Regional Waveforms Research Articles

Shallow Focal Depths of the 2022 Ms 6.8 Luding Earthquake and Its M ≥3 Aftershocks

Abstract The accurate determination of earthquake focal depths is crucial, yet notably challenging. The 2022 Ms 6.8 Luding earthquake sequence is a typical example for which focal depths remain controversial. In this study, we conducted a comprehensive regional waveform analysis, including depth-phase waveform modeling and relative location methods, to determine the focal depths of the Ms 6.8 mainshock and its M ≥ 3 aftershocks. Specifically, we first inverted the focal mechanisms of these events using the generalized cut-and-paste method. We then determined their absolute focal depths using array-based stacking and Pn depth-phase modeling. For events without clear depth phases, we calculated their relative depths with respect to the known ones using differential travel times of Pg, Sg, and Pn phases. Our results indicate that the mainshock’s hypocenter lies at a depth of 11 km, whereas its centroid appears at a shallower depth of ∼6.5 km. All M ≥ 3 aftershocks are located within a narrow depth range of 3–7 km, in contrast to the deeper focal depths previously obtained using conventional travel-time-based location methods. This shallow seismogenic depth aligns well with the rheological behavior of the crust along the southern Xianshuihe fault, in which the brittle–ductile transition depth is estimated to be about 10km. Our results reveal that the mainshock nucleated at the base of the seismogenic zone and ruptured toward the shallower portion, consistent with the geodetically constrained mainshock slip distribution. Our study also emphasizes the necessity of caution when interpreting focal depths determined by routine travel-time-based methods, especially in regions with sparse seismic station coverage.

Joint Inversion of Regional Waveform, First-Motion Polarity, and Synthetic Aperture Radar Surface Displacement for the Fourth and Sixth North Korean Declared Nuclear Explosions

ABSTRACT This study analyzed the Democratic People’s Republic of Korea’s (DPRK) fourth (DPRK4, 6 January 2016 Mw 4.49) and sixth (DPRK6, 7 September 2017 Mw 5.2) declared nuclear tests, employing a joint seismic and Interferometric Synthetic Aperture Radar (InSAR) inversion to improve understanding of these events and enhance moment tensor (MT) inversion capabilities. The recent efforts have focused on employing seismic waveform and InSAR geodetic deformation data separately to analyze these and the previous nuclear tests (e.g., Chiang et al., 2018; Myers et al., 2018; Wang et al., 2018). Building upon our previous work (Chi-Durán et al., 2021), we performed a joint regional waveform, first-motion (FM) polarity, and surface displacement inversion, which demonstrated improved source-type discrimination, a revised MT solution with reduced scalar moment uncertainty, and an independently constrained location. In this article, we build on the previous results for DPRK6 by including an analysis using a four-layered velocity model with free-surface topography to compute the near-source static deformation Green’s functions. The model consists of a 50 m basalt layer (VP=2.07 km/s, VS=1.2 km/s), a 250 m stratified volcanic deposit layer (VP=1.73 km/s, VS=1.0 km/s), a 700 m weathered granodiorite layer (VP=2.5 km/s, VS=1.3 km/s), and a granodiorite half-space (VP=5.35 km/s, VS=3.09 km/s). The half-space shares the velocity of the regional MDJ2 velocity model (Ford et al., 2010), which has proven effective for waveform inversion in the region. This model considers the range of reported values for various lithologies and weathering effects. Our findings show that using the layered velocity model enhances the recovery of source location and depth for both the explosions by improving fits and reducing uncertainties. The joint inversion also improves source-type discrimination and better constrains the scalar seismic moment necessary for downstream yield estimation.

