Quantifying earthquake source parameter uncertainties associated with
local site effects using a dense array
Abstract
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 Peak-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.