Three strong aftershocks (M(S)6+) occurred in the northeastern rupture zone of the 2008 M(W)7.9 Wenchuan earthquake within three months. No surface ruptures were observed, and the seismogenic faults remain unclear. Resolving the source parameters and seismogenic structures of these strong aftershocks is essential for clarifying the rupture termination mechanism of the mainshock and for future seismic hazard assessment. In this study, we determined the point source parameters of eight moderate to strong aftershocks and the rupture directivity of three strong aftershocks through regional and teleseismic waveform modeling. The focal mechanisms of these aftershocks are diverse, including both strike-slip and thrust-slip types, with centroid depths ranging from the middle crust (12-19 km) to the shallow part (3-5 km), highlighting the complexity in the rupture termination zone. The rupture directivity analysis shows that the strike-slip May 25 event (Mw6.0) ruptured from SW to NE along the right-lateral plane (60 degrees/81 degrees/173 degrees) for similar to 7 km, the strike-slip July 24 event (Mw5.5) on ruptured from NNE to SSW along the right-lateral plane (16 degrees/67 degrees/147 degrees) for similar to 6 km, and the thrust-slip August 5 event (Mw5.9) ruptured upwards along the northeast dipping plane (339 degrees/56 degrees/83 degrees) for 6-8 km. The strike of ruptured faults changes from NE to NNE, differing from the Qingchuan fault. The estimated stress drop of the event in the middle crust (similar to 19 km, 9.3 MPa) is larger than that of the shallower event (similar to 4 km, 1.9 MPa), possibly due to the low strength of the shallow crust. Moreover, the rupture direction of the July 24 event is opposite to that of the mainshock, potentially due to the Bikou block's differing bi-material contrast, which may have hindered the northeastward extension of the mainshock's rupture.

Continental rejuvenation results from the tectonic reactivation of crustal structures and lithospheric reworking by mantle flow. Geochemical observations and field mapping have traditionally provided the primary evidence for the secular evolution of crustal composition and tectonic processes during continental rejuvenation. Nonetheless, the impact of continental rejuvenation on the observed present-day strain rate and orogenic-scale lithospheric structure has not been well constrained. The pre-existing E-W−trending Central China Orogenic Belt has been overprinted by the N-S−trending Central Longitudinal Seismic Belt and constitutes the intracontinental West Qinling Syntaxis in central China, where the tectonic setting changes eastward from contraction to extension. Combining updated global positioning system data and high-resolution crustal seismic tomography, we reveal a modern continental rejuvenation process within the West Qinling Syntaxis in central China. The northward extrusion of the Tibetan Plateau’s weak lithospheric layer (middle-lower crust and lithospheric mantle) of southwestern China relative to the rigid Sichuan Basin/Ordos Block of the eastern West Qinling Syntaxis results in regional dextral shearing that shapes the Central Longitudinal Seismic Belt and defines the eastern Tibetan Plateau margin. The pre-existing E-W−trending Central China Orogenic Belt has been preserved above the brittle-ductile transition zone, and the northward movement of the deep lithospheric layer drives the deformation of the upper crust in the West Qinling Syntaxis. Our results, along with previous studies, suggest the presence of an intracontinental lithospheric interchange structure in central China. The continental rejuvenation of the West Qinling Syntaxis results from a combination of fault reactivation in the upper crust (Stage I, Eocene−Oligocene) and reworking of the deep lithosphere (Stage II, middle−late Miocene) related to the plateau-wide shift in stress accommodation ultimately driven by the redistribution of mass outward from the central Tibetan Plateau. At present, the transition zone between the high- and low-velocity anomalies along the Central Longitudinal Seismic Belt not only shapes the landscape boundary but controls the size and recurrence interval of earthquakes within the West Qinling Syntaxis in central China.

Focal depth of earthquakes is essential for studies of seismogenic processes and seismic hazards. Surface waves are usually the strongest seismic phases at local and regional distances, and its excitation is sensitive to source depth. We observe that the optimal period (the period corresponding to the maximum amplitude) of Rayleigh waves at local distances shows an almost linear correlation with focal depth, based on which we propose a method for resolving the focal depth of local earthquakes. We propose an automated data processing workflow, and applications to earthquakes in diverse tectonic settings demonstrate that reliable focal depth with uncertainty of 1~2 km can be determined even with one or a few seismic stations. Then, we use the Longmenshan region as a case study to systematically assess the impact of the 3D velocity model on the results through forward simulation. A total of 191 events at depths ranging from 5 to 20 km are simulated. The standard deviation between the focal depths determined by this method and the input values is approximately 1.5 km, with 95% events having errors within 2 times the standard deviation. This indicates that the method exhibits good applicability even in regions with complex velocity structures, and highlights the applicability of the method in scenarios characterized by sparse network coverage or historical events.

