U.S. Geological Survey
Open-File Report 96-705

Database of Potential Sources For Earthquakes
Larger than Magnitude 6 in Northern California

By

The Working Group on Northern California Earthquake Potential

The Northern California Earthquake Potential (NCEP) working group, composed of many contributors and reviewers in industry, academia and government, has pooled its collective expertise and knowledge of regional tectonics to identify potential sources of large earthquakes in northern California. We have created a map and database of active faults, both surficial and buried, that forms the basis for the northern California portion of the national map of probabilistic seismic hazard. The database contains 62 potential sources, including fault segments and areally distributed zones. The working group has integrated constraints from broadly-based plate tectonic and VLBI models with local geologic slip rates, geodetic strain rate, and microseismicity. Our earthquake source database derives from a scientific consensus that accounts for conflict in the diverse data. Our preliminary product, as described in this report brings to light many gaps in the data, including a need for better information on the proportion of deformation in fault systems that is aseismic.

INTRODUCTION

The working group began in late 1994, initiated by the regional program coordinator of the National Earthquake Hazards Reduction Program of USGS for the Northern California region, to assist in the preparation of National Seismic Hazard Maps [Frankel and others, 1996]. To meet the requirements for national maps of probabilistic seismic hazard, we set out to develop a map and database of potential sources of magnitude >6 earthquakes for the San Andreas fault system and northeastern California faults, using all available geologic and geophysical data north of 36¡ N and west of 120¡W. McCrory [1996] has reported on the database for the Cascadian subduction zone separately. In later stages of the review process, we merged these USGS databases with a similar one prepared by the California Division of Mines and Geology (CDMG) [Petersen and others, 1996a,b]. It is this jointly developed CDMG-USGS database that was used to create both the USGS national and the CDMG state hazard maps. In turn these hazard maps form the basis for the design-ground-motion maps of the 1997 edition of the National Hazard Reduction Program Recommended Provisions for Seismic Regulations for New Buildings. To assure public acceptance of resulting hazard maps and building codes, a scientific consensus was required on key parameters of potential earthquake sources such as magnitude and recurrence. The working group includes geologists and geophysicists from government, academia and industry who have worked with USGS toward a consensus fault model, which is a reasonable simplification of abundant, diverse scientific data. The model weights historical and paleoseismologic data heavily when available, otherwise accepted empirical scaling relationships are used. Regional strain data and plate tectonic models agree well with geologically measured slip rates on major faults, so regional hazard estimates are likewise well-constrained. In the sections that follow, we will first discuss how in general we determined the key parameters in the database, then we present our consensus results for each major fault zone or region, and finally we will discuss the internal consistency of our results in comparison to broader regional models such as Nuvel-1A, VLBI

More information about earthquake probabilties in Northern California.

This report consists of 27 pages of text , 6 tables, and 13 figures

For printing, download the complete report as a PDF document (requires Adobe Acrobat Reader).

For GIS applications, download the map files (DOS version also available).

____________________________________

This report is also available from

USGS Information Services, Box 25286
Federal Center, Denver, CO 80225
Telephone: 303-202-4210; Email: infoservices@usgs.gov

More information on ordering products from the U.S. Geological Survey