Improving Earthquake Locations in Northern California Using Waveform Based Differential Time Measurements

We simultaneously re-analyzed two decades (1984-2003) of the digital seismic

archive of Northern California using waveform cross correlation (CC) and double-difference

(DD) methods to improve the resolution in hypocenter locations in the existing earthquake

catalog generated at the Northern California Seismic Network (NCSN) by up to three orders of

magnitude. We used a combination of ~3 billion CC differential times measured from all

correlated pairs of events that are separated by less than 5 km and ~7 million P-wave arrival time

picks listed in the NCSN bulletin. The data is inverted for precise relative locations of 311,273

events using the DD method. The relocated catalog is able to image the fine-scale structure of

seismicity associated with active faults, and reveals characteristic spatio-temporal structures such

as streaks and repeating earthquakes. We find that 90% of the earthquakes have correlated P- and

S-wave trains at common stations, and 12% are co-located repeating events. An analysis of the

repeating events indicates that uncertainties at the 95% confidence level in the existing network

locations are on average 0.7 km laterally and 2 km vertically. Correlation characteristics and

relative location improvement are remarkably similar across most of Northern California,

implying the general applicability of these techniques to image high-resolution seismicity caused

by a variety of plate-tectonic and anthropogenic processes.

We have developed a real-time procedure (DD-RT) that rapidly relocates new

earthquakes relative to nearby events in the new DD catalog. The DD-RT software currently runs

on a test-bed at Lamont, using near real-time parametric and waveform data feeds from the

NCSN and the NCEDC for new events, and a locally stored archive of seismic data for past

events. We evaluated the performance of the new monitoring system by back-testing it with

events that occurred in the past. This work demonstrates that consistent long-term seismic

monitoring and data archiving practices, as followed at the NCSN and NCEDC, are key to

increase resolution in existing hypocenter catalogs, and to estimate the precise location of future

events on a routine basis.