DATA AND METHODS
This catalog is based on a simultaneous re-analysis of 25 years (1984-2008) of the digital seismic archive of Northern California. Waveform cross correlation (CC) and double-difference (DD) methods are used 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 measured about 50 billion CC differential times with Cf>=0.6 (6.2 billion with Cf>=0.7) from all correlated pairs of events that are separated by less than 5 km (see Schaff and Waldhauser, 2005 for details) and combined them with 10 million P-wave arrival time picks listed in the NCSN bulletin. The data was inverted for precise relative locations of 445,493 events using the DD algorithm (for details see Waldhauser and Schaff, 2008).

CORRELATION RESULTS
About 90% of the earthquakes have correlated P- and S-wave trains at common stations, and 12% are co-located repeating events. Correlation characteristics and 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. These results indicate that consistent long-term seismic monitoring and data archiving practices are key to increase resolution in existing hypocenter catalogs, and to estimate the precise location of future events on a routine basis.

RELOCATION RESULTS
The relocated catalog is able to image the fine-scale structure of seismicity associated with active faults, and reveals spatio-temporal characteristics such as streaks and repeating earthquakes. Median relative location uncertainties are ~30 m for events constrained with correlation data and ~70 m for those constrained by phase picks only.

Figures showing epicenters before (NCSN, left) and after (DD, right) relocation demonstrate the location improvment achieved over existing locations. Click on thumbnail for full-size figure. Events in red are the ones added to this update.

CATALOG GROWTH
The DD catalog grows at an average annual rate of ~20,000 events and ~500,000 phase picks. The figures to the right show the year-over-year increase in the number of correlated events (right) and the number of correlation measurements (Cf>=0.7) (far right) for various tectonic regions. Click on figure for full size view.

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]

DOWNLOADS:
README file for the 2008 DD catalog : README
Download the 2008 DD catalog in ASCII format: NCAeqDD.v200812.1.1

Click HERE to access/download the old 2003 DD catalog.

FUTURE 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.