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Poster #190, Seismology

Quantifying earthquake source parameter uncertainties associated with local site effects using a dense array

Hilary Chang, Rachel E. Abercrombie, Nori Nakata, Colin N. Pennington, Kilian B. Kemna, Elizabeth S. Cochran, & Rebecca M. Harrington
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Poster Presentation

2021 SCEC Annual Meeting, Poster #190, SCEC Contribution #11244 VIEW PDF
We investigate the influence of local site effects on earthquake source parameter estimates using the LArge-n Seismic Survey in Oklahoma (LASSO). The LASSO array consisted of 1825 stations in a 25 km x 32 km region with extensive wastewater injection and recorded more than 1500 local events (M < 3) during spring 2016. We analyze the site amplification dependence on earthquake corner frequency (fc), seismic moment (M0), and stress drop estimated by modeling individual spectra. We evaluate and correct these site effects and compare the effectiveness of the correction to results using the spectral ratio method.
We estimate local site amplification at each station using the average Pea...
k-Ground-Velocity (PGV) of 14 regional earthquakes (~130 km away). The fc from the single spectrum method negatively correlates with site amplification, whereas M0 from the single spectrum method positively correlates with site amplification. This suggests the source parameters calculated by modeling individual spectra are biased by the local site effects. The high amplifications are typically located on young alluvial sedimentary deposits. We correct site effects by removing the trend between PGV and these two parameters in the regression analysis, which reduces the standard deviation of these parameters across the array and makes the calculated stress drop less site dependent. We compare corrections using other site-effect proxies such as the Root-Mean-Square (RMS) amplitude, surface geological formation, P-arrival-delay, and topographic slope. The PGV and the RMS corrections provide the greatest reduction of the spatial deviation of source parameters. In comparison, the spectral ratio method effectively removes the site effects using the Empirical Green’s Function (EGF) approach. The trends being removed by EGF are close to the apparent trends between the single spectrum estimated parameters and the PGV, which suggests the consistency of these different correction approaches. Our results provide a potential way to remove the site effects when only the main event spectrum is available and demonstrates the effectiveness of using the EGF approach for removing site effects. The resulting inter-station variability provides an estimate of the likely uncertainty in source parameters estimated from smaller numbers of stations.