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##### U.S. Geological SurveyMiscellaneous Field Studies Map MF-21961992

# Map of Recently Active Traces of the Hayward Fault, Alameda and Contra Costa Counties, California

### *By* James J. Lienkaemper

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| Introduction

The purpose of this map (pl. 1) is to show the locationof and evidence for recent movement on active faulttraces within the Hayward Fault Zone. The mapped tracesrepresent the integration of three different types of data:(1) geomorphic expression, (2) creep (aseismic faultslip), and (3) trench exposures. The location of themapped area is shown on figure 1 on plate 1.

A major scientific goal of this mapping project was tolearn how the distribution of fault creep and creep ratevaries spatially, both along and transverse to the fault.The results related to creep rate are available inLienkaemper and others (1991), and are not repeatedhere. Detailed mapping of the active fault zone contributesto a better understanding of the earthquake sourceprocess by constraining estimates of: (1) the probablerecurrence times of major earthquakes, (2) the size ofexpected surface displacements, and (3) the expectedlength of ruptures accompanying these earthquakes.Now that the 1989 Loma Prieta earthquake has occurredon the San Andreas Fault, the Working Group onCalifornia Earthquake Probabilities (1990) considers theHayward Fault the most probable source of a major earthquake(magnitude 7 or larger) in the San Francisco Bayregion in the next few decades.

This map also is of general use to engineers and landuseplanners. The traces shown on the map are those thatcan be expected to have the most intense ground rupturingfrom fault slip in future major earthquakes on theHayward Fault. However, the small scale of the map(1:24,000) does not provide sufficient details of thelocal complexities of the fault zone for site developmentpurposes. The term "recently active fault trace" isdefined here as a fault trace that has evidence of movementin the Holocene or approximately the last 10,000years. This definition also satisfies the legal definitionof active fault used in the implementation of the Alquist-Priolo Act of 1972 (Hart, 1990).

This map provides a starting point for planners byshowing locations of fault creep and trench exposures ofactive traces. However, minor fault traces may not berecognized because many sections of the fault werealready urbanized by 1939, the date of the earliest aerialphotography for the entire fault. Thus, geomorphicfeatures indicative of active faulting have been degradedor destroyed by human activity, especially secondarytraces that have minor cumulative slip. For this reason,subsurface investigations will continue to be the mainmethod of recognizing and precisely locating active faultstrands in sites that lack reliable creep evidence.Mapping of creep evidence and monitoring of fault creepcan be the most definitive methods to precisely locateactive traces. Because important subsurface and creepmonitoring investigations are now in progress orplanned, this map must be considered an interim reportof data available on January 1, 1992. |
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Note: Note: MF2196 has been superceded by USGS Data Series 177. The map was originally printed with two colors. This site allows download of grayscale scanned images of the original paper map. Some variation from the original 1:24,000 scale may result from the scanning of a non-scale-stable materials.

For questions about the content of this report, contact Jim Lienkaemper

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 | U.S. Geological Survey | Privacy Statement | Disclaimer | Accessibility |  URL of this page: https://pubs.usgs.gov/mf/1992/2196/  Maintained by: Michael Diggles  Created: March 6, 2006  Last modified: March 7, 2006 (mfd)