The eastern Himalayan syntaxis is located at the front of the collision between the Indian and Eurasian continents. This region is affected by the interaction of the Himalayan, Lhasa, Qiangtang, and Sichuan-Yunnan blocks and the Indian plate and is characterized by strong tectonic deformation with frequent earthquakes primarily distributed linearly. In this study, various seismological methods were used to reveal the seismicity, seismogenic mechanism, and tectonic stress field in this region. First, we used the double-difference location method (HypoDD) to relocate 65663 earthquakes with M ≥1.0 during 2008-2018. Then, the Cut-And-Paste (CAP) method was adopted to invert the focal mechanism solutions of 163 events with M ≥3.5 from 2009 to 2021. Combining the focal mechanisms inverted in this study and 1156 solutions collected from the GlobalCMT catalog and other published studies, we obtained the regional stress field with the damped regional-scale stress tensor inversion method. The results show that the earthquakes in this region are mainly distributed along mapped faults, among which the eastern Himalayan syntaxis, the extensional rift in the middle of the plateau, the Sichuan-Yunnan block, and the Yunnan-Burma block experience significant seismic activity. The earthquakes are distributed in the upper and middle crust (5-25 km), and there is a significant increase in the number and dominant depth distribution of earthquakes within the Sichuan-Yunnan and Yunnan-Myanmar blocks from those in the Lhasa and Qiangtang blocks. Earthquakes of various mechanisms occur frequently at the eastern Himalayan syntaxis; strike-slip earthquakes are mainly distributed along large boundary faults; normal earthquakes primarily occur along the western boundary faults of the Sichuan-Yunnan block, and thrust earthquakes are concentrated at the front of the collision between the Indian and Eurasian plates. The horizontal direction of the principal compressive stress axis rotates nearly clockwise around the eastern Himalayan syntaxis, from the Himalayas, Lhasa, Qiangtang, Sichuan-Yunnan to Yunnan-Burma blocks. Moreover, strong local inhomogeneity in the stress fields are found in the shallow eastern Himalayan syntaxis and northwest Sichuan-Yunnan block.

The Mw 6.6 Menyuan earthquake occurred on January 8th of 2022 on the northern margin of Menyuan Basin, Qinghai. It was followed by over 600 aftershocks with magnitudes of up to M 5.1. The mainshock was located on the Lenglongling (LLLF) segment of the Qilian-Haiyuan (QLHYF) sinistral fault system, a 127-km-long active fault with a characteristic event of Mw 7.3-7.5. In this study, we used seismic and geodetic data for the characterization of this seismic event and its large aftershocks. Using the seismic data and the CAPjoint inversion method, we obtained the centroid depths and focal mechanisms of the mainshock and the 17 largest (Ms >= 3.0) aftershocks. We determined that the mainshock was a strike-slip event with a moment magnitude of 6.58, centroid depth of 4 km, and the two nodal planes of 197 degrees/83 degrees/-162 degrees and 104 degrees/72 degrees/-7 degrees. Most of the estimated aftershocks were also strike-slip events located at depths ranging from 3-9 km at the periphery of the earthquake centroid. We analyzed the rupture directivity of the two large aftershocks (Ms 5.2 and Ms 4.8) with the azimuthal variation of source duration and found that both events ruptured 1.5-2.0 km segments along the sinistral fault plane. We also processed Sentinel-1 SAR data acquired on tracks 026, 033, and 128 using an automated InSAR processing package, pSAR. The SAR-derived results that include subpixel offsets, coherence maps, and differential interferometry consistently reveal two major surface rupture segments that correlate with a step-over of Tuolaishan fault and the western segment of LLLF, respectively. We performed a geodetic inversion of the ascending and descending coseismic InSAR observations using a geodetic inversion package PSOKINV in order to determine geometric parameters and subsurface slip on the fault. During the inversion, the surface fault traces were fixed based on the observations from the subpixel offsets and coherence maps produced from the SAR data. The geodetic inversion indicates that the sinistral strike-slips on the two steep-dipping fault segments are responsible for the mainshock, having three distinct slip patterns with a maximum slip of about 4 m at a depth of 4 km, which is consistent with the seismic solutions. The geodetic moment from the slip model was 1.58x10(19) Nm, corresponding to Mw 6.68, which is slightly greater than Mw 6.58 estimated from the seismic data. We also compared the seismic source solutions produced by the different seismic methods to the multiple earthquake fault segments inferred from the InSAR results obtained in this study. We found that the significant variations of the seismic solutions from P first motion polarizations, CAPjoint, GCMT and USGS W-phase are mainly due to the different periods of the seismic data used in the inversions. The solution derived from the P first motions is consistent with the fault F1 determined from the offset maps presented in this study that corresponds to the step-over of Tuolaishan fault. With longer periods, the seismic focal mechanism solutions turn out closer to the epicenter of the earthquake. This could help with the understanding of seismic solutions for other earthquakes, particularly those with significant strike variations along the rupture. Combining with regional tectonic structures and historical earthquakes, e.g., the 1986 and 2016 Ms 6.4 thrust-slip Menyuan earthquakes, we suggest that an asymmetrical flower structure along QLHYF may have been a key model for unleashing the regional strain. The coseismic slip-derived stress analysis indicates that the earthquake significantly increased the Coulomb stress in the vicinity of the hypocenter, particularly at the western end of the earthquake rupture, where few aftershocks were observed, drawing our attention to the increased seismic risk in that region.

