John O. Langbein
Noha Sameh Ahmed Farghal
Andrew J. Barbour
2021
<p><span>Dynamic strains have never played a role in determining local earthquake magnitudes, which are routinely set by displacement waveforms from seismic instrumentation (e.g., </span><span class="inline-formula no-formula-id"><span id="MathJax-Element-1-Frame" class="MathJax" data-mathml="<math xmlns=">M<sub>L</sub></span><sub></sub></span><span>). We present a magnitude scale for local earthquakes based on broadband dynamic strain waveforms. This scale is derived from the peak root‐mean‐squared strains (</span><span class="inline-formula no-formula-id"><span id="MathJax-Element-2-Frame" class="MathJax" data-mathml="<math xmlns=">A</span></span><span>) in 4589 records of dynamic strain associated with 365 crustal earthquakes and 77 borehole strainmeters along the Pacific‐North American plate boundary on the west coast of the United States and Canada. In this data set, catalog moment magnitudes range from </span><span class="inline-formula no-formula-id"><span id="MathJax-Element-3-Frame" class="MathJax" data-mathml="<math xmlns=">3.5≤<i>M</i><sub>w</sub><span id="MathJax-Span-9" class="math"><span><span id="MathJax-Span-10" class="mrow"><span id="MathJax-Span-16" class="mo">≤</span><span id="MathJax-Span-17" class="mn">7.2</span></span></span></span></span></span><span>, and hypocentral distances range from </span><span class="inline-formula no-formula-id"><span id="MathJax-Element-4-Frame" class="MathJax" data-mathml="<math xmlns=">6≤<i>R</i>≤500 km</span></span><span>. The 1D representation of geometrical spreading and attenuation of </span><span class="inline-formula no-formula-id"><span id="MathJax-Element-5-Frame" class="MathJax" data-mathml="<math xmlns=">A</span></span><span> common to all strain data is </span><span class="inline-formula no-formula-id"><span id="MathJax-Element-6-Frame" class="MathJax" data-mathml="<math xmlns=">log<i>A<sub>0</sub></i>(<i>R</i>)=−0.00072R−1.45log(R)</span></span><span>. After correcting for instrument gain, site terms, and event terms, the magnitude scale, </span><span class="inline-formula no-formula-id"><span id="MathJax-Element-7-Frame" class="MathJax" data-mathml="<math xmlns="><i>M</i><sub>DS</sub>=log <i>A</i>−log <i>A</i><sub>0</sub>(<i>R</i>)−log(3×10−9)</span></span><span>, scales as </span><span class="inline-formula no-formula-id"><span id="MathJax-Element-8-Frame" class="MathJax" data-mathml="<math xmlns=">≈0.92<i>M</i><sub>w</sub></span></span><span> with a residual standard deviation of 0.19. This close association with </span><span class="inline-formula no-formula-id"><span id="MathJax-Element-9-Frame" class="MathJax" data-mathml="<math xmlns="><i>M</i><sub>w</sub></span></span><span> holds for events east of the −124° meridian; west of this boundary, however, a constant correction of 0.41 is needed to adjust for additional along‐path attenuation effects. As a check on the accuracy of this magnitude scale, we apply it to dynamic strain records from three strainmeters located in the near field of the 2019 </span><strong><span class="inline-formula no-formula-id"><span id="MathJax-Element-10-Frame" class="MathJax" data-mathml="<math xmlns=">M</span></span></strong><span> 6.4 and 7.1 Ridgecrest earthquakes. Results from these six records are in agreement to within 0.5 magnitude units, and five out of six records are in agreement to within 0.34 units.</span></p>
application/pdf
10.1785/0120200360
en
Seismological Society of America
Earthquake magnitudes from dynamic strain
article