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  Improving Earthquake Doublet Frequency Predictions by Modified Spatial Trigger Kernels in the Epidemic-Type Aftershock Sequence (ETAS) Model

Grimm, C., Käser, M., Hainzl, S., Pagani, M., Küchenhoff, H. (2022): Improving Earthquake Doublet Frequency Predictions by Modified Spatial Trigger Kernels in the Epidemic-Type Aftershock Sequence (ETAS) Model. - Bulletin of the Seismological Society of America, 112, 1, 474-493.
https://doi.org/10.1785/0120210097

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 Creators:
Grimm, Christian1, Author
Käser, Martin1, Author
Hainzl, S.2, Author              
Pagani, Marco1, Author
Küchenhoff, Helmut1, Author
Affiliations:
1External Organizations, ou_persistent22              
22.1 Physics of Earthquakes and Volcanoes, 2.0 Geophysics, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum, ou_146029              

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 Abstract: Earthquake sequences add a substantial hazard beyond the solely declustered perspective of common probabilistic seismic hazard analysis. A particularly strong driver for both social and economic losses are so‐called earthquake doublets (more generally multiplets), that is, sequences of two (or more) comparatively large events in spatial and temporal proximity. Without differentiating between foreshocks and aftershocks, we hypothesize three main influencing factors 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 with observations in historical catalogs. In combination with that the simulated catalogs show a smoother spatiotemporal clustering compared with the observed counterparts. Focusing on the impact on direct aftershock productivity and total cluster sizes, we propose two modifications of the ETAS spatial kernel 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 Båth’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 log‐likelihood values, which would favor the conventional, unrestricted models.

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 Dates: 2021-09-072022
 Publication Status: Finally published
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 Identifiers: DOI: 10.1785/0120210097
GFZPOF: p3 PT4 Natural Hazards
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Title: Bulletin of the Seismological Society of America
Source Genre: Journal, SCI, Scopus
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Pages: - Volume / Issue: 112 (1) Sequence Number: - Start / End Page: 474 - 493 Identifier: CoNE: https://gfzpublic.gfz-potsdam.de/cone/journals/resource/journals59
Publisher: Seismological Society of America (SSA)