Tectonic stress regime and faulting style analysis due to sequence of earthquakes in Western Nepal

The current study examines the source mechanism and the regional stress regime of a sequence of moderate-sized earthquakes between October 2022 and October 2023. ISOLA-GUI-a Matlab coded program has been used to estimate Moment Tensor Solution to understand the source mechanisms of the earthquakes in the region. The earthquakes are recorded at the broadband station arrays of the regional seismological network around the Tehri region, installed by the Indian Institute of Technology Roorkee in India. The study focuses on earthquakes with epicentral distances of less than 10°. The investigation aims to comprehend the source process by estimating the focal mechanisms utilizing regional waveform data and then inverting the acquired focal mechanism of these earthquakes to comprehend the active stress regime in the region. The obtained source mechanism reveals a predominance of a double-couple mechanism in faulting and hence, suggests that the majority of the seismic energy is released through shear motion along a defined fault plane. By revealing the complex interaction of geological forces in the Himalayan belt, this investigation will help us to better understand the distribution of stress and deformation patterns beneath the Earth's crust. The main emphasis lies on assessing stress distribution through the utilization of earthquake focal mechanism data and employing the Stress Inversion technique using the WinTensor program. The result of stress inversion suggests that the maximum compressive stress is plunging horizontally, while the minimum compressive stress is oriented vertically. Hence revealing a thrust stress regime in Western Nepal, trending in the NNE- SSW direction corroborating the ongoing tectonic processes associated with the collision of the Indian plate with the Eurasian plate.

Imaging the Sources of the March 2021 Seismic Sequence in Thessaly Basin (Central Greece) from Kinematic Slip Inversion and Backprojection of Waveform Envelopes

ABSTRACT In March 2021, a series of three moderate events with moment magnitudes Mw 6.3, 6.0, and 5.5 occurred within a span of 10 days in northern Thessaly, Greece, resulting in a stop–start pattern. The moment tensors (MTs) obtained from the events suggested normal faulting along distinct yet adjacent southeast–northwest-trending faults, indicating a sequential triggering process. We applied two methods to investigate the spatial and temporal characteristics of the source process of the triplet. This approach includes linear slip inversion of regional seismic waveforms and static Global Positioning System offsets, along with backprojection of regional waveforms using the Source-Scanning Algorithm technique. The detailed modeling suggests that three adjacent, subparallel (∼300°), and low-angle, detachment-type structures were responsible for the earthquakes. The stress axes orientation was determined by inverting a dataset of recomputed MTs combined with published solutions of the sequence. The results indicated a clockwise rotation of the local extensional axis by ∼25° compared to the regional ∼north–south extension. This rotation is consistent with the orientation of the modeled seismogenic structures. Inversions for both nodal planes and grid-searching fault geometry and orientation showed that for the first-two strongest events, the activated fault planes dipped to the northeast, whereas an antithetic (southwest dipping) fault was activated during the last weaker event. For the Mw 6.3 event, both the independent analyses indicated an overall rupture duration of ∼10 s, energy bursts above and near the hypocenter, up-dip (toward the southwest) rupture propagation in the early stages of the source process, and a bilateral rupture at later stages.

Rupture imaging of the 2021 Ms 6.4 Yangbi, China, earthquake: Implications for the diffuse deformation in the northern region of the Red River fault

In May 2021, a Ms 6.4 earthquake occurred in the Yangbi Country, Yunnan, China, located in the northern region of the Red River fault, resulting in three deaths. It is a typical foreshock-mainshock-aftershock sequence. However, the kinematic rupture process and the interplay between foreshocks, the mainshock, and aftershocks associated with the Yangbi event remain controversial. Here, we decipher the detailed rupture process associated with this moderate event by jointly inverting the teleseismic body waves, three-component regional waveform, near-field Global Positioning System offsets, and Interferometric Synthetic Aperture Radar data. Results show that the rupture expands as a narrow slip-pulse that propagates southeastward. This event is dominated by the dextral movements with minor normal components, cohere with the tectonic shear and dilation strain partitioning. The high slip is concentrated within the depth range of 5–13 km, spanning ∼12 km along strike. A significant shallow slip deficit is identified, perhaps related to the fault being immature. The foreshocks, coseismic slips, and aftershocks reveal a complementary pattern, together releasing the accumulated stresses on the fault. Their distribution areas and Coulomb stress changes suggest that the Yangbi seismic sequence follows the rupture cascade. The absence of large earthquakes, dispersed seismicity, and diffuse strain rate patterns indicate that strain accumulates over a wide area around the northern region of the Red River fault, leading to small-scale ruptures distributed over the area and a low possibility of M ≥ 6.5 events in the near future.

