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October 8, 2008 > Letter to the Editor: Are mapped Hayward Fault zones accurate?

Letter to the Editor: Are mapped Hayward Fault zones accurate?

We live in the East Bay, one of the most seismically active regions in the world. Scientists say the Hayward fault or any of its many splays are expected to crack the earth's surface within the next 30 years. When the break occurs, homes lying on the fault trace could be shattered. The location of the expected ground breaks could be in the flat plain extending out from the base of the East Bay Hills from about Warm Springs in Fremont to Suisun Bay north of Pinole.

Because of the very real possibility of ground rupture, in 1979, the state of California established earthquake fault zones. By state law, if a new home is proposed in one of these fault zones, a registered geologist must examine a trench excavated across the property and look for the presence or absence of a fault trace.

This required subsurface exploration has led to the discovery of previously unrecognized fault traces by geologists as they investigate for new homes and other buildings. Trenches explored by personnel with Clark GeoTechnical of Fremont over the past few years in the Cities of Fremont and Hayward and as far north as Pinole have discovered active fault traces that up until now were unknown. In some cases, the newly found fault traces were outside of the state's designated fault zones.

Discoveries of new active faults have led Alameda County to expand the fault-zone boundary in at least one area and to look closely at other areas. The expansion of the fault zones means some buildable lots that were not required by state law to be investigated for active fault traces now must be explored by a geologist. The County requirement has added new expense to building near the fault but is necessary to prevent building on an active fault trace. Anyone considering building in the East Bay should consult city or county planning agencies or a geotechnical firm in advance to find out if their property is in the state's fault zone or in an indistinct and un-published fault zone set by local authorities.

Michael Clark

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