Multiscale Earthquake Simulator, Using Rate and State Friction and Fast Multipoles, Focused on Parkfield, California
Abstract
We developed a multiscale grid and corresponding distribution of fault constitutive parameters to simulate earthquake sequences on the San Andreas over a wide range of scales at Parkfield, CA. The distributions of elements and constitutive parameters are based on the spatial distribution of microseismicity at Parkfield, including earthquakes ranging from magnitude 1 to magnitude 6. Our intent is to understand the interplay between earthquakes of a wide range of magnitudes, in particular what conditions allow small earthquakes to grow into larger ones, whether detectable accelerating seismicity presages larger events, and to simulate the target earthquakes of the SAFOD drilling experiment?. Because a detailed time-space history of microseismicity and larger events at Parkfield exists, it is possible to make comparisons between the properties and history of simulated events and actual events. The smallest elements in the multiscale grid are 7 m in dimension, values small enough to represent a continuum with laboratory values of Dc and the other constitutive parameters. The multiscale grid is used so that only the areas having experienced earthquakes are represented by the smallest elements. The largest elements are used in areas where microearthquakes do not occur, and these are 200 m in dimension. The total model area is 47 km long by 15 km deep and is based on the observed distribution of 4966 microearthquakes and has 1,464,433 elements. Running the total model for sufficient time steps, is beyond the range of existing computers. Consequently we are starting with subsets having a range of sizes and numbers of actual earthquakes, to gain experience with the behavior of the simulations. Note that although the distribution of constitutive parameters and of the smallest elements may restrict the simulated earthquakes in the model to be spatially similar to actual earthquakes at Parkfield, the time histories of the simulated earthquakes will occur spontaneously. Although this is a work in progress at an early stage, several results are notable. Some previously recognized issues relevant to computational efficiency worthy of emphasis include: 1) because, for fixed constitutive parameters a and b, the degree of instability increases as the ratio of normal stress to Dc, if effective normal stress actually increases with depth as we assume, then either Dc needs to increase with depth or the grid size needs to decrease with depth in order to be able to represent the behavior equally at all depths, and 2) with the same values of constitutive parameters, simulations using the slowness law for state evolution proceed farther in simulated time per computational time step than with the slip law, but the slip velocities are smaller with the slowness law. Although lab data suggest the slip law better represents behavior above slip velocities of about 0.01 microns/s, the slowness law may proxy for actual behavior during earthquakes better than the slip law because it involves larger fracture energy that during earthquakes may partially result from off-fault damage. Preliminary scientific observations include 1) dynamic events that are internally complex and interact with other events, and 2) accelerating seismicity generally occurs prior to larger events, suggesting that it might allow short-term earthquake prediction.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.S32A..08T
- Keywords:
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- 1217 Time variable gravity (7223;
- 7230);
- 1242 Seismic cycle related deformations (6924;
- 7209;
- 7223;
- 7230);
- 4255 Numerical modeling (0545;
- 0560)