The 2022 Ms6.0 Maerkang earthquake sequence, Sichuan, China, occurred in an unexpected area with historically rare seismicity in the Bayan Har block. Here we relocated the earthquake sequence, inverted for the focal mechanisms of the larger events, and calculated the rupture directivity of the earthquake sequence to reveal the seismogenic structures and mechanisms of this sequence. The high-precision relocations indicate that the seismogenic structures consist of several clusters that are generally parallel to the nearby NW-trending Songgang fault, and relatively small-scale conjugate faults are also identified. The seismicity migrated from cluster one in the south to cluster two in the north during the sequence. Furthermore, the hypocenters were largely located at 5–10 km depth, thereby highlighting that the seismogenic structures are buried. The vertical fault planes of the seismogenic structures are consistent with the high-dip focal mechanism solutions from seven events. A stress field inversion based on the focal mechanisms indicates that the sequence occurred in a strike-slip environment that was controlled by a NNW–SSE-striking principal compressive stress. The different rupture directivities of the M s5.8 (southwestward) and M s6.0 (southeastward) events prove the existence of conjugate faults. The M s5.8 event induced a coseismic Coulomb stress change of 1.6 MPa where the M s6.0 event subsequently occurred, thereby highlighting that the M s5.8 event triggered the M s6.0 event and produced the spatiotemporal seismicity pattern of the sequence. We therefore conclude that the seismogenic structures of the 2022 M s6.0 Maerkang earthquake sequence are previously unknown concealed conjugate structures associated with the main Songgang fault. The complex seismogenic structures and their potential to generate large earthquakes warrant the need to better understand the seismogenesis of this area and the seismic risks that may be present.

SUMMARY Ambient noise tomography (ANT) is a widely used method to obtain shear wave velocity structure in the crust and upper mantle. Usually, the topography is assumed to have negligible effect on the resulting models. This, however, might not be proper in regions with large topographic variation, such as plateau edges, submarine slopes and volcanic islands. In this study, we use synthetics from waveform-based numerical simulation to quantify the topography effect on ANT in the Longmen Shan area, eastern Tibetan Plateau margin. Three kinds of models are used in forward simulation to obtain theoretical waveforms, including Case1: the layered model, Case2: the layered model with topographic variation and Case3: the flattened model of Case2. The final inversion results show that the bias of ANT is negligible in the blocks with relatively flat topography, such as the interior regions of the Tibetan Plateau and the Sichuan Basin. However, for the Longmen Shan boundary zone with significant topographic variation (∼4 km), the shear wave velocity image has an obvious negative bias that can reach up to −4 per cent. The maximum depth of bias is ∼5 km, which is mirrored with the maximum topographic elevation difference of the region, and the average bias disappears as the depth decreases to the surface (0 km) or increases to three times of the maximum influence depth (∼15 km). The horizontal distribution of the tomographic bias is almost linearly related to the topographic elevation difference with a slope of −1.04 and a correlation coefficient of 0.90 at maximum influence depth. According to this first-order correction formula and the decreasing trend of average bias with depth, the topography effect on ANT can be suppressed to a certain extent.

