Seismic-Wave Attenuation Determined from Tectonic Tremor in Multiple Subduction Zones

Abstract

Tectonic tremor provides a new source of observations that can be used to constrain the seismic attenuation parameter for ground-motion prediction and hazard mapping. Traditionally, recorded earthquakes of magnitude ∼3-8 are used to develop ground-motion prediction equations; however, typical earthquake records may be sparse in areas of high hazard. In this study, we constrain the distance decay of seismic waves using measurements of the amplitude decay of tectonic tremor, which is plentiful in some regions. Tectonic tremor occurs in the frequency band of interest for ground-motion prediction (i.e., ∼2-8 Hz) and is located on the subducting plate interface, at the lower boundary of where future large earthquakes are expected. We empirically fit the distance decay of peak ground velocity from tremor to determine the attenuation parameter in four subduction zones: Nankai, Japan; Cascadia, United States-Canada; Jalisco, Mexico; and southern Chile. With the large amount of data available from tremor, we show that in the upper plate, the lower crust is less attenuating than the upper crust. We apply the same analysis to intraslab events in Nankai and show the possibility that waves traveling from deeper intraslab events experience more attenuation than those from the shallower tremor due to ray paths that pass through the subducting and highly attenuating oceanic crust. This suggests that high pore-fluid pressure is present in the tremor source region. These differences imply that the attenuation parameter determined from intraslab earthquakes may underestimate ground motion for future large earthquakes on the plate interface.

Magnitudes and moment‐duration scaling of low‐frequency earthquakes beneath southern Vancouver Island

“…These values range between −1.0 and 0.6 log 10 units. Figure shows a histogram of all path correction terms that displays a distribution skewed to larger values as previously reported by Yabe et al [] for tremor data.…”

“…It is conceivable that strong along‐path attenuation might produce pulse broadening similar to that in Figure if LFE corner frequencies for the range of nominal magnitudes M W =1.0–2.6 were markedly higher than the observation pass band of 1–8 Hz. We consider this possibility unlikely since (i) seismograms of intraslab events with overlapping and longer paths retain significant energy to frequencies of 40 Hz or more and (ii) attenuation modeling of tremor and intraslab events in Cascadia and other subduction zones has shown that the latter are more severely attenuated than the former due to their additional transit through a low‐ Q layer at the top of the subducting plate [ Yabe et al , ].…”

“…using a nominal value of ( Q S ∼650) based on the tremor studies of Baltay and Beroza [] and Yabe et al [] for S waves and assume Q P = Q S (or ) for P waves. The unit moment tensor is represented by the average double couple mechanism for LFE templates on southern Vancouver Island reported by Royer and Bostock [].…”

“…Tremor data in northern Cascadia were exploited by Baltay and Beroza [] to quantify peak ground acceleration and velocity and develop an anelastic attenuation parameter for that region. Yabe et al [] extended this work to Nankai, Mexico, and Chile. They demonstrated that lower crust in the overriding plate is more attenuating than upper crust and is underlain by a highly attenuating, subducted oceanic crust.…”

Temporal variation of tectonic tremor activity in southern Taiwan around the 2010 ML6.4 Jiashian earthquake

“…Because the amplitude of tremor is generally small and varies only slightly with size [ Vidale et al ., ], we set a threshold of 500 nm/s to separate tremor from earthquake signals. This empirical amplitude threshold is based on visual examinations of tremor waveforms during the 6 month (January 2010 to June 2010) period, resulting in an average of 251 ± 136 nm/s, similar to the amplitudes from another study [ Yabe et al ., ]. After applying the amplitude criterion, we rejected another 2640 events.…”