Seismogenic Structures Research Articles

The Volcano-Tectonics of the Northern Sector of Ischia Island Caldera (Southern Italy): Resurgence, Subsidence and Earthquakes

The island of Ischia, an active volcanic field emerging in the western sector of the Gulf of Naples (Southern Italy), represents an archetypal case of caldera that underwent a very large resurgence related to the intrusion of a shallow magma body. The resurgence culminated with the formation of a structural high in the central sector of the island, i.e., the Mt. Epomeo block. This is bordered by a system of faults along which volcanic activity occurred up to 1302 A.D., and damaging earthquakes were generated in historical and recent time. The seismicity is located prevalently in the northern sector of the island and appears to be correlated with the most recent phase (<5 ka) of ground movement (subsidence), although the mechanism of earthquakes’ generation is still debated. By jointly analyzing offshore and onshore data (seismic profile and stratigraphy wells, respectively) and new petrological and geochemical data related to the most recent phase of volcano-tectonic activity, we develop a geological and structural layout of the northern sector of the island. In particular, we identify the seismogenic fault associated with the historical and recent destructive earthquakes of Ischia. This fault formed in the northern sector of the island during the final stage of the resurgence. We also propose a conceptual volcano-tectonic model of the northern sector of the Ischia Island, depicting the displacement of the fault zones in the off-shore area and the possible mechanism of stress loading and release in the on-shore zone, which is mainly driven by the subsidence of the Mt. Epomeo block. Our results are crucial for evaluating the dynamics of the seismogenic structures in the framework of the general subsidence of the island, as well as the related seismic hazard.

A Shallow and Left‐Lateral Rupture Event of the 2021 Mw 5.3 Baicheng Earthquake: Implications for the Diffuse Deformation of Southern Tianshan

AbstractOn 24 March 2021, an Mw 5.3 earthquake struck northwest Baicheng, located in the Kuqa fold‐and‐thrust belt (FTB), northwest China. In the current study, interferometric synthetic aperture radar (InSAR) data were used to investigate the associated fault rupture solution (dip, dip direction, and slip sense), to determine the geometry of the seismogenic structure; the high‐resolution images of the surface rupture were obtained using an unmanned aerial vehicle (UAV). This geometric model, along with the co‐seismic slip distribution from the InSAR data, and deformation characteristics of co‐seismic surface rupture revealed that: (a) it was a shallow event, the focal depth was 0.5–2 km, and the co‐seismic slip was distributed in the 0–7 km range with high dip angles (66°, 70°). (b) A 4‐km long surface rupture with obvious left‐lateral strike‐slip was distributed on the surface, and the maximum strike‐slip displacement and width were 0.79 and 0.7 m, respectively. This left‐lateral strike‐slip event indicates that the relative motion between the Tianshan and Tarim blocks is a continuous and diffuse deformation process. The study shows that, when evaluating the earthquake risk of the Kuqa FTB, we should consider both the front position of the fault outburst and the internal compressional salt‐related structures, which can also produce moderate to strong earthquakes.

Resolving co- and early post-seismic slip variations of the 2021 MW 7.4 Maduo earthquake in east Bayan Har block with a block-wide distributed deformation mode from satellite synthetic aperture radar data

On 21 May 2021 (UTC), an <i>M</i>_W 7.4 earthquake jolted the east Bayan Har block in the Tibetan Plateau. The earthquake received widespread attention as it is the largest event in the Tibetan Plateau and its surroundings since the 2008 Wenchuan earthquake, and especially in proximity to the seismic gaps on the east Kunlun fault. Here we use satellite interferometric synthetic aperture radar data and subpixel offset observations along the range directions to characterize the coseismic deformation of the earthquake. Range offset displacements depict clear surface ruptures with a total length of ~170 km involving two possible activated fault segments in the earthquake. Coseismic modeling results indicate that the earthquake was dominated by left-lateral strike-slip motions of up to 7 m within the top 12 km of the crust. The well-resolved slip variations are characterized by five major slip patches along strike and 64% of shallow slip deficit, suggesting a young seismogenic structure. Spatial–temporal changes of the postseismic deformation are mapped from early 6-day and 24-day InSAR observations, and are well explained by time-dependent afterslip models. Analysis of Global Navigation Satellite System (GNSS) velocity profiles and strain rates suggests that the eastward extrusion of plateau is diffusely distributed across the east Bayan Har block, but exhibits significant lateral heterogeneities, as evidenced by magnetotelluric observations. The block-wide distributed deformation of the east Bayan Har block along with the significant co- and post-seismic stress loadings from the Madoi earthquake imply high seismic risks along regional faults, especially the Tuosuo Lake and Maqên–Maqu segments of the Kunlun fault that are known as seismic gaps.

