Tectonic Settings of Great Outer-Rise/Outer-Trench-Slope (OR/OTS) Earthquakes in the Instrumental Record

American Geophysical Union Fall Meeting
December 14-18, 2009, San Francisco, CA

Paper U21D-06 in Special Late-Breaking Session: The 2009 Samoan and Sumatran Earthquakes: Origins, Impacts and Consequences II

S Kirby

USGS

Menlo Park, CA

skirby@usgs.gov

R Hino

Research Center for Earthquake and Volcanic Eruption Prediction, Faculty of Sciences

Tohoku University

Sendai, JAPAN

E Geist

USGS

Menlo Park, CA

DJ Wright

Dept of Geosciences, Oregon State Univ

Corvallis, OR

dawn@dusk.geo.orst.edu

E Okal

Dept of Geological Sciences, Northwestern Univ

Evanston, IL

J Wartman

USGS

Menlo Park, CA

Great off-trench earthquakes in subduction zones are rare in the instrumental seismic record. Only five, or possibly six such events are presently known or suspected (2 May 1917 in vicinity of Kermadec Trench; 26 June 1917 in vicinity of Tonga Trench; 2 March 1933 in Sanriku, Japan under outer-trench slope; 19 August 1977 near Sunda Trench off Sumbawa Island, Indonesia, 13 January 2007 seaward of Kuriles in Russia, and the recent 29 September 2009 in northern Tonga that spawned destructive tsunami waves that swept the Samoan Islands). This number compares with 59 great mega-thrust earthquakes that have occurred since 1900. Based on recent research on the great 1933 Sanriku earthquake (Kirby et al., Fall 2008 AGU) we have identified some features common to great off-trench shocks: (i) All have occurred in subduction systems in which convergence rates are greater than about 65 mm/a and the incoming bending oceanic plate is old (Mesozoic), thermally mature, and, by inference, mechanically thick. Seafloor bending strains and strain rates estimated from these observations are high. (ii) Large outer-rise gravity anomalies attest to high bending stresses in these source regions. (iii) Focal mechanisms indicate that the ruptures cross ocean spreading fabric at angles greater than 30 degrees and, in the two cases for which there are high-resolution swath maps available, fault scarps with significant relief are evident. (iv) For those events for which seismological constraints on depth and/or focal mechanisms are available, they show that the earthquakes were very shallow normal-faulting ruptures (less than 30 km). These common features of great off-trench earthquakes are consistent with shallow seismic deformation by large-scale bending at high stresses and strain rates. That such events occur in deep water and involve steeply-dipping (≥45°) dip-slip ruptures gives insight into the large tsunami runups, damage, and loss of life that can occur on a regional scale near such earthquake sources compared to mega-thrust earthquakes of comparable magnitudes. Potential sites for future great off-trench earthquakes are identified that share the above attributes. Methods are suggested that may also help us estimate rates of slip accumulation and average recurrence times of such events. Great variability may be found in the physical expressions of plate bending even among plates of comparable age and convergence rates. This variability may be associated with the effects of subduction of seamounts, volcanic ridges, and fracture zones.

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