Earthquake Research Advances最新文献

The development of machine learning technology enables more robust real-time earthquake monitoring through automated implementations. However, the application of machine learning to earthquake location problems faces challenges in regions with limited available training data. To address the issues of sparse event distribution and inaccurate ground truth in historical seismic datasets, we expand the training dataset by using a large number of synthetic envelopes that closely resemble real data and build an earthquake location model named ENVloc. We propose an envelope-based machine learning workflow for simultaneously determining earthquake location and origin time. The method eliminates the need for phase picking and avoids the accumulation of location errors resulting from inaccurate picking results. In practical application, ENVloc is applied to several data intercepted at different starting points. We take the starting point of the time window corresponding to the highest prediction probability value as the origin time and save the predicted result as the earthquake location. We apply ENVloc to observed data acquired in the southern Sichuan Basin, China, between September 2018 and March 2019. The results show that the average difference with the catalog in latitude, longitude, depth, and origin time is 0.02°, 0.02°, 2 ​km, and 1.25 ​s, respectively. These suggest that our envelope-based method provides an efficient and robust way to locate earthquakes without phase picking, and can be used in earthquake monitoring in near-real time.

On December 18, 2023, the Jishishan area in Gansu Province was jolted by a M_S 6.2 earthquake, which is the most powerful seismic event that occurred throughout the year in China. The earthquake occurred along the NW-trending Lajishan fault (LJSF), a large tectonic transformation zone. After this event, China Earthquake Networks Center (CENC) has timely published several reports about seismic sources for emergency responses. The earthquake early warning system issued the first alert 4.9 ​s after the earthquake occurrence, providing prompt notification that effectively mitigated panics, injuries, and deaths of residents. The near real-time focal mechanism solution indicates that this earthquake is associated with a thrust fault. The distribution of aftershocks, the rupture process, and the recorded amplitudes from seismic monitoring and GNSS stations, all suggest that the mainshock rupture predominately propagates to the northwest direction. The duration of the rupture process is ∼12 ​s, and the largest slip is located at approximately 6.3 ​km to the NNW from the epicenter, with a peak slip of 0.12 ​m at ∼8 ​km depth. Seismic station N0028 recorded the highest instrumental intensity, which is 9.4 on the Mercalli scale. The estimated intensity map shows a seismic intensity reaching up to IX near the rupture area, consistent with field survey results. The aftershocks (up to December 22, 2023) are mostly distributed in the northwest direction within ∼20 ​km of the epicenter. This earthquake caused serious casualties and house collapses, which requires further investigations into the impact of this earthquake.

We have developed an automatic regional focal mechanism inversion system based on the Earthquake Rapid Report (ERR) system and the real-time three-component seismic waveform stream of 1 000 broadband seismic stations provided by the China Earthquake Networks Center (CENC). The system can rapidly provide a double couple solution and centroid depth within 5–15 ​min after receiving earthquake information from the ERR system. The data processing is triggered by earthquake information obtained from the ERR system. The system is capable of determining the focal mechanism of all shallow-depth earthquakes in the Chinese mainland with a magnitude of 5.5–6.5. It utilizes waveform data recorded by seismic stations located within 500 ​km from the epicenter, enabling the reporting of a focal mechanism solution within 5–15 ​min of an earthquake occurrence. Additionally, the system can assign a corresponding grade (A B C) to the focal mechanism solution. We processed a total of 301 earthquakes that occurred from 2021 to June 2022, and after the quality control, 166 of them were selected. These selected solutions were manually checked, and 160 of them were compiled in a focal mechanism catalog. This catalog can be conveniently downloaded online via the Internet. The automatic focal mechanism solution of earthquakes in eastern China exhibits a good agreement with that provided by the Global Centroid Moment Tensor (GCMT), when available. The average Kagan angle between this catalog and GCMT is 22°, and the average difference in M_W is 0.17. Furthermore, compared with GCMT, the minimum magnitude of our catalog has been reduced from approximately 5.0 to 4.0. The correlation between the centroid depth and crustal thickness in the Chinese mainland confirms the distribution of the centroid depth.

Displacement control algorithms commonly used to evaluate axial force-bending moment (PM) diagrams may lead to incorrect interpretations of the strength envelopes for asymmetric sections. This paper aims to offer valuable insights by comparing existing displacement control algorithms with a newly proposed force control algorithm. The main focus is on the PM diagrams of three example sections that exhibit varying degrees of asymmetry. The comparative study indicates that conventional displacement control algorithms inevitably introduce non-zero out-of-plane bending moments. The reported PM diagram in such cases erroneously neglects the out-of-plane moment and fails to represent the strength envelope accurately. This oversight results in significant and unconservative errors when verifying the strength of asymmetric sections.

