Aftershock Hypocenters Research Articles

Study of the aftershock sequence of the Oued Djer (Algeria) January 2nd, 2018 earthquake (Mw = 4.7) using a single station

The locality of Oued Djer, located about 40 km southwest of the city of Algiers, was hit by moderate earthquake Mw = 4.7 on January 2nd, 2018 at 20:59. We study this seismic event using locally recorded accelerograms. We located the main shock at (36.383°N; 2.568°E; 4 km) and we estimated its size that gives M0 = 1.2 E+16 N m (Mw = 4.7) using the inversion procedure. The obtained focal mechanism from the moment tensor estimation yields a reverse faulting solution with a small lateral component and a compression axis oriented NW–SE. We also, located 84 aftershocks events, using a single station, that gives a 2 km-long, 3 km-wide swarm which NE-SW trending and SE dipping. The aftershock hypocenters range from 4.4 km to 5.7 km. Our results are consistent with the seismotectonic context which suggest that the earthquake was related to the south westernmost edge of the Mitidja basin, underlined by a segment of the south Mitidja Fault System represented by the Blida Fault.

Structural complexities and tectonic barriers controlling recent seismic activity in the Pollino area (Calabria–Lucania, southern Italy) – constraints from stress inversion and 3D fault model building

Abstract. We reconstruct the 3D fault model of the structures causative of the 2010–2014 Pollino seismic activity by integrating structural–geological and high-resolution seismological data. We constrained the model at the surface with fault-slip data, and at depth, by using the distributions of selected high-quality relocated hypocenters. Relocations were performed through the non-linear Bayloc algorithm, followed by the double-difference relative location method HypoDD applied to a 3D P-wave velocity model. Geological and seismological data highlight an asymmetric active extensional fault system characterized by an E- to NNE-dipping low-angle detachment, with high-angle synthetic splays, and SW- to WSW-dipping, high-angle antithetic faults. Hypocenter clustering and the time–space evolution of the seismicity suggest that two sub-parallel WSW-dipping seismogenic sources, the Rotonda–Campotenese and Morano–Piano di Ruggio faults, are responsible for the 2010–2014 seismicity. The area of the seismogenic patches obtained projecting the hypocenters of the early aftershocks on the 3D fault planes, are consistent with the observed magnitude of the strongest events (Mw=5.2, and Mw=4.3). Since earthquake-scaling relationships provide maximum expected magnitudes of Mw=6.4 for the Rotonda–Campotenese and Mw=6.2 for the Morano–Piano di Ruggio faults, we may suppose that, during the sequence, the two structures did not entirely release their seismic potential. The reconstructed 3D fault model also points out the relationships between the activated fault system and the western segment of the Pollino Fault. The latter was not involved in the recent seismic activity but could have acted as a barrier to the southern propagation of the seismogenic faults, limiting their dimensions and the magnitude of the generated earthquakes.

VAN GÖLÜ BÖLGESİNDE 2011 YILI DEPREMLERİNİN ARTÇI ŞOK SERİSİ KIRILMA KARMAŞIĞINA YAPISAL BAĞLANTI(DOĞU ANADOLU, TÜRKİYE)

This study presents an analysis of the structural coupling to the two large destructive earthquakes that occurred in Lake Van area on October 23 (Mw 7.1), and November 9 (Mw 5.6), 2011, together with the azimuth-dependent distribution of the seismic activity and microseismicity clusters after the mainshocks, associated with the complex rupture processes of the aftershock sequence. The aftershock sequences after the two mainshocks were linked to the local crustal faults beneath Lake Van area, followed successively and produced unusually intense activity and significant damage in the area. The main purposes of this study are to document the spatial and temporal distribution and evolution of the October 23, 2011 aftershock hypocenters and the azimuth-dependent distribution of seismic activity, and to understand the structural character of the aftershock sequence using the distributional and evolutional patterns of the aftershock hypocenters. A total of 10,000 aftershocks were obtained from seismic data with a high signal-to-noise ratio collected over three years from October 23, 2011 to March 17, 2014. These aftershocks were relocated for the time periods from November 2011 through March 2012 to March 2014 and ≈ 5000 aftershocks were retained in the depth versus distance cross-sections to detect the clusters in the first step of study (November 2011-March 2012). The focal depth distribution of the aftershock clusters, the migration of hypocenter activity and microseismicity clusters were analyzed and the distributional patterns of the detected clusters were assessed using the geometric distribution of the aftershock hypocenters. The spatial and temporal distribution of aftershocks reveals interesting key features of the deep rupture complexity of the Van earthquake. This study presents the results of the first detailed hypocentral observations and this is important data for future seismic hazard analyses in the study area.

