Overview

This periodically updated catalog of earthquake locations is computed from a simultaneous re-analysis of 27 years (1984-2011) of the digital seismic archive of Northern California. Waveform cross correlation (CC) and double-difference (DD) methods are used to improve the resolution in the existing earthquake catalog generated at the Northern California Seismic Network (NCSN) by up to three orders of magnitude. The catalog is based on 1.7 billion CC differential time measurements with correlation coefficients, Cf>=0.7 from all correlated pairs of events that are separated by less than 5 km (see Schaff and Waldhauser, 2005 for details). These data, combined with 11.4 million arrival time picks from the NCSN bulletin, were inverted for the precise relative locations of 512,059 events using the hypoDD algorithm (see Waldhauser and Schaff, 2008). Click here for a summary of the results.

Correlation characteristics and location improvements 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. These results indicate that consistent long-term seismic monitoring and data archiving practices are key to increase the resolution in existing hypocenter catalogs, and to estimate the precise location of future events on a routine basis.

Location Improvement

The figures below show epicenters before (NCSN, left subpanel) and after (DD, right subpanel) relocation for four regions of different tectonic origin. Red dots are events that were added to the most recent catalog update. Click on figures for full-size view.

Location Stats

The figures below show distributions of error estimates and location differences to previous DD catalog and original locations. Click on figures for full-size view.

Distribution of relative location errors estimated from bootstrap resampling analysis.

Location differences between corresponding events in the 2011 and 2009 DD catalogs.

Location differences between corresponding events in the 2011 DD and the 2011 network (NCSN) catalogs.

Correlation Stats

The figures below show the distribution of correlated events and cross-correlation delay time measurements. Click on figures for full-size view.

Percentage of correlated events within bins of 20 x 20 km across Northern California. About 91% of all earthquakes with waveforms have correlated P- or S-wave trains at common stations.

Year-over-year increase in the number of correlated events, for various tectonic regions.

Year-over-year increase in the number of correlation based delay time measurements (in billions).

References:

Waldhauser, F. and D.P. Schaff (2008), Large-scale relocation of two decades of Northern California seismicity using cross-correlation and double-difference methods, J. Geophys. Res., 113, B08311, doi:10.1029/2007JB005479. [PDF]

Schaff, D.P. and F. Waldhauser (2005), Waveform cross-correlation-based differential travel-time measurements at the Northern California Seismic Network, Bull. Seism. Soc. Am., 95, 2446-2461. [PDF]

Download Catalog:

READ this file first before downloading/using the 2011 DD catalog: README
Download the 2011 DD catalog in ASCII format: NCAeqDD.v201112.1

Go to older catalogs: 2003 | 2008 | 2009.

Contact and Future Updates:

Contact: Felix Waldhauser (felixw@ldeo.columbia.edu)
Updates: If you wish to receive an email when future updates become available please send email to felixw@ldeo.columbia.edu and include "NCAeqDD UPDATE" in the subject line.