Characteristics of Oceanic Strike-Slip Earthquakes Differ Between Plate Boundary and Intraplate Settings

The April 11, 2012 Mw 8.6 earthquake that occurred off the coast of Sumatra was the largest ever recorded strike-slip event. This earthquake ruptured the oceanic lithosphere more than 100 km from the nearest plate boundary and was followed by 86 aftershocks of mb ≥ 4.8 within 200 km and 30 days of the mainshock, including a Mw 8.2 event. While the recent Great Sumatra strike-slip earthquake was extraordinary in its seismic moment release, we find that in terms of aftershock productivity it had the characteristics of an ordinary intraplate oceanic strike-slip event. Using catalog data from the International Seismological Centre (ISC) and Global Centroid Moment Tensor (CMT) Project from January 1976 through June 2012, we have investigated the rates, maximum magnitudes, and aftershock productivities of 15,519 Mw ≥ 5.5 strike-slip earthquakes in the oceanic lithosphere. We compared our results from intraplate regions to those on mid-ocean ridge transform faults (RTFs) and find that the seismic characteristics differ between the two tectonic settings. The rate of Mw ≥ 5.5 events is 35 times greater on RTFs, while the moment release rate is an order of magnitude greater in intraplate settings. Aftershock statistics of both regions can be quantified using the Epidemic Type Aftershock Sequence model with a triggering exponent, alpha, of 0.9. We find RTF earthquakes have almost an order of magnitude fewer aftershocks than do strike-slip intraplate oceanic earthquakes. Our previous work showed that the maximum magnitude of RTF earthquakes is very small (less than 40%) compared with the total fault area above the 600C isotherm, AT. The largest recorded mid-ocean ridge transform earthquake was a Mw 7.1 on the Romanche RTF. Through a statistical analysis of catalog data from 1964 through 2001 we found that the largest expected earthquake, i.e. the corner moment in a tapered Gutenberg-Richter distribution, Mc, does not grow linearly with fault area, but instead scales as AT to the 1/2 power. This scaling relation is so robust that it can be used, in combination with a scaling relation for seismic coupling, to accurately obtain the global mid-ocean ridge transform fault seismicity distribution from 2002 through the present. On a number of moderate and fast slipping RTFs, the largest earthquakes are known to re-rupture the same fault patch in relatively regular and short (5-20 year) seismic cycles. These small sections of the fault appear to be fully-coupled, such that the tectonic moment rate can be used to constrain the timing of the next maximum magnitude rupture. The much larger earthquakes and more abundant aftershocks following oceanic intraplate strike-slip earthquakes, suggest that fault zone composition alone is unlikely to be the primary cause of the small maximum magnitudes, short recurrence intervals, and few aftershocks observed on RTFs.