A Feasibility Study on Monitoring Crustal Structure Variations by Direct Comparison of Surface Wave Dispersion Curves from Ambient Seismic Noise

K Muhumuza

doi:10.1155/2020/5269537

Abstract

This work assesses the feasibility of the direct use of surface-wave dispersion curves from seismic ambient noise to gain insight into the crustal structure of Bransfield Strait and detect seasonal seismic velocity changes. We cross-correlated four years of vertical component ambient noise data recorded by a seismic array in West Antarctica. To estimate fundamental mode Rayleigh wave Green’s functions, the correlations are computed in 4-hr segments, stacked over 1-year time windows and moving windows of 3 months. Rayleigh wave group dispersion curves are then measured on two spectral bands—primary (10–30 s) and secondary (5–10 s) microseisms—using frequency-time analysis. We analyze the temporal evolution of seismic velocity by comparing dispersion curves for the successive annual and 3-month correlation stacks. Our main assumption was that the Green’s functions from the cross-correlations, and thus the dispersion curves, remain invariant if the crustal structure remains unchanged. Maximum amplitudes of secondary microseisms were observed during local winter when the Southern Ocean experiences winter storms. The Rayleigh wave group velocity ranges between 2.1 and 3.7 km/s, considering our period range studied. Interannual velocity variations are not much evident. We observe a slight velocity decrease in summer and increase in winter, which could be attributed to the pressure melting of ice and an increase in ice mass, respectively. The velocity anomalies observed within the crust and upper mantle structure correlate with the major crustal and upper mantle features known from previous studies in the area. Our results demonstrate that the direct comparison of surface wave dispersion curves extracted from ambient noise might be a useful tool in monitoring crustal structure variations.

Highlights

Passive seismic methods as tools for seismological studies provide excellent opportunities to use continuously recorded seismic noise data, eliminating the need for earthquakes or artificial sources
We investigate whether Rayleigh-wave group dispersion curves based on ambient noise surface wave data can be compared directly to monitor seasonal variations in seismic velocities. is is on the assumption that if there are no changes, noise measurements performed on the same earth medium at different seasons yield the same dispersion characteristics
We used vertical component continuous seismic data from 4 stations in the Argentine Antarctica region from Antarctic Seismographic Argentinean Italian Network (ASAIN) (Antarctic Seismographic Argentine-Italian Network) in West Antarctica (Figure 1). e stations Carlini—formerly Jubany (JUBA)—station, Esperanza (ESPZ), Orcada (ORCD), and San Martin (SMAI) are part of the 5 seismological stations that have been set under ASAIN collaborative agreement. e seismologic network provides essential data to study the internal structure of the Antarctic continent, monitor seismicity, and to monitor large-scale phenomena, such as melting of the Antarctic ice sheet, among others

Summary

Introduction

Passive seismic methods as tools for seismological studies provide excellent opportunities to use continuously recorded seismic noise data, eliminating the need for earthquakes or artificial sources. E seismic noise wavefield between a pair of receivers is estimated by creating a virtual source at a receiver location In principle, both surface waves and body waves can be extracted from the cross correlation of noise. Because noise continuously illuminates the subsurface, and we can reconstruct waves with virtual sources using only receivers, this technique is useful to detect temporal variations of the earth structure. With this methodology, one obtains some advantages for subsurface monitoring that include repeatability, avoiding the uncertainty in earthquake source locations and origin times, cancelling the complexity of wave propagation to a virtual source, and using measurements (ambient noise) unusable with active source methods. We obtain insight into the feasibility of the direct use of Rayleigh wave dispersion curves to study the crustal structure of the Bransfield Strait and monitor seasonal velocity variations

Data and Measurement Methods

Estimation of Green’s Function

Seasonal Variations of Microseismic Signal

Group Velocity Dispersion Measurements

Velocity Variations

Conclusions