The 2022 Delingha, China, Earthquake Sequence and Implication for Seismic Hazard near the Western End of the Qilian–Haiyuan Fault

Abstract Although the Qilian–Haiyuan fault is known to be responsible for major earthquakes up to M 8, the potential of damaging earthquakes near its western end is not well understood. Since January 2022, three moderate earthquakes (M 5.8, M 6.0, and M 5.4) occurred around the Halahu region of Delingha, China, near the western end of the Qilian–Haiyuan fault. These earthquakes are unusual M 5+ events in this low-seismicity region, and both the U.S. Geological Survey and Global Centroid Moment Tensor solutions suggest that the focal mechanisms of the three mainshocks are distinct from the activity characteristics of the nearby mapped faults. Thus, determining the precise source parameters and identifying the causative fault of this earthquake sequence are important to analyze its seismogenic settings and seismic hazard in this region. In this article, we determined the point-source parameters of the three moderate events via regional waveform modeling and found that these earthquakes are strike-slip events with the nodal planes striking nearly north–south and east–west directions. We then resolved its seismogenic faults by analyzing the aftershock distribution and the rupture directivity. The results show that both the M 5.8 and M 6.0 events ruptured along the north–south nodal plane and expanded toward the south for ∼4 km and ∼3 km, respectively, and the ruptured fault of the M 5.4 is difficult to distinguish. Furthermore, we constrained the relative location using the Interferometric Synthetic Aperture Radar observation, and the result is consistent with that obtained from seismic waveform data. We proposed that this earthquake sequence ruptured along an unmapped dextral fault, which forms a conjugate fault system with the sinistral strike-slip Qilian–Haiyuan fault. Its deformation mechanism may be controlled by long-term, protracted, nearly north–south-trending, right-lateral simple shear in the Qilian Shan fold-thrust belt. The accumulated static Coulomb stress changes resulted in the northwest of this ruptured fault being closer to failure in the future.

Determination of intraplate focal mechanisms with the Brazilian Seismic Network: A simplified Cut-and-Paste approach

The feasibility of a simplified Cut-and-Paste approach (CAP) for routine determination of intraplate earthquake mechanisms in Brazil is demonstrated with data acquired by the Brazilian Seismographic Network (RSBR). Our approach models three-component seismic recordings through: (i) computation of synthetics seismograms with a single, average velocity model, and (ii) independent alignment of observed and synthetic waveforms through waveform cross-correlation. No partitioning of regional waveforms into their corresponding body- and surface-wave trains is considered. The empirical alignment successfully accounts for lateral variations along specific source-station paths, along with location uncertainties and timing errors, and effectively augments the number of waveforms available for inversion. The performance of the proposed approach is demonstrated through modeling of two recent intraplate earthquakes: the Mw 4.4 2017 Maranhão earthquake and the Mw 4.1 2020 Amargosa earthquake. For the Maranhão event, we obtain a strike-slip focal mechanism (340° ± 0°, 70° ± 0°, 0° ± 0°) and a centroid depth of 9 km with magnitude of Mw 4.4, fully consistent with previous studies. For the Amargosa earthquake, we obtain a normal-fault mechanism (20° ± 4.6°, 60° ± 16.9°, -30° ± 14.4°) with a preferred centroid depth of 10 km, which is in general agreement with P-wave polarity data and the orientation of the nearby Salvador Shear Zone. We thus conclude that our simplified CAP approach is an effective tool for modeling regional waveforms in Brazil and, most likely, in sparse networks in general.

