The great 1933 Sanriku-oki earthquake: reappraisal of the main shock and its aftershocks and implications for its tsunami using regional tsunami and seismic data

Geophysical Journal International Geophys. J. Int. (2016) 206, 1619–1633 Advance Access publication 2016 June 27 GJI Geodynamics and tectonics doi: 10.1093/gji/ggw234 The great 1933 Sanriku-oki earthquake: reappraisal of the main shock and its aftershocks and implications for its tsunami using regional tsunami and seismic data Naoki Uchida,1 Stephen H. Kirby,2 Norihito Umino,1 Ryota Hino1 and Tomoe Kazakami3 1 Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan. E-mail: 2 U.S. Geological Survey, Menlo Park, CA 94025, USA 3 National Research Institute for Earth Science and Disaster Prevention, Tsukuba 305-0006, Japan SUMMARY The aftershock distribution of the 1933 Sanriku-oki outer trench earthquake is estimated by using modern relocation methods and a newly developed velocity structure to examine the spatial extent of the source-fault and the possibility of a triggered interplate seismicity. In this study, we first examined the regional data quality of the 1933 earthquake based on smokedpaper records and then relocated the earthquakes by using the 3-D velocity structure and double-difference method. The improvements of hypocentre locations using these methods were confirmed by the examination of recent earthquakes that are accurately located based on ocean bottom seismometer data. The results show that the 1933 aftershocks occurred under both the outer- and inner-trench-slope regions. In the outer-trench-slope region, aftershocks are distributed in a ∼280-km-long area and their depths are shallower than 50 km. Although we could not constrain the fault geometry from the hypocentre distribution, the depth distribution suggests the whole lithosphere is probably not under deviatoric tension at the time of the 1933 earthquake. The occurrence of aftershocks under the inner trench slope was also confirmed by an investigation of waveform frequency difference between outer and inner trench earthquakes as recorded at Mizusawa. The earthquakes under the inner trench slope were shallow (depth 30 km) and the waveforms show a low-frequency character similar to the waveforms of recent, precisely located earthquakes in the same area. They are also located where recent activity of interplate thrust earthquakes is high. These suggest that the 1933 outer-trenchslope main shock triggered interplate earthquakes, which is an unusual case in the order of occurrence in contrast with the more common pairing of a large initial interplate shock with subsequent outer-slope earthquakes. The off-trench earthquakes are distributed about 80 km width in the trench perpendicular direction. This wide width cannot be explained from a single high-angle fault confined at a shallow depth (depth 50 km). The upward motion of the 1933 tsunami waveform records observed at Sanriku coast also cannot be explained from a single high-angle west-dipping normal fault. If we consider additional fault, involvement of highangle, east-dipping normal faults can better explain the tsunami first motion and triggering of the aftershock in a wide area under the outer trench slope. Therefore multiple off-trench normal faults may have activated during the 1933 earthquake. We also relocated recent (2001– 2012) seismicity by the same method. The results show that the present seismicity in the outer-trench-slope region can be divided into several groups along the trench. Comparison of the 1933 rupture dimensions based on our aftershock relocations with the morphologies of fault scarps in the outer trench slope suggest that the rupture was limited to the region  C The Authors 2016. Published by Oxford University Press on behalf of The Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. 1619 Accepted 2016 June 20. Received 2016 June 14; in original form 2015 November 6 1620 N. Uchida et al. where fault scarps are largely trench parallel and cross cut the seafloor spreading fabric. These findings imply that bending geometry and structural segmentation of the incoming plate largely controls the spatial extent of the 1933 seismogenic faulting. In this shallow rupture model for this largest outer trench earthquake, triggered seismicity in the forearc and structural control of faulting represent an important deformation styles for off-trench and shallow megathrust zones. Key words: Earthquake dynamics; Earthquake source observations; Seismicity and tectonics; Subduction zone processes; Dynamics: seismotectonics; Pacific Ocean. 1 I N T RO D U C T I O N 2 T H E S O U RC E M O D E L S O F T H E G R E AT 1 9 3 3 S A N R I K U - O K I E A RT H Q UA K E To model observed long-period surface waves and near-field displacement waveforms, Kanamori (1971) constructed a seismic rupture model as a 185 × 100 km seismogenic fault dipping 45◦ towards N90◦ W (Fig. 1b) that explains the 1933 main-shock waveforms with a uniform slip of 3.3 m. The fault length (185 km) was based on the one-day aftershock distribution and the dip angle was based on a first motion fault plane solution (45◦ ). The 100-km fault width was based on a 70-km horizontal width of the aftershocks and the fault dip angle (70/cos45◦ ≈ 100 km) suggesting the fault rupture extended to a maximum depth of about 70 km (Fig. 1b). From an estimated thickness of the oceanic lithosphere of 70 km (Kanamori & Press 1970), he concluded that rupture cut through the entire thickness of the Pacific-Plate lithosphere. Although, some recent studies suggest thicker lithosphere for the study region (80–100 km; Zhao et al. 1994; Tonegawa et al. 2006; Kawakatsu et al. 2009), the depth extent of the rupture is still large fraction of the lithosphere thickness. However, observations of focal mechanisms of recent events (2000–2006) near the 1933 earthquake suggest a stress-state reversal from a shallower level (normal faulting at 10–20 km depth) to deeper level (reverse faulting at 25–40 km depth) (Gamage et al. 2009). Therefore, a full lithosphere rupture during the 1933 main shock is unlikely to have occurred given the present-day the stress state. From the point of view of the initial motion of tsunami waves, Abe (1978) noted that the Kanamori’s (1971) fault model could not explain the tsunami first motions at the Sanriku coast, to the west of the main-shock epicentre (Fig. 1a)—Kanamori’s model predicted a downward first motion, but all of the four Sanriku-coast tidegauges in the Abe’s paper show an upward tsunami first motions. To fit observed tsunami waveforms, Abe proposed the same westdipping fault but with a smaller dip angle (30◦ ) than the Kanamori’s model (45◦ ) (Fig. 1b). Aida (1977) suggests from tsunami wave The 1933 Sanriku-oki earthquake o (...truncated)