2019 M6.7 Yamagata-Oki earthquake in the stress shadow of 2011 Tohoku-Oki earthquake: Was it caused by the reduction in fault strength?

Earthquake occurrence in the stress shadow provides a unique opportunity for extracting information about the physical processes behind earthquakes because it highlights processes other than the ambient stress change in earthquake generation. In this study, we examined the fault structure and the spatiotemporal distribution of the aftershocks of the 2019 M6.7 Yamagata-Oki earthquake, which occurred in the stress shadow of the 2011 M9.0 Tohoku-Oki earthquake, to better understand the earthquake generation mechanism. The detection and hypocenter relocation of the aftershocks led to the delineation of three planar earthquake structures consitent with their focal mechanisms. The results suggest that individual aftershocks were caused by a slip on these macroscopic planar structures. Aftershock hypocenters rapidly migrated upward from the deeper part of the major plane (fault), similar to the recent earthquake swarm sequences following the Tohoku-Oki earthquake in the upper plate. Moreover, we investigated the temporal evolution of the surface strain rate distribution in the source region using GNSS data. The east–west contraction strain rate in the source region of the Yamagata-Oki earthquake, with an E–W compressional reverse fault mechanism, changed to an E–W extension as a result of the Tohoku-Oki earthquake, and it continued until the occurrence of the Yamagata-Oki earthquake. The upward hypocenter migrations, together with the earthquake occurrence in the stress shadow and in the E–W extension strain rate field, suggest that a reduction in the fault strength due to uprising fluids contributed to the occurrence of this earthquake sequence. Localized aseismic deformations, such as aseismic creeps, beneath the fault zone may also have contributed to the earthquake occurrence. The results support the hypothesis that aseismic processes in the deeper part of the fault play crucial roles in the occurrence of shallow intraplate earthquakes.

Hypocenter and Magnitude Analysis of Aftershocks of the 2018 Lombok, Indonesia, Earthquakes Using Local Seismographic Networks

Abstract The island of Lombok in Indonesia is located between the Indo-Australian and Eurasian subduction trenches and the Flores back-arc thrust, making it vulnerable to earthquakes. On 29 July 2018, a significant earthquake Mw 6.4 shook this region and was followed by series of major earthquakes (Mw>5.8) on 5, 9, and 19 August, which led to severe damage in the northern Lombok area. In this study, we attempt to reveal the possible cause of the sequences of the 2018 Lombok earthquakes based on aftershock monitoring data. Twenty stations were deployed to record earthquake waveform data from 4 August to 9 September 2018. In total, 3259 events were identified using 28,728 P- and 20,713 S-wave arrival times during the monitoring. The aftershock hypocenters were determined using a nonlinear approach and relocated using double-difference method. The moment magnitude (Mw) of each event was determined by fitting the displacement spectrum amplitude using a Brune-type model. The magnitudes of the aftershocks range from Mw 1.7 to 6.7. The seismicity pattern reveals three clusters located in the Flores oceanic crust, which fit well with the occurrences of the four events with Mw>6. We interpret these events as the main rupture area of the 2018 Lombok earthquake sequence. Furthermore, an aseismic zone in the vicinity of Rinjani extending toward the northwestern part of Lombok was observed. We propose that the crust in this area has elevated temperatures and is highly fractured thus inhibiting the generation of large earthquakes. The aseismic nature is therefore an artifact of the detection threshold of our network (Mw 4.6).

