两次近距离大震前成都台视电阻率重现性、相似性和各向异性变化

本文评价了四川汶川MS8.0、芦山MS7.0地震前后成都台地电观测环境,研究了该台视电阻率变化.结果为:(1)两次大震发生在该台以西的龙门山断裂带、震源机制和震源深度接近,是近距离大震,相应地,在两次地震前该台两个正交测道中的每一测道视电阻率变化均显示了中期异常及其变化过程的重现性和异常幅度的相似性;(2)在每次地震前,两个测道表现了异常变化形态、幅度和起始时间的差异性.其重现性、相似性证明这些异常与两次大震晚期孕育有关;差异性主要展示了与震源机制有直接联系的视电阻率各向异性变化,揭示了震前该台地下介质经历了强烈的电性各向异性变化的过程.

Repeatability,similarity and anisotropy changes in apparent resistivity recorded by station Chengdu at near distances before two great earthquakes

The precursor observations, such as electromagnetism, underground fluid, geodesy, etc., for earthquake monitoring and prediction have been internationally implemented for decades. However, nowadays it is highly debatable whether the earthquake-related precursory anomalies can be observed or not, owing to the lack of both the reproducibility and similarity of these anomalies. The anomalies of direct current apparent resistivity (AR, for short; denoted as ρ_s) recorded by station Chengdu before the M_S8.0 Wenchuan quake on May 12, 2008, and the M_S7.0 Lushan quake on Apr. 20, 2013 provide examples to address this issue. The reasons for this are as follows. First, this station is only 35 km from the epicenter in Wenchuan and 99 km from that in Lushan. Second, both the two events happened along the NE-strike Longmen Shan fault zone west of the station. Third, the two focal mechanisms and depths are almost the same. Whether did the reproducibility and similarity of the AR anomalies appeared before the two great shocks? The primary curve of AR observation data and the annual-variation-free curve from which the annual variation of AR is eliminated by using the moving Fourier method (MFM, for short) are traditionally adopted to analyze the anomalies in China. As for the traditional approaches, it is reliable for identifying the relative AR change with the amplitude of no less than 1% because the measurement accuracy is much better than 0.3% in the observation of AR. But, it is sometimes difficult to distinguish the normal and anomalous changes using the traditional approaches, which will result in the uncertainty of anomalies. To solve this problem, the normalized variation rate method (NVRM) is put forward and its principle is briefly described as follows. The long-term decreasing or decreasing variation in the AR time series {ρ_s} (with the length N), if any, is eliminated by the linear regression and the annual variation in it is also removed by using the MFM. The ξ continuous data in {ρ_s} are selected to form the i^th time sub-series and the variation rate is calculated from ρ_si^·'=K_i×R_i (where K_i is its slope coefficient and R_i is linear correlation coefficient). Like this, the initial variation rate time series ρ_s^·' is generated with the step length ξ fixed. Finally, the NVRM time series ρ_s^· (dimensionless) with its mean value m→0 and RMSE σ_n-1→1 is obtained from ρ_si^·' after the processing such as the normalization, centralization, and noise reduction. The index of anomalies is consistently defined as ±2.4 on ρ_s^· for each station or channel. In this paper the traditional approaches and the NVRM are applied to process the AR data observed by station Chengdu. At station Chengdu, two measurement channels are respectively in N58°E and N49°W orientations at the same location and each channel employs the fixed Schlumberger array. Before the M_S8.0 Wenchuan and M_S7.0 Lushan events, the two orthogonal channels recorded pre-seismic anomalies with the repeatability, similarity and difference. (1) The two anomalies recorded by each channel showed the repeatability and similarity. The AR anomalies of channel NE, with the relative amplitude of ~-7.0% on the daily mean curve, before the Wenchuan event persistently decreased for 19 months, and then started a recovery (increasing) change about 3.5 months before the shock. Another decrease anomaly, -5.9%, before the M_S7.0 Lushan event continued for 8 months and started a recovery change immediately following the maximum drop. The earlier decreasing and then increasing changes had distinctly exhibited the reproducibility in their changing forms and processes, which tallied with the change process of the electrical resistivity within the focal area predicted by the dilatancy-diffusion model. Furthermore, they also exhibited the similarity in the large-amplitude decrease in the medium-term before the two events. The two anomalies recorded by channel NW in the medium-term clearly showed their reproducibility of the two positive anomalies on the NVRM curves. Their change forms were identical, their starting times were nearly the same, and only the amplitude was larger before the near-distance and greater event in Wenchuan than that before the slightly farther and smaller shock in Lushan. (2) Before each quake the two anomalies recorded by the two channels showed differences in their change forms, amplitudes and starting times. The channel NE recorded the two prominent decreasing anomalies in the medium-term while channel NW recorded two NVRM positive anomalies that indicated the increasing changes of AR, which manifested as the anisotropic changes in the reverse directions. The two anomalies on the monthly and daily mean curves of channel NE were larger in amplitude while those of channel NW appeared only on the NVRM curves that indicated the small-amplitude changes of AR, which showed the apparent anisotropic changes in amplitude. The two medium-term anomalies of channel NE started earlier while those on channel NW started later. Furthermore, the channel NE is obviously superior to channel NW in the responsiveness to the two quakes. (3) The possible reasons for the anomalies of AR is theoretically explained based on the anisotropic medium as follows. During the later preparation of the two great earthquakes, there occurred such the processes as the nonlinear growth and development of the micro cracks within the medium below the station, with the micro-crack strikes in the dominant arrangement roughly along the NW-SE direction and the penetration of low-resistance water within the medium. As a result, the processes brought about the anisotropic changes in electrical resistivity within the medium, with the change in the direction being larger in amplitude than that perpendicular to direction. It resulted in the apparent anisotropic changes, with the AR change of channel NE being larger than that of channel NW. Furthermore, the anisotropic changes in the electrical resistivity were rather strong due to the near distance and large magnitude, so the reverse changes of AR appeared; and before the Wenchuan quake the underground medium below this station had undergone the process mostly characterized by the NW-SE direction compression since August 2006 at latest, and before the Lushan quake the compression had been formed since August 2012. The M_S8.0 Wenchuan and M_S7.0 Lushan earthquakes all occurred on the Longmen Shan fault, very near to station Chengdu. Their focal characters and depths are almost the same, and the two measuring channels in the station record the AR anomalies with the following features: (1) Before the two events, the two anomalies recorded by each channel clearly exhibited the repeatability in the medium-term feature and changing form, and the similarity in the amplitude. Especially, the two anomalies recorded by channel NE show the repeatability in the "earlier decreasing and then recovery (increasing) change" process. (2) Before each quake, the two anomalies recorded by the two channels show the differences in the change form, amplitude and starting time. The repeatability and similarity prove the anomalies to be directly associated with the two great earthquakes, and the differences show that the anisotropic AR changes are directly related to the two source characters.