ABSTRACT The Longmen Shan fault zone that was shocked by the 12 May 2008 M 8.0 Wenchuan earthquake acts as the boundary between the western edge of the Sichuan basin and the steep eastern margin of the Songpan-Ganze block. In this study, continuous seismic data recorded by 176 temporary short-period seismic stations between 22 October and 20 November 2017 are used to study the shallow crustal structure of the Longmen Shan fault zone by applying ambient-noise tomography and horizontal-to-vertical spectral ratio (HVSR) analysis. From ambient-noise analysis, fundamental-mode Rayleigh-wave dispersion curves between 0.25 and 1 Hz are extracted. Then, the direct surface-wave tomographic method is used to invert surface-wave dispersion data for the 3D shallow shear-wave velocity structure. Our results show that low shear-wave velocities are mainly distributed around the surface rupture trace of the Wenchuan earthquake at least down to 2 km. From the HVSR method, the sites are sorted into two types according to the pattern of HVSR curves with single peak or double peak. By converting frequency to depth, the results show that the sediments are thicker near the surface rupture. The low-velocity zone based on ambient-noise tomography agrees well with the distribution of sedimentary cover estimated from HVSR, which are generally consistent with geological information. Our results provide high-resolution shallow crustal velocity structure for future detailed studies of the Longmen Shan fault.
The Red River fault is a Holocene active fault between the Indo-China plate and Yangtze plate, its great earthquake risk has gained attention. The middle-north section and nearby areas of the fault can be divided into three tectonic units. In the study, the crustal velocity structure images and accurate earthquake locating results were obtained for various zones by using the tomoDD method. The results showed that the northern and middle section of the region were characterized by low-velocity anomalies in NW and NS directions, and such feature became more obvious as the depth increased. On the west section, large-scale low-velocity anomalies were correlated with the Kainozoic volcanic zone. In addition, from the results of stress drop and velocity structure, the correlation between them is not obvious. Our study may help to understand seismogenic structures of complex fault systems in the Middle-north section of Red River Fault and adjacent areas.
A high-resolution two-dimensional Pg-wave velocity model is obtained for the upper crust around the epicenters of the April 20, 2013 Ms7.0 Lushan earthquake and the May 12, 2008 Ms8.0 Wenchuan earthquake, China. The tomographic inversion uses 47235 Pg arrival times from 6812 aftershocks recorded by 61 stations around the Lushan and Wenchuan earthquakes. Across the front Longmenshan fault near the Lushan earthquake, there exists a strong velocity contrast with higher velocities to the west and lower velocities to the east. Along the Longmenshan fault system, there exist two high velocity patches showing an “X” shape with an obtuse angle along the near northwest-southeast (NW-SE) direction. They correspond to the Precambrian Pengguan and Baoxing complexes on the surface but with a ~20 km shift, respectively. The aftershock gap of the 2008 Wenchuan and the 2013 Lushan earthquakes is associated with lower velocities. Based on the theory of maximum effective moment criterion, this suggests that the aftershock gap is weak and the ductile deformation is more likely to occur in the upper crust within the gap under the near NW-SE compression. Therefore our results suggest that the large earthquake may be hard to happen within the gap.
A compact filtering antenna with multi-adjustable radiation nulls is designed. Initially, a conventional aperture-coupled antenna is crafted by fine-tuning positions of the gap and radiation patch, achieving a radiation null. Additionally, an aperture-coupled filtering antenna is created by cascading a resonator with a microstrip feedline. Subsequently, the integration of the aperture-coupled antenna and the filtering antenna results in a dual-slot aperture cross-coupling filtering antenna, with two radiation nulls at both ends of the passband. Finally, a pair of slot lines is etched on the radiation patch, introducing an extra radiation null to enhance selectivity further. The measured results show that the proposed filtering antenna has a bandwidth from 2.44 to 2.64 GHz for |S11|<−10 dB, a peak gain of 5.5 dBi, and three radiation nulls at 2.3, 2.75, and 3.15 GHz enhancing the selectivity.
Abstract At 16:03 on 9 June 2022 (UTC), an Mw 5.5 earthquake followed by several Mw>4 events, including the largest event of Mw 5.8 within a few hours, occurred in the Maerkang area near the Caodeng Hot Spring Town, located in the south-central part of the Bayan Har plate on the eastern margin of the Qinghai–Tibet plateau. The earthquake swarm allows understanding the tectonic stress environment of the Bayan Har plate and is an example of a typical moderate-to-strong intraplate earthquake swarm. This article comprehensively analyzes the detailed seismogenic fault structure of the swarm by means of precise hypocenter relocation, focal mechanism inversion for Mw>4 earthquakes, inversion of the tectonic stress field in different regions of the Bayan Har plate, tidal strain calculation, and seismicity statistics. The results show that the swarm was not directly related to the nearby mapped Songgang fault, but rather resulted from the successive activation of a series of unknown faults. The precise hypocenter distribution, together with focal mechanism solutions of major earthquakes, illuminates five major seismogenic faults with conjugate relationships and stepover. Spatial and temporal migration of hypocenters, stress transfer, and tidal correlations demonstrate that cascade triggering, afterslip, and overpressured fluid might have jointly played a role in causing the earthquake swarm. As an output of this research, a set of verifiable datasets are provided as a basis for further in-depth research.
Abstract On April 20th, 2013, an earthquake of magnitude M w 6.6 occurred at Lushan of Sichuan on the southern segment of the Longmenshan fault zone, with no typical coseismic surface rupture. This work plotted an isoseismal map of the earthquake after repositioning over 400 post–earthquake macro‐damage survey points from peak ground acceleration (PGA) data recorded by the Sichuan Digital Strong Earthquake Network. This map indicates that the Lushan earthquake has a damage intensity of IX on the Liedu scale, and that the meizoseismal area displays an oblate ellipsoid shape, with its longitudinal axis in the NE direction. No obvious directivity was detected. Furthermore, the repositioning results of 3323 early aftershocks, seismic reflection profiles and focal mechanism solutions suggests that the major seismogenic structure of the earthquake was the Dayi Fault, which partly defines the eastern Mengshan Mountain. This earthquake resulted from the thrusting of the Dayi Fault, and caused shortening of the southern segment of the Longmenshan in the NW–SE direction. Coseismal rupture was also produced in the deep of the Xinkaidian Fault Based on the above seismogenic model and the presentation of coseismic surface deformation, it is speculated that there is a risk of more major earthquakes occurring in this region.
SUMMARY Stress drop is a proxy of understanding earthquake source process, and it is controversial whether the stress drops of induced earthquakes associated with hydraulic fracturing and injection activities are similar to those of tectonic earthquakes. The measurement of stress drops is usually biased due to the limitations of observation means, or hidden issues in the estimation approaches. Utilizing a local short-period seismic network, we investigate the stress drops of induced earthquakes in Weiyuan Shale Gas Field in Sichuan Province, China from 2019 to 2020. Totally 11 844 earthquakes are involved in the analysis, and their stress drops are obtained using an improved approach on the basis of the traditional spectral decomposition method combined with a global optimization algorithm to avoid stacking of spectra that is found leading to source parameter underestimation. We divide the studied area into three subareas, and the results show strong stress drop heterogeneity across the entire region. We obtain an average stress drop of 2.29 MPa, piecewise stress drop dependence to earthquake magnitude, and complex depth dependence pattern. Our results indicate that stress drops of induced earthquakes are overall consistent with the induced earthquakes in other areas as well as tectonic earthquakes in different environments. Meanwhile, the complexity in the stress drop dependence to depth possibly reflects the variability of stress drops for different earthquake triggering mechanisms.
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