Advancing the ETAS Model to Improve Forecasts of Earthquake Sequences and Doublets
Authors: Grimm Christian , Hainzl Sebastian , Käser Martin , Pagani Marco , Küchenhoff Helmut 
Published: March 2022

&lt;p&gt;&lt;span&gt;Earthquake sequences typically show distinct spatiotemporal patterns, characterized by a power-law decay&lt;/span&gt;&lt;br&gt;&lt;span&gt;of aftershock times and elongated aftershock distributions around the (extended) rupture.&lt;/span&gt; &lt;span&gt;A prominent approach to&lt;/span&gt;&lt;br&gt;&lt;span&gt;model seismic clustering in space and time is the&lt;/span&gt; &lt;span&gt;Epidemic Type Aftershock Sequence (ETAS)&lt;/span&gt; &lt;span&gt;model that differentiates&lt;/span&gt;&lt;br&gt;&lt;span&gt;an independent background seismicity process from a branching tree process for triggered events.&lt;/span&gt; &lt;span&gt;The conventional&lt;/span&gt;&lt;br&gt;&lt;span&gt;ETAS&lt;/span&gt; &lt;span&gt;approach&lt;/span&gt; &lt;span&gt;shows&lt;/span&gt; &lt;span&gt;three&lt;/span&gt; &lt;span&gt;substantial&lt;/span&gt; &lt;span&gt;biases:&lt;/span&gt; &lt;span&gt;(1)&lt;/span&gt; &lt;span&gt;The&lt;/span&gt; &lt;span&gt;assumption&lt;/span&gt; &lt;span&gt;of&lt;/span&gt; &lt;span&gt;isotropic&lt;/span&gt; &lt;span&gt;spatial&lt;/span&gt; &lt;span&gt;distributions&lt;/span&gt; &lt;span&gt;of&lt;/span&gt; &lt;span&gt;aftershock&lt;/span&gt;&lt;br&gt;&lt;span&gt;locations stands in contrast to observations and geophysical models for large mainshocks.&lt;/span&gt; &lt;span&gt;(2) The unlimited spatial&lt;/span&gt;&lt;br&gt;&lt;span&gt;distribution&lt;/span&gt; &lt;span&gt;allows&lt;/span&gt; &lt;span&gt;small&lt;/span&gt; &lt;span&gt;events&lt;/span&gt; &lt;span&gt;to&lt;/span&gt; &lt;span&gt;trigger&lt;/span&gt; &lt;span&gt;aftershocks&lt;/span&gt; &lt;span&gt;at&lt;/span&gt; &lt;span&gt;unrealistically&lt;/span&gt; &lt;span&gt;large&lt;/span&gt; &lt;span&gt;distances.&lt;/span&gt; &lt;span&gt;(3)&lt;/span&gt; &lt;span&gt;Short-term&lt;/span&gt; &lt;span&gt;incomplete&lt;/span&gt;&lt;br&gt;&lt;span&gt;event records after large mainshock events suggest supposedly smaller aftershock productivity and cluster sizes.&lt;/span&gt; &lt;span&gt;The&lt;/span&gt;&lt;br&gt;&lt;span&gt;above&lt;/span&gt; &lt;span&gt;biases&lt;/span&gt; &lt;span&gt;can&lt;/span&gt; &lt;span&gt;lead&lt;/span&gt; &lt;span&gt;to&lt;/span&gt; &lt;span&gt;an&lt;/span&gt; &lt;span&gt;underestimation&lt;/span&gt; &lt;span&gt;of&lt;/span&gt; &lt;span&gt;the&lt;/span&gt; &lt;span&gt;aftershock&lt;/span&gt; &lt;span&gt;productivity&lt;/span&gt; &lt;span&gt;of&lt;/span&gt; &lt;span&gt;strong&lt;/span&gt; &lt;span&gt;events,&lt;/span&gt; &lt;span&gt;and&lt;/span&gt; &lt;span&gt;in&lt;/span&gt; &lt;span&gt;consequence&lt;/span&gt; &lt;span&gt;to&lt;/span&gt;&lt;br&gt;&lt;span&gt;underpredicted cluster sizes, and to a miss-specification of the spatial aftershock distribution in the case of clearly ex-&lt;/span&gt;&lt;br&gt;&lt;span&gt;tended ruptures. Here, we combine an ETAS-Incomplete model, accounting for short-term aftershock incompleteness,&lt;/span&gt;&lt;br&gt;&lt;span&gt;with an ETAS approach applying anisotropic,&lt;/span&gt; &lt;span&gt;spatially restricted distributions of aftershock locations.&lt;/span&gt; &lt;span&gt;We evaluate&lt;/span&gt;&lt;br&gt;&lt;span&gt;the benefits of these models by running forecast experiments for the 2019 Ridgecrest sequence and analyzing the oc-&lt;/span&gt;&lt;br&gt;&lt;span&gt;currence frequencies of so-called&lt;/span&gt; &lt;span&gt;Earthquake Doublets&lt;/span&gt;&lt;span&gt;,&lt;/span&gt; &lt;span&gt;i.e.,&lt;/span&gt; &lt;span&gt;sequences of two or more similarly strong earthquakes&lt;/span&gt;&lt;br&gt;&lt;span&gt;within a small time-space window. The new model provides more realistic sequence forecasts and doublet predictions&lt;/span&gt;&lt;br&gt;&lt;span&gt;and might be of particular interest for (short term) risk assessment units.&lt;/span&gt;&lt;/p&gt;