Abstract

For earthquakes that are large enough to break the earth’s surface, slip can be measured directly, providing model-independent information of spatially varying behavior in earthquakes. Here new techniques are developed and applied to extract robust measures of surface slip. In particular, I examine how differences in slip scale with differences in separation. Examining slip distributions of seven large earthquakes in a digital database, I find that the curves collapse into a common behavior over kilometer to tens-of-kilometer length scales. Distributions of differences of slip are found to be reasonably well fit by normal distributions, with the variance of the distributions scaling with separation distance. In particular, average slip differences are seen to be increasing linearly out to length scales of the seismogenic crust, but with a nonzero intercept when extrapolated back to a zero separation of around 1 m offset. The variability of slip extrapolated to zero separation, the mean offset, of around 1 m (.96±.15 m) is a remarkable feature of the observations, holding for all seven of the large earthquakes analyzed. Leaving aside the offset and looking at the increase as a function of separation, the slope or lateral strain has a value consistent with large scale average strains. Thus, behavior consistent with constant stress drop is seen at length scales smaller than the event size, revealing a further invariant of earthquake dynamics. Finally, taking into account the noisy environment by looking for coherent structures unlikely to be noise related, I find structures which have moderate values of lateral strains, on the order of a factor of 10 times mean values, with values appearing to be independent of length scale and magnitude.

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