The contribution of the NEMO-SN1 seafloor observatory to improve the seismic locations in the Ionian Sea (Italy)

The Western Ionian Sea is characterised by an active and diffuse seismicity, directly related to the convergence of the European and African Plates and by gravitational sinking and rollback of the oceanic lithosphere. In this area, the location of earthquakes is characterised by considerable uncertainties due to large azimuthal gaps, resulting in notable location errors. This problem was partially overcome with the use of data recorded by NEMO-SN1 seafloor observatory (October 2002 - February 2003; June 2012 - May 2013). We relocated 1130 crustal and sub-crustal earthquakes using land network and NEMO-SN1 data. As most events occurred on Mt. Etna, we focused on 358 earthquakes in the offshore area and near the coasts of Sicily and Calabria. The use of the combined land-marine networks has improved the earthquake locations in terms of azimuthal GAP, as well as in horizontal and vertical errors. The comparison between locations performed with and without NEMO-SN1 data shows that differences in latitude, longitude and depths are more evident in the Western Ionian Sea and in the coast of Sicily, where values of the differences over 5 km correspond to structural heterogeneities. The increased number of seismic stations deployed on land from 2003 to 2012 did not influence the location of events occurring offshore, where NEMO-SN1 continued to be the distinctive tool in the location process. Moreover, the new 73 focal mechanisms computed with P-wave polarities from NEMO-SN1 and land stations are in agreement with the regional structural model, showing a prevalent normal, normal/oblique, and strike-slip kinematics. The similarity of two new focal solutions with the mechanisms of the main shock and aftershock of the 1990 earthquake demonstrates that the seismic structures are still active and potentially dangerous. The P-wave travel time residual analysis confirms the activity along the main structural alignments. A single point of observation in the Ionian Sea can significantly improve the quality of locations, giving an opportunity to focus on the seismogenic structures responsible for the occurrence of medium-to-high magnitude earthquakes.

Improving Absolute Hypocenter Accuracy With 3D Pg and Sg Body‐Wave Inversion Procedures and Application to Earthquakes in the Central Alps Region

AbstractAccuracy of hypocenter location, in particular focal depth, is a precondition for high‐resolution seismotectonic analysis of natural and induced seismicity. For instance, linking seismicity with mapped fault segments requires hypocenter accuracy at the sub‐kilometer scale. In this study, we demonstrate that inaccurate velocity models and improper phase selection can bias absolute hypocenter locations and location uncertainties, resulting in errors larger than the targeted accuracy. To avoid such bias in densely instrumented seismic networks, we propose a coupled hypocenter‐velocity inversion procedure restricted to direct, first‐arriving, mainly upper‐crustal Pg and Sg phases. On the basis of synthetic tests and selected ground‐truth events we demonstrate that a sub‐kilometer hypocenter accuracy can be achieved by regional‐scale, three‐dimensional Pg and Sg velocity models combined with dynamic phase selection and a non‐linear location algorithm. The tomographic inversion uses about 60,000 Pg and 30,000 Sg quality‐checked phases of local earthquakes in the Central Alps region. The derived models image the VP and VS structure of the Central Alps upper crust at unprecedented resolution, including small‐scale anomalies such as those caused by Subalpine Molasse units below the Alpine front. The relocation procedure is applied to more than 18,000 earthquakes and the relocated hypocenters reveal previously unrecognized seismogenic structures, for instance in the Swiss Molasse basin south of Bern. The ML 4.6 Urnerboden earthquake of 2017 is used as an example to demonstrate how the derived 3D velocity structure and relocated hypocenters can be jointly interpreted to constrain the lithology hosting upper‐crustal seismicity in the Central Alps.

