Earthquake Research Advances最新文献

Large earthquakes can cause both casualties and economic losses, but they also provide invaluable opportunities for earthquake scientific research. Geofluids, due to their wide distribution, sensitive response to underground conditions, and ease of observation, are widely applied in field investigations after earthquakes. Analyzing the origin of fluids, energy transfer processes, temperature/pressure conditions, and the spatial-temporal evolution of geofluids can provide valuable information concerning the mechanism of earthquake precursor anomalies, short-term prediction methods, identification of the seismogenic faults, determination of earthquake risk, and the environmental impact of post-earthquake fluids. This article details post-earthquake scientific expeditions and research on fluid geochemistry in China and abroad, aimed at providing ideas and guidance for future scientific expedition work and geochemistry-related earthquake studies.

Decoding the variation laws of the deformation field before strong earthquakes has long been recognized as an essential issue in earthquake prediction research. In this paper, the temporal and spatial distribution characteristics of deformation anomalies in the northeastern margin of the Qinghai-Tibetan Plateau before and after the Menyuan M_S 6.9 earthquake were studied by using the Fisher statistical test method. By analyzing the characteristics of these anomalies, we found that: 1) The deformation anomalies are mainly distributed in the marginal front area of the Qinghai-Tibetan Plateau, where short-term deformation anomalies are prone to occur due to a high gradient of gravity; 2) The deformation anomalies along the northeastern margin of the Qinghai-Tibetan Plateau are characterized by spatial propagation, and the migration rate is about 2.4 ​km/d. The propagation pattern is counterclockwise, consistent with the migration direction of M_S ​≥ ​6.0 earthquakes; 3) The time and location of the Menyuan earthquake are related to the group migration of earthquakes with M_S ​≥ ​6.0. Finally, based on the results of gravity field variation and the theory of crust stress wave, the law of deformation anomaly distribution was discussed. We suggest that both the deformation propagation along the northeastern margin of the Qinghai-Tibetan Plateau and the earthquake migration are possibly associated with the variation of the stress field caused by subsurface mass flow.

The central-southern part of the eastern border of the Sichuan-Yunnan rhombic block provides the research strategy of ‘trade space for time’ with an interesting fault system, where the segments have similar focal mechanisms and cover almost continuous spectra of elapse rates. We experiment to study the seismological characteristics of different segments with different elapse rates. We employed the de-clustered earthquake catalog for the calculation of b values for each segment. The analysis revealed that different segments have similar b values, which implies that, although different segments have different periods of earthquake recurrence, the 'natural time' for the whole fault system elapses with a homogeneous pace. We extended the earthquake potential score (EPS) for nowcasting earthquakes to a quasi-EPS (qEPS). It is found that qEPS increases with the increase of elapse rates, albeit for those fault segments whose elapse rates have exceeded 1, qEPS may better reflect the seismic hazard.

After the occurrence of destructively strong earthquakes, rapid acquisition of the source rupture process can provide important reference information for post-earthquake disaster relief and aftershock trend determination. An M 6.9 earthquake occurred in Menyuan County, Qinghai Province on January 8, 2022. The epicenter is located in the seismic gap in the middle section of the Haiyuan fault belt. Such a typical strong earthquake was taken as an example to investigate the rupture process of strong earthquakes. Three days after the earthquake, the InSAR (Interferometric Synthetic Aperture Radar) coseismic deformation field was obtained by Sentinel radar, indicating that the surface ruptured obviously. The southern block of the earthquake faces towards the satellite about 95 ​cm along the LOS (line of sight) direction, and the northern block is away from the satellite by ​∼ ​74 ​cm, consistent with the characteristic of left-lateral strike-slip motion. In this study, InSAR coseismic deformation data and far-field waveform data were used to jointly invert the earthquake rupture process, and a four-segment finite fault model was constructed by referring to the surface deformation. The inversion results show that the focal depth of the Menyuan earthquake is about 7 ​km, and the strike of the seismogenic fault is 89.0°, 104.0°, 119.0° and 131.0° from west to east, respectively. It is a high-dip left-lateral strike-slip earthquake event lasting about 14 ​s. The rupture propagation mode is a bilateral extension. The maximum slip along the fault is about 380 ​cm, and the seismic moment magnitude is 6.7. The surface rupture length is about 24 ​km, which is consistent with that measured in the field survey. The detailed seismic source model can provide basic data for the aftershock trend determination and seismic risk analysis of the adjacent active faults.

