摘要：Scientists demystify stress changes within tens of days before a mainshockand often utilize its foreshocks as an indicator. Typically, foreshocks aredetected near fault zones, which may be due to the distribution ofseismometers. This study investigates changes in seismicity far frommainshocks by examining tens of thousands of M≥2 quakes that weremonitored by dense seismic arrays for more than 10 years in Taiwan andJapan. The quakes occurred within epicentral distances ranging from 0 to400km during a period of 60d before and after the mainshocks that areutilized to exhibit common behaviors of seismicity in the spatiotemporaldomain. The superimposition results show that wide areas exhibit increasedseismicity associated with mainshocks occurring more than several times to areas of the fault rupture. The seismicity increase initially concentrates in the fault zones and gradually expands outward to over 50km away from the epicenters approximately 40d before the mainshocks. The seismicity increases more rapidly around the fault zones approximately 20d before the mainshocks. The stressed crust triggers ground vibrations at frequencies varying from ∼5×10-4 to ∼10-3Hz (i.e., variable frequency) along with earthquake-related stress that migrates from exterior areas to approach the fault zones. The variable frequency is determined by the observation of continuous seismic waveforms through the superimposition processes and is further supported by the resonant frequency model. These results suggest that the variable frequency of ground vibrations is a function of areas with increased seismicity leading to earthquakes.
