Co-Seismic Response of Bedrock Temperature to the Ms6.3 Kangding Earthquake on 22 November 2014 in Sichuan, China

Abstract

In this paper, we report the co-seismic temperature response of the Ms6.3 Kangding earthquake in Sichuan, China, which occurred within a bedrock temperature monitoring station network constructed in the east of the Qinghai-Tibet Plateau. Results indicate that two kinds of co-seismic responses of bedrock temperature, exponential and step-rise ones, were recorded at different stations. As for the former, the amplitude of co-seismic response is about 5.6–11.5 mK (the average value is 7.33 mK), while the latter is about 0.31–0.98 mK (the average value is 0.52 mK). These two kinds of responses would be indicative of the two different mechanisms for change in temperature. On the one hand, the exponential responses represent the effects of fluid advection along the borehole wall, which are associated with a finite quantity of heat produced by earthquake-driven transient fluid flow. The step-rise responses, on the other hand, represent the direct response of temperature to change in the crustal stress accompanying with earthquake. We still tested relationship between stress variation and temperature response on the rock samples taken from the boreholes, and the average value is 0.77 mK/MPa. Upon thus, these step-rise temperature responses imply that co-seismic change in mean stress is about 0.40 MPa. We simplify effects of fluid by presuming a one-dimensional model where the flow is either up or down based on a pressure change due to compression or tension. The stress state, compression or tension, deduced by the direction of the fluid flow is qualitatively accordant with the direct response of bedrock temperature to stress change. In summary, characteristic changes in bedrock temperature occurred coincidentally with the earthquake at different stations, agreeing generally with regional stress adjustments accompanying the earthquake. Changes in bedrock temperature do seem to offer a useful means of monitoring dynamic changes in shallow crustal stress.

Appendix A: Uniaxial Tests of Temperature Response to Stress in Change on the Rock Samples Taken From the Boreholes

Uniaxial tests on the rock samples are carried out in a laboratory. The rock sample, taken from the borehole, is a cylinder with a diameter of 70 mm and a height of 40 mm. The loading is performed by the mode of the triangle wave, and the loading rate is 1.5 kN/s. The room temperature is synchronously recorded with the temperature of rock. The sensors are the platinum resistance–temperature transmitter (Pt1000). Figure 10 shows an experimental case demonstrating the variation of temperature during the loading of rock sample from station 105. To verify the repeatability of the experiment, the loading triangle waves have two amplitudes, 40 and 50 MPa, separately, and the same loading amplitude is performed over three cycles. As shown in Fig. 10, the temperature variations are in line with loading. Temperature increases as the load increases while the temperature decreases as the load decreases. According to tests on the rock samples from stations 103, 105, and 106, the magnitudes of the relationship between temperature and stress change at room temperature are 0.82, 0.74 and 0.73 mK/MPa, respectively. The average value is 0.77 mK/MPa. Note that the rock in station 108 is mudstone that is fractured (Fig. 1c). The intact rock in station 108 was not obtained and as such, it is not to conduct this kind of mechanical experiment.

Fig. 10

Variation of temperature during loading of Marble (Station 105)

The rock sample is a cylinder with a diameter of 70 mm and a height of 40 mm; Room temperature at the beginning of test (0 s) is 23.9 °C; Loading rate is 1.5 kN/s; the relationship between temperature and stress change is 0.74 mK/MPa.