Multi-Method Study on Seismogenic Structure of Rushan Earthquake Sequence in Shandong Province, China

Multi-Method Study on Seismogenic Structure of Rushan Earthquake Sequence in Shandong Province, China

Reevaluation of the Seismogenic Fault of the 1654 M8.0 Tianshui Earthquake: Evidence from Geology, Geomorphology, and Chronology along the Lixian-Luojiapu Fault

AbstractA historically documented M8.0 earthquake occurred in Tianshui, Gansu, China, in 1654. Its epicenter was located near the Lixian-Luojiapu fault in the triangular area at the conjunction of the Tibetan Plateau, North China block and South China block. Due to the lack of geologic and geomorphologic evidence and precise chronologic constraints, the seismogenic fault of this large event remains controversial. This work attempted to further address this issue. Based on data from unmanned aerial vehicle measurements, field investigations, analysis of typical fault profiles, and 14C dating, we determined the age of the latest slip on the Lixian-Luojiapu fault and the corresponding surface rupture range. In combination with historical earthquake data for this region, our work suggests that two sections of the Lixian-Luojiapu fault, the Tanchang-Lixian subsegment and the eastern portion of the Lixian-Luojiapu subsegment, were indeed involved in the surface rupturing of the 1654 Tianshui M8.0 earthquake. The scratches on the fault plane indicate that this seismic rupturing exhibited a left-lateral slip with a normal component and the maximum vertical displacement by this shock on the Tanchang-Lixian subsegment was 1.7±0.2 m. We suggest that the Tanchang-Lixian subsegment and the eastern portion of the Lixian-Luojiapu subsegment of the Lixian-Luojiapu fault and the West Qinling fault in the north jointly constituted the seismogenic structure of the Tianshui M8.0 earthquake based on a previous trench study. Located in the triangular zone between the Tibetan Plateau and the North China and South China blocks, tectonic stress accumulates rapidly on the Lixian-Luojiapu fault, resulting in a high potential of moderate to strong earthquakes, and this issue should be given more attention in the future.

The Seismogenic Structure of the 2017 Mw 6.9 Milin, Tibet, Earthquake: A Possible Newly Active Fault at the Eastern Himalayan Syntaxis

AbstractOn 18 November 2017, an Mw 6.9 earthquake occurred in Milin, Tibet, with the epicenter at the top of the Namche Barwa syntaxis. This event did not produce surface ruptures, and its seismogenic structure remains unclear or controversial. Using the locations of the Milin mainshock and aftershocks, locations of regional small earthquakes and focal mechanism solutions from 2007 to 2009, this work analyzed the causative fault and tectonic setting of the Milin earthquake and assessed the regional seismic risk. The results suggest that the seismogenic structure of the Milin earthquake was a secondary fault, the southern branch of the XiXingla fault (XXLF). Within 28 hr after the mainshock, the aftershocks of the Milin event spread northeastward to the secondary north branch fault of the XXLF and the secondary south branch fault of the Palong–Pangxin fault. Across the top of the Namche Barwa syntaxis (Namche Barwa block) and the Chayu block in the southeast, an earthquake dense belt (EDB) has developed. This EDB has similar deep structures beneath the two blocks, in which several northeast-dipping structural planes exit, and different portions of the EDB imply a unified tectonic stress field. Combining these data with the foreshock–mainshock–aftershock data for the 1950 Mw 8.6 Chayu, Tibet, earthquake, we speculate that the structural planes produced by the EDB at depth in the two blocks have already been connected or tended to connect, resulting in a new fault system trending northwest and approximately 280 km long. The 2017 Mw 6.9 Milin earthquake occurred at the northwestern end of this fault system. At present, the development stage, maturity, and fine structure of this new fault system remain unclear but should receive additional attention. Based on its maximum rupture area, this new fault system is capable of generating an Mw 7.7 earthquake in the future.

