake

AbstractThe 30 May 2015 Mw 7.9 Bonin earthquake, one of the largest and deepest earthquakes ever recorded by modern seismology, provides a unique opportunity to study the source process and physical mechanisms of deep‐focus earthquakes. We develop a novel back projection technique that allows source imaging in full three‐dimensional space with a high‐depth resolution. Our results indicate an initial SW‐NE bilateral source propagation followed by a northwest source extension. The multiple‐source inversion reveals a two‐step source process with propagating directions nearly perpendicular to each other, consistent with the 3‐D back projection result. The spatial distribution and focal mechanisms of the subevents cannot be modeled by a single planar rupture, which may display a curved rupture plane or subevents crossing multiple fault interfaces. The complex source process can be best explained by stress or structure heterogeneity within the deep slab.

Foreshocks, b Value Map, and Aftershock Triggering for the 2011 Mw 5.7 Virginia Earthquake

AbstractThe 2011 Mw 5.7 Virginia earthquake and subsequent dense deployment provide us an unprecedented opportunity to study in detail an earthquake sequence within stable continental United States. Here we apply the waveform‐based matched filter technique to obtain more complete earthquake catalogs around the origin time of the Virginia mainshock. With the enhanced earthquake catalogs, we conclude that no foreshock activity existed prior to the Virginia mainshock. The b value map shows significant variations across the aftershock zone, suggesting strong heterogeneity in stress distribution or crustal material in the study region. We also investigate multiple earthquake triggering mechanisms, including static and dynamic triggering, afterslip, and atmospheric pressure changes. We find that dynamic triggering best explains aftershock distributions close to the mainshock's rupture plane, while the activation of off‐fault seismicity is consistent with static Coulomb stress changes. Moreover, an off‐fault earthquake swarm occurred as the Category 2 Hurricane Irene passed by the aftershock zone 5 days after the mainshock, which might be promoted by stress changes associated with atmospheric pressure drop. Our observations suggest that multiple mechanisms may be responsible for triggering aftershocks following one earthquake.

Source Parameter Studies on the 8 January 2017 Mw 6.1 Resolute, Nunavut, Canada, Earthquake

On 8 January 2017, a strong earthquake with Mw 6.1 occurred 8° north of the Arctic Circle and 90 km southeast of Resolute, Nunavut, Canada. Because the epicenter was in a very remote region, the seismic station coverage was not good. As such, the errors in the source parameters determined using conventional procedures were not small. In this article, some results obtained on the source parameters by processing waveform records and modeling procedures are introduced. The teleseismic depth phase sP was used to determine the focal depths of the mainshock and the two principal aftershocks, because the nearest seismic station was at a distance of ∼90 km. The selected mantle Rayleigh-wave records surrounding the epicenter were used to invert for the moment tensor of the mainshock, and the nodal plane corresponding to the rupture plane was selected by analyzing the arrival-time differences between the Sg and Pg or Sn and Pn phases at the three close stations. Based on the focal depth values, it was found that the mainshock and its two principal aftershocks occurred within the lower crust. The fault plane was inferred to strike southeast and dip to the southwest, from the relocated hypocenters of the mainshock and the two principal aftershocks. A procedure using the calculated sP-P time duration to quickly determine focal depth was established. An average crustal model surrounding the epicenter was retrieved using Rayleigh-wave dispersion data.

Simulation of orthogonal horizontal components of near‐fault ground motion for specified earthquake source and site characteristics

SummaryA procedure to generate horizontal pairs of synthetic near‐fault ground motion components for specified earthquake source and site characteristics is presented. Some near‐fault ground motions contain a forward directivity pulse; others do not, even when the conditions for such a pulse are favorable. The proposed procedure generates pulse‐like and non‐pulse‐like motions in appropriate proportions. We use our recent stochastic models of pulse‐like and non‐pulse‐like near‐fault ground motions that are formulated in terms of physically meaningful parameters. The parameters of these models are fitted to databases of recorded pulse‐like and non‐pulse‐like motions. Using these empirical “observations,” predictive relations are developed for the model parameters in terms of the earthquake source and site characteristics (type of faulting, earthquake magnitude, depth to top of rupture plane, source‐to‐site distance, site characteristics, and directivity parameters). The correlation coefficients between the model parameters are also estimated. For a given earthquake scenario, the probability of occurrence of a directivity pulse is first computed; pulse‐like and non‐pulse‐like motions are then simulated according to the predicted proportions using the empirical predictive models. The resulting time series are realistic and reproduce important features of recorded near‐fault ground motions, including the natural variability. Moreover, the statistics of their elastic response spectra agree with those of the NGA‐West2 dataset, with the additional feature of distinguishing between pulse‐like and non‐pulse‐like cases and between forward and backward directivity scenarios. The synthetic motions can be used in addition to or in place of recorded motions in performance‐based earthquake engineering, particularly when recorded motions are scarce.