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Laboratory Earthquakes
Ares Rosakis (California Institute of Technology)

29 juin 2005

Recently, seismologists have discovered that some earthquakes are very slow with almost no energy radiation, while others excite strong and damaging seismic waves with extremely high rupture speed, sometimes believed to be super-shear (faster than shear-wave speed). This variation reflects the difference in rupture physics in different tectonic environments, and it has become very important to understand what causes the dramatic variation of rupture speed, and rupture behavior. We have embarked on this problem by constructing laboratory models of earthquakes and by using high-speed imaging of photo-elastic rupture patterns. We have designed a series of experiments to determine the behavior of rupture under spontaneous loading similar to that of natural earthquakes. With these experiments we have demonstrated that under reasonable loading conditions similar to those for natural earthquakes, super-shear rupture propagation can occur. This is probably the first experimental demonstration of super-shear rupture propagation under spontaneous loading.
Motivated by such experiments we extended the work to observations of spontaneously nucleated events occurring on frictionally held bi-material interfaces. Previously, it was generally thought that if there is a velocity contrast the rupture preferentially goes toward the direction of the slip in the lower-speed medium. In contrast to this, we have found that this is not necessarily the case; the rupture can propagate in both directions. In one direction, rupture always propagates at the Generalized Rayleigh wave speed (i.e., sub-shear) whereas in the opposite direction it may either be sub-shear or may transition to super-shear. This behaviour could explain why the rupture in the recent Parkfield earthquake propagated to southeast whereas it propagated northwest in the previous two Parkfield earthquakes. It can also be used to explain field observations during the 1999 Izmit earthquake in Turkey. We hope that these results will help seismologists understand the basic physics of earthquakes and contribute to a better understanding of the vast diversity of earthquake characteristics.

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Ares Rosakis Ares Rosakis (California Institute of Technology)
Director, GALCIT - Graduate Aeronautical Laboratories