Stress evolution before the 2014 Ludian M6.5 earthquake: Insights from groundwater and geodetic measurement

Because the meta-instability stage before an earthquake can be found in rock-deformation experiments in the laboratory, great expectations are placed on the detection of possible precursors in earthquake forecasting and prediction. However, varietal observations are difficult to compare, and it is very difficult to distinguish to which stage the observed value belongs. The top priority is to study the relationship between the observations and the meta-instability stage in a uniform system of measurement. The 2014 Ludian M6.5 earthquake, which occurred in the southeastern region of the Qinghai-Tibet Plateau, was surrounded by 122 observatories for groundwater, electromagnetism, and surface deformation within a 300-km radius. Many data were observed before and after the Ludian earthquake, which provided a great case to study the relationship between the observations, stress evolutions, and meta-instability stage before the earthquake. We determined the detailed spatiotemporal variations in groundwater temperature, water level, chemical ion measurement, and crust deformation by strain meter and tilt meter. Following the previous study on the relationship between the observations and stress field, we simply classified all these data from 78 observatories into three groups: stress increased, stress disturbed, and uninfluenced. The results show that the stress field displayed a four-quadrant pattern around the seismogenic structure of the 2014 Ludian earthquake, which is called the Baogunao-Xiaohe Fault and is known as a buried branch fault of the Daliangshan fault. Along with the orientation of fault-block movement, the stress decreased at the back while increasing at the front. In addition, some stress perturbations were found along nearby large active faults, such as the Xianshuihe-Anninghe-Zemuhe-Xiaojiang fault zone, which is a deep-cutting active fault belt confining the Sichuan-Yunnan block. Based on the spatiotemporal evolution of the stress field and distribution of aftershocks, we simplified the seismogenic structure to a set of conjugated strike–slip faults. The nonlinear friction finite-element method was applied to simulate the stress variations before and after the extensive disengagement and slip of nodes on the fault plane. The computed stress field is consistent with the observed phenomenon and the meta-instability stage could be distinguished out from our simulation. Our research can be used to help detach earthquake precursors and determine whether the earthquake system is in the meta-instability stage.

Focal mechanism and seismogenic structure of the Shiqu MS4.4 earthquake

The focal mechanism solution of the Shiqu MS 4.4 earthquake occurred on May 16th, 2017 in Sichuan Province is studied by the gCAP method using the waveform data from the regional seismic networks in Sichuan, Qinghai, Tibet and Gansu provinces. The strike/dip/dipping angle of the first nodal plane are 214°/80°/167° and those of the second nodal plane are 306°/77°/10°, the optimal centroid depth is 7.3 ​± ​0.6 ​km and the moment magnitude is MW 4.5. Furthermore, the study investigates the robustness of the results against the error of crustal velocity structure, location, data quality and difference of seismic parameters, subsequently obtaining a stable resolved focal mechanism. According to the geological structure in the seismogenic area, spatial distribution of aftershock sequenceof the regional tectonic stress field, and the focal mechanism of the main shock, we suggest that the Shiqu earthquake is induced by a left-lateral strike-slip mechanism and the second nodal plane is inferred to be the seismogenic fault, consistent with the geometry of the Changshagongma fault which is the secondary fault of the northwest part of the Xianshuihe fault zone.

