[S-SS09] Crustal Structure

[SSS09-03] Time-lapse Seismic Monitoring of Structural Deformations and Fluid Flow Caused by the 2011 Tohoku-Oki Earthquake (M 9.0)

*Ehsan Jamali Hondori^1,2, Jin-Oh Park¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Geoscience Enterprise Inc. )

Keywords:Time-lapse, Reverse Time Migration, Fluid Flow, Tohoku-Oki Earthquake

The 2011 Tohoku-Oki earthquake (M 9.0) occurred offshore northeast Japan, where the Pacific plate subducts beneath the Okhotsk plate at the Japan Trench margin. Although extensive research has been done using various geoscientific data after the earthquake, there are no detailed structural comparisons on seismic depth images before and after the earthquake. We applied Reverse Time Migration (RTM) on two seismic survey lines acquired on the same trajectory, but before and after the earthquake, to perform a 2D time-lapse analysis of the structural evolutions and possible fluid flow paths through the faults and fractures. The baseline dataset along survey line MY101 has been acquired in 1999 using a shot spacing of 50 m and streamer cable of 132 live channels at 25 m intervals with a maximum offset of 3400 m. The monitor dataset of line D13 has been acquired in May 2011, just two months after the earthquake, using a similar shot spacing but a more advanced seismic acquisition system including a streamer cable of 444 channels at 12.5 m intervals with a maximum offset of 5700 m. Both surveys have been conducted by Japan Agency for Marine-Earth Science and Technology (JAMSTEC). We selected a subset of the monitor dataset which covers the same offset range as the baseline survey and skipped every other channel to have the same CMP (Common Mid-Point) fold coverage. We used a similar pre-processing flow for both surveys and applied RTM with a maximum frequency of 20 Hz.

The results show several distinct differences between the two seismic datasets, which could be summarized in three major features. (1) The plate boundary fault (i.e., decollement) reflection near the trench is continuously visible in the baseline but seems to be weak and partially broken in the monitor survey. (2) A number of thrust faults with strong reflectivity appear in the monitor line, which are not clearly visible in the baseline survey. (3) The sedimentary units in the forearc of monitor line are partially deformed with different dips compared to the baseline survey. Previously we have calculated pore-fluid pressures along the decollement and backstop interface, and observed that the backstop interface and several thrust faults in the accretionary wedge contribute to the fluid drainage from the underthrust sediments to the seafloor. The sharp reflections observed in the monitor data may be caused by the impedance contrast of the fluid drainage paths along those faults. Several seismic reflectors with a reverse polarity compared to the seafloor reflection polarity are observed in both baseline and monitor datasets. However, an anticline-shaped reflector seems to have a dramatically different dip after the earthquake compared to the baseline survey. We propose that not only the structural deformations in the upper plate, but also the fluid flow through the faults changed the reflectivity pattern of these reflectors. We calculated RMS amplitudes of the RTM depth sections and noticed that the strong reflectivity along the backstop interface and several reverse faults supports the hypothesis of fluid migration via the faults and fracture zones.