While understanding the long-term slip rate of active normal faults is essential for the comprehensive assessment of seismic activity, it is difficult due to the absence of age control in the erosional bedrock region. The preserved sequence of wave-cut platforms in granite allows exploration of the long-term slip rate in the footwall of some normal faults. We investigated wave-cut platforms in the southern Pearl River Delta (PRD), a coastal delta transected by the seismically active Littoral Fault Zone (LFZ) in the northern South China Sea, to derive slip rates and their impacts on the seismic hazard potential. We mapped a flight of four wave-cut platforms (T 1 –T 4 ), dated the T 2 and T 4 platforms by 10 Be cosmogenic nuclide dating, and used the absolute age to correlate the un-dated platform to global sea-level highstands. Our results allocate the ages of 128 ka, 197 ka, and 239 ka to the upper three wave-cut platforms and yield temporally various uplift rates ranging from 0.30 to 0.38 mm/a during 239–128 ka to 0.09 mm/a since 128 ka. A decrease in the uplift rate, which coincided with a decreased subsidence rate within the PRD in previous work, implied a weakened differential uplift onshore of the LFZ system. Our findings infer that the transgression event occurred as early as marine isotope stage (MIS) 7 in the PRD, consistent with the view that Pleistocene sedimentation began in MIS 5 or earlier in the PRD.

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 under-stood. 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 identi-fying the causative fault of this earthquake sequence are important to analyze its seis-mogenic 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 consis-tent with that obtained from seismic waveform data. We proposed that this earthquake sequence ruptured along an unmapped dextral fault, which forms a conjugate fault sys-tem 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.

On 30 October 2020, an Mw 6.9 normal faulting earthquake occurred off the northern coasts of Samos Island, Greece. The earthquake with moderate magnitude generated the largest tsunami in the eastern Mediterranean since the 1956 Amorgos event. Here, we investigate the earthquake source characteristics and examine the factors contributing to the exacerbated tsunami using an integrated approach, including the source inversion from InSAR and GPS data, tsunami simulation, and spectral analysis of tsunami waveforms. The results show that a set of conjugate faults, determined in this study, can explain the geodetic surface deformation equally well. Our tsunami modeling results indicate that the shallow north‐dipping fault was most likely responsible for the mainshock. With the key features of our preferred source models, we infer that the tsunami was intensified by several conditions: (a) the spatial distribution of major coseismic slips coincided with the deepest portion of the Samos Basin which served as an energy reservoir for the tsunami generation; (b) the tsunami was generated in a semi‐closed bay which trapped most of the tsunami energy; (c) the east‐west orientation of the mainshock fault and the location of the concentrated coseismic slips directed the majority of the tsunami energy toward Sığacık Bay, which formed the extremely high tsunami of 3.8 m; (d) the primary tsunami wave period, close to natural oscillation frequency of the harbor, resulted in the observed significant oscillation. We conclude that the tsunami of the 2020 Samos earthquake was exacerbated by both the source characteristics and tsunami resonance property.

In foreland thrust systems, the complex structural geometry creates greater difficulties in constraining the seismotectonics of moderate magnitude earthquakes without pronounced surface ruptures and in assessing future seismic risk. The 2013 M 5.1 Urumqi earthquake was a moderate event that occurred in the Urumqi foreland thrust system. Based on surface investigations, geological mapping, and interpretations of deep seismic reflection profiles, we suggest that the Urumqi foreland thrust belt is a typical active wedge thrust system. Two connected fault segments of the gently south-dipping blind fault ramp and the north-dipping Xishan back-thrust fault merge at a depth of similar to 11 km and bound the wedge-shaped fault block. Analyses of the focal mechanism and aftershock distribution indicate that the seismogenic fault responsible for the M 5.1 Urumqi event is the south-dipping Xishan fault ramp with a dip angle of similar to 10-30 degrees, which highlights the potential seismic hazards of the unruptured north-dipping Xishan back-thrust ramp. The four fault branches on the hanging wall of the Xishan fault are interpreted as flexural-slip thrusts. Our results indicate that multiple surface ruptures over a 6 km-wide area may form if a large earthquake occurs along the Xishan fault system.

