Study on the regional characteristics of the subsidence seismic records in the northern Shaanxi

This paper selects 287 records of 18 natural earthquake events with magnitudes of M_L2.7—3.1 in Shaanxi Province from 2011 to 2018 and 185 records of 20 subsidence seismic events in northern Shaanxi with the same magnitude range from 2013 to 2018. Through the analysis and comparison in time and frequency and time-frequency domain, the characteristics of subsidence seismic records in the northern Shaanxi different from natural earthquake events are summarized as follows: ① In the time domain, the P-wave initial motion of subsidence is weak and gentle, while that of natural earthquake is strong and sharp. To the subsidence, only a few vertical direction of P-wave initial motions are clear, while that of natural earthquake is clear, and it has the characteristics of quadrant distribution. The body wave period of the subsidence is larger than the natural earthquake. The development of surface wave is one of the most significant characteristics that could distinguish subsidence from natural earthquake. Even if the epicenter distance is very small, subsidence seismic records still have significant short-period surface wave Rg. When the epicenter distance is less than 50 km, the amplitude ratio value A_S/A_P of subsidence is larger than that of natural earthquake. With the increase of epicenter distance, the amplitude ratio value A_S/A_P of subsidence becomes smaller and similar to that of earthquake under the same epicenter distance. With the same magnitude, the subsidence has a longer coda duration and slower energy decay than that of the earthquake. ② In the frequency domain, the frequency domain of subsidence is narrower, mainly low frequency signal, concentrated in 0−3 Hz. The frequency domain of the natural earthquake is wider, but the low frequency signal is less. The ratio of spectral variance to the mean in the frequency band was defined as the spectral difference coefficient α, and the α of the subsidence is generally higher than that of the natural earthquake. In the range of epicenter distance 200 km, corner frequency f_c of the subsidence is smaller than that of the natural earthquake. Corner frequency f_c of the subsidence is concentrated in 1−3 Hz, while f_c of the natural earthquake is distributed in 3−15 Hz. ③ From the comparison of the time-frequency spectrum, it can be seen that the main frequency component of the subsidence is low-frequency surface waves, and the natural earthquake frequency component is most abundant in the shear wave band.