Two subevents across the Japan Trench during the 7 December 2012 off Tohoku earthquake (M_w 7.3) inferred from offshore tsunami records

Abstract

A tsunamigenic earthquake with a moment magnitude of 7.3 occurred near the Japan Trench, off Tohoku, northeast Japan, on 7 December 2012. Operational seismic monitoring inferred that the earthquake was composed of doublet sources of comparable magnitudes: the first event was reverse faulting and the second event, which occurred 10-20 s later, was normal faulting. An associated tsunami was observed at offshore stations, having an amplitude of 10 À1 to 10 1 cm. Inverse modeling using the observed tsunami records reveals possible vertical seafloor deformations. We can then constrain, in terms of the source location, that the tsunamigenic earthquake involves two subfaults across the Japan Trench: one is the outer trench reverse faulting as the first event and the second is the inner trench normal faulting. The present study shows that tsunami analysis with seismograph analyses effectively constrains the epicenter locations, even in the case of complex earthquakes like doublets.

In situ observations of earthquake-driven fluid pulses within the Japan Trench plate boundary fault zone

“…During this time, a M w 7.3 intraslab doublet on 7 December 2012 occurred very close to the observatory, involving slip on a normal fault within the downgoing Pacific plate (Lay et al, 2013;Inazu and Saito, 2014). This earthquake resulted in an easily observable change in the temperature pattern.…”

Assessment of GNSS-based height data of multiple ships for measuring and forecasting great tsunamis

“…The tsunami forecast method is based on the tsunami Forecasting based on Inversion for initial sea-Surface Height (tFISH) (Tsushima et al 2009(Tsushima et al , 2011. The following procedure was introduced by Inazu and Saito (2014). Based on a linear inversion, we use offshore tsunami observation data to estimate the initial tsunami height as the tsunami source for the forecasting.…”

“…1d) are added to the GPS buoy observation. No noise is added to the deep-sea pressure observation which is sensitive to subcentimeter tsunamis (Filloux 1982;Hino et al 2001;Inazu and Saito 2014).…”

“…The first 3-min data are omitted because the sea surface and bottom are expected to be significantly affected by elastic wave effects immediately after the earthquake (Nosov and Kolesov 2007;Saito 2013). In the tsunami forecast tests, λ is empirically fixed, respectively, for using the data from the operating offshore observatories, and for using the PPP ship height data (Inazu and Saito 2014).…”

Estimate of tsunami source using optimized unit sources and including dispersion effects during tsunami propagation: The 2012 Haida Gwaii earthquake

“…The longwave assumption breaks down and dispersion effect becomes significant at short wavelengths for earthquakes with a steep dipping fault plane [e.g., Gusman et al, 2009;Inazu and Saito, 2014] or submarine mass failures [e.g., Synolakis et al, 2002;Watts et al, 2003;Grilli and Watts, 2005;Løvholt et al, 2005;Tappin et al, 2014]. The longwave assumption breaks down and dispersion effect becomes significant at short wavelengths for earthquakes with a steep dipping fault plane [e.g., Gusman et al, 2009;Inazu and Saito, 2014] or submarine mass failures [e.g., Synolakis et al, 2002;Watts et al, 2003;Grilli and Watts, 2005;Løvholt et al, 2005;Tappin et al, 2014].…”