To enhance the understanding of the geometry and characteristics of seismogenic faults in the Beijing-Tianjin-Hebei region, we relocated 14 805 out of 16 063 earthquakes (113°E−120°E, 36°N–43°N) that occurred between January 2008 and December 2020 using the double-difference tomography method. Based on the spatial variation in seismicity after relocation, the Beijing–Tianjin–Hebei region can be divided into three seismic zones: Xingtai–Wen'an, Zhangbei–Ninghexi, and Tangshan. (1) The Xingtai–Wen'an Seismic Zone has a northeast-southwest strike. The depth profile of earthquakes perpendicular to the strike reveals three northeast-striking, southeast-dipping, high-angle deep faults (>10 ​km depth), including one below the shallow (<10 ​km depth) listric, northwest-dipping Xinghe fault in the Xingtai region. Two additional deep faults in the Wen'an region are suggested to be associated with the 2006 M 5.1 Wen'an Earthquake and the 1967 M 6.3 Dacheng earthquake; (2) The Zhangbei-Ninghexi Seismic Zone is oriented north-northwest. Multiple northeast-striking faults (10–20 ​km depth), inferred from the earthquake-intensive zones, exist beneath the shallow (<10 ​km depth) Xiandian Fault, Xiaotangshan Fault, Huailai-Zhuolu Basin North Fault, Yangyuan Basin Fault and Yanggao Basin North Fault; (3) In the Tangshan Seismic Zone, earthquakes are mainly concentrated near the northeast-striking Tangshan-Guye Fault, Lulong Fault, and northwest-striking Luanxian-Laoting Fault. An inferred north-south-oriented blind fault is present to the north of the Tangshan-Guye Fault. The 1976 M 7.8 Tangshan earthquake occurred at the junction of a shallow northwest-dipping fault and a deep southeast-dipping fault. This study emphasizes that earthquakes in the region are primarily associated with deep blind faults. Some deep blind faults have different geometries compared to shallow faults, suggesting a complex fault system in the region. Overall, this research provides valuable insights into the seismogenic faults in the Beijing–Tianjin–Hebei region. Further studies and monitoring of these faults are essential for earthquake mitigation efforts in this region.

This study analyzed and summarized in detail the spatial and temporal distributions of earthquakes, tidal responses, focal mechanisms, and stress field characteristics for the M 7.3 Haicheng earthquake sequence in February 1975. The foreshocks are related to the main fault and the conjugate faults surrounding the extension step-over in the middle. The initiation timing of the foreshock clusters and the original time of the mainshock were clearly modulated by the Earth's tidal force and coincided with the peak of dilational volumetric tidal strain. As a plausible and testable hypothesis, we proposed a fluid-driven foreshock model, by which all observed seismicity features can be more reasonably interpreted with respect to the results of existing models. Together with some other known examples, the widely existing step-over along strike-slip faults and associated conjugate faults, especially for extensional ones in the presence of deep fluids, favor the occurrence of short-term foreshocks. Although clustered seismicity with characteristics similar to those of the studied case is not a sufficient and necessary condition for large earthquakes to occur under similar tectonic conditions, it undoubtedly has a warning significance for the criticality of the main fault. Subsequent testing would require quantification of true/false positives/negatives.

To enable the experimental assessment of the seismic performance of full-scale nonstructural elements with multiple engineering parameters (EDPs), a three-layer testbed named Nonstructural Element Simulator on Shake Table (NEST) has been developed. The testbed consists of three consecutive floors of steel structure. The bottom two floors provide a space to accommodate a full-scale room. To fully explore the flexibility of NEST, we propose a novel control strategy to generate the required shake table input time histories for the testbed to track the target floor motions of the buildings of interest with high accuracy. The control strategy contains two parts: an inverse dynamic compensation via simulation of feedback control systems (IDCS) algorithm and an offline iteration procedure based on a refined nonlinear numerical model of the testbed. The key aspects of the control strategy were introduced in this paper. Experimental tests were conducted to simulate the seismic responses of a full-scale office room on the 21^st floor of a 42-story high-rise building. The test results show that the proposed control strategy can reproduce the target floor motions of the building of interest with less than 20% errors within the specified frequency range.