Abstract
We conduct seismic tomography using arrival time data picked by NIED Hi-net, including earthquakes off the coast, outside the seismic network. For these offshore events, we use the NIED F-net focal depth. We detect two low-V. zones in the uppermost subducting oceanic crust. The landward low-V zone with a large anomaly corresponds to the western edge of the coseismic slip zone of the 2011 off the Pacific coast of Tohoku Earthquake. The asperities of the previously known Off-Miyagi and Off-Fukushima earthquakes with magnitudes around 7.0 are also located at the boundary of the low-V and the eastern high-V zones. The initial break point (hypocenter) is associated with the edge of a slightly low-V and low-V p /V s zone. The trenchward low-V and low-V p /V s zone extending southwestward from the hypocenter may indicate the existence of a subducted seamount. The high-V zone and low-V p /V s zone might have accumulated the strain and resulted in the huge coseismic slip zone of the 2011 Tohoku Earthquake. The low-V and low-V p /V s zone is a slight fluctuation within the high-V zone and might have acted as the initial break point of the 2011 Tohoku Earthquake.
Highlights
The 2011 off the Pacific coast of Tohoku Earthquake occurred on March 11, 2011
The trenchward low-V and low-Vp/Vs zone is a slight fluctuation within the high-V zone and might have acted as the initial break point of the 2011 Tohoku Earthquake
The landward low-V zone with a large anomaly is consistent with the western edge of the coseismic slip zone of the 2011 off the Pacific coast of Tohoku Earthquake
Summary
The 2011 off the Pacific coast of Tohoku Earthquake occurred on March 11, 2011. There are few studies (e.g. Zhao et al, 2007) of the velocity structure near the Japan trench using data from inland seismic stations. We use the events off the coast whose focal depths are determined by NIED F-net. Sometimes the NIED Hi-net system determines the depths of offshore earthquakes deeper than the true depths because of the lack of stations above the hypocenters and traveltime data of P- and S-waves observed at stations distributed only on the west side of the hypocenters. Those events are located deeper as they get close to the trench in spite of the plate boundary becoming shallower.
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