Indo-South China lying between the Indian Plate and the rigid South China Block is characterized by intense crustal deformation with severe seismic hazards. How deformation is partitioned through slip on various structural features provides key insights into understanding the regional geodynamic processes. Based on the upto-date GPS velocity data together with the existing active tectonic, seismological, and geophysical data, we find that the relative motion between India and South China is mainly accommodated by two parts: -47 mm/yr dextral slip and 15-18 mm/yr contraction to the west of the Shan Plateau, and - 10 mm/yr sinistral slip and 10 mm/yr extension to the east of it. The Burmese Range takes up - 22 mm/yr dextral slip and 10-13 mm/yr contraction, while the Sagaing Fault accommodates - 18 mm/yr dextral slip and 5-6 mm/yr contraction. The Shan Plateau Shear Zone absorbs - 8 mm/yr dextral slip and 5-6 mm/yr extension. The Chuandian Terrane from east of the Red River Fault to the Xiaojiang Fault manifests 8-10 mm/yr sinistral slip and 5-6 mm/yr extension. The block motion inversion reveals that the clockwise rotation and deformation are caused by the northeastward motion of the India Plate and the clockwise lithospheric flow around the Eastern Himalayan Syntaxis. The Shan Plateau Shear Zone may be a newly formed dextral shear zone at depth of the middle and lower crust, driving the sinistral slip and extension along the ENE-trending faults in upper crustal level. The Shan Plateau Shear Zone might have replaced the Red River Fault as the western boundary to cause the southeastward coherent movement of the Chuandian Terrane. The complex pattern of active tectonic deformation in the region results from interactions among hyper-oblique subduction beneath the Burmese Range, clockwise rotation of lithospheric material around the Eastern Himalayan Syntaxis, and the apparently rigid Sundaland Block separating the two systems in between.

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.

On 4 January 2020, an ML 3.5 earthquake occurred in the Pearl River Estuary (PRE) and was felt at a distance of more than 200 km. According to the China Earthquake Networks Center, this event has been the only M > 3 earthquake within the PRE since 1900. The Guangdong?Hong Kong?Macau Bay Area (GHMBA) surrounding the PRE is one of China?s most critical financial circles, and coastal earthquake hazard has become an increasing concern. Investigating the source parameter and causative fault of this earthquake is helpful for seismic hazard estimation and mitigation in the GHMBA. In this study, we first determined the focal mechanism of the mainshock using the cutand-paste method. We then used the sliding-window cross-correlation method to detect foreshocks and aftershocks before relocating the earthquakes. Finally, we conducted forward modeling to retrieve the rupture directivity of the mainshock, using waveforms of one aftershock as empirical Green?s functions. The results demonstrate that this earthquake was an Mw 3.7 strike-slip event, with a focal depth of 10 km. The rupture direction of the mainshock was 78?, consistent with the northeast-east-trending fault system in the region. The identified source fault confirmed a seismogenic segment of the northeast-east-trending fault system in the PRE, which is the primary source of seismic hazard in the area.

甘肃夏河M5.7地震的震中位于临潭-宕昌断裂与西秦岭北缘断裂之间.文中采用gCAP方法及P波初动方法求解了主震的震源机制解,并利用双差定位方法对夏河地震及其余震序列进行了重定位,分析了地震的发震断层.结果显示:节面Ⅰ的走向、倾角和滑动角分别为185°、56°和127°;节面Ⅱ的走向、倾角和滑动角分别为312°、48°和48°.重定位后,余震序列的震源呈NE倾、宽2～4km的条带状分布,深度集中于3～9km.余震的分布特征与主震震源机制解的节面Ⅱ相符,由此推断夏河M5.7地震的发震断裂为临潭-宕昌断裂的次级断裂,该断裂的走向为312°,倾向NE.最后,初步探讨了该次地震的发震模式.

