关键词: 定日M_S6.8地震, 热红外异常, 西藏南部, 特征研究

Abstract:

On January 7, 2025, a magnitude 6.8 earthquake struck Dingri County in Xizang. To investigate pre-seismic signals, we applied the relative power spectrum variation method and analyzed brightness temperature data from the FY geostationary meteorological satellite to detect thermal infrared(TIR)anomalies preceding the event. Building on this approach, the present study offers a more in-depth analysis of TIR anomalies prior to both the Dingri earthquake and a subsequent M_S5.5 earthquake that occurred in Maduo on January 8, 2025. The Dingri M_S6.8 earthquake occurred in the southern Tibetan Plateau, a region situated at the forefront of the ongoing collision and compression between the Indian and Eurasian plates. This area features major tectonic structures, including the Himalayan Frontal Thrust, the large-scale Karakoram-Jiali strike-slip fault, and seven nearly north-south-trending rift valleys that developed between them. The Dingri earthquake, a normal-faulting event, occurred within one of these rift valleys. To further understand TIR anomalies in this tectonically active region, we selected three additional earthquakes with similar geological settings and magnitudes for comparative analysis.

The TIR anomalies associated with the Dingri earthquake were primarily distributed within the region bounded by multiple faults, covering a maximum area of approximately 210 000km². The anomaly persisted for 80 days without complete dissipation and gradually evolved into a localized, high-intensity anomaly migrating in a northeastward direction. The Maduo M_S5.5 earthquake occurred at the edge of this localized anomaly one day after the Dingri event. These two stages—widespread anomaly and localized concentration—are interpreted as part of a continuous anomaly evolution process, with the anomaly migration direction pointing toward the epicenter of the Maduo earthquake. Analysis of the time-series relative power spectrum prior to the Dingri earthquake revealed three significant episodes where the anomaly amplitude exceeded six times the background level. The first two episodes lasted 15 days and 22 days, respectively, while the third, which immediately preceded the Dingri event, persisted for 54 days, indicating a marked difference in duration and intensity. The relative power spectrum peaks were 12.4 for the Dingri event and 12.9 for the Maduo event, occurring 123 and 111 days, respectively, prior to the earthquakes.

The spatial distribution of TIR anomalies associated with multiple earthquakes in southern Xizang appears closely linked to the extensional rift systems and active tectonic structures of the region. The directional evolution of these anomalies correlates with the eventual earthquake epicenters, which were generally located at the leading edge of the migrating anomaly zones. This finding is consistent with previous studies that have observed similar migration characteristics of TIR anomalies preceding earthquakes. Among the four examined earthquakes in southern Xizang, relative power spectrum peaks ranged from 12 to 18 times of the background level, appearing 28 to 123 days prior to the events. The maximum extent of anomalous areas varied between 170 000 and 210 000km², with the duration of days exceeding the sixfold threshold ranging from 34 to 54 days. Despite some variation in these parameters, all events displayed common features of high-amplitude, large-area, and persistent anomalies, predominantly occurring during the short-term and imminent pre-seismic periods. Notably, in all four cases, no significant anomalies were observed directly at the epicenters; instead, the epicentral locations were consistently positioned at the margins of pronounced anomalous zones.

The tectonic regime of the southern and central Tibetan Plateau is characterized by east-west extensional stress, resulting from the regional compressive stress field. This has led to the development of numerous north-south-oriented normal faults, which act as conduits for the upwelling of geothermal fluids. Additionally, the region experiences intense hydrothermal activity and significant CO₂ degassing. Drawing on previous research, we propose that the combination of extensional rifting and active hydrothermal systems facilitates the ascent of geothermal fluids and greenhouse gases(including CO₂)to the surface. This process likely contributes to the enhanced surface thermal radiation observed in satellite data and may explain the large-scale, fault-aligned TIR anomalies detected prior to these earthquakes in southern Xizang.

Key words: Dingri M_S6.8 earthquake, thermal infrared anomalies, southern Xizang, feature research