No. 1073

Title:

Seismicity shadows in space-time aftershock activity

Author(s):

Ogata, Yosihiko (Institute of Statistical Mathematics)

Key words:

Aseismic slip, Coulomb failure stress, Focal mechanisms, Omori-Utsu formula, Aftershock deficit, Stress shadows

Abstract:

    This paper retrospectively examines space-time aftershock coordinates before a large aftershock that enlarges the primary aftershock region significantly, triggering a large number of secondary aftershocks. For this analysis, the time elapsed from the main shock is transformed using the standard aftershock decay rate in the entire aftershock area. Here, it is assumed that, for the period until such a large aftershock, the occurrence rates in all aftershock sub-regions obey the same Omori-Utsu function except for the parameter K that depends only on the location. Then, in the typically normal aftershock activity, each location should have a constant occurrence rate with respect to the transformed time. Otherwise, there are space-time zones where the aftershocks appear more frequently or less frequently than those are in the previous or succeeding period. The more frequent anomaly includes secondary aftershock activity or triggered swarms. The less frequent anomaly, which we call the seismicity shadow in this paper, indicates aftershock deficit relative to the normal decay rate in a region and a period. We investigate a number of recent aftershock sequences in and around Japan in conjunction with the fault location and orientations of the focal large aftershocks. The possible cause of the seismicity shadow of the considered aftershock activity is aseismic slip on the large aftershock fault or its adjacent extension. According to the present case studies, they are not only preseismic slips of the large aftershock but also postseismic slips of the main shock.