25,000 Years long seismic cycle in a slow deforming continental region of Mongolia

The spatial distribution of large earthquakes in slowly deforming continental regions (SDCR) is poorly documented and, thus, has often been deemed to be random. Unlike in high strain regions, where seismic activity concentrates along major active faults, earthquakes in SDCR may seem to occur more erratically in space and time. This questions classical fault behavior models, posing paramount issues for seismic hazard assessment. Here, we investigate the M7, 1967, Mogod earthquake in Mongolia, a region recognized as a SDCR. Despite the absence of visible cumulative deformation at the ground surface, we found evidence for at least 3 surface rupturing earthquakes during the last 50,000 years, associated with a slip-rate of 0.06 ± 0.01 mm/year. These results show that in SDCR, like in faster deforming regions, deformation localizes on specific structures. However, the excessive length of return time for large earthquakes along these structures makes it more difficult to recognize earthquake series, and could conversely lead to the misconception that in SDCR earthquakes would be randomly located. Thus, our result emphasizes the need for systematic appraisal of the potential seismogenic structures in SDCR in order to lower the uncertainties associated with the seismogenic sources in seismic hazard models.

One Year of Seismicity Recorded Through Ocean Bottom Seismometers Illuminates Active Tectonic Structures in the Ionian Sea (Central Mediterranean)

Seismological data recorded in the Ionian Sea by a network of seven Ocean Bottom Seismometers (OBSs) during the 2017–2018 SEISMOFAULTS experiment provides a close-up view of seismogenic structures that are potential sources of medium-high magnitude earthquakes. The high-quality signal-to-noise ratio waveforms are observed for earthquakes at different scales: teleseismic, regional, and local earthquakes as well as single station earthquakes and small crack events. In this work, we focus on two different types of recording: 1) local earthquakes and 2) Short Duration Events (SDE) associated to micro-fracturing processes. During the SEISMOFAULTS experiment, 133 local earthquakes were recorded by both OBSs and land stations (local magnitude ranging between 0.9 and 3.8), while a group of local earthquakes (76), due to their low magnitude, were recorded only by the OBS network. We relocated 133 earthquakes by integrating onshore and offshore travel times and obtaining a significant improvement in accuracy, particularly for the offshore events. Moreover, the higher signal-to-noise ratio of the OBS network revealed a significant seismicity not detected onshore, which shed new light on the location and kinematics of seismogenic structures in the Calabrian Arc accretionary prism and associated to the subduction of the Ionian lithosphere beneath the Apennines. Other signals recorded only by the OBS network include a high number of Short Duration Events (SDE). The different waveforms of SDEs at two groups of OBSs and the close correlation between the occurrence of events recorded at single stations and SDEs suggest an endogenous fluid venting from mud volcanoes and active fault traces. Results from the analysis of seismological data collected during the SEISMOFAULTS experiment confirm the necessity and potential of marine studies with OBSs, particularly in those geologically active areas of the Mediterranean Sea prone to high seismic risk.

Seismogenic structure along the deformation front off SW Taiwan revealed by the aftershocks of the 2017 Tainan near-shore earthquake with ocean bottom seismometers

Located in the northern end of the Manila Trench, the deformation front (DF) off southwestern Taiwan is considered as a potential area for generating hazardous earthquakes and tsunamis. However, this area has been relatively seismically quiet, and the seismogenic structure has been unclear, particularly in the middle-to-lower crust. On February 10, 2017, an Mw5.3 earthquake occurred in the near-shore area of Tainan in SW Taiwan, which provides us with a good opportunity to study the seismogenic structure of this area. Five ocean bottom seismometers (OBSs) were deployed to cover the mainshock source area from February 13 to March 1, 2017, to record the aftershock data. All the OBSs were successfully recovered and, the P- and S-wave arrival were manually identified. In collaboration with arrival times from the Central Weather Bureau of Taiwan, we simultaneously determined the 1-D optimal Vp and Vs velocity models and hypocenter locations. The inversion results show that the upper crust (<15 km) has a relatively low velocity, and most events are confined between 10 and 15 km depth. The relocated epicenters were all in areas with positive gravity anomalies, and most of them formed a NE-SW striking seismogenic structure that was approximately parallel to the surface trace of the DF and did not link to any known active faults in SW Taiwan. The hypocenters illustrate a conjugate fault system comprising of a SW-dipping NE-SW striking fault and an SE-dipping NW-SE striking fault. Based on focal mechanisms and aftershock distribution, we demonstrate that the 2017 Tainan near-shore earthquake may result from the reactivation of pre-existing faults, and the driving force comes from the bending stresses of the subducting Eurasia Plate at the Manila Trench. Given the active subduction processes offshore of Taiwan, such slab-flexural-related earthquakes must be considered in seismic and tsunami hazard assessments.

