Using a Time‐Based Subarray Method to Extract and Invert Noise‐Derived Body Waves at Long Beach, California

Jorge C Castellanos,Alan Juarez,Robert W Clayton

doi:10.1029/2019jb018855

Abstract

AbstractThe reconstruction of body waves from the cross‐correlation of random wavefields has recently emerged as a promising approach to probe the fine‐scale structure of the Earth. However, because of the nature of the ambient noise field, the retrieval of body waves from seismic noise recordings is highly challenging and has only been successful in a few cases. Here, we use seismic noise data from a 5,200‐node oil‐company survey to reconstruct body waves and determine the velocity structure beneath Long Beach, California. To isolate the body wave energy from the ambient noise field, we divide the entire survey into small‐aperture subarrays and apply a modified double‐beamforming scheme to enhance coherent arrivals within the cross‐correlated waveforms. The resulting beamed traces allow us to identify clear refracted P waves traveling between different subarray pairs, which we then use to construct a high‐resolution 3D velocity model of the region. The inverted velocity model reveals velocity variations of the order of 3% and strong lateral discontinuities caused by the presence of sharp geologic structures such as the Newport‐Inglewood fault (NIF). Additionally, we show that the resolution that is achieved through the use of high‐frequency body waves allows us to illuminate small geometric variations of the NIF that were previously unresolved with traditional passive imaging methods.

Highlights

Detailed knowledge of the subsurface velocity structure is essential for predicting ground motion and, earthquake-hazard assessment
We show that the resolution that is achieved through the use of high-frequency body waves allows us to illuminate small geometric variations of the Newport-Inglewood fault (NIF) that were previously unresolved with traditional passive imaging methods
The average velocity structure of this feature has been imaged in previous tomography studies (e.g., Chang et al, 2016; Lin et al, 2013), the resolution that is achieved with high-frequency body waves allow us to illuminate some of its geometric variations

Summary

Introduction

Detailed knowledge of the subsurface velocity structure is essential for predicting ground motion and, earthquake-hazard assessment. Over the past few decades, significant advancements in imaging the Earth's interior have been possible with the advent of ambient noise cross-correlation This method goes beyond the spatial limitations of classical earthquake seismology and uses the Earth's background vibrations (i.e., ambient noise field) recorded at a pair of synchronous seismic stations to reconstruct Green's function between the two stations (Campillo & Paul, 2003; Lobkis & Weaver, 2001; Shapiro & Campillo, 2004). In a recent study, Nakata et al (2015) used a 2,500-station array adjacent to the Long Beach survey to extract body waves and performed the first-ever body wave tomography using pure ambient noise recorded at the ground surface This investigation, used a series of selection filters on individual waveforms to isolate the body wave energy, which, based on a set of quality control criteria, only allowed the use of a small portion of the entire data set (about 35%).

Ambient Noise Correlation and Body Waves

Double Beamforming and Body Wave Extraction

Traveltime Measurements and Body Wave Tomography

Results and Discussion

Conclusions