Seafloor geodetic observations for detection of various slow-slips before and after the 2011 Tohoku-oki earthquake: Review and prospect

Wednesday, 24 February 2016: 3:50 PM
Ryota Hino, Tohoku University, Graduate School of Science, Sendai, Japan, Motoyuki Kido, Tohoku University, International Research Institute of Disaster Science, Sendai, Japan, Yoshihiro Ito, Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan and Masanao Shinohara, University of Tokyo, Bunkyo-ku, Japan
Abstract:
A number of important aspects of the 2011 Tohoku-oki earthquake (Mw 9.0) were clarified by the seafloor geodetic observation above the rupture area of the earthquake. Besides the extraordinarily large coseismic displacements, various kinds of slow slip phenomena on the plate boundary fault were identified by near field seafloor geodetic observations.

The Tohoku-oki earthquake was preceded by evident foreshock activity with a spatial expansion of this seismicity. The activity became significantly intense after the occurrence of the largest foreshock two days before the mainshock rupture. During the period, clear continuous seafloor deformation was identified caused by the aseismic slip following the largest foreshock. Another different type of aseismic slip event had occurred before this pre-imminent activity had started about a month before the largest foreshock happened. The observed increased seismicity associated with aseismic slip suggests that there must have been some chain reaction like interplay of seismic and interseismic slips before the large earthquake broke out. However, no evident deformation signals indicating acceleration of fault slip were observed immediately before the mainshock.

The postseismic deformation around the rupture area of the Tohoku-oki earthquake shows complex spatial pattern due to significant viscoelastic relaxation induced by the huge coseismic slip. The complexity makes it difficult to identify the deformation associated with the afterslip and requires us to enhance the abilities of seafloor monitoring to detect the slip activities on the fault. We started an array of arrays observation including broad-band seismographs to detect and locate slow-slip events and low-frequency tremors. Another observation we started is direct-path acoustic ranging across the trench axis. Slip rate of the shallow fault can be measured by monitoring the change in distance between the benchmarks on the incoming and overrding plates.

<< Previous Abstract | Next Abstract >>