A review of geohazards on the northern continental margin of the South China Sea

Crustal instability to induce the geohazards often results from both the internal tectonic dynamics and external sediment processes. Thus, the detailed geodynamics and kinematics of geohazards are important for understanding the tectonic evolutionary process and evaluating potential geohazards. This paper focuses on the active faults, submarine landslides, earthquakes as well as submarine canyons geomorphology description and kinetics explanation in the northern South China Sea (SCS) continental margin. Firstly, we have estimated the spatial extent, fault-plane geometry and faulting behaviors of the submarine active faults, such as the Littoral Fault Zone, based on cumulative deformation recorded by geophysical seismic reflections and geomorphic markers. We also analyzed the reactivation of preexisting structures, as well as the spatial and temporal distribution and migration of the submarine landslides and canyons. Then, several conceptual models and a comprehensive study of some of the different factors that contributed to the geohazard chain are conducted based on pre-existing theory and available literature. This review study indicates: (i) geohazards of different phases and geohazard chain are generally triggered by both early-phase tectonics-dominated and later-phase sedimentation-dominated processes; (ii) the fault geometric and stress segmentation may have played an important role in controlling on the uneven and asymmetric distribution of earthquakes and submarine canyons of the northern SCS, Especially, the ENE-striking preexisting Littoral Fault Zone is the major seismogenic structure along the Southchina coastline; (iii) the tectono-sedimentary coupling induced landslide and slope failure that has help to shape the modern coastal and slope geomorphology.

Campi Flegrei, Vesuvius and Ischia Seismicity in the Context of the Neapolitan Volcanic Area

Studying seismicity in a volcanic environment provides important information on the state of activity of volcanoes. The seismicity of the Neapolitan volcanoes, Campi Flegrei, Vesuvius, and Ischia, shows distinctive characteristics for each volcano, covering a wide range of patterns and types. In this study we relocated some significant volcano-tectonic earthquake swarms that occurred in Campi Flegrei and Vesuvius. Moreover, we compared the earthquake occurrence evolution, the magnitude and the seismic energy release of the three volcanoes. Also, we considered the results of seismic analysis in the light of geochemical and ground deformation data that contribute to defining the state of activity of volcanoes. In Campi Flegrei, which is experiencing a long term unrest, we identified a seismogenic structure at shallow depth in Pisciarelli zone that has been activated repeatedly. The increasing seismicity accompanies an escalation of the hydrothermal activity and a ground uplift phase. At Vesuvius a very shallow seismicity is recorded, which in recent years has shown an increase in terms of the number of events per year. Earthquakes are usually located right beneath the crater axis. They are concentrated in a volume affected by the hydrothermal system. Finally, Ischia generally shows a low level of seismicity, however, in Casamicciola area events with a moderate magnitude can occur and these are potentially capable of causing severe damage to the town and population, due to their small hypocentral depth (typically &amp;lt; 2.5 km). After the seismic crisis of August 21, 2017 (mainshock magnitude M = 4), the seismicity returned to a low level in terms of occurrence rate and magnitude of earthquakes. The seismicity of these three different volcanic areas shows some common aspects that highlight a relevant role of hydrothermal processes in the seismogenesis of volcanic areas. However, while the main swarms in Campi Flegrei and most of the Vesuvian earthquakes are distributed along conduit-like structures, the seismicity of Ischia is mainly located along faults. Furthermore, the temporal evolution of seismicity in Neapolitan volcanic area suggests a concomitant increase in the occurrence of earthquakes both in Campi Flegrei and Vesuvius in recent years.

