Utsu aftershock productivity law explained from geometric operations on the permanent static stress field of mainshocks

作者：Mignan Arnaud^*

来源：Nonlinear Processes in Geophysics, 2018, 25(1): 241-250.

DOI：10.5194/npg-25-241-2018

The aftershock productivity law is an exponential function of the form K alpha exp(alpha M), with K being the number of aftershocks triggered by a given mainshock of magnitude M and alpha approximate to ln (10) being the productivity parameter. This law remains empirical in nature although it has also been retrieved in static stress simulations. Here, we parameterize this law using the solid seismicity postulate (SSP), the basis of a geometrical theory of seismicity where seismicity patterns are described by mathematical expressions obtained from geometric operations on a permanent static stress field. We first test the SSP that relates seismicity density to a static stress step function. We show that it yields a power exponent q = 1.96 +/- 0.01 for the power-law spatial linear density distribution of aftershocks, once uniform noise is added to the static stress field, in agreement with observations. We then recover the exponential function of the productivity law with a break in scaling obtained between small and large M, with alpha = 1.5 ln(10) and ln (10), respectively, in agreement with results from previous static stress simulations. Possible biases of aftershock selection, proven to exist in epidemic-type aftershock sequence (ETAS) simulations, may explain the lack of break in scaling observed in seismicity catalogues. The existence of the theoretical kink, however, remains to be proven. Finally, we describe how to estimate the solid seismicity parameters (activation density delta(+), aftershock solid envelope r(*) and background stress amplitude range Delta o(*)) for large M values.