The seismic performance of medical systems is crucial for the seismic resilience of communities. The report summarizes the observed damage to twelve hospital buildings in the area affected by the M_W 7.8 and M_W 7.5 earthquakes on February 6, 2023 in Turkey. They include five base-isolated buildings and seven fixed-base buildings in southcentral Turkey's seven most heavily affected provinces. By relating the post-quake occupancy statuses of the hospitals with the estimated seismic demands during the earthquake doublet, the report offers the following observations: (1) the base-isolated hospital buildings on friction pendulum bearings generally exhibited superior performance of achieving the goal of immediate occupancy and provided better protection for nonstructural elements than fixed-base counterparts did; (2) the fixed-base hospital buildings of reinforced concrete structures constructed after 2001 successfully achieved the goal of collapse prevention even under very high seismic demands; (3) some fixed-base hospitals also remained operational even if they were very close to the fault rupture and were subjected to higher-than-design-level earthquake ground motions.

Based on the seismic data recorded by the China Earthquake Networks Center (CENC) in the Luxian area from January 2009 to October 2021, the 3D V_P, V_S, V_P/V_S structures and seismic locations of the area are obtained by joint inversion using the V_P/V_S model consistency-constrained double-difference tomography method (tomoDDMC). The earthquakes in the study area are mainly concentrated at a depth of 2–6 ​km, and the focal depth is generally shallow. The M_S 6.0 Luxian earthquake occurred at the transition zone of high- and low-velocity anomalies and the aftershock sequence was distributed along the edge of the low-V_P zone. A small number of foreshocks occurred on the west side of the M_S 6.0 Luxian earthquake, while most of the aftershocks were distributed on the east side of the M_S 6.0 Luxian earthquake. The aftershock sequence consisted of three seismic bands with different trends, and the overall distribution was in a NWW direction, which was inconsistent with the spatial distribution of the main active faults nearby. In addition, the spatiotemporal distribution of earthquakes and the variation of b-values are closely related to the industrial water injection activities in the study area, reflecting the activation of pre-existing hidden faults under certain tectonic and stress environments leading to seismic activities in the area.

To reveal the seismogenic mechanism of the Luding earthquake, we employed the 118 China Seismic Network stations to collect the P-wave polarity data from each station, which was then used in the P-wave first motion approach to calculate the focal mechanism solution of the M6.8 Luding earthquake that occurred on September 5, 2022. We have also studied the loading effect of tectonic stress on the Luding earthquake fault based on the stress field data for the research area. The results indicate that this earthquake was a strike-slip type, the nodal plane I: strike 167°, dip Angle 78°, slip Angle 2°; Nodal plane Ⅱ: strike 77°, dip Angle 88°, slip Angle 168°. The two fault planes’ instability coefficients of the Luding earthquake are examined considering the region’s background stress field’s condition. The nodal plane I in the Moho circle is discovered to practically coincide with the Coulomb failure line and the tangent point of the Moho circle, indicating that this nodal plane has a high instability coefficient compared to the nodal plane II. The conclusion is that the nodal plane I has a higher likelihood of being the seismogenic fault plane, which is congruent with the seismogenic fault plane suggested by the aftershock distribution, the earthquake radiation energy distribution of a single station, and seismic intensity distribution. The Luding earthquake’s focal mechanism is highly like the theoretical focal mechanism of the fault situated at the location where the Coulomb failure line intersects the Mohr circle, demonstrating that background stress is what caused the earthquake. The substantial fault instability and similarity between the solved and theoretical focal mechanisms make it easier to comprehend the loading effect of tectonic stress on the Luding earthquake fault.

The relationship of the crustal contact between the Indian and Eurasian plates is a key issue in understanding crustal thickening and the subduction of the Indian lithosphere beneath the Qinghai-Tibetan Plateau. Across the middle of the Yarlung-Zangbo Suture (YZS), we deployed an ∼450-km-long SN-trending wide-angle reflection/refraction profile to observe the P-wave velocity (v_P) structure beneath the northern Himalaya and the southern plateau. Our results show that, 1. the high v_P (∼7.1 ​km/s) indicates that the Indian lower crust extends no more than 50 ​km north of the YZS. 2. The lower crust beneath the southern part of the plateau features an extremely low v_P (<6.7 ​± ​0.2 ​km/s). 3. Compared with the velocities of several typical crustal lithologies in different temperature regimes, the low v_P in the lower crust can be explained by felsic-intermediate granulite, which has prevented the lower crust from further eclogitization. We propose that the dip angle of the Indian lithospheric slab beneath the YZS is partly controlled by the composition of the lower crust of the plateau. In the northern middle YZS, the crust of the southern plateau is too thick and blocks the northward advancement of the Indian lower crust, resulting in the subduction of the Indian lithospheric slab into the upper mantle. The lower crust in western and eastern Lhasa is dominated by a mafic composition, and it was delaminated after eclogitization before the Miocene. The void zone generated by delamination favors the flattening and underthrusting of the Indian lower crust.