Fault Investigation in Western Washington Using 2D Ambient Noise Tomography

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

Geophysical investigations were carried out at Centralia, WA, to delineate geological structure associated with an major fault. The geophysical methods include seismic reflection, active and passive surface wave methods. This paper summarizes data acquisition and analysis of 2D passive surface wave data. Thirteen cableless seismic data acquisition units with vertical component 2 Hz geophones were used to record ambient noise data. Each unit includes a GPS clock so that all units can be synchronized over any distance without cables. Data acquisition mainly used a linear array with geophone spacing of approximately 100 m and four geophones were moved up forward every 45 min. along approximately 2500 m length lines. Two T-shaped arrays were also measured to confirm the omni directional propagation of ambient noise. Total data acquisition took 2 days. Recorded ambient noise data were processed using the common midpoint spatial autocorrelation method (CMP-SPAC) and clear dispersion curves were obtained at all CMP locations. Minimum frequency ranges from 0.4 to 0.7 Hz and maximum frequency ranges from 5 to 15 Hz depending on the CMP. Dispersion curves obtained from the linear array are generally consistent with those obtained from T-shaped arrays. Non-linear inversion was performed and 2D S-wave velocity (VS) models were obtained. The method penetrated to a depth of 1500 m and provided 2500 m cross sections along the survey line. The interpretation focused on identifying the change of bedrock depth associated with the fault. Resultant VS profiles are generally consistent with seismic reflection method and existing geological information.

Presenting Author: Recep Cakir

Authors