关键词: 定日地震, 矩张量解, 发震节面, 地震精定位, 发震构造

Abstract:

This study investigates the M_S6.8 earthquake that occurred in Dingri, Xizang(Tibet), on January 7, 2025, using the azimuth spectrum fast detection method. The moment tensor solution, relative position between the seismogenic point and moment centroid, and other key parameters were determined. The seismogenic nodal plane was further constrained using this approach. The method involves dividing the multi-dimensional parameter space into subspaces and applying a combination of grid search and gradient descent to identify the optimal solution with the least parameter misfit in each subspace. Waveform data were collected from 28 fixed and mobile seismic stations, with relevant station and instrument response data provided by the Institute of Earthquake Forecasting of the China Earthquake Administration. Twelve stations with epicentral distances ranging from 100km to 450km and a filtering range of 0.02-0.05Hz were selected for analysis. A time-domain full-band inversion was employed to incorporate more waveform information and enhance constraints on source parameters such as spatial location and rupture direction. Observed and theoretical three-component waveforms were compared, and records with low signal-to-noise ratios or poor fit were excluded. Final calculations were performed using data from seven stations. The resulting moment tensor solution indicates two nodal planes with strike/dip/rake values of(188°, 46°, -90°)and(9°, 44°, -90°), a focal depth of 8.9km, and a moment magnitude(M_W)of 7.0. To test solution stability, multiple initial input combinations(strike, dip, and rake)were examined, yielding consistent inversion results. Comparison with results from the USGS, GFZ, and ZHANG Zhe(China Earthquake Administration)shows discrepancies within 24°, 6°, and 13°, respectively, for strike, dip, and rake. Further analysis of the source geometry yielded a boundary radius of 10km, relative rupture velocity of 0.7, and relative distances of 4.5km along strike and 0km along dip between the seismogenic point and centroid. The rupture propagated both upward and downward from the hypocenter. The seismogenic fault plane was identified with strike/dip/rake parameters of 188°, 46°, -90°.

To further investigate the earthquake sequence, we analyzed phase reports of 3, 545 events from January 7 to 14, 2025, provided by the China Earthquake Networks Center. The HypoDD method was used for relocation, with events recorded within 300km of the cluster center. Parameters included a maximum inter-event distance of 10km and a minimum of 8 links per pair. A total of 197, 898 P-wave and 252, 651 S-wave differential times were successfully used, representing 80% and 81% of the total available data, respectively. Relocation was performed using the conjugate gradient method and a one-dimensional velocity model by Monsalve et al. for southern Tibet. Quality control parameters ranged from 40 to 80, resulting in successful relocation of 3, 155 events. The relocated hypocenters reveal that the sequence can be divided into three segments—southern, central, and northern. The southern swarm extends NW-SE from Guojia Town to Cuoguo, intersected by the Cuoguo and Dengmecuo faults. The central segment trends NNE-SSE from the mainshock along the Cuoguo fault toward Qiugu Village, with relatively sparse seismicity and a seismic gap near Changsuo Town. The northern segment continues NW-SE toward Xingdang and is intersected by the Nongqu fault. While the central swarm aligns with the Cuoguo fault, the southern and northern segments deviate from mapped fault trends, suggesting the presence of NW-SE-trending subsidiary faults.

Depth profile analysis indicates that all three swarms occurred on west-dipping fault planes. The southern and central segments show clear layering with focal depths of 6-14km and 20-30km, while the northern segment shows less stratification. The spatial pattern suggests a complex, segmented fault system with a possible Z-shaped branch fault in the Shenzha-Dingjie normal fault zone. The Dingri earthquake sequence is therefore attributed to rupture within a complex fault network.

Key words: Dingri earthquake, moment tensor, seismogenic nodal, precise earthquake relocation, seismogenic structure