High-precision, absolute earthquake location based on waveform similarity between events and application to imaging foreshocks, fault complexity and damage zones for recent western US earthquakes.

We present a high-precision, absolute earthquake location procedure (NLL-SSST-coherence) based on waveform similarity between events and using the probabilistic, global-search NonLinLoc (NLL) location algorithm. NLL defines a posterior probability density function (PDF) in 3D space for absolute hypocenter location and invokes the equal differential-time (EDT) likelihood function which is very robust in the presence of outlier data. For NLL-SSST-coherence location we take initial NLL locations and iteratively generate smooth, 3D, source-specific, station travel-time corrections (SSST) for each station and phase type and an updated set of locations. Next, we greatly reduce absolute location, aleatoric error by combining location information across events based on waveform coherency between the events. This absolute coherency relocation is based on the concept that if the waveforms at a station for two or more 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. The NLL coherency relocation for a target event is a stack over 3D space of the event’s SSST location PDF and the SSST PDF’s for other similar events, each weighted by the waveform coherency between the target event and the other event. Absolute coherency relocation requires waveforms from only one or a few stations, allowing precise relocation for sparse networks, and for foreshocks and early aftershocks of a mainshock sequence or swarm before temporary stations are installed.

We apply the NLL-SSST-coherence procedure to the Mw5.8 Lone Pine CA, Mw5.7 Magna UT and Mw6.4 Monte Cristo NV earthquake sequences in 2020 and compare with other absolute and relative seismicity catalogs for these events. The NLL-SSST-coherence relocations generally show increased organization, clustering and depth resolution over other absolute location catalogs. The NLL-SSST-coherence relocations reflect well smaller scale patterns and features in relative location catalogs, with evidence of improved depth precision and accuracy over relative location results when there are no stations over or near the seismicity.

For all three western US sequences in 2020 the NLL-SSST-coherence relocations show mainly sparse clusters of seismicity. We interpret these clusters as damage zones around patches of principal mainshock slip containing few events, larger scale damage zone and splay structures around main slip patches, and background seismicity reactivated by stress changes from mainshock rupture. The Monte Cristo Range seismicity (Lomax 2020) shows two, en-echelon primary slip surfaces and surrounding, characteristic shear-crack features such as edge, wall, tip, and linking damage zones, showing that this sequence ruptured a complete shear crack system. See presentation EGU21-13447 for more details.

Lomax (2020) The 2020 Mw6.5 Monte Cristo NV earthquake: relocated seismicity shows rupture of a complete shear-crack system. Preprint: https://eartharxiv.org/repository/view/1904