ИЛЬПЫРСКОЕ ЗЕМЛЕТРЯСЕНИЕ 13 марта 2013 г. с Mwрег=5.8, ML=6.2, КS=13.9, I0p=8 (Камчатский перешеек)

The strong (Mwreg=5.8, ML=6.2) near-surface seismic event (Ilpyrskoye earthquake) occurred at 03h12m on 13 March, in the Kamchatka Isthmus. It was the strongest earthquake between 1962 and 2013 for this area. The greatest macroseismic effect was observed at a distance of ~30 km, I=6–7 on the scale MSK-64. We used two independent methods for determining its regional focal mechanism: 1) regional moment tensor in-version using broadband waveforms; 2) solution based on polarities of the P waves. The results are similar: the focal mechanism of Ilpyrskoye earthquake is thrust faulting with strike-slip component; the compression axis is subhorizontal and is oriented in the north-east – south-west direction. The mechanisms for the two strongest aftershocks were also identified, as a result, a change in focal movements during the aftershock process was revealed.The analysis of the aftershock process which consists of two stages with different de-cay character was performed. The process lasted ~ 75 days. About 200 aftershocks ML=3.0–5.7 (КS=7.5–12.9) were recorded, hypocenter depth estimations vary from 0 to 10 km for about 80 % of them. The strongest aftershock was on May 6, 2013 with ML=5.7, Mwreg=4.8, at which the change in focal movements occurred. According to the results of near real time processing, aftershock cloud of Ilpyrskoye earthquake had a pronounced linearity and a great length, which was an artifact. The main cause of the artifact is the minimum number of stations involved in determining the hypocenters of most aftershocks and their quasi-linear disposition. The confidence areas within which solutions are equivalent are shown. We concluded that Ilpyrskoye earthquake is a serious argument that the area of compression between the Okhotsk and North American plates is extended further to the east and the border passes through the Kamchatka Isthmus

Crustal Fault Connectivity of the Mw 7.8 2016 Kaikōura Earthquake Constrained by Aftershock Relocations

AbstractThe 14 November 2016 Mw7.8 Kaikōura earthquake in the northern South Island, New Zealand, involved highly complex, multifault rupture. We combine data from a temporary network and the permanent national seismograph network to repick and relocate ~2,700 aftershocks of M≥3 that occurred between 14 November 2016 and 13 May 2017. Automatic phase‐picking is carried out using REST, a newly developed hybrid method whose pick quality is assessed by comparing automatic picks for a subset of 138 events with analysts' picks. Aftershock hypocenters computed from high‐quality REST picks and a 3‐D velocity model cluster almost exclusively in the shallow crust of the upper plate and reveal linkages at depth between surface‐rupturing fault segments. Only eight aftershocks are relocated on a deeper structure positioned between patches of geodetically detected afterslip. This indicates that afterslip has not triggered significant earthquake activity on the subduction interface during the period of aftershock activity analyzed.

The nature of the subduction wedge in an erosive margin: Insights from the analysis of aftershocks of the 2015 Mw 8.3 Illapel earthquake beneath the Chilean Coastal Range

Aftershocks of the 16 September 2015 M8.3 Illapel earthquake in central Chile were recorded for a period of one year by the Chile-Illapel Aftershock Experiment (CHILLAX) seismic network. An initial catalog of about 100,000 events was generated by a novel automated picking algorithm that combines an auto-regressive detection/onset estimation method with some recently developed windowing techniques. We combine arrival times of P and S waves from about 9,000 of the best recorded of these events with relative arrival times of teleseismic P waves and phase delays of Rayleigh waves recovered from ambient noise to generate a three-dimensional image of P and S wavespeeds in that part of the Andean margin located beneath the network. Hypocenters of aftershocks located in the final model appear to be grouped into four distinct zones: two lower parallel zones that dip about 20° to the east, an upper, highly active zone that dips about 30° to the east, and a diffuse zone of smaller events located between this upper zone and the surface. Nearly all of the events with M > 4 are located at the bottom edge of the highly active zone and define a plane dipping about 26° to the east. Most of these events have almost identical thrust mechanisms and are separated from the lower zones by a steep gradient in increasing wavespeeds. The lower end of the upper zone, at about 55 km depth, is marked by another velocity gradient dipping to the west that bounds the diffuse shallow activity and projects to the surface west of the high Andes. We postulate that the triangular region delimited by these two high wavespeed gradients is a subduction wedge, generated by the removal of material from the leading edge of the upper plate by subduction erosion, and subsequently underplated back on to the overriding plate at depth. This model explains many of the features we observe in the seismicity and wavespeeds and is consistent with geologic observations of uplift and extension in this part of the Chilean coastal margin. Characteristics of this wedge, such as its shape and the lack of correlation with the coeval back-arc tectonic setting, suggest that high Andes are not directly coupled with the behavior of the subduction interface.

