Variation in Seismic-Wave Attenuation Along the Cascadia Subduction Zone Determined from Tectonic Tremor
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Many subduction zones worldwide are known to host devastating large earthquakes, such as the 2011 M9 Tohoku-Oki earthquake. In addition to fast, seismic slip many subduction zones also host slow, largely aseismic slip. These “slow earthquakes” occur on timescales of weeks to months and are often accompanied by a weak seismic signal known as “tectonic tremor,” or simply “tremor.” Tremor behaves differently than regular earthquakes in that it is comprised of many small earthquakes that radiate low-frequency seismic energy and originate at the plate interface downdip of where large earthquakes typically occur. This behavior is thought to reflect variation in frictional properties, effective stress, or both in between the aseismic and seismic sections of the plate interface. Seismic-wave attenuation is a parameter that quantifies the decrease in amplitude of seismic waves as a function of distance from the earthquake source. Estimates of attenuation are commonly used in ground-motion prediction equations (GMPEs) that quantify ground motion during an earthquake. Because tremor occurs frequently when compared to regular earthquakes in Cascadia, it presents an opportunity to better define attenuation parameters used for GMPEs in earthquake engineering. Our goal is to quantify seismic wave attenuation in Cascadia and determine its spatial variations using tectonic tremor. By inverting tremor ground motion data for the attenuation parameter, we can analyze if and how the results vary spatially in Cascadia and attempt to relate these variations to lithology and/or other physical properties. Changes in seismic-wave attenuation along the Cascadia Subduction Zone could result in significantly different ground motions in the event of a very large earthquake, hence quantifying attenuation may help to better estimate the severity of shaking in densely populated metropolitan areas such as Vancouver, Seattle and Portland.