TY - JOUR
T1 - The Showa Sanriku earthquake of 1933 March 2
T2 - A global seismological reassessment
AU - Okal, Emile A.
AU - Kirby, Stephen H.
AU - Kalligeris, Nikos
N1 - Funding Information:
ACKNOWLEDGEMENTS We are grateful to Hiroo Kanamori for sending us a preliminary write up of his solution for the moment of the 1933 earthquake, and to Naoki Uchida for collaboration and a data set of Uchida et al. (2016) relocations. We thank Roger Buck for discussions on stress release in the upper plate, Norihito Umino for access to T. Matuzawa's collection of original seismograms and to the Omori records from the Mizusawa archives, and Takeo Ishibe for a data set of intensity values during the 2011 Tohoku earthquake. EAO was partially supported by the National Science Foundation, under subcontract from the University of Pittsburgh's Hazards SEES Grant number OCE-1331463; NK enjoyed support from the National Science Foundation under Grant CMI-1538624 to the University of Southern California. The paper was improved through the comments of two anonymous reviewers. Some figures were plotted using the GMT software (Wessel & Smith 1991).
Publisher Copyright:
© The Authors 2016. Published by Oxford University Press on behalf of The Royal Astronomical Society.
PY - 2016/9/1
Y1 - 2016/9/1
N2 - After 83 yr, the great normal-faulting earthquake of 1933 March 2, which took place off the Japan Trench and produced a devastating tsunami on the Sanriku coast and damaging waves in Hawaii, remains the largest recorded normal-faulting earthquake. This study uses advanced methods to investigate this event using far-field seismological and tsunami data and complements a sister study by Uchida et al. which used exclusively arrival times at Japanese stations. Our relocation of the main shock (39.22°N, 144.45°E, with a poorly constrained depth of less than 40 km) places it in the outer trench slope, below a seafloor depth of ~6500 m, in a region of horst-and-graben structure, with fault scarps approximately parallel to the axis of the Japan Trench. Relocated aftershocks show a band of genuine shallow aftershocks parallel to the Japan Trench under the outer trench slope and a region of post-mainshock events landward of the trench axis that occur over roughly the same latitude range and are thought to be the result of stress transfer to the interplate thrust boundary following the normalfaulting rupture. Based on a combination of P-wave first motions and inversion of surface wave spectral amplitudes, we propose a normal-faulting focal mechanism (φ = 200°, δ = 61° and λ= 271°) and a seismic moment M0 = (7 ± 1) × 1028 dyn cm (Mw = 8.5). A wide variety of data, including the distribution of isoseismals, the large magnitudes (up to 8.9) proposed by early investigators before the standardization of magnitude scales, estimates of energy-to-moment ratios and the tentative identification of a T wave at Pasadena (and possibly Riverside), clearly indicate that this seismic source was exceptionally rich in high-frequency wave energy, suggesting a large apparent stress and a sharp rise time, and consistent with the behaviour of many smaller shallow normal-faulting earthquakes. Hydrodynamic simulations based on a range of possible sources consistent with the above findings, including a compound rupture on two opposite-facing normal-faulting segments, are in satisfactory agreement with tsunami observations in Hawaii, where run-up reached 3 m, causing significant damage. This study emphasizes the need to include off-trench normal-faulting earthquake sources in global assessments of tsunami hazards emanating from the subduction of old and cold plates, whose total length of trenches exceed 20 000 km, even though only a handful of great such events are known with confidence in the instrumental record.
AB - After 83 yr, the great normal-faulting earthquake of 1933 March 2, which took place off the Japan Trench and produced a devastating tsunami on the Sanriku coast and damaging waves in Hawaii, remains the largest recorded normal-faulting earthquake. This study uses advanced methods to investigate this event using far-field seismological and tsunami data and complements a sister study by Uchida et al. which used exclusively arrival times at Japanese stations. Our relocation of the main shock (39.22°N, 144.45°E, with a poorly constrained depth of less than 40 km) places it in the outer trench slope, below a seafloor depth of ~6500 m, in a region of horst-and-graben structure, with fault scarps approximately parallel to the axis of the Japan Trench. Relocated aftershocks show a band of genuine shallow aftershocks parallel to the Japan Trench under the outer trench slope and a region of post-mainshock events landward of the trench axis that occur over roughly the same latitude range and are thought to be the result of stress transfer to the interplate thrust boundary following the normalfaulting rupture. Based on a combination of P-wave first motions and inversion of surface wave spectral amplitudes, we propose a normal-faulting focal mechanism (φ = 200°, δ = 61° and λ= 271°) and a seismic moment M0 = (7 ± 1) × 1028 dyn cm (Mw = 8.5). A wide variety of data, including the distribution of isoseismals, the large magnitudes (up to 8.9) proposed by early investigators before the standardization of magnitude scales, estimates of energy-to-moment ratios and the tentative identification of a T wave at Pasadena (and possibly Riverside), clearly indicate that this seismic source was exceptionally rich in high-frequency wave energy, suggesting a large apparent stress and a sharp rise time, and consistent with the behaviour of many smaller shallow normal-faulting earthquakes. Hydrodynamic simulations based on a range of possible sources consistent with the above findings, including a compound rupture on two opposite-facing normal-faulting segments, are in satisfactory agreement with tsunami observations in Hawaii, where run-up reached 3 m, causing significant damage. This study emphasizes the need to include off-trench normal-faulting earthquake sources in global assessments of tsunami hazards emanating from the subduction of old and cold plates, whose total length of trenches exceed 20 000 km, even though only a handful of great such events are known with confidence in the instrumental record.
KW - Earthquake source observations
KW - Intra-plate processes
KW - Tsunamis
UR - http://www.scopus.com/inward/record.url?scp=84988799199&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84988799199&partnerID=8YFLogxK
U2 - 10.1093/gji/ggw206
DO - 10.1093/gji/ggw206
M3 - Article
AN - SCOPUS:84988799199
SN - 0956-540X
VL - 206
SP - 1492
EP - 1514
JO - Geophysical Journal International
JF - Geophysical Journal International
IS - 3
ER -