Revisiting the Laalam (Eastern Algeria) March 20, 2006 (Mw 5.1) Earthquake and its Seismotectonic Implication

Hypocenters of main shock and aftershocks of the March 20, 2006 Laalam earthquake are relocated using HypoDD double-difference technique. We combined accelerogram and seismogram data of the National Center of Applied Research in Earthquake Engineering (CGS). Among about 191 aftershocks, recorded at least by 4 stations, 141 aftershocks of Md 1.2–2.7 were relocated using HypoDD. The obtained swarm of epicenters occupying a crustal volume of 5 km × 3 km × 5 km and the focal mechanism corresponds to an unknown pure left lateral strike slip, trending N174°E. We were able to calculate focal mechanisms for only seven aftershocks with more than seven polarity readings, which give a P axis oriented NNW–SSE. The waveform inversion also provided values of Mw 5.1, M0 5.9 10−16 Nm and the depth 4.9 km. The dislocation and the stress drop were estimated to 90 cm and 16.5 bars, respectively.

Aftershock Patterns in Recent Central Apennines Sequences

AbstractDuring the last 20 years, three seismic sequences affected the Apenninic belt (central Italy): Colfiorito (1997–1998), L'Aquila (2009), and Amatrice Visso‐Norcia Campotosto (2016–2017). They lasted for a long time, with a series of moderate‐to‐large earthquakes distributed over 40‐ to 60‐km‐long Apenninic‐trending segments. Their closeness in space and time suggested to study their aftershock sequences to highlight similarities and differences. Aftershock space migration and the distribution of aftershock interarrival times were studied. Mathematical Morphology and nonparametric statistics were applied to reduce the effect of spatial noise. Parametric analysis in time domain and spectral analysis were performed. Two different types of aftershock sequences were found. The L'Aquila sequence presented a continuous and periodic temporal variation (period ≃120 days) of aftershock activity center along the sequence axis, while the other two sequences showed a piecewise continuous pattern and a shorter duration. We also found two different types of temporal evolution of the mean radial distance between the aftershock hypocenters and the one of a reference event corresponding to the start of a large and fast increase of daily energy release. One type was well described by a simple exponential model, while a power law model was more appropriate for the other one. Furthermore, in the first case, the aftershock interarrival time was very well fitted by an exponential model, while noticeable deviations were present in the other case. A possible explanation was provided in terms of the local geological and hydrogeological properties, which depend on the region location with respect to the Ancona‐Anzio tectonic lineament.

Evolution of views on the structure of sources of strong earthquakes
 at the end of XX and beginning of XXI centuries

The paper addresses the evolution of scientific views on the structure of the sources of strong earthquakes at the end of the 20th and beginning of the 21th century in Russia. The scientific concepts that emerged in the main developed countries initially typically lacked a clear and consistent understanding of the structure of sources of the strongest seismic events. In the 1950s, at the Schmidt Institute of Physics of the Earth of the USSR Academy of Sciences, G.A. Gamburtsev formulated a hypothesis of a long-term (a few hundred years) stability of seismic regime of a system of seismic sutures. The recently studied earthquakes have their sources in the regions of the large faults. The earthquakes of larger magnitudes have more extended and structurally more complex sources. Some sources in the considered cases are relatively simple to reconstruct (they encompass the fault planes of the large faults, e.g., the Spitak source, M = 6.8). Other sources are more complex; they are formed in the disjunctive nodes or encompass the crustal blocks. For example, the seismic source of the Altai earthquake (M = 7.3) has a volumetric structure and is developed along the boundaries of the large seismogenic blocks. The Wenchuan earthquake (M = 7.9) has a most complicated source which looks as a three-dimensional (3D) structure composed of a few crustal blocks framed by two extended northeast striking faults and separated by the northwesterly trending transverse fault. The structurally different sources differently manifest themselves in the pattern of seismic dislocations on the surface and in the distribution of aftershock hypocenters at depth. The anomalously low velocity “pockets” identified by local seismic tomography in the source areas of the Spitak and Altai earthquakes which accompany the main and secondary faults at depth are likely to be the zones of dynamic control of these faults. The breaked near-fault zones abundant with cracks and fractures are the severely looze inclusions in the crustal rocks hampering the propagation of seismic waves. Therefore, the P-waves in these pockets propagate at lower velocities than in the undamaged geological medium. The paleoseismological studies of seismic faults in trenches have shown that the strong earthquakes have occurred in the same sources in the past and the recurrence period of the strongest seismic events ranges from a few hundred to a few thousand years. Thus, the combined studies of the source zones of the strongest earthquakes conducted in the past decades in the different regions of Eurasia have shown that Gamburtsev’s hypothesis has remained relevant.