Assessing effect of ground reflections on gunshot noise exposure

Investigation of weapons noise exposure outdoors necessitates including the impact of ground reflections. When analyzing the reflections’ impact on A-weighted equivalent level (LAeq), the contribution of the reflected waveform to the total level can often be acceptably modeled using a Friedlander equation-based source model and an ideal reflection based on the measurement geometry. However, the Friedlander model does not adequately model the negative phase of a gunshot waveform, making it inaccurate for locations where the waveform of the direct blast and the waveform of the reflection begin to overlap temporally. This merging of the waveforms for locations where the direct and ground-reflected path length difference is relatively small also makes it difficult to separate the waveforms and calculate the reflected wave’s contribution to noise exposure. As such, this paper investigates different approaches to estimating the contribution of the ground reflected wave to the LAeq, using direct and reflected waveforms in regions where they are well separated and then adjusting timing and amplitude for different geometries where merging takes place. Results reveal regions where the ground reflection contributes negligibly or even negatively to the total LAeq, which is not predicted by the Friedlander model nor by simple energy-addition approaches. [Work supported by Air Force Research Laboratory through Ball Aerospace Technology Corporation.]

The 2014 Zigui Earthquake Sequence near the Three Gorges Dam in China

AbstractSeismicity in the Three Gorges Reservoir (TGR) region increased noticeably as the water level of the reservoir rises, since the Three Gorges Dam (TGD) was built in 2003. Here, we determined moment tensors of six earthquakes in the 2014 Zigui earthquake sequence (ZGS) in the TGR region using local and regional broadband seismic waveform data. We also determined the focal depth of the mainshock using its teleseismic waveform data and then relocated the epicenter using its travel times at local stations. High-precision locations of 64 events in the sequence were obtained by combining the double-difference relative location result and the mainshock’s absolute location. The event locations and moment tensor solutions indicate that the ZGS occurred between depth 5 and 9 km on a previously unknown fault striking southwest and dipping ∼80° to the northwest. The event depth distribution and coulomb stress change estimation suggest that the ZGS were not induced directly by the reservoir water but triggered by the stress change from the annual reservoir water level variation. We estimated that the newly found fault has the potential for a magnitude 5.7 earthquake for which ground motion has a 16% probability to exceed the designed maximum intensity level at the TGD, though it would take more than 100 yr to accumulate the needed amount of slip.

New insight into the 24 January 2020, Mw 6.8 Elazığ earthquake (Turkey): An evidence for rupture-parallel pull-apart basin activation along the East Anatolian Fault Zone constrained by Geodetic and Seismological data

In this study, we investigated the main features of the causative fault of the 24 January 2020, Mw 6.8 Elazığ earthquake (Turkey) using seismological and geodetic data sets to provide new insight into the East Anatolian Fault Zone (EAFZ). We first constrained the co-seismic surface deformation and the rupture geometry of the causative fault segment using Interferometric Synthetic Aperture Radar (InSAR) interferograms (Sentinel-1A/B satellites) and teleseismic waveform inversion, respectively. Also, we determined the centroid moment tensor (CMT) solutions of focal mechanisms of the 27 aftershocks using the regional waveform inversion method. Finally, we evaluated the co-seismic slip distribution and the CMT solutions of the causative fault as well as of adjacent segments using the 27 focal solutions of the aftershocks, superimposed on the surface deformation pattern. The CMT solution of the 24 January 2020Elazığ earthquake reveals a pure strike-slip focal mechanism, consistent with the structural pattern and left-lateral motion of the EAFZ. The rupture process of the Elazığ event indicated that the rupture is started at 12 km around the hypocenter, and then propagated bilaterally along the NE-SW but mainly toward the southwest. The rupture slip has initially propagated toward the southwest (first 10 s) and northeast (4 s), and again toward the southwest (9 s). Maximum displacement is calculated as 1.3 m about 20 km southwest of the hypocenter at 6 km depth (centroid depth). The rupture stopped to down-dip around 20 km depth toward the southwest. The distribution of the slip vectors indicates that the rupture continued mostly through a normal oblique movement. Most of the moment release was released SW of the hypocenter and the rupture reached up to around 50 km. The focal mechanisms of analyzed 27 aftershocks show strike-slip, but mostly normal and normal oblique-slip faulting with an orientation of the tensional axes (NNE-SSW), indicating a normal oblique-slip, “transtensional” stress regime, parallel-subparallel to the strike of the EAFZ, consistent with SW-rupture directivity and co- seismic deformation pattern. Finally, based on the co-seismic surface deformation compatible with the distributional pattern of normal focal solutions, normal and normal oblique-slip focals of the aftershocks evidence the rupture-parallel pull-apart basin activation as a segment boundary of the left-lateral strike-slip movement of the EAFZ.

