Temporal Change in Near-source Attenuation probably due to the pore
pressure diffusion in the Source Region of the Intense Earthquake Swarm
in the Yamagata-Fukushima border, NE Japan
Abstract
Existence of fluids in the crust is key to understanding the occurrence
of earthquakes because it affects the fault strength. Given that fluids
are intensely distributed in fault zones, anelastic attenuation of
seismic waves may be locally high in these regions. The present study
examined near-source attenuation in the focal region of the intense
swarm activity in the Yamagata-Fukushima border region of Japan by a new
simple approach. This earthquake swarm exhibits a distinctive migration
behavior of hypocenters similar to fluid-injection induced seismicity
and was estimated to be caused by the pore pressure change. Near-source
attenuation was estimated by examining the decay of amplitude ratios of
nearby earthquake pairs with travel time differences. The obtained was
high during the initial ~50 days (with a median value of
0.040 for 2-4 Hz), and significantly decreased to become almost constant
for the later period (with a median value of 0.011). This pattern is
similar to those independently obtained for background seismicity rate,
b-value, stress drop, and fault strength. These patterns can be
explained in a consistent manner by the hypothesis that the swarm in
question was triggered by fluid movement following the 2011 Tohoku-Oki
earthquake, and the source and seismicity characteristics were also
affected by this temporal change in pore pressure. Attenuation was high
near the earthquake sources than that in the surrounding crust in the
initial period of the swarm, indicating the importance of considering
the near-source attenuation to correctly estimate the source-effect of
an earthquake.