Characteristics of Seismicity in the Eagle Ford Shale Play, Southern Texas, Constrained by Earthquake Relocation and Centroid Moment Tensor Inversion

Abstract Analysis of earthquake locations and centroid moment tensors (CMTs) is critical in assessing seismogenic structures and connecting earthquakes to anthropogenic activities. The objective of this study was to gain insights into the seismotectonics of the Eagle Ford Shale play (EF), southern Texas, through relative relocation of earthquakes, assessment of CMT solutions, and investigation of the background stress field. Using Texas Seismological Network (TexNet) data from 2017 through 2019, we were able to relocate 326 earthquakes and obtain CMT solutions for 37 ML≥2.0 earthquakes. These earthquakes are located in the sedimentary basin and uppermost crust, with depths ranging from 2 to 10 km. The earthquake groups in the northeastern EF are linearly distributed along the Karnes fault zone, whereas the southern and western groups are spatially scattered around mapped or unmapped faults. CMT solutions identified 32 normal fault earthquakes and five strike-slip earthquakes. The orientation of the fault plane of most normal fault earthquakes is southwest–northeast, whereas the possible fault plane of the strike-slip fault is from north-northwest to south-southeast, which is roughly perpendicular to the normal faults. Normal and strike-slip faults in the EF are of high dip angles, with the dip angles of the most faults ranging from 60° to 80°. Stress inversion results show that the major orientation of maximum horizontal stress (SHmax) is southwest–northeast, with minor local stress-field rotations. We further estimated earthquake energy release in the EF region using moment magnitude from the CMT solutions, and the cumulative earthquake energy release curve reveals three notable increases in cumulative seismic moment, which occurred in January–July 2018 and January–March 2019, and May–August 2019. Whether these energy releases were caused by anthropogenic activities is a matter for further investigation.

The 2020 M6.4 Jiashi Earthquake: An Event that Occurred Under the Décollement on the Kaping Fold-and-Thrust Belt in the Southwestern Tien Shan Mountains, China

The 2020 Jiashi M6.4 earthquake occurred in the Kaping fold-and-thrust belt, a major south-verging active thin-skin system in the southwestern Chinese Tien Shan Mountain, north of the Tarim Basin. Within 50 km from the epicentral area, seismic hazard is high, as suggested by the occurrence of the 1902 Mw 7.7 Artux (Kashgar) earthquake and 1997 Jiashi strong earthquake swarm. The seismogenic structure responsible for the 2020 event is not well constrained and is a subject of debate. We relocated the 2020 Jiashi earthquake sequence and assessed the relocation uncertainties, using eight seismic velocity models and based on detailed local and regional subcrustal structures from seismic profiles. Then we compared the temporal variation in the Gutenberg–Richter b-values of the 2020 sequence with those of the 1997, 1998, and 2003 earthquake sequences. Our results show that most events cluster at depths greater than 10 km, suggesting that the events most likely occurred beneath the décollement and inside the Tarim Craton. The spatiotemporal evolution of the sequence suggests that two groups of structures at depth were involved in the 2020 sequences: NW–SE-trending lateral strike-slip faults and E–W-trending reverse faults. The b-values of the 2020 sequence exhibits relatively stable temporal evolution, unlike those of the multi-shock sequence that occurred inside the Tarim Craton. It indicates that the 2020 sequence perhaps was influenced by the stress interaction with the 10 km thick overlying strata. Our study provides a new perspective on the seismogenic structure of the earthquakes that occurred because of reactivation of ancient structures developed in a stable craton.

Focal Mechanism and Seismogenic Structure of the MS 5.1 Qingbaijiang Earthquake on February 3, 2020, Southwestern China

