Wide sensitive area of small foreshocks

Scientists demystify stress changes within tens of days before a mainshock and often utilize its foreshock as an indicator.  Typically, foreshocks are detected near fault zones, which may be due to the distribution of seismometers.  This study investigates changes in seismicity far from mainshocks by examining tens of thousands of M >= 2 quakes that were monitored by dense seismic arrays for more than 10 years in Taiwan and Japan.  The quakes occurred within epicentral distances ranging from 0 km to 400 km during a period of 60 days before and after the mainshocks that are utilized to exhibit common behaviors of seismicity in the spatiotemporal domain.  The superimposition results show that wide areas exhibit increased seismicity associated with mainshocks being more than 50 times to areas of the fault rupture.  The seismicity increase initially concentrates in the fault zones, and gradually expands outward to over 50 km away from the epicenters approximately 40 days before the mainshocks.  The seismicity increases more rapidly around the fault zones approximately 20 days before the mainshocks.  The stressed crust triggers resonance at frequencies varying from ~3×10-4 Hz to ~10-3 Hz (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.