MIGRATING EARTHQUAKES AND FAULTS SWITCHING ON AND OFF: A COMPLEX SYSTEM VIEW OF INTRACONTINENTAL EARTHQUAKES

Intracontinental seismic zones have traditionally been treated like slowly deforming (< 2 mm/yr) plate boundaries. In that model, one expects steady deformation focused in narrow zones, such that the past rates and locations shown by geology and the earthquake record would be consistent with present and future deformation and seismicity. However, data from China, North America, NW Europe, and Australia reveal a different picture: earthquakes migrate between faults, which remain inactive for long periods and then have pulses of activity. A 2000-year record from North China shows that large (M>7) earthquakes migrated, with none repeating on the same fault segment. In addition, GPS studies in the New Madrid and other intracontinental seismic zones still fail to detect significant strain accumulation, also in contrast with a slow plate boundary-type model.

This time- and space-variable behavior arises because in mid-continents tectonic loading is slow and stress in the crust is strongly influenced by mechanical interaction among a network of widespread faults. Slow loading also causes aftershock sequences to continue for hundreds of years, much longer than at plate boundaries. As a result, the past earthquake history can be a poor predictor of the future. Conventional seismic hazard assessment, which assumes steady behavior over 500-2500 years, can overestimate risks in regions of recent large earthquakes and underestimate them elsewhere. For example, the May 2008 Sichuan earthquake occurred on a fault system that was considered to be at low level of hazard, due to the lack of recent seismicity and low slip rates.

In contrast to a plate boundary fault that gives quasi-periodic earthquakes, the interacting fault networks in midcontinents predict complex variability of earthquakes. Approaching intracontinental seismic zones as a complex system is necessary to improve our understanding of midcontinental tectonics, the resulting earthquakes, and the hazards they pose.

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Session No. 33

T3B. The Challenge of Understanding Continental Intraplate Earthquakes

Tuesday, 6 September 2011: 14:00-17:05

A 016 (Ludwig-Maximilians-Universität München)

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