IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1W_2F-P18

Date/Time: July 21 Poster

Spatiotemporal strain distribution around Kussharo caldera, eastern Hokkaido, Japan,
measured by GPS
Mako Ohzono, Hiroaki Takahashi
ISV, Hokkaido University, Japan
E-mail: m.ohzono@mail.sci.hokudai.ac.jp

The Kussharo caldera in eastern Hokkaido, Japan, is one of the active caldera in Japan. This caldera is atypical island-arc volcanic complex associated with the subducting Pacific plate. Post-caldera volcanism stated justafter the caldera forming (35ka) and has still continued. Sub-caldera formations in main caldera by large volumeeruptions with more than 10 km3 deposits were recorded at 20ka, 12ka and 7ka. Although episodic expanding(1993-1995) and contraction (1995-1998) has been detected from InSAR at the center of caldera, no significant
magmatic eruption has been recorded during recent a thousand years.
This area has high shallow seismicity relative to other area in Hokkaido. Four moderate (∼M6) earthquakes arerecorded in the past one hundred years. In addition, subsurface structure surveys showed low seismic velocity
zone in upper mantle and low resistivity body in middle to upper crust beneath caldera. Ground temperature profile
along deep boreholes also indicated high geothermal gradient around this area. These facts implied that weak
structure in crust and upper mantle induces high stress build-up in the upper crust and generates active seismicity.
In order to investigate the strain accumulation/release process in and around Kussharo caldera, we analyzed GPS
data obtained from continuous and dense campaign-mode surveys. From continuous data, we picked up three
periods and estimated velocity and strain rate fields in each terms (term1: 1998-2011, interseismic period without
any large earthquake, term2: 2007-2009 postseismic period of the 2003 Tokachi-oki earthquake (M8.0), term3:
postseismic period of the 2011 off the Pacific coast of Tohoku Earthquake (M9.0)). In term1 and term2, regional
compressional strain field around the caldera, which is driven by interplate coupling between the subducting Pacificplate and the overriding plates, was dominant. Term3 also indicates compressional strain field with a little higher
rate. Some effect of postseismic deformation of the 2011 earthquake is attributed. The strain rate map clearly
indicated a stable, high dilatational strain rate of 0.2 microstrain per year over a decade only in Kussharo caldera.
Data from the dense campaign GPS observation provides one coordinate set every year from 1997 to 2001,
and 2012. We estimated 11 years displacement from the coordinate difference between 2001 and 2012 tosee recent crustal deformation. Although deformation rate is not stable for 11 years, because there are someinterplate earthquakes near Kurile trench and Japan trench, total deformation and strain field indicates local strain
concentration toward the center of caldera appears.
These results show that the strain concentration process in Kussharo caldera is in progress for at least a decade,even the large earthquake occurred around Hokkaido. Therefore, it is important to continue monitoring the tectonic
behavior using multi geophysical data.

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