Evolution of Views on the Structure of Sources of Strong Earthquakes at the End of XX and Beginning of XXI Centuries

The evolution of scientific views on the structure of the sources of strong earthquakes at the end of the 20th and beginning of the 21th century in Russia is considered. The lack of a clear, consistent understanding of the structure of sources of the strongest seismic events was initially typical of the scientific concepts that emerged in the main developed countries. In the 1950s, at the Schmidt Institute of Physics of the Earth of the USSR Academy of Sciences, G.A. Gamburtsev formulated a hypothesis about the stability of the seismic regime of a system of seismic sutures over a long period of time (a few hundred years). The seismic sources of the recently studied earthquakes are located in the regions of large faults. With the increase in magnitude, they become more extended and structurally complex. In the considered cases, there are sources that are relatively simply to reconstruct, which encompass the fault planes of the large existing faults (Spitak source, M = 6.8), as well as sources that are more complex, formed in the disjunctive nodes, or those that encompass the crustal blocks. For example, the seismic source of the Altai earthquake (M = 7.3) is characterized by a volumetric structure and is developed along the boundaries of the large seismogenic blocks. The source of the Wenchuan earthquake (M = 7.9) is most complicated. It appears as a three-dimensional (3D) structure composed of a few crustal blocks framed by two extended northeast striking faults and separated by a transverse fault of the northwestern orientation. The sources having a different focal structure differently manifest themselves in the structure of seismic dislocations on the surface and in the distribution of aftershock hypocenters at depth. The anomalously low velocity “pockets” that were identified by the method of local seismic tomography in the source areas of the Spitak and Altai earthquakes and that accompany the main and secondary faults at depth are likely to be the zones of the dynamic influence of these faults. The damaged near-fault zones, with abundant cracks and fractures, are the severely destroyed inclusions in the crustal rocks, and they hamper the propagation of seismic waves. Therefore, within these pockets, the P-waves propagate at a lower velocity than the undamaged geological medium. The results of the paleoseismological study of seismic faults in the trenches showed that strong earthquakes also occurred in the same sources in the past, and the period of recurrence of the strongest seismic events ranges from a few hundred to the first thousand years. Thus, the integrated studies of the source zones of the strongest earthquakes that were conducted in the past decades in different regions of Eurasia have shown that the hypothesis of Gamburtsev has remained relevant.