Continental Tectonics Inferred From High‐Resolution Imaging of the Mantle Beneath the United States, Through the Combination of USArray Data Types

AbstractA comprehensive North American upper mantle seismic‐tomographic model, NA13, was inferred from a combination of several decades of seismic data from early North American seismic networks and USArray data. This data‐driven modeling inverted a combination of regional waveform fits, teleseismic S delay times, and point constraints on Moho depths. The joint inversion model combines the contrasting and overlapping resolving power of the different data sets, and demonstrates enhanced resolution over regional models created with a single geophysical data set. Resolution in the upper mantle is achieved on the scale of ∼100 km, although travel time delay studies suggest that anomalies in parts of the western United States are smaller. In NA13, velocities beneath the Yellowstone plume are low enough to require the presence of partial melt. NA13 also models a variety of smaller scale velocity variations beneath the Central and Eastern United States that reveal remnant velocity anomalies related to lithospheric variations in the upper mantle between Proterozoic units.

The March 2021 Tyrnavos, central Greece, doublet (Μw6.3 and Mw6.0): Aftershock relocation, faulting details, coseismic slip and deformation

On 3 March 2021, the Mw6.3 Tyrnavos earthquake shook much of the Thessalia region, leading to extensive damage in many small towns and villages in the activated area. The first main shock was followed in the next day, on 4th of March 2021, by an “equivalent” main shock with Mw6.0 in the adjacent fault segment. These are the largest earthquakes to strike the northeastern part of Thessalia since the M6.3, 1941 Larissa earthquake. The main shocks triggered extensive liquefaction mainly along the banks of the Titarisios tributary where alluvial flood deposits most probably amplified the ground motions. Our seismic monitoring efforts, with the use of recordings of the regional seismological network along with a dense local network that was installed three days after the seismic excitation initiation, led to the improved understanding the geometry and kinematics of the activated faults. The aftershocks form a north–northwest–trending, east–northeast–dipping, ~40 km long distribution, encompassing the two main ruptures along with minor activated structures, consistent with the rupture length estimated from analysis of regional waveform data and InSAR modeling. The first rupture was expanded bilaterally, the second main shock nucleated at its northern tip, where from this second rupture propagated unilaterally to the north–northwest. The focal mechanisms of the two main shocks support an almost pure normal faulting, similar to the aftershocks fault plane solution determined in this study. The strong ground motion of the March 3 main shock was computed with a stochastic simulation of finite fault model. Coseismic displacements that were detected using a dense GPS / GNSS network of five permanent stations located the Thessaly region, have shown an NNE–SSW extension as expected from the nature and location of the causative fault. Coulomb stress changes due to the coseismic slip of the first main shock, revealed that the hypocentral region of the second main shock was brought closer to failure by more than 10 bars.

