On June 17, 2013, the Tianshui central seismic station submitted an earthquake prediction report based on observed anomalies in downhole georesistivity, groundwater radon concentrations from Wushan Well No.22 and Wushan Spring No.1, and borehole tilt measurements from the Wushan vertical pendulum. On July 22 of the same year, an earthquake with M_S6.6 struck the border region between Minxian and Zhangxian counties in Gansu Province. This event confirmed that the four sets of geophysical field observations from the Tianshui station had effectively captured pre-seismic changes within the regional subsurface medium. In this study, we analyze anomalies from these four datasets using the normalized rate method, subordinate function method, and tidal factor method, respectively. We systematically summarize the anomalous characteristics of each observation item and examine their correlation with the Minxian-Zhangxian earthquake. Finally, incorporating the focal mechanism solution, we investigate the spatio-temporal characteristics of the geophysical anomalies based on a fault virtual dislocation model.

The Tianshui georesistivity observation station is located in Yawan Village, Mapaoquan Town, Maiji District. The current downhole observation system, implemented as part of a post-disaster reconstruction project, commenced formal operation in January 2012, with electrodes installed at a depth of 100 m. From April 9 to May 9, 2013, the hourly georesistivity values along the NS, EW, and N45°W directions at the Tianshui station exhibited synchronous high-frequency disturbances, with maximum variation amplitudes of 1.83%, 1.00%, and 4.28%, respectively. The Lushan M_S7.0 earthquake in Sichuan Province on April 20, 2013 occurred during this anomalous interval. A second episode of synchronous high-frequency disturbances was recorded between June 12 and August 8, 2013, with peak amplitudes of 1.41%, 0.85%, and 0.94%, respectively, during which the Minxian-Zhangxian M_S6.6 earthquake on July 22, 2013, took place. Notably, the disturbance amplitudes were more pronounced prior to the Lushan event, which may be attributed to differences in deep geological structure and seismogenic environment between the two earthquake sequences. Furthermore, the daily variation pattern of georesistivity at Tianshui aligns broadly with the source region resistivity changes described by the DD （dilatancy-diffusion） mode, in which pore fluids play a critical role.

Wushan Spring No.1, Well No.22, and the vertical pendulum borehole tiltmeter are all situated at the Wushan seismic station in Wenquan Town, Wushan County. The spring is naturally exposed, while the well is fully confined; the two are spaced approximately 100 meters apart. Their background radon concentrations are 240 Bq/L and 470 Bq/L, respectively. Radon variations in both the well and the spring were largely synchronous. Starting in April 2012, concentrations generally followed a “low – high – low” pattern, with the elevated phase lasting about one year. A short-term pre-seismic anomaly began on June 25, 2013, exhibiting a “rise – earthquake – decline” sequence. The maximum pre-seismic variations reached 6.36% in Well No.22 and 9.59% in Spring No.1. Following the earthquake, radon levels decreased but did not fully return to pre-anomaly background values. In contrast, the NS and EW components of the borehole tiltmeter recorded opposing trends: the NS component decreased while the EW component increased. The tilt rate accelerated from 3.11×10⁻³/d during July 2012–February 2013 to 5.66×10⁻³/d from February to June 2013. Post-seismic deformation was marked by a clear co-seismic strain step.

Prior to the Minxian-Zhangxian M_S6.6 earthquake, the normalized rate values for all three georesistivity components exhibited synchronous increasing anomalies, with a predefined threshold of 0.8. The monthly average radon concentration at Wushan Well No.22 increased from March to August 2012, reaching a maximum variation of 13.6%, while its 13-point moving average rose from February to September 2012, with a peak variation of 8.7%. The subordinate function μ for this well exceeded the threshold three times before the earthquake, starting 14 months prior to the event. Similarly, the monthly average radon concentration at Wushan Spring No.1 began to increase from March 2012, with a maximum variation of 15.7%, and its 13-point moving average rose from February to November 2012. The μ value for the spring also surpassed the threshold three times, commencing 14 months before the earthquake. Following both the Lushan M_S7.0 and Minxian-Zhangxian M_S6.6 earthquakes, the μ values began to recover in August 2013. Significant changes in the tidal factor were also observed before the Minxian-Zhangxian earthquake. The tidal factor γ values for the NS and EW components showed a marked step-like decrease from April 2012 to January 2013, indicating a medium- to long-term trend change. The earthquake occurred approximately six months after the γ values began to rebound.

The following conclusions can be drawn: ① All four observation items are situated within a region characterized by compressive stress enhancement; ② The geophysical field anomalies recorded at the Tianshui station were mainly distributed within 200 km of the epicenter. Their evolution and amplitude exhibited clear “long-term–intermediate-term–short-term–imminent-term” staging characteristics relative to the epicentral distance, demonstrating notable spatio-temporal correspondence with the Minxian-Zhangxian earthquake; ③ The seismogenic process is reflected in the geophysical field through long-term background trends, intermediate-term anomalies, and short-term/imminent changes. After the occurrence of regional earthquakes, it is necessary to systematically analyze observation items with anomalous data, accurately identify precursory signals based on their spatio-temporal distribution, intensity, and waveform morphology, and build a representative database of typical regional earthquake cases.