Signature and physical mechanism of borehole dilatometers in response to super typhoon Lekima in southeastern coastal area of China

The aim of this study is to investigate how volumetric strain and super typhoon Lekima interact with each other. By combining the intensity and spatio-temporal evolution of Lekima, we systematically analyze the extracted volumetric deformation of shallow crust at five stations in the southeastern coastal area of China induced by Lekima using a decomposition analysis called the empirical mode decomposition （EMD）. Furthermore, we discuss the physical mechanism of typhoon-induced volumetric strain. The results show that: ① The super typhoon’s signature consists in a ground dilatation due to barometric pressure drop drastically, generally followed by a ground compression due to the barometric pressure recovery. The dynamic patterns for the volumetric strain and the barometric pressure are both similar to the symme-trical funnel, and the response of volumetric strain to the barometric pressure is almost instant and linear. In addition, the durations are nearly equivalent for both of them. ② The maximum magnitude in volumetric strain induced by barometric pressure with fluctuation of −18.2 hPa can reach up to −112.1×10⁻⁹ in the 62 m deep borehole, and the corresponding coefficient of barometric pressure response is about 6.2×10⁻⁹/hPa at the 103-hour period. ③ Obviously, the dilatometer records can be remotely disturbed by super typhoon at a distance of approximately 980 km away from the borehole site. When the typhoon center approaches or moves away from the borehole site, the dilatational magnitude of borehole strain will increase or decrease correspondingly. The obtained results can be used not only for identifying and determining reasonably the physical mechanism of anomalous changes induced by typhoon in low-frequency range for southeastern coastal and inland areas in China, but also for contributing the reliably observational evidences to the theoretical model research for the volumetric strain in response to barometric pressure in the low-frequency range.