Preface to the Focused Issue on Earthquake Simulators

Terry E. Tullis

Published 2012, SCEC Contribution #1658

This focused issue of Seismological Research Letters describes some earthquake simulators and some results of using them on a fault model that encompasses a fairly comprehensive set of faults in California, excluding the Cascadia megathrust. Earthquake simulators are computer programs that model long histories of earthquake occurrence and slip using various approximations of what is known about the physics of stress transfer due to fault slip and the rheological properties of faults. The purpose of these large simulations of earthquake history is to learn about the statistical behavior of earthquakes within the simulated region in the hope that the simulated earthquake catalogs can demonstrate to us some aspects of earthquake behavior that observations cannot. The difficulty with basing our knowledge of earthquake behavior only on observations is that the record of earthquake occurrence from the combination of instrumental, historical, and paleoseismic data is too short and incomplete. As the results of the simulation studies described herein show, and as is recognized by everyone studying earthquake occurrence, the observation time needed is longer than is represented in our limited data, especially for the largest earthquakes that represent the greatest seismic hazard. Clearly, for earthquake simulators to be considered valid, they need to be consistent with our temporally-constrained observations. This is necessary, if not sufficient, for us to have faith in the result of earthquake simulators. Presumably, the greater the extent of known physics that can be included in an earthquake simulator is, the more likely it is to correctly represent actual behavior. However, limitations on computational power presently constrain our ability to include as much physics as is
known into earthquake simulators while, at the same time, including a large number of faults that are represented with sufficiently detailed resolution. Consequently, the four simulators included in the study represented by the papers in this focus section make a variety of approximations of known physics. The fact that true elastodynamics is omitted by all four simulators is a notable example of the sacrifices made by each simulator in order to allow many faults and long histories to be treated. Some aspects of elastodynamics can be approximated
by including such items as radiation damping and parameters that encourage rupture propagation, but other aspects cannot. For example, remote triggering by seismic waves cannot occur in these simulations.

Citation
Tullis, T. E. (2012). Preface to the Focused Issue on Earthquake Simulators. Seismological Research Letters, 83(6), 957-958. doi: 10.1785/0220120122.