The 3 February 2020 MS 5.1 Qingbaijiang earthquake, southwestern China, is the closest recorded MS ≥ 5.0 event to downtown Chengdu City to date, with an epicentral distance of only 38 km. Here we analyze seismic data from the Sichuan and Chengdu regional seismic networks, and employ a multi-stage location method to relocate the earthquakes that have occurred along the central and northern segments of the Longquanshan fault zone since 2009, including the MS 5.1 Qingbaijiang earthquake sequence, to investigate the seismogenic structure of the region. The relocation results indicate that the seismicity along the central and northern segments of the Longquanshan fault zone has occurred mainly along the eastern branch since 2009, with the hypocentral distribution along a vertical cross-section illustrating a steep, NW-dipping parallel imbricate structure. The terminating depth of the eastern branch is about 12 km. The distribution of the MS 5.1 Qingbaijiang earthquake sequence is along the NE–SW-striking Longquanshan fault zone. The aftershock focal depths are in the 3–6 km range, with the mainshock located at 104.475°E, 30.73°N. Its initial rupture depth of 5.2 km indicates that the earthquake occurred above the shallow decollement layer of the upper crust in this region. The hypocentral distribution along the long axis of the aftershock area highlights that this earthquake sequence occurred along a fault dipping at 56° to the NW. Our surface projection of the inferred fault plane places it near the eastern branch of the Longquanshan fault zone. We infer the MS 5.1 mainshock to be a thrust faulting event based on the focal mechanism solution via the cut-and-paste waveform inversion method, with strike/dip/rake parameters of 22°/36°/91° and 200°/54°/89° obtained for nodal planes I and II, respectively. We identify that the seismogenic fault of the MS 5.1 Qingbaijiang earthquake lies along the eastern branch of the Longquanshan fault zone, and nodal plane II represents the coseismic rupture plane, based on a joint analysis of the event relocation results, mainshock focal mechanism, and regional geological information. Our study provides vital information for assessing the seismic hazard of the Longquanshan fault zone near Chengdu City.

First-Motion Focal Mechanism Solutions for 2015–2019 M ≥ 4.0 Italian Earthquakes

A list of 100 focal mechanism solutions that occurred in Italy between 2015 and 2019 has been compiled for earthquakes with magnitude M ≥ 4.0. We define earthquake parameters for additional 22 seismic events with 3.0 ≤ M &amp;lt; 4.0 for two specific key zones: Muccia, at the northern termination of the Amatrice–Visso–Norcia 2016–2018 central Italy seismic sequence, and Montecilfone (southern Italy) struck in 2018 by a deep, strike-slip Mw 5.1 earthquake apparently anomalous for the southern Apennines extensional belt. First-motion focal mechanism solutions are a good proxy for the initial rupture and they provide important additional information on the source mechanism. The catalog compiled in the present paper provides earthquake parameters for individual events of interest to contribute, as a valuable source of information, for further studies as seismotectonic investigations and stress distribution maps. We calculated the focal mechanisms using as a reference the phase pickings reported in the Italian Seismic Bulletin (BSI). We visually checked the reference picks to accurately revise manual first-motion polarities, or include new onsets when they are not present in the BSI dataset, for the selected earthquakes within the whole Italian region, with a separate focus on the Amatrice–Visso–Norcia seismic sequence area from August 24, 2016 to August 24, 2018. For the Montecilfone area, we combined the information on the geometry and kinematics of the source of the 2018 Mw 5.1 event obtained in this study with available subsurface and structural data on the Outer Apulia Carbonate Platform to improve understanding of this intriguing strike-slip sequence. Our analysis suggests that the Montecilfone earthquake ruptured a W–E trending strike-slip dextral fault. This structure is confined within the Apulia crystalline crust and it might represent the western prolongation of the Mattinata Fault–Apricena Fault active and seismogenic structures. The calculated focal mechanisms of the entire catalog are of good quality complementing important details on source mechanics from moment tensors and confirming the relevance of systematically including manually revised and more accurate polarity data within the BSI database.

Disaster mechanism of Donghekou landslide in the Ms8.0 Wenchuan earthquake

During the “512” Wenchuan earthquake, Donghekou, which is located at the end of the seismogenic structure, experienced serious collapse and slide, resulting in serious disasters. In this paper, based on the analysis of near-fault strong ground motion records of Wenchuan earthquake, the collection of seismogenic structural data and near-fault structural damage data, the disaster mechanism of Donghekou landslide is studied. The earthquake damage of Donghekou was caused by pulse-like ground motion and the surface rupture along the seismogenic structure. The surface rupture at the far end of Donghekou participated in the formation of the pulse-like ground motion.