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Higher Earthquake Intensity Attenuation Rates in the Urbanized Southern Puget Lowland Than Elsewhere Along the Cascadia Forearc

Thomas M. Brocher

Published August 15, 2016, SCEC Contribution #6875, 2016 SCEC Annual Meeting Poster #280

The attenuation of seismic intensity with distance for 15 magnitude M4.8 to M6.8 earthquakes between 1949 and 2015 shows significant variation along the Cascadia forearc. Felt intensities for these earthquakes were taken from NOAA’s U.S. Earthquake Intensity Database (1635-1985), USGS Open-File Reports, and a 2015 Did You Feel It? report. I follow the approach of Bakun et al. (2002), and fit linear intensity-attenuation functions to the median distances at which the earthquakes were reported felt. With the exception of the coefficients of the distance-dependent term, I used the same form-of-equation and constants as used by those authors. Attenuation rates for 4 M5.1 to M5.8 crustal earthquakes in the Cascadia forearc south of Puget Lowland are well-resolved and lie between 0.018 and 0.023 ± 0.002 MMI/km, close to that determined by Bakun et al. (2002) and Bakun and Wentworth (1997) for crustal earthquakes west of the Cascades and in California. In contrast, poorly resolved attenuation rates for 4 M4.8 to M5.2 crustal earthquakes within Puget Lowland lie between 0.032 and 0.038 ± 0.005 MMI/km, 45 to 72% larger. Attenuation rates for 5 M4.8 to M6.8 inslab earthquakes beneath Puget Lowland and the southeastern end of Vancouver Island are well-resolved and show no dependence on magnitude: they range between 0.0205 and 0.030 ± 0.002 MMI/km, intermediate between these two crustal rates. The attenuation rate for one inslab event, the M5.8 1999 Satsop earthquake located to the west of the Lowland, was noticeably higher at 0.034 ± 0.003 MMI/km. Although magnitude dependence of the attenuation rates may explain some of the observed variations, well-resolved higher attenuation rates of felt intensities for 3 M5.4 to M6.8 crustal and inslab earthquakes in southern Puget Lowland are associated with the presence of large glaciated Cenozoic basins and with the northward thinning of the Crescent Formation, a unit having high seismic velocities and presumably high Q.

Brocher, T. M. (2016, 08). Higher Earthquake Intensity Attenuation Rates in the Urbanized Southern Puget Lowland Than Elsewhere Along the Cascadia Forearc . Poster Presentation at 2016 SCEC Annual Meeting.

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