High-precision, absolute earthquake location using source-specific
station terms and inter-event waveform similarity
Abstract
Earthquake monitoring and many seismological studies depend on absolute
earthquake locations from phase arrival-times. We present an absolute
earthquake location procedure (NLL-SSST-coherence) which approaches the
precision of waveform-based, relative location 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 absolute 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, absolute
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.