HR: 16:15h
AN: S24A-02    [Abstracts]
TI: A Quantitative Test for the Spatial Relationship Between Aftershock Distributions and Mainshock Rupture Properties
AU: * Woessner, J
EM: jochen.woessner@sed.ethz.ch
AF: ETH Zurich, Swiss Seismological Service, Institute of Geophysics, ETH Hoenggerberg HPP P3, Zurich, 8093 Switzerland
AU: Ripperger, J
EM: j.ripperger@sed.ethz.ch
AF: ETH Zurich, Swiss Seismological Service, Institute of Geophysics, ETH Hoenggerberg HPP P3, Zurich, 8093 Switzerland
AU: Mai, M P
EM: m.mai@sed.ethz.ch
AF: ETH Zurich, Swiss Seismological Service, Institute of Geophysics, ETH Hoenggerberg HPP P3, Zurich, 8093 Switzerland
AU: Wiemer, S
EM: stefan.wiemer@sed.ethz.ch
AF: ETH Zurich, Swiss Seismological Service, Institute of Geophysics, ETH Hoenggerberg HPP P3, Zurich, 8093 Switzerland
AB: Correlating the properties of the mainshock rupture with the location of corresponding aftershocks may provide insight into the relationship between mainshock-induced static stress changes and aftershock occurrence. In this study, we develop a rigorous statistical test to quantify the spatial pattern of aftershock locations with the corresponding distributions of coseismic slip and stress-drop. Well-located aftershock hypocenters are projected onto the mainshock fault plane and coseismic slip and stress drop values are interpolated to their respective location. The null hypothesis H0 for the applied test statistic is: Aftershock hypocenters are randomly distributed on the mainshock fault plane and are not correlated with mainshock properties. Because we want to maintain spatial earthquake clustering as one of the important observed features of seismicity, we synthesize slip distributions using a random spatial field model from which we then compute the respective stress-drop distributions. For each simulation of earthquake slip, we compute the test statistic for the slip and stress-drop distribution, testing whether or not an apparent correlation between mainshock properties and aftershock locations exists. Uncertainties in the aftershock locations are accounted for by simulating a thousand catalogues for which we randomize the location of the aftershocks within their given location error bounds. We then determine the number of aftershocks in low-slip or negative stress-drop regions for simulated slip distributions, and compare those to the measurements obtained for finite-source slip inversions. We apply the test to crustal earthquakes in California and Japan. If possible, we use different source models and earthquake catalogues with varying accuracy to investigate the dependence of the test results on, for example, the location uncertainties of aftershocks. Contrary to the visual impression, we find that for some strike-slip earthquakes or segments of the mainshock rupture plane in California, the null hypothesis cannot be rejected. However, H0 can be rejected for the Homestead Valley fault segment of the Landers 1992 earthquake and the Morgan Hill 1984 earthquake. The test results so far imply that there is only for a fraction of the analyzed earthquakes a statistically significant correlation between the occurrence of aftershocks and low-slip or increased shear-stress regions on the mainshock rupture plane. If this result is confirmed by additional cases studies that are being performed right now, the common understanding of aftershock genesis may have to be rethought.
DE: 7299 General or miscellaneous
DE: 7200 SEISMOLOGY
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting