indo–us workshop on “intraplate seismicity†- The CERI Blog
indo–us workshop on “intraplate seismicity†- The CERI Blog
indo–us workshop on “intraplate seismicity†- The CERI Blog
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1 .Department of Earth and Planetary Sciences, 1850 Campus Drive, Northwestern<br />
University, Evanst<strong>on</strong>, IL 60208, USA, 2 .101 Geology Building, Dept. of Geological Sciences,<br />
University of Missouri, Columbia, MO 65211-‐1380, USA, 3 .Department of Earth and<br />
Atmospheric Sciences, Purdue University, West Lafayette, IN 47906, USA.<br />
Intrac<strong>on</strong>tinental seismic z<strong>on</strong>es have traditi<strong>on</strong>ally been treated like slowly deforming (< 2<br />
mm/yr) plate boundaries. In that model, <strong>on</strong>e expects steady deformati<strong>on</strong> focused in narrow<br />
z<strong>on</strong>es, such that the past rates and locati<strong>on</strong>s shown by geology and the earthquake record<br />
would be c<strong>on</strong>sistent with present and future deformati<strong>on</strong> and seismicity. However, data<br />
from China, North America, NW Europe, and Australia reveal a different picture:<br />
earthquakes migrate between faults, which remain inactive for l<strong>on</strong>g periods and then have<br />
pulses of activity. A 2000-‐year record from North China shows that large (M>7) earthquakes<br />
migrated, with n<strong>on</strong>e repeating <strong>on</strong> the same fault segment. In additi<strong>on</strong>, GPS studies in the<br />
New Madrid and other intrac<strong>on</strong>tinental seismic z<strong>on</strong>es still fail to detect significant strain<br />
accumulati<strong>on</strong>, also in c<strong>on</strong>trast with a slow plate boundary-‐type model.<br />
This time-‐ and space-‐variable behavior arises because in mid-‐c<strong>on</strong>tinents tect<strong>on</strong>ic loading is<br />
slow and stress in the crust is str<strong>on</strong>gly influenced by mechanical interacti<strong>on</strong> am<strong>on</strong>g a<br />
network of widespread faults. Slow loading also causes aftershock sequences to c<strong>on</strong>tinue<br />
for hundreds of years, much l<strong>on</strong>ger than at plate boundaries. As a result, the past<br />
earthquake history can be a poor predictor of the future. C<strong>on</strong>venti<strong>on</strong>al seismic hazard<br />
assessment, which assumes steady behavior over 500-‐2500 years, can overestimate risks in<br />
regi<strong>on</strong>s of recent large earthquakes and underestimate them elsewhere. For example, the<br />
May 2008 Sichuan earthquake occurred <strong>on</strong> a fault system that was c<strong>on</strong>sidered to be at low<br />
level of hazard, due to the lack of recent seismicity and low slip rates.<br />
In c<strong>on</strong>trast to a plate boundary fault that gives quasi-‐periodic earthquakes, the interacting<br />
fault networks in midc<strong>on</strong>tinents predict complex variability of earthquakes. Approaching<br />
intrac<strong>on</strong>tinental seismic z<strong>on</strong>es as a complex system is necessary to improve our<br />
understanding of midc<strong>on</strong>tinental tect<strong>on</strong>ics, the resulting earthquakes, and the hazards they<br />
pose.
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