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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy<br />

(S) - <strong>IASPEI</strong> - International Association of Seismology and Physics of the Earth's<br />

Interior<br />

JSS005 Oral Presentation 1905<br />

Empirical relations among magnitude and rupture characteristics, through<br />

mechanical modeling of interacting faults and fault patches<br />

Mr. Olaf Zielke<br />

SESE ASU<br />

Ramon J Arrowsmith<br />

Determining the magnitude of paleo-earthquakes is as difficult as it is important. These data are crucial<br />

components to assess seismic hazard: knowing the slip and date of the last major earthquake that<br />

occurred on a specific fault, the derived long-term slip-rate can be used to estimate the timing of the<br />

next large event assuming that they are time predictable. Furthermore, long-term slip-rates define the<br />

slip-distribution within a fault system constraining the regional stress field a crucial component to run<br />

increasingly realistic fault models. Such models may characterize the seismic hazard at fault system<br />

level by incorporating processes such as fault interaction In order to determine paleo-magnitudes,<br />

earthquake-related geomorphic features such as rupture length, average surface displacement, or<br />

maximum surface displacement are utilized, assuming that an earthquake of a specific size will cause<br />

surface features of correlated size. The well known Wells & Coppersmith (1994) paper defined empirical<br />

relationships between these and other parameters, based on events with known magnitudes and<br />

rupture characteristics. However, because their study depended on a limited number of observed<br />

events, the uncertainties on their correlations are rather large, coefficients ranging from 0.71 to 0.95. In<br />

addition, they were only able to differentiate between the three general fault types (strike, normal,<br />

reverse). Therefore, the effect of oblique slip on rupture characteristics was not addressed affecting the<br />

derived relationships. We have developed a boundary element model, based on derivations by Okada<br />

(1992) that simulates faulting and fault interaction of any number of arbitrarily oriented, located, and<br />

loaded faults. For the study presented here, we simulate faulting along a single fault divided into a large<br />

number of (~5000) elements and utilize a simple Coulomb static/dynamic friction law to create synthetic<br />

seismic catalogs: Each time step the faults are tectonically loaded. When the stress on an element<br />

exceeds its static frictional strength, it starts to slip, corresponding to the stress drop to its dynamical<br />

frictional strength. Due to a fault interaction algorithm, this may cause other elements to fail as well,<br />

cascading into events of all sizes, populating the synthetic seismic catalog. This catalog will be used to<br />

develop a new set of more complete magnitude vs. rupture characteristics plots in the sense of Wells &<br />

Coppersmith (1994). Several enhancements justify this approach: A) We can use seismic catalog of any<br />

length. B) Our catalog will be not limited to large, rare, surface rupturing earthquakes but contain the<br />

full spectrum of possible magnitudes. C) The uncertainty in the correlations will be small, providing a<br />

more realistic assessment of variability. D) How different parameters such as oblique slip, fault<br />

sinuosity, or fault roughness affect the magnitude vs. rupture characteristics relationships can be<br />

explored. In addition to a full exploration of relevant parameters from Wells & Coppersmith, we will also<br />

study system level properties such as fault sinuosity, fragmentation, slip obliquity, etc. Since these are<br />

parameters that can be constrained in the field, our experiment will help paleo-seismologist to interpret<br />

their findings in a more complete way that treats faults not as individual, isolated entities but as part of<br />

a system of interacting structures.<br />

Keywords: mechanical modeling, interaction, synthetic seismicity

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