New Data on Aftershocks of the December 7, 1988, Spitak Earthquake

The paper addresses the collection and analysis of new data on aftershocks that occurred within 20 days of the main shock of the December 7, 1988, Spitak earthquake, Mw = 6.8. The data were used to improve the location of aftershock hypocenters and magnitudes. Available data concerning this 20-day period were the least reliable in terms of completeness, representativeness, and the accuracy of hypocenter location and, in particular, estimation of energy classes and magnitudes. New data were retrieved from the records and bulletins of the seismic stations of the regional and global networks. Hypocenter parameters were determined by means of the minimization of wave travel-time residuals and subsequent double-difference hypocenter relocation. Digital records of the Obninsk and Arti seismic stations (Δ = 15°–18°) and five more distant stations (Δ = 34°–53°) were used to more accurately estimate the surface-wave magnitude of the main shock and strongest aftershock. The aftershock catalog of the Spitak earthquake was substantially revised. First, the previous hypocenter locations (Aref’ev et al., 1991) were improved using the double-difference method; second, new data were retrieved from the bulletins of Caucasian seismic stations. The minimum magnitude of completeness (M c = 1.9) of the new catalog for the first 20 days after the main shock (when there were no epicentral observations) is the same as that for the period from December 7, 1988, to December 31, 1989. The new catalog contains information on 2090 aftershocks with magnitude M = 1.9 and more for the period from December 7, 1988, to December 31, 1989. The double-difference method allowed the location of the epicenters of clustered earthquakes to be reliably estimated with a longitude error of no more than 4.6 km, a latitude error of 4 km, and a depth error of 5 km. The new spatial distribution of the aftershock hypocenters is better correlated with the tectonic setting than the old data. The new catalog can be used to assess seismic hazard after strong earthquakes in the region.

Hypocenter relocations and tsunami simulation for the 15 November 2014 Northern Molucca Sea earthquake in Indonesia

A reverse fault earthquake (Mw 7.1) occurred in the Northern Molucca Sea, Indonesia, on 15 November 2014 at 2:31:40 UTC. The earthquake produced small tsunami waves that are recorded at Jailolo (9 cm), Tobelo (1 cm), and Menado (3 cm) tide gauges. The Indonesian Agency for Climatology, Meteorology, and Geophysics (BMKG) issued a timely (5 minutes after the earthquake) tsunami warning for the event. We used the teleseismic double‐difference seismic tomography method (teletomoDD) to relocate the hypocenters of the mainshock and the aftershocks. The relocated hypocenter of the mainshock for the 2014 Northern Molucca Sea earthquake is located at 1.923°N, 126.539°E, and depth of 48.87 km. In general, the relocated aftershock hypocenters are shallower than those from the BMKG catalog. The relocated hypocenters are distributed within a depth range of 6 to 64 km. The aftershock area from the relocated hypocenters is 80 km long and 55 km wide. The estimated seismic moment from the Global CMT solution (GCMT) was 4.75 × 1019 Nm. We simulated the tsunami from fault model of each GCMT nodal plane to find a fault model that can best explain the observed tsunami heights at Jailolo, Tobelo, and Menado tide gauges. The best single fault model for this event is dipping to the west, has fault length, width, and slip amount of 47 km, 25 km, and 1.16 m, respectively. The K value calculated using the observed and simulated tsunami heights for this best model is 1.026, suggests a very good fit to tsunami observations.

3-D dynamic rupture simulations of the 2016 Kumamoto, Japan, earthquake

Using 3-D dynamic rupture simulations, we investigated the 2016 Mw7.1 Kumamoto, Japan, earthquake to elucidate why and how the rupture of the main shock propagated successfully, assuming a complicated fault geometry estimated on the basis of the distributions of the aftershocks. The Mw7.1 main shock occurred along the Futagawa and Hinagu faults. Within 28 h before the main shock, three M6-class foreshocks occurred. Their hypocenters were located along the Hinagu and Futagawa faults, and their focal mechanisms were similar to that of the main shock. Therefore, an extensive stress shadow should have been generated on the fault plane of the main shock. First, we estimated the geometry of the fault planes of the three foreshocks as well as that of the main shock based on the temporal evolution of the relocated aftershock hypocenters. We then evaluated the static stress changes on the main shock fault plane that were due to the occurrence of the three foreshocks, assuming elliptical cracks with constant stress drops on the estimated fault planes. The obtained static stress change distribution indicated that Coulomb failure stress change (ΔCFS) was positive just below the hypocenter of the main shock, while the ΔCFS in the shallow region above the hypocenter was negative. Therefore, these foreshocks could encourage the initiation of the main shock rupture and could hinder the propagation of the rupture toward the shallow region. Finally, we conducted 3-D dynamic rupture simulations of the main shock using the initial stress distribution, which was the sum of the static stress changes caused by these foreshocks and the regional stress field. Assuming a slip-weakening law with uniform friction parameters, we computed 3-D dynamic rupture by varying the friction parameters and the values of the principal stresses. We obtained feasible parameter ranges that could reproduce the characteristic features of the main shock rupture revealed by seismic waveform analyses. We also observed that the free surface encouraged the slip evolution of the main shock.

