High-precision earthquake location using source-specific station terms
and inter-event waveform similarity
Abstract
Earthquake monitoring and many seismological studies depend on
earthquake locations from phase arrival-times. We present an extended,
arrival-time earthquake location procedure (NLL-SSST- coherence) which
approaches the precision of differential-timing based, relative location
methods and is applicable with few seismic stations. NLL-SSST-coherence
is based on the probabilistic, global-search NonLinLoc (NLL) location
algorithm which defines a probability density function (PDF) in 3D space
for hypocenter location and is highly robust to outlier data.
NLL-SSST-coherence location first reduces velocity model error through
iteratively generated, smooth, source-specific, station travel-time
corrections (SSST). Next, arrival-time error is reduced by consolidating
location information across events based on inter-event waveform
coherency. If the waveforms at a station for multiple events are very
similar (have high coherency) up to a given frequency, then the distance
separating these “multiplet” events is small relative to the seismic
wavelength at that frequency. NLL-coherence relocation for a target
event is a stack over 3D space of the NLL-SSST location PDF for the
event and the PDF’s for other multiplet events, each weighted by its
waveform coherency with the target. NLL-coherence relocation requires
waveforms from only one or a few seismic stations, enabling precise
relocation with sparse networks, for foreshocks and early aftershocks of
significant events before installation of temporary stations, and for
older data sets with few waveform data. We show the behavior and
performance of NLL-SSST-coherence with synthetic and ground-truth tests,
and through application and comparison to relative locations for
California earthquake sequences with dense and sparse station coverage.