Fault model of the intraplate doublet on Dec. 7 2012 derived from near- and far-field tsunami records and its implication for the post-2011 stress state

Abstract:

On December 7, 2012, an earthquake doublet (two consecutive earthquakes) occurred in the subducting Pacific slab near the rupture area of the 2011 Tohoku earthquake. A reverse faulting event with a focal depth of ~ 60 km occurred, which was followed by a normal faulting event at a depth of ~ 20 km. The size of the two events were almost the same, Mw 7.2 and the time interval was ~ 10 seconds. When the doublet occurred, 19 ocean bottom pressure gauges were deployed along the trench slope of the Japan Trench, near the epicenter of the doublet and obtained high quality tsunami records. The analyses of the local tsunami waveforms are supposed to improve the fault model and help to understand the rupture process of the doublet, strongly affected by the post-2011 stress state in the subducting slab.

As the first step of the analyses, we inverted tsunami waveforms for estimating the initial sea-surface height distribution. Comparing the tsunami source with the Global CMT solution, we found that the subsided area can be reproduced only by the 2nd normal faulting event, but the 1st deep thrust event substantially contributed to form the uplift area. Then we estimated the fault model comprised of two planer rectangles based on the result of the initial sea-surface height inversion of the first step. The depths of the lower edge of the shallower fault plane and the upper edge the deeper fault plane were almost identical, about 40 km. This depth corresponds with the lower limit of the focal depth distribution of the normal faulting intraplate small earthquakes after the 2011 Tohoku Earthquake (Obana et al., 2012). The estimated fault extent of the shallower event of the doublet is consistent with the normal faulting aftershocks activity in depth extent. Seno and Gonzalez (1987) reported a reverse faulting earthquake of Mw 4.7 occurred at a depth of 41 km in 1967 around this area. Since the depth to the upper edge of the fault plane of the compressional event corresponds with the focal depth of this old compressional earthquake, we interpret that a compressional stress field below ~ 40 km have been persistent. Our future study using seafloor observation will clarify the hypocenter location on the rupture plane extent to reveal the relationship between the post-2011 stress state and the rupture processes of the doublet.