Propagation of crust deformation anomalies related to the Menyuan M_(S) 6.9 earthquake

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.

Stress Field Changes before the <i>M</i>_S8.0 Wenchuan Earthquake

The apparent lack of pre-seismic crustal deformation preceding the 2008 Ms8.0 Wenchuan earthquake has been the subject of debate. In this study, tiltmeter data recorded close to the earthquake epicenter were analysed using spectrum and wavelet analysis. Changes in the stress field before the earthquake were analyzed based on the Benioff creep release and changes in regional Global Navigation Satellite System (GNSS) baselines. The characteris-tics of far- and near-field seismicity and deformation processes were investigated using rock fracture experiments. The results show that during pre-seismic strain energy accumu-lation, there was a synergy between stress field changes both proximal and distal to the ep-icenter;moreover, we identified a strong correlation between Benioff creep release and shortening of the LUZH-GUAN GNSS baseline. During the sub-instability stage, the de-formation characteristics of different structural sectors differed;faults near the epicenter were in a highly locked state, and the deformation rate and wave spectra of main period waves obviously decreased. This reflects fixed point deformation driven by deep stress. These results are of great significance for understanding geophysical field observations, for clarifying pre-seismic deformation and for earthquake prediction.

Correlation between the tilt anomaly on the vertical pendulum at the Songpan station and the 2021 MS7.4 Maduo earthquake in Qinghai province,China

Understanding the relationship between precursory deformation anomalies and strong earthquakes is vital for physical earthquake prediction. Six months before the 2021 MS7.4 Maduo earthquake in Qinghai province, China, the vertical pendulum at the Songpan station was observed to tilt southward with a high rate and large amplitude. Studies conducted before the 2021 MS7.4 Maduo earthquake inferred the tilt anomaly to be an earthquake precursor. However, after the earthquake, the relation between the earthquake and the anomaly became controversial, partly because the Songpan station is located at a great distance from the epicenter. In this study, based on the deformation anomaly characteristics, relationship between the seismogenic fault and the fault near the anomaly, and associated quantitative analyses, we concluded that this anomaly may be associated with the 2021 MS7.4 Maduo earthquake. The duration and amplitude of this anomaly matched with the magnitude and epicenter distance of the Maduo earthquake. We have also interpreted the reason why the anomaly occurred near a fault that is obliquely intersected with the seismogenic fault and why the anomaly is located far from the earthquake epicenter.