Anomalously large complete stress drop during the 2016 Mw 5.2 Borrego Springs earthquake inferred by waveform modeling and near‐source aftershock deficit

AbstractThe 2016 Mw 5.2 Borrego Springs earthquake occurred in the trifurcation area of the San Jacinto Fault Zone and generated more than 23,000 aftershocks. We analyze source properties of this earthquake along with 12,487 precisely located aftershock hypocenters to obtain an unusually detailed view of the rupture process and energy budget for this moderate earthquake. Source time functions are obtained using an empirical Green's function approach and are inverted for a slip distribution on the fault plane. The rupture propagated unilaterally to the northwest over a distance of 1.8 km, resulting in clear directivity signals. Two asperities are identified and the maximum slip is 2.54 m, resulting in a static stress drop of 78.2 MPa. Over 97% of the aftershocks occur more than 1 rupture length from the slip area. We conclude that the Borrego Springs earthquake had a complete stress drop and estimate the seismic efficiency to be 15–26%.

Integrated geophysical characteristics of the 2015 Illapel, Chile, earthquake

AbstractOn 16 September 2015, a Mw 8.3 earthquake ruptured the subduction zone offshore of Illapel, Chile, generating an aftershock sequence with 14 Mw 6.0–7.0 events. A double source W phase moment tensor inversion consists of a Mw 7.2 subevent and the main Mw 8.2 phase. We determine two slip models for the mainshock, one using teleseismic broadband waveforms and the other using static GPS and InSAR surface displacements, which indicate high slip north of the epicenter and west‐northwest of the epicenter near the oceanic trench. These models and slip distributions published in other studies suggest spatial slip uncertainties of ~25 km and have peak slip values that vary by a factor of 2. We relocate aftershock hypocenters using a Bayesian multiple‐event relocation algorithm, revealing a cluster of aftershocks under the Chilean coast associated with deep (20–45 km depth) mainshock slip. Less vigorous aftershock activity also occurred near the trench and along strike of the main aftershock region. Most aftershocks are thrust‐faulting events, except for normal‐faulting events near the trench. Coulomb failure stress change amplitudes and signs are uncertain for aftershocks collocated with deeper mainshock slip; other aftershocks are more clearly associated with loading from the mainshock. These observations reveal a frictionally heterogeneous interface that ruptured in patches at seismogenic depths (associated with many aftershocks) and with homogeneous slip (and few aftershocks) up to the trench. This event likely triggered seismicity separate from the main slip region, including along‐strike events on the megathrust and intraplate extensional events.

Source parameters of the 27th of June 2015 Gulf of Aqaba earthquake

On the 27 June 2015, at 15:34:03 UTC, a moderate-sized earthquake of M w 5.0 occurred in the Gulf of Aqaba. Using teleseismic P waves, the focal mechanism of the mainshock was investigated by two techniques. The first technique used the polarities of the first P wave onsets, and the second technique was based on the normalized waveform modeling technique. The results showed that the extension stress has a NE orientation with a shallow southward plunge while the compression stress has a NW trend with a nearly shallow westward plunge, obtaining a strike-slip mechanism. This result agrees well with the typical consequence of crustal deformation resulting from the ongoing extensional to shear stress regime in the Gulf of Aqaba (NE-SW extension and NW-SE compression). The grid search method over a range of focal depths indicates an optimum solution at 15 ± 1 km. To identify the causative fault plane, the aftershock hypocenters were relocated using the local waveform data and the double-difference technique. Considering the fault trends, the spatial distribution of relocated aftershocks demarcated a NS-oriented causative fault, in consistence with one of the nodal planes of the focal mechanism solution, emphasizing the dominant stress regime in the region. Following the Brune model, the estimates of source parameters exhibited fault lengths of 0.29 ≤ L ≤ 2.48 km, moment magnitudes of 3.0 ≤ M w ≤ 5.0, and stress drops of 0.14 ≤ Δσ < 1.14 MPa, indicating a source scaling similar to the tectonic earthquakes related to plate boundaries.