On October 30 2020 11:51 UTC, a Mw 6.9 normal fault earthquake occurred off the northern coasts of Samos Island, Eastern Aegean, Greece. Over a 120 people were killed and more than 1000 people were injured during the seismic sequence. The quake produced a moderate tsunami that swapped the coastal areas of Izmir (Turkey) and Samos (Greece) with inundation heights up to ~3 m. Finding the source of such a tsunami has been puzzling as a normal fault earthquake with Mw 6.9 would not be considered significant enough to generate metric-scale waves. Furthermore, the lack of near-field observations has made the identification of the seismogenic fault responsible for the mainshock difficult. In this study, we infer the source characteristics from multiple observation data, including InSAR, GPS, teleseismic waves and tsunami waves. We first generate two Sentinel-1 co-seismic interferograms with a maximum Line of Sight (LOS) change of 8 cm on the coastal areas at the Samos island. We obtain a north-dipping fault model, which can slightly better explain the geodetic observations and teleseismic P waves. To understand the potential tsunami source, we use several earthquake slip models collected from different research groups to conduct tsunami simulations. Comparing simulated tsunami waveforms with those measured at 6 local tide gauges, we show that the north-dipping fault can fit tsunami records better than the south-dipping fault. The north-dipping fault hypothesis is also further supported by the spatial distributions of the aftershocks. The spectral analysis of tsunami waveforms at selected tide gauges suggests that the tsunami period band is within 4.6 ~ 21.3 min and the primary wave period is ~14.2 min. Using this wave period as an indirect constraint, we show that the source dimension of our slip model can produce tsunami waveforms with similar wave period. We also find high-energy wave of the Samos earthquake that lasted 20 h, and fundamental oscillation periods of Sığacık Bay are remarkably close to some dominating tsunami periods. We infer the coseismic seafloor displacement alone is not enough to create disastrous effects on coastal cities; therefore we suggest that the tsunami waves may have been amplified by local coastline and tsunami resonance with local bay, or another source, e.g. triggered landslides.

2016年1月21日青海省门源县发生MS6.4级地震,震中及周围地区震感明显,大量房屋损坏.虽然地震发生在人烟稀少地区但仍有9人受伤.本文开展了门源地震震源参数及破裂过程参数的研究,以期为理解门源地震的发震机理,完善震中区域震后灾害评估体系,进一步探究中等强度逆冲地震的破坏程度空间分布等提供基础信息.首先,利用青海、甘肃省固定台站和部分青海省流动台站记录的地震波形数据,拾取近震震相 Pg、Pn、Sg 到时,并使用 HYPOINVERSE-2000和 Pn/Pg 相对定位方法,以2016年8月13日门源MS 4.7余震作为参考事件对主震进行重新定位.然后,结合两次地震CAP方法反演过程中的质心深度和时移信息,利用最小二乘法拟合得到门源地震沿走向和倾向的破裂方向性特征.结果显示主震沿断层走向破裂长度为9.6 km,呈现单侧破裂特征;主震质心深度和起始破裂深度分别为9 km和10 km,沿断层倾向方向上属于双侧破裂.对于门源地震的深部孕震过程,推测可能是青藏高原东北缘受到印度板块与欧亚板块碰撞和阿拉善地块与鄂尔多斯地块刚性阻挡作用下,经过长期应力积累导致地震的发生.

