Dynamical triggering of tremors near Nanao, Taiwan, by the 2011 Tohoku
earthquake: Slab-related fluid-induced seismicity
Abstract
Dynamic triggering has been documented in many places in both seismic
active and inactive regions, and most triggered events are tremors.
These tremors provide a scaling relationship that bridges natural
earthquakes and laboratory experiments. In particular, dynamic
triggering may help understand the rupture mechanism of natural
earthquakes. The Nanao array, a small aperture array composed of 4 dual
broadband and strong motion seismic stations in Taiwan, recorded the
2011 Tohoku M9 earthquake and locally triggered tremors, in addition to
the ambient tremors. Using Spudich’s method to derive shallow crustal
shear strain, dilation, and rotation during tremor episodes, we found
that tremors occurred when dilation was larger than
10–8, similar to Nankai Trough cases. Previous
tomographic studies have shown partial melting coming from the
dehydration of the subducting Ryukyu slab and the slab edge corner. Such
a partial melt zone extends to shallow depth near the Nanao array and
could potentially elevate the pore fluid temperature. A systematic check
of all the seismic stations in Northern Taiwan shows clear increased
triggered tremors only near Nanao right after the Tohoku earthquake. We
applied array processing methods to the Nanao array data and derived a
NE to SW back azimuth directions of the tremors, filling a seismic gap
in northeastern Taiwan seismicity six months after Tohoku earthquake.
Analogous to fluid-related acoustic emission lab experiments, we propose
that this is among the first field examples of dynamically triggered
tremors associated with moving fluids from a slab and its edge.