Estimating shallow shear-wave velocity profiles in Alaska using the initial portion of P waves from local earthquakes

Abstract

The Alaska Regional Network and Transportable Array provide an invaluable waveform data set for studying ground motions in Alaska. However, the data set is useful only after the site effects at each station are well understood. Considering the large number of stations associated with these networks, it would be onerous to measure the sub-surface velocity structure beneath every station using geophysical exploration techniques involving arrays, such as active-source or passive-source non-invasive array methods. Instead, it is more economical to estimate the site conditions using waveforms recorded at the seismic stations. Most of the methods for estimating site response from recorded waveforms use the frequency-dependent ratio between the horizontal and vertical component of either ambient noise or S waves from earthquakes. We instead use the horizontal and vertical component of P waves to infer the sub-surface velocity structure. It has been demonstrated that the ratio of radial to vertical P waves is mostly sensitive to sub-surface shear velocity. Therefore, the sub-surface velocity structure can be estimated using an approach similar to teleseismic P receiver functions, but at much smaller scale and higher frequency. The results from this method are in good agreement with results from active-source or passive-source non-invasive array methods and have been widely used in the Central Eastern United States. The Alaska Regional Network and Transportable Array have recorded numerous earthquakes in the magnitude range of interest and at shallow depth, which provides an ideal opportunity for this study. VS30 is used to represent the site amplification of ground motions in all ground motion models that are used in generating the US Geological Survey’s National Seismic Hazard Maps, as well as in the professional practice of seismic hazard analysis. The results of this work provide a basis for improved site-specific hazard estimates in Alaska.