Path Calibration of the Democratic People’s Republic of Korea 3 September 2017 Nuclear Test

Abstract The recovery of seismic source parameters requires the use of calibrated velocity models. Although at long periods it is possible to use relatively simple plane-layered models, calibration is still needed to better constrain source parameters. For the Democratic People’s Republic of Korea (DPRK) nuclear tests, this is particularly true when considering the use of seismic stations in Japan where paths cross the Sea of Japan. In this study, we perform forward waveform modeling to calibrate velocity structures. We show that we can arrive at a set of models that minimizes Rayleigh group-velocity anomaly at stations located in China, South Korea, and Japan. Overall moment tensor capability with the new models is improved such that cross-correlation analysis of the 20–50 s period waveforms recovers the true time alignment of the data, enabling better automated source analysis. We performed source-type inversions for the moment tensor jointly for long-period regional waveforms and regional and teleseismic P-waves polarity data for the latest and largest DPRK nuclear test and examine event discrimination/screening capability, scalar moment, and yield estimates with uncertainty. We present the models for improved seismic source modeling of the region.

Loreto Intermediate Depth Earthquake of 26 May 2019 (Northeast Peru): Source Parameters by Inversion of Local to Regional Waveforms and Strong-Motion Observations

AbstractThe Loreto earthquake of 26 May 2019 occurred below the extreme northeast part of Peru at a depth of 140 km within the subducting Nazca plate at a distance of 700 km from the trench Peru–Chile. The orientation of the seismic source was obtained from waveform inversion in the near field using velocity and strong-motion data. The rupture occurred in normal faulting corresponding to a tensional process with T axis oriented in east–west direction similar to the direction of convergence between the Nazca and South America plates. The analysis of the strong-motion data shows that the levels of ground shaking are very heterogeneous with values greater than 50 Gal up to distances of 300 km; the maximum recorded acceleration of 122 Gal at a distance of 100 km from the epicenter. The Loreto earthquake is classified as a large extensional event in the descending Nazca slab in the transition from flat-slab geometry to greater dip.

Joint Regional Waveform, First-Motion Polarity, and Surface Displacement Moment Tensor Inversion of the 3 September 2017 North Korean Nuclear Test

AbstractThe 3 September 2017 Mw 5.2 North Korean underground nuclear test (DPRK2017) is the largest man-made explosion with surface displacements observed by Synthetic Aperture Radar (SAR) and showed as much as 3.5 m of horizontal permanent deformation. Although regional distance waveform-based seismic moment tensor (MT) inversion methods successfully identify this event as an explosion, the inverted solutions do not fit the SAR displacement field well. To better constrain the source, we developed an MT source-type inversion method that incorporates surface ground deformation (accounting for free-surface topography), regional seismic waveforms, and first-motion polarities. We applied the source-type inversion over a grid of possible source locations to find the best-fitting location, depth, and point-source MT for the event. Our best-fitting MT solution achieves ∼70% horizontal geodetic fit, ∼80% waveform fit, and 100% fit in the first-motion polarities. The joint inversion narrows the range of acceptable source types improving discrimination, and reduces the uncertainty in scalar moment and estimated yield. The method is transportable and can be applied to other types of events that may have measurable geodetic signals such as underground mine collapses and volcanic events.

The 2019 Ms 4.2 and 5.2 Beiliu Earthquake Sequence in South China: Complex Conjugate Strike-Slip Faulting Revealed by Rupture Directivity Analysis

Abstract The South China block has been one of the most seismically quiescent regions in China, and the geometries and activities of the Quaternary faults have remained less studied due to the limited outcrops. Thus, source parameters of small-to-moderate earthquakes are important to help reveal the location, geometry distribution, and mechanical properties of the subsurface faults and thus improve the seismic risk assessment. On 12 October 2019, two earthquakes (the Ms 4.2 foreshock and the Ms 5.2 mainshock) occurred within 2 s and are located in southern South China block, near the junction region of the large-scale northeast-trending fault zones and the less continuous northwest-trending fault zones. We determined the point-source parameters of the two events via P-wave polarity analysis and regional waveform modeling, and the resolved focal mechanisms are significantly different with the minimum 3D rotation angle of 52°. We then resolved the rupture directivity of the two events by analyzing the azimuth variation of the source time duration and found the Ms 4.2 foreshock ruptured toward north-northwest for ∼1.0 km, and the Ms 5.2 mainshock ruptured toward east-southeast (ESE) for ∼1.5 km, implying conjugate strike-slip faulting. The conjugate causative faults have not been mapped on the regional geological map, and we infer that the two faults may be associated with the northwest-trending Bama-Bobai fault zone (the Shiwo section). These active faults are optimally oriented in the present-day stress field (northwest-southeast) and thus may now be potentially accumulating elastic strain to be released in a future large earthquake.

