Abstract
YRY-4 borehole strainmeters have been installed in Sichuan Province, China, since 2008, aimed at monitoring the crustal activities associated with earthquakes. In this study, data from six YRY-4 strainmeters at the southwestern endpoint of the Longmenshan fault zone were analysed, to study the relationship of tectonic strain changes with the 2013 Lushan earthquake. We developed a state-space model to remove the strain response due to air pressure, solid tides and the changes in the water level to preferentially isolate non-tectonic disturbances. Strain responses to each influencing factor were estimated using the environmental coefficients computed in the state-space model by an adaptive Kalman filter with measurement noise. The results were consistent with the expected response of the strainmeter systems. The corrected strain considered to originate from underground tectonics provides new insights into the changes in the pre-earthquake strain. Approximate negentropy (ApNe) and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${b}$ </tex-math></inline-formula> value were introduced to quantify the probability distribution of the corrected borehole strain and compared with the local seismic activities. The nearest station and two further stations, almost simultaneously recorded short-term ApNe anomalies six to four months before the earthquake. The anomalous region also had a correspondingly low <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${b}$ </tex-math></inline-formula> value. Moreover, the anomaly acceleration rates of each station were dependent on the epicentral distances. Further comparison with the strain of random periods illustrated the significance of the extracted anomalies. Our results indicate that the corrected strain may contain seismogenic information and reflect the accelerated strain accumulation of focal areas before the earthquake.
Highlights
The preparatory process and occurrence of a major shallow earthquake are typically accompanied by crustal deformation [1], [2]
To verify the existence of the strain precursors preceding the 2013 Lushan earthquake, we applied an anomaly detection method based on a space-state model and Approximate negentropy (ApNe) analysis to the borehole strain data from 2011−2014 along the southwestern end of the Longmenshan fault zone
As for the environmental disturbances, we constructed a statespace model that uses air pressure, earth tides, and water level data based on the observations recorded using YRY-4 borehole strainmeters
Summary
The preparatory process and occurrence of a major shallow earthquake are typically accompanied by crustal deformation [1], [2] (e.g., the 2011 Tohoku earthquake [3] and the 2004 Kii Peninsula earthquake in Japan [4]). For sequences of observations that can be used to monitor the changes in underground deformation, Chi et al [25] detected anomalies in the YRY-4 borehole strain by investigating the principal stress direction deduced from the abnormal strain excluding the interference from teleseismic waves and human factors According to their results, anomalies occurred for six to four months and four days before the Lushan earthquake from the nearest GZ station to the epicentre. To eliminate the possibility that the changes in the strain occur due to the environmental disturbances, we applied a statespace model to analyse the observed strain data, including the areal strain Sna and shear strain Sns, by assuming that the induced strain from solid tides, En, air pressure, Pn, and water level, Wn are additive. These parameters are related to the response of the strainmeter system and help the assessment of the installation of the sensors or the quality of the mathematical inversion
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