HR: 17:15h
AN: NG54A-06    [Abstracts]
TI: Detailed Multi-scale Earthquake Modeling Using Rate and State Friction and the Fast Multipole Method on Parallel Computers
AU: * Tullis, T E
EM: Terry\_Tullis@brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912-1846 United States
AB: If short- and intermediate-term earthquake prediction is eventually possible, it is likely to result from remote detection of nonlinear processes occurring near the earthquake hypocenter. We know that such processes occur in the laboratory during the failure of intact rocks as well as prior to unstable slip during repeated cycles of stick-slip frictional sliding. In the case of repeated unstable slip the nonlinear behavior is well described by rate and state friction, and for most major earthquakes it is it repeated slip on well-established faults that we would most like to be able to predict, so models using rate and state friction may be the most useful. One of the difficulties in predicting major damaging earthquakes is that their initial stages may be very similar or identical to those of the much more numerous small earthquakes. What determines whether a small earthquake grows into a large one, and is it possible to detect in advance some aspect of the behavior of the system that will indicate when this will happen? The pessimistic view is that at any time a small earthquake can grow into a large one. The optimistic view is that this will only happen when the stresses in the region have sufficiently recovered from the last earthquake. In this case, accelerating moment release in the region may signal sufficient recovery of the regional stresses that small earthquakes are more likely to grow into large ones. Sufficiently realistic models can help us understand the behavior of fault systems and can help determine whether the seismicity patterns may allow prediction of a large event. Moment rate acceleration proportional to 1/(time-to-earthquake) was shown in a model of earthquakes at Parkfield using rate and state friction [{\it Tullis}, 1996], but only for a very coarse model in which microseismicity could not occur and smooth acceleration was occurring on the model element that was to become the eventual hypocenter. What remained unclear was whether this pattern of accelerating slip would occur or could be recognized when the moment release occurs via discrete events with a wide range of sizes. Until recently such modeling employing realistic rate and state friction could not be done with a sufficiently large number of elements to allow a wide range in the sizes of modeled earthquakes, and so realistic patterns and sequences of earthquakes could not be modeled. A NASA-funded CT project has allowed development of a parallel computer code that uses the Fast Multipole method and thus allows at least hundreds of thousands of boundary elements to be employed that lie on one or many fault surfaces. Elements do not have to be of uniform size and consequently it is possible to choose elements in a way that allows many of them to have dimensions of a few meters while modeling a large area. The code is available (http://www.servogrid.org/slide/GEM/PARK/) although it is still being improved. The scaling of compute time with number of elements and number of processors show that the code will be useful for studying earthquake interactions and prediction.
DE: 7260 Theory and modeling
DE: 8010 Fractures and faults
DE: 8159 Rheology--crust and lithosphere
DE: 7209 Earthquake dynamics and mechanics
DE: 3230 Numerical solutions
SC: Nonlinear Geophysics [NG]
MN: 2004 AGU Fall Meeting