Rupture directivity of the 25 November 2018 Taiwan Strait Mw5.8 earthquake and its tectonic implications

Taiwan Strait is located between the actively deforming Taiwan Island and stable South China Craton. While strong earthquakes have occurred on the east and west sides of the Taiwan Strait and their seismotectonic are relatively well understood, the earthquake risk and seismogenic structure in the strait still need more investigations. On November 25, 2018, a rare moderately strong earthquake with a magnitude of Mw5.8 occurred inside the strait, and it caused broad ground motions in the near regions. From the waveform inversion of the seismic data, this earthquake is found to be a high-dip angle strike-slip event, with the two nodal planes striking east-west (EW) and north-south (NS) directions. As there are mapped faults striking EW and NS in the epicentral region, further studies are needed to determine the ruptured fault plane. To determine the ruptured fault plane, we analyzed the rupture directivity of the mainshock with the relative location between the hypocenter and the centroid. We chose several M4+ aftershocks as reference events and relocated the reference earthquakes with the Pn travel time difference. Then, we used regional waveform data to invert centroid location and source parameters of earthquakes. Thereafter, we estimated the strike of the mainshock ruptured fault and rupture length, via fitting the azimuthal variation of the Love wave time shift difference between the mainshock and the reference earthquakes. Our results showed that the mainshock ruptured along NS strike with the rupture length about 3.6–5.6 km. Combined with the tectonics, we inferred that the seismogenic fault is the Dongshan uplift eastern fault (DSEF) and the 1994 event may promote the occurrence of the 2018 Mw5.8 earthquake.

New focal mechanisms reveal fragmentation and active subduction of the Antalya slab in the Eastern Mediterranean

In the Eastern Mediterranean, the convergence of Nubia and Eurasia is accommodated along the Hellenic and Cyprus Trenches. The plate geometry and mode of convergence to the east of the Hellenic Trench remains poorly understood. Specifically, along the west of Cyprus, tomography models reveal NW striking high velocity anomaly, which is interpreted as a slab named as the Antalya slab. However, near surface observations pertaining to the top 15 km of the convergence zone show no evidence of an active subduction in the region. In this study, we investigate the seismic activity along the Antalya slab in order to constrain the mode of convergence and the geometry along this plate boundary. Towards this end, we obtain a new focal mechanism catalogue with reliable centroid depths for earthquakes Mw > 4 from regional waveforms by using path dependent seismic velocity models. Our results indicate that the subduction is still active along the plate boundary segment between Cyprus and Anaximander seamount. The earthquakes along the slab interface reveal a NE-dipping slab with a dip angle of ~35°. Slip vectors show a pure convergent motion which is consistent with relative motion of Anatolia with respect to Nubia. We also identify a number of intraslab earthquakes down to a depth of 120 km along the Antalya slab. The T-axis of intraslab earthquakes show down-dip extension consistent with negatively buoyant subducting slab. The Paphos fault bounds the Antalya slab to the east where previously large right-lateral earthquakes occurred, which marks the relative motion of the Antalya slab with respect to the eastern Cyprus slab. Based on slab isodepth contours of the slab interface, we calculate ~100 km offset between the 80 km depth contours of the Antalya and Cyprus slabs. The western boundary of the Antalya slab possibly lies between the Anaximander and Anaximenes seamounts, where several left-lateral earthquakes are obtained. The western boundary of the Antalya slab can also be identified from orientation change of normal fault earthquakes along overriding plate which shows the limit of N-S extension due to roll-back of the neighboring Hellenic slab.

