ASSESSING THE MAXIMUM MAGNITUDE OF INTRAPLATE STRIKE-SLIP EARTHQUAKES IN THE OCEANIC LITHOSPHERE

The April 11, 2012 Mw 8.6 earthquake located west of Sumatra was the largest ever recorded strike-slip event. This complex earthquake ruptured the oceanic lithosphere of the Indo-Australia plate more than 100 km from the nearest plate boundary. Additionally, the Mw 8.6 was followed by 91 aftershocks of mb ≥ 4.8 within 700 km and 30 days, including the second largest recorded strike-slip earthquake— a Mw 8.2 event. These great Off-Sumatra intraplate earthquakes resulted in 10 deaths and provide an example of seismic hazard in an intraplate setting.

Using catalog data from the International Seismological Centre (ISC) and Global Centroid Moment Tensor (CMT) Project from January 1976 through November 2012, we have investigated the rates, maximum magnitudes, and aftershock productivities of ~12,000 Mw ≥ 5.5 strike-slip earthquakes in the oceanic lithosphere. We separated our dataset into intraplate and plate boundary events and fit the tapered Gutenberg Richter distribution to the population of intraplate strike-slip earthquakes. A beta-value of 2/3 (equivalent to a b-value of 1) fits well at the 99% confidence level and the distribution of Mw ≤ 6.6 lie directly on the linear slope. Above Mw 6.6 the magnitude-frequency distribution displays characteristic earthquake behavior, where the largest events occur at a higher rate than would be expected based on the more frequent smaller events. The maximum-likelihood fit to the distribution prior to April 2012, shows the maximum probable magnitude for intraplate oceanic strike-slip earthquake was Mw 8.6, with no constrained upper bounds. The recent great earthquakes off-Sumatra were therefore not unexpected in terms of their size. By contrast, the same technique shows that the maximum probable magnitude of oceanic transform fault earthquakes is very low, Mw ≤ 7.1. Thus, their is significant seismic hazard from strike-slip earthquakes in the oceanic lithosphere in intraplate settings.