Delaney Park Geotechnical Array Dynamic Properties Inferred from the Magnitude 7.1 2018 Anchorage, Alaska, Earthquake Sequence

Abstract

We used waveforms from the 2018 MW 7.1 Anchorage earthquake and six selected aftershocks, ranging from MW 4.2 to 5.7, to quantify site properties including shear-wave velocity profile, predominant frequencies, borehole amplification, soil-damping ratio, and shear modulus at Delaney Park in downtown Anchorage, Alaska. The waveforms were recorded by surface and six borehole (up to 61-m depth) three-component accelerometers. The deconvolution of the waveforms at various borehole depths on horizontal sensors with respect to the corresponding waveform at the surface provides incident and reflected traveling waves within the soil column. The shear-wave velocities determined from these events are consistent, and generally agree well with the in situ measurements. The borehole amplification based on surface-to-borehole traditional standard spectral ratio (SSR) and surface-to-borehole response spectral ratio (RSR) and cross-spectral ratio (c-SSR) were also evaluated. Based on c-SSR, we computed the borehole amplification as 4.8 at 1.35 Hz (0.74 s), close to the predominant frequency of the soil column.