7. Spatial/Time-lapse Data Management -Invited-
Fluid flow within the plate boundary shear zones at Japan and Kuril Trenches
Tetsuro Tsuru
Cosmo Energy Exploration & Development Ltd., Japan.
Abstract
When micro-scale physical-and-chemical reactions within rocks affect macro-scale physical properties, we may be able to observe what dynamic reactions occur in the deep earth interior by geophysical exploration techniques. Here we discuss on fluid flow within the plate boundary shear zone, an evidence of which was observed by a time-lapse seismic survey that conducted before and after the 2003 Tokachioki interplate earthquake of MW8.0. The datasets of the survey demonstrated a spatio-temporal evolution in reflectivity near the source region of the earthquake. From the evolution pattern and the aftershock observations, we attribute the change in reflectivity to gradual fluid flow, and estimate the subsequent permeability in the plate boundary shear zone to be 10-15 m2. The magnitude of the reflectivity change implies a 1.6 % increase in porosity and a 2.2-5.6 MPa reduction in shear strength. The shear strength reduction estimated from the time-lapse survey is one order greater than the temporal change in shear stress on the plate boundary due to afterslip estimated from GPS data, being not as small as it can be disregarded. Thus the low-frictional expanded porous patches are observed by the tome-lapse survey. Interestingly they are located on the subasperity nearest the initial break of the 2003 Tokachioki earthquake, being suggestive that the seismic rupture has propagated toward the low frictional patches (LFP). As a hypothesis to test, we propose that the LFP guided the seismic rupture propagation in the Tokachi-oki earthquake. Considering the earthquake recurrence and the gradual growth of LFP, we may be observing invasion of barriers within the plate boundary shear zone.
Last modified: Sat May 27 09:17:14 2006