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INTRODUCTION

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Trenching Evidence of Active Faulting

[image of trench log][image of trenches on golf course]

Exploratory trenches provide a valuable means to precisely locate and establish the recency of movement of particular fault traces. Locations of trenches are shown on the maps as thin blue lines and each is labeled using codes (T) and abbreviated descriptions of the evidence found in the trench. ABBREVIATED REFERENCES are used on the map. Full references are in the PAMPHLET.

Most trenches across the Hayward fault were excavated to meet the requirements of the Alquist-Priolo Earthquake Fault Zoning Act, to insure that structures are not built over active fault traces. Other trenches have been used by scientists to learn about the fault's capability as a source of earthquakes. Earthquake geologists probe in to the fault's past behavior using the tools of paleoseismology.

PALEOSEISMOLOGY: Paleoseismologists, geologists who trench across faults, document evidence of paleoearthquakes, prehistoric earthquakes large enough to rupture the fault at the ground surface. Using radiocarbon analysis to date these past earthquakes, scientists have shown that these large earthquakes occur roughly every 100 to 200 years on the Hayward fault.

 Recurrence interval, time between large earthquakes (click image to download 0.3 Mb PDF )
[recurrence diagram]
 Hayward Fault Paleoseismology research papers:

 1) Northern Hayward fault, past 2000 yr
      [Hayward Fault Paleoearthquake Group, 1999]

 2) Southern Hayward fault, past 500 yr
         [Lienkaemper et al., 2002]

 3) Southern Hayward fault, past 1650 yr [in progress, see version 060105 above]
           [Lienkaemper et al., in press 2006, abstract]

image of trench being dug
  Visit a trench site

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Comments to Jim Lienkaemper (jlienk@usgs.gov)

                         (Last updated July 30, 2007 )