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Improving earthquake doublet frequency predictions by modified spatial trigger kernels in the ETAS model

Grimm, Christian, Martin Käser, Sebastian Hainzl, Marco Pagani, and Helmut Küchenhoff (2021), Improving earthquake doublet frequency predictions by modified spatial trigger kernels in the ETAS model, BSSA, in press.

Abstract
Earthquake sequences add a substantial hazard beyond the solely declustered
perspective of common probabilistic seismic hazard analysis (PSHA). A particularly
strong driver for both social and economic losses are so-called earthquake doublets
(more generally multiplets), i.e. sequences of two (or more) comparatively large events
in spatial and temporal proximity. Without differentiating between foreshocks and
aftershocks, we hypothesize three main drivers of doublet occurrence: (1) the number
of direct and secondary aftershocks triggered by an earthquake; (2) the occurrence of
independent clusters and seismic background events in the same time-space window;
and (3) the magnitude size distribution of triggered events (in contrast to independent
events). We tested synthetic catalogs simulated by a standard epidemic type
aftershock sequence (ETAS) model for both Japan and Southern California. Our
findings show that the common ETAS approach significantly underestimates doublet
frequencies compared to observations in historical catalogs. In combination with that,
the simulated catalogs show a smoother spatiotemporal clustering compared to the
observed counterparts. Focusing on the impact on direct aftershock productivity and
total cluster sizes, we propose two modifications of the ETAS spatial kernel in order to
improve doublet rate predictions: (a) a restriction of the spatial function to a maximum
distance of 2.5 estimated rupture lengths; and (b) an anisotropic function with contour
lines constructed by a box with two semicircular ends around the estimated rupture
segment. These modifications shift the triggering potential from weaker to stronger
events and consequently improve doublet rate predictions for larger events, despite
still underestimating historic doublet occurrence rates. Besides, the results for the
restricted spatial functions fulfill better the empirical Bath's law for the maximum
aftershock magnitude. The tested clustering properties of strong events are not
sufficiently incorporated in typically used global catalog scale measures, such as loglikelihood
values, which would favor the conventional, unrestricted models.
BibTeX
@article{id2701,
  author = {Christian Grimm and Martin K{\"a}ser and Sebastian Hainzl and Marco Pagani and Helmut K{\"u}chenhoff},
  journal = {BSSA},
  note = {in press},
  title = {{Improving earthquake doublet frequency predictions by modified spatial trigger kernels in the ETAS model}},
  year = {2021},
}
EndNote
%0 Journal Article
%A Grimm, Christian
%A Käser, Martin
%A Hainzl, Sebastian
%A Pagani, Marco
%A Küchenhoff, Helmut
%D 2021
%J BSSA
%T Improving earthquake doublet frequency predictions by modified spatial trigger kernels in the ETAS model
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Printed 04. Dec 2021 08:04