The Normal-Faulting 2020 Mw 5.8 Lone Pine, Eastern California, Earthquake Sequence

AbstractThe 2020 Mw 5.8 Lone Pine earthquake, the largest earthquake on the Owens Valley fault zone, eastern California, since the nineteenth century, ruptured an extensional stepover in that fault. Owens Valley separates two normal-faulting regimes, the western margin of the Great basin and the eastern margin of the Sierra Nevada, forming a complex seismotectonic zone, and a possible nascent plate boundary. Foreshocks began on 22 June 2020; the largest Mw 4.7 foreshock occurred at ∼6 km depth, with primarily normal faulting, followed ∼40 hr later on 24 June 2020 by an Mw 5.8 mainshock at ∼7 km depth. The sequence caused overlapping ruptures across a ∼0.25 km2 area, extended to ∼4 km2, and culminated in an ∼25 km2 aftershock area. The mainshock was predominantly normal faulting, with a strike of 330° (north-northwest), dipping 60°–65° to the east-northeast. Comparison of background seismicity and 2020 Ridgecrest aftershock rates showed that this earthquake was not an aftershock of the Ridgecrest mainshock. The Mw–mB relationship and distribution of ground motions suggest typical rupture speeds. The aftershocks form a north-northwest-trending, north-northeast-dipping, 5 km long distribution, consistent with the rupture length estimated from analysis of regional waveform data. No surface rupture was reported along the 1872 scarps from the 2020 Mw 5.8 mainshock, although, the dipping rupture zone of the Mw 5.8 mainshock projects to the surface in the general area. The mainshock seismic energy triggered rockfalls at high elevations (>3.0 km) in the Sierra Nevada, at distances of 8–20 km, and liquefaction along the western edge of Owens Lake. Because there were ∼30% fewer aftershocks than for an average southern California sequence, the aftershock forecast probabilities were lower than expected. ShakeAlert, the earthquake early warning system, provided first warning within 9.9 s, as well as subsequent updates.

The 25 October 2018 Zakynthos (Greece) earthquake: seismic activity at the transition between a transform fault and a subduction zone

SUMMARYThe properties of the Mw = 6.7 earthquake that took place on 25 October 2018, 22:54:51 UTC, ∼50 km SW of the Zakynthos Island, Greece, are thoroughly examined. The main rupture occurred on a dextral strike-slip, low-angle, east-dipping fault at a depth of 12 km, as determined by teleseismic waveform modelling. Over 4000 aftershocks were manually analysed for a period of 158 d. The events were initially located with an optimal 1D velocity model and then relocated with the double-difference method to reveal details of their spatial distribution. The latter spreads in an area spanning 80 km NNW–SSE and ∼55 km WSW–ENE. Certain parts of the aftershock zone present strong spatial clustering, mainly to the north, close to Zakynthos Island, and at the southernmost edge of the sequence. Focal mechanisms were determined for 61 significant aftershocks using regional waveform modelling. The results revealed characteristics similar to the main shock, with few aftershocks exhibiting strike-slip faulting at steeper dip angles, possibly related to splay faults on the accretionary prism. The slip vectors that correspond to the east-dipping planes are compatible with the long-term plate convergence and with the direction of coseismic displacement on the Zakynthos Island. Fault-plane solutions in the broader study area were inverted for the determination of the regional stress-field. The results revealed a nearly horizontal, SW–NE to E–W-trending S1 and a more variable S3 axis, favouring transpressional tectonics. Spatial clusters at the northern and southern ends of the aftershock zone coincide with the SW extension of subvertical along-dip faults of the segmented subducting slab. The main shock occurred in an area where strike-slip tectonics, related to the Cephalonia Transform Fault and the NW Peloponnese region, gradually converts into reverse faulting at the western edge of the Hellenic subduction. Plausible scenarios for the 2018 Zakynthos earthquake sequence include a rupture on the subduction interface, provided the slab is tilted eastwards in that area, or the reactivation of an older east-dipping thrust as a low-angle strike-slip fault that contributes to strain partitioning.