Large intraslab earthquake (2011 April 7, M 7.1) after the 2011 off the Pacific coast of Tohoku Earthquake (M 9.0): Coseismic fault model based on the dense GPS network data

Abstract

We propose a source fault model for the 2011, April 7, earthquake (M 7.1) deduced from a dense GPS network. The coseismic displacements obtained by GPS data analysis clearly show the spatial pattern specific to intraslab earthquakes, not only in the horizontal components but also the vertical ones. A rectangular fault with uniform slip was estimated by a non-linear inversion approach. The results indicate that the simple rectangular fault model can explain the overall features of the observations. The amount of moment released is equivalent to M w 7.17. The hypocenter depth of the main shock estimated by the Japan Meteorological Agency is slightly deeper than the neutral plane between down-dip compression (DC) and down-dip extension (DE) stress zones of the doubleplaned seismic zone. This suggests that the depth of the neutral plane was deepened by the huge slip of the 2011, M 9.0, Tohoku Earthquake, and the rupture of the thrust M 7.1 earthquake was initiated at that depth, although more investigations are required to confirm this idea. The estimated fault plane has an angle of ∼60 degrees from the surface of the subducting Pacific plate. It is consistent with the hypothesis that intraslab earthquakes are thought to be a reactivation of the pre-existing hydrated weak zones produced in the bending process of oceanic plates around outer-rise regions.

Seismic evidence for reactivation of a buried hydrated fault in the Pacific slab by the 2011 M9.0 Tohoku earthquake

“…The distribution of aftershocks suggests that the rupture associated with this event did not propagate into the oceanic crust beyond the Moho. These conclusions are also supported by the fault plane solution of the M7.1 event inferred from geodetic data [ Ohta et al , 2011].…”

“…On 7 April, an earthquake (M7.1) occurred in the Pacific slab at a depth of 66 km, located near the down‐dip limit of the large interplate slip of the M9 event (Figure 1). GPS data indicate slips of 2.5 m during the M7.1 event and the source area of 30 km by 30 km [ Ohta et al , 2011]. The M7.1 event was probably triggered by stress change resulting from the large fault slip by the M9 event.…”

Quasi real‐time fault model estimation for near‐field tsunami forecasting based on RTK‐GPS analysis: Application to the 2011 Tohoku‐Oki earthquake (Mw 9.0)

“…This method can put the constraints of fault parameters as prior information. The estimated parameters occasionally depend on the initial values assumed in the recursive procedure [e.g., Ohta et al , 2012]. To estimate all the fault parameters (location, depth, length, width, strike, dip, rake, and slip amount), we assumed initial values based on the two rectangular fault models provided by the GSI (Geospatial Information Authority of Japan, http://www.gsi.go.jp/cais/topic110422-index-e.html, 2011).…”

Ground Shaking and Seismic Source Spectra for Large Earthquakes around the Megathrust Fault Offshore of Northeastern Honshu, Japan

“…We analyze the ground shaking and earthquake source spectra for 11 large (M w 6.0-7.6) earthquakes from 2003 to 2011 (Table 1), including a large thrust event (M w 7.2, 7 April 2011) that occurred in the Pacific slab below the down-dip limit of the 2011 Tohoku earthquake and an outer-rise normal-faulting event (M w 7.6, 11 March 2011) that occurred 40 min after the great 2011 event. Both events were probably triggered by stress changes resulting from the large fault slip in the great 2011 event Ohta et al, 2011). Nakajima et al (2011) suggest that the thrust event possibly involved reactivation of a preexisting hydrated fault with overpressurized fluids or serpentine minerals that had originally formed during bending near the trench.…”