On May 21, 2021, two earthquakes, with magnitudes of 5.6 and 6.4 respectively, struck the Yangbi County in Yunnan, China. The epicenters of the two earthquakes were about 7 km apart, and both were located in the western Yunnan region. This region is located in the southeastern margin of the Qinghai-Tibet Plateau and the southern segment of the north-south seismic belt. Preliminary analysis shows the Yangbi M(S)6.4 earthquake sequence is with foreshock-mainshock-aftershock type, and the M(S)5.6 earthquake is the foreshock of the M(S)6.4 earthquake. In this study, using the seismic phase report provided by Yunnan Earthquake Networks, we relocated the early events (with local magnitude ML larger than 0.0 from May 18 to 25, 2021) of the M(S)6.4 Yangbi earthquake sequence using the double-difference relocation method. Meanwhile, the focal mechanism solutions and centroid depths of 31 M-S >= 3.0 events in the sequence were determined using the Cut-And-Paste (CAP) waveform inversion method. The seismogenic structure of the earthquake sequence based on the inverted results is also discussed. The results indicate: (1) The relocated 2159 M-L >= 0.0 events are distributed in a narrow zone with a length of similar to 25 km in the NW-SE direction, and a width of 5 similar to 10 km. The focal depth of the main shock is 8.9 km, and the depth range of the sequence is between 4 and 10 km with an average depth of 7.5 km. (2) The foreshock sequence shows the rupture starts from the middle, then goes to NW and finally to the SE direction. The epicenter of the main shock is located in the NW end of the aftershock region, and the epicenter of the largest aftershock (M(S)5.2 earthquake) is located in the SE end of the region. (3) The centroid depths of all the 31 events with M-S >= 3.0 from CAP method range from 4 to 11 km with a mean value of around 6.5 km, which is consistent with the predominant focal depth of the whole sequence from double-difference method. The agreement of the results of the two methods verify that the double-difference relocation result is reliable. (4) The focal mechanism solutions of the 31 M-S >= 3.0 events are mostly of strike-slip type, while some events show an obvious normal-fault component. The inverted stress result based on the focal mechanism solution is consistent with the regional horizontal principal compressional stress field, which indicates that the tectonic activity in this region is mainly controlled by the regional tectonic stress field. According to the distribution of relocated earthquake sequence, combined with the focal mechanism solutions and the regional structures, we conclude that the seismogenic structure for the Yangbi M(S)6.4 earthquake sequence is a secondary fault of the Weixi-Qiaohou-Weishan fault system.

文中针对1990—2018年间发生在鄂尔多斯西缘的地震事件,采用双差定位法获得其中4417个事件(M≥1.0)的精确定位结果;利用CAP方法求解了54个地震(M≥3.5)的震源机制解,并收集了15个前人获取的震源机制结果,综合研究了区域内地震事件的空间分布规律、主要活动断裂的深部几何结构和区域构造应力场特征.小震精定位结果揭示M≥3.5地震震中位于主要活动断裂的边缘,刻画出较为清晰的断裂几何学特征.震源机制解反映海原断裂、香山-天景山断裂和烟筒山断裂以压扭性质为主;黄河断裂以伸展为主;西秦岭断裂表现为压扭性质.综合小震精定位和震源机制解结果,并结合研究区内已发表的活动构造和地球物理资料,证实了鄂尔多斯西缘受青藏高原、阿拉善和鄂尔多斯3个地块的共同作用,表现出不同的构造变形模式,同时活动断裂之间的次级块体也存在明显的运动差异.香山-天景山断裂以南的区域自第四纪早期整体向SEE运动,而银川盆地以及黄河断裂东缘的块体向SE运动.

The Jinghe M(s)6. 6 earthquake on August 9, 2017 is the largest earthquake in northwest Tianshan in recent years. The earthquake is located within the Kusongmuqike fault zone, and there are several active thrust faults near the epicenter region, including Chahantu fault, Kusongmuqike Frontal fault and Jinghenan fault. Due to the deep source, complicated tectonic deformation and sparse seismic network, it is difficult to directly determine the seismogenic structure of the earthquake based on focal mechanism, aftershock distribution and InSAR observation. In this paper, a method for determining the rupture directivity based on regional seismic waveforms is developed for dip-slip earthquakes. Aftershocks are chosen as reference events, which can be applied as path calibration, and the rupture lengths in the horizontal and depth directions are determined based on the waveform time-shift and Pn-Pg arrival time difference. Then, the rupture plane and rupture length can be inferred. We first invert the point source parameters of the mainshock, then apply the new algorithm to determine the rupture directivity. The results show that the moment magnitude of the mainshock is about 6. 2, the centroid depth is 21 km, the source duration is 5. 5 s, and the two double-coupled planes are 102 degrees/45 degrees/106 degrees (NP1) and 259 degrees/47 degrees/74 degrees (NP2) , respectively. The rupture plane is the south-dipping NP1, and the earthquake ruptured towards southwest and downdip for about 11. 5 km. The rupture range along depth is about 8 km, the horizontal rupture range is about 9 km, and the average rupture speed is about 2. 1 km . s(-1). Furthermore, we combine the seismological results with regional geological data, satellite imagery, etc. , and infer the seismogenic fault of this earthquake is the Jinghenan fault, and the earthquake may only rupture the deeper section of the fault (17 similar to 25 km) without breaking the surface.