Statistics of Recent Aftershock Sequences in Eastern North America and their Implications for Declustering

Session: Seismicity and Tectonics of Stable Continental Interiors [Poster]
Type: Poster
Date: 4/30/2020
Time: 08:00 AM
Room: Ballroom
Description: 

The spatiotemporal behavior of recent moderate-magnitude (M4.1–5.7) earthquake sequences in intraplate eastern North America is quantified with a simple empirical approach. First, baseline seismicity within ~100 km of the mainshock epicenter and over timescales of decades is described in terms of Gutenberg-Richter a- and b-values. Post-mainshock a-values are next computed over finer radius bins and timescales ranging from days to a few years, then normalized by catalog duration and area. The difference represents aftershock productivity as a function of space and time, and the ratio gives the empirical probability that a given event is an aftershock. Omori decay parameters can easily be computed for a chosen aftershock area, but this work finds that the region of elevated seismicity rates is not stable in time. Instead, the affected radius decays with a typical halving-time between several months and two or three years for the events studied. Once fully fleshed out, these results may have two implications for declustering of intraplate catalogs. First, aftershock regions about a mainshock of a given magnitude appear to initially be slightly smaller than, for example, given by the Gardner-Knopoff relation; to quickly shrink well below Gardner-Knopoff radii; and to persist longer at these small distances, with rates often increased by a factor of ~0.5 to 5 for several years within ~5–15 km of the epicenter. Second, it is not necessary to make a binary choice between aftershock and independent event. The time- and distance-dependent ratio of background to post-mainshock rates yields a probability that each event should be removed that can be propagated through probabilistic hazard assessments and forecasts. Such empirical region- or even sequence-specific treatment could aid in optimizing declustering protocols for stable and active regions and for natural and induced earthquake sequences.

Presenting Author: Will Levandowski

Authors