The 75th Anniversary of the Great Sanriku-oki, Japan earthquake of March 2nd, 1933: New Observations and New Insights into the Largest Recorded Outer-Rise Earthquake
Abstract
The shallow M8.4 outer-rise earthquake of March 2, 1933 is the largest recorded event of its kind. It produced strong ground motions and tsunami waves along the coast of Honshu and more than 3000 fatalities. Kanamori [EPSL, 1971] showed that this event was a normal-faulting rupture with ~N-S-striking nodal planes and proposed that this event represented the tectonic effect of bending of the Pacific Plate in the Japan Trench (JT). Aftershocks occurred far west of the main shock and the JT. This suggested to Kanamori that rupture occurred on a 100-km-wide plane dipping at about 45° toward the JT, implying a depth of seismogenic slip of about 70 km. However, phase data from the larger aftershocks are inconsistent with such depths. A recent investigation of the sP phase data from recent earthquakes recorded by land stations [Gamage et al., AGU 2005 and GJI, in press] indicates that small offshore aftershocks east of the JT are still occurring on this rupture plane 75 years after the main shock, consistent with known aftershock decay properties of this earthquake. None of these aftershocks in this shallow zone occur deeper than about 25 km below the seafloor. A Mw7.1 shallow normal-faulting earthquake in 2005 occurred off the Japan Trench south of the 1933 epicenter. An OBS investigation of its aftershocks confirms that rupture occurred on conjugate normal faults to depths of no more than about 20 km below the seafloor [Hino, Fall AGU, 2007]. The regional pattern of hypocenters of recent offshore events near the JT show a double seismic zone parallel to the seafloor with a zone of shallow normal faulting events within about 20 km of the seafloor and a zone of reverse-faulting earthquakes about 25-30 km deeper, both consistent with convex-upward flexure. These findings cast doubt on the original interpretation that the main shock rupture extended to a depth of 70 km. Moreover, S-P delays of the 1933 aftershocks and their recorded waveforms at Mizuzawa Station suggest that the events east of the JT represent seismic activity on this normal-fault rupture and the aftershocks west of the JT represented normal faulting or interplate thrust faulting near the top of the subducted plate. As such, the aftershocks west of the JT probably represent stress transfer from the normal- faulting rupture of main shock, similar to the pattern observed in aftershocks of the large, shallow outer-rise event in the Kuriles in 2007. A large gravity high exists in the source region of the 1933 main shock. Multibeam swath mapping of the bathymetry there shows long, relatively straight normal fault scarps that obliquely cross cut the magnetic anomalies [Nishizawa, 2001]. The aftershock distribution east of the JT and the above findings suggest source dimensions of L = 220 km, W = 35 km, average displacement of u = 8 m, and rigidity of 0.7×1012 dyn/cm2 could satisfy the scalar moment of 4.3× 1028 dyn-cm estimated by Kanamori from long-period wave amplitudes. The large-angle, cross cutting of seafloor fabric of the fault scarps, and the large gravity anomaly in the source region are consistent with unusually large bending stresses and dynamic displacements.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.S14A..05K
- Keywords:
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- 1240 Satellite geodesy: results (6929;
- 7215;
- 7230;
- 7240);
- 7215 Earthquake source observations (1240);
- 7230 Seismicity and tectonics (1207;
- 1217;
- 1240;
- 1242)