SISMIKO: emergency network deployment and data sharing for the 2016 central Italy seismic sequence

&lt;p&gt;At 01:36 UTC (03:36 local time) on August 24th 2016, an earthquake Mw 6.0 struck an extensive sector of the central Apennines (coordinates: latitude 42.70° N, longitude 13.23° E, 8.0 km depth). The earthquake caused about 300 casualties and severe damage to the historical buildings and economic activity in an area located near the borders of the Umbria, Lazio, Abruzzo and Marche regions. The Istituto Nazionale di Geofisica e Vulcanologia (INGV) located in few minutes the hypocenter near Accumoli, a small town in the province of Rieti. In the hours after the quake, dozens of events were recorded by the National Seismic Network (Rete Sismica Nazionale, RSN) of the INGV, many of which had a ML &amp;gt; 3.0. The density and coverage of the RSN in the epicentral area meant the epicenter and magnitude of the main event and subsequent shocks that followed it in the early hours of the seismic sequence were well constrained. However, in order to better constrain the localizations of the aftershock hypocenters, especially the depths, a denser seismic monitoring network was needed. Just after the mainshock, SISMIKO, the coordinating body of the emergency seismic network at INGV, was activated in order to install a temporary seismic network integrated with the existing permanent network in the epicentral area. From August the 24th to the 30th, SISMIKO deployed eighteen seismic stations, generally six components (equipped with both velocimeter and accelerometer), with thirteen of the seismic station transmitting in real-time to the INGV seismic monitoring room in Rome. The design and geometry of the temporary network was decided in consolation with other groups who were deploying seismic stations in the region, namely EMERSITO (a group studying site-effects), and the emergency Italian strong motion network (RAN) managed by the National Civil Protection Department (DPC). Further 25 BB temporary seismic stations were deployed by colleagues of the British Geological Survey (BGS) and the School of Geosciences, University of Edinburgh in collaboration with INGV. All data acquired from SISMIKO stations, are quickly available at the European Integrated Data Archive (EIDA). The data acquired by the SISMIKO stations were included in the preliminary analysis that was performed by the Bollettino Sismico Italiano (BSI), the Centro Nazionale Terremoti (CNT) staff working in Ancona, and the INGV-MI, described below.&lt;/p&gt;

Mapping b-value for 2009 Harrat Lunayyir earthquake swarm, western Saudi Arabia and Coulomb stress for its mainshock

The Harrat Lunayyir (HL) earthquake swarm of 2009 originated in the HL volcanic field and attracted global attention mainly due to three factors: (i) its relatively short life span that ushered a large frequency of the swarm population (30,000 events in <2years), (ii) the swarm epicenter zone was contained within a small crustal volume under the HL and (iii) the migratory nature of the swarm following the tectonic trend of a normal fault zone beneath HL. The HL belongs to the Large Igneous Province of Saudi Arabia (LIP-SA) where it correlates to the Great Dikes locally. Our aim in this study is to describe the spatial distribution of the hypocenters, b-value character, and Coulomb stress failure (CSF) in an attempt to analyze the underlying geodynamic process that caused the swarm. We utilize the relocated hypocenters monitored by local networks to examine the b-value characteristics for the swarm. This is best represented in a cross section showing two domains of higher b-value anomalies: two patches occurring at shallow depth and at the deeper crust to the SE from the mainshock originated at the shallower depth northwestward. Consistently positive ΔCFF pattern with a large percentage of aftershocks imply how the mainshock rupture controlled the aftershocks activity. This implies that the failure along the NNW fault trend is due to the prevailing ambient stress field imparted to the swarm. We model this by CSF associated with the mainshock for three time dependent situations: (a) foreshock and aftershock epicenters, (b) foreshock hypocenters, and (c) aftershock hypocenters. In actuality, multiple factors might have controlled the aftershock activity as we speculate that positive Coulomb stress was associated in an area where the higher b-value prevails. The CSF produced by the mainshock illustrates how the stress dissipated along the NNW normal fault zone that interrupts the Great Dykes along the Red Sea coast. These results further suggest that the crustal heterogeneity under HL act as an asperity in the epicentral area, whose origin may relate to magma intrusion into upper crust. However, seismic survey is required for detailing this geologic inference.