Andrew J. Michael
Andrea L. Llenos
2020
<p><span>We use an epidemic‐type aftershock sequence (ETAS) based approach to develop a regionally optimized background earthquake rates from ETAS (ROBERE) method for probabilistic seismic hazard assessment. ROBERE fits parameters to the full seismicity catalog for a region with maximum‐likelihood estimation, including uncertainty. It then averages the earthquake rates over a suite of catalogs from which foreshocks and aftershocks have been removed using stochastic declustering while maintaining the same Gaussian smoothing currently used for the U.S. Geological Survey National Seismic Hazard Model (NSHM). The NSHM currently determines these rates by smoothing a single catalog from which foreshocks and aftershocks have been removed using the method of </span><a class="link link-ref link-reveal xref-bibr" data-open="rf12">Gardner and Knopoff (1974</a><span>; hereafter, GK74). The parameters used in GK74 were determined from subjectively identified aftershock sequences, unlike ROBERE, in which both background rate and aftershock triggering parameters are objectively fitted. A major difference between the impacts of the two methods is GK74 significantly reduces the </span><i><span class="inline-formula no-formula-id"><span id="MathJax-Element-1-Frame" class="MathJax" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi xmlns="">b</mi></math>"><span id="MathJax-Span-1" class="math"><span><span id="MathJax-Span-2" class="mrow"><span id="MathJax-Span-3" class="mi">b</span></span></span></span></span></span></i><span>‐value, a critical value for seismic hazard analysis, whereas ROBERE maintains the original </span><i><span class="inline-formula no-formula-id"><span id="MathJax-Element-2-Frame" class="MathJax" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi xmlns="">b</mi></math>"><span id="MathJax-Span-4" class="math"><span><span id="MathJax-Span-5" class="mrow"><span id="MathJax-Span-6" class="mi">b</span></span></span></span></span></span></i><span>‐value from the full catalog. We apply these methods to the induced seismicity in Oklahoma and Kansas and tectonic activity in the San Francisco Bay Region. Using GK74 gives lower overall earthquake rates but estimates higher hazard due to the reduction in the </span><i><span class="inline-formula no-formula-id"><span id="MathJax-Element-3-Frame" class="MathJax" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi xmlns="">b</mi></math>"><span id="MathJax-Span-7" class="math"><span><span id="MathJax-Span-8" class="mrow"><span id="MathJax-Span-9" class="mi">b</span></span></span></span></span></span></i><span>‐value. ROBERE provides higher earthquake rates, at the magnitude of completeness, but lower hazard because it does not alter the </span><i><span class="inline-formula no-formula-id"><span id="MathJax-Element-4-Frame" class="MathJax" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi xmlns="">b</mi></math>"><span id="MathJax-Span-10" class="math"><span><span id="MathJax-Span-11" class="mrow"><span id="MathJax-Span-12" class="mi">b</span></span></span></span></span></span></i><span>‐value. We test two other declustering methods that produce results closer to ROBERE but do not use objectively fit parameters, include uncertainty, and may not work as well in other areas. We suggest adopting ROBERE for the NSHM so that our hazard estimates are based on an objective analysis, including uncertainty, and do not depend strongly on potentially biased </span><i><span class="inline-formula no-formula-id"><span id="MathJax-Element-5-Frame" class="MathJax" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi xmlns="">b</mi></math>"><span class="MJX_Assistive_MathML">b</span></span></span></i><span>‐values, which was never the goal of the existing methodology.</span></p>
application/pdf
10.1785/0120190279
en
Seismological Society of America
Regionally Optimized Background Earthquake Rates from ETAS (ROBERE) for probabilistic seismic hazard assessment
article