HAYWARD -- An observant engineer who spotted an irregularity in the soil at a housing development site more than 10 years ago led to the discovery of a new fault in the Hayward fault earthquake zone.
The 1.4-mile-long Ashland fault is one of two small ones that the state Geological Survey recently added to the Hayward area earthquake fault zone map, which has been updated for the first time since 1982.
The second small newly mapped fault, which is unnamed, is about 1.25 miles long and lies west of Lake Chabot's western shore, said Bill Bryant, California Geological Survey senior engineering geologist, who heads up the state's quake zone mapping program.
Both that fault, found at an old quarry, and the Ashland fault are offshoots of the Hayward fault.
While neither newly discovered fault is likely to cause a major quake and they don't add danger to the already dangerous Hayward fault, their discovery widens the Hayward fault zone and gives scientists a better idea of where seismic activity has occurred.
According to Bryant, the Ashland fault could sustain up to a 4.5 magnitude earthquake; the smaller fault, a 2 or 3.
"Seismically, they won't rupture independently of the Hayward fault with anything significant," Bryant said. Although the quake maps shows many fault traces, those traces are not considered independent of the Hayward fault, he said.
The Hayward fault extends for 62 miles from San Jose to San Pablo Bay through a densely populated area. Scientists warn that the fault is overdue for a major earthquake; the last one was in 1868, with an estimated magnitude of 7.1.
The updated hazard zone map also shows zones prone to liquefaction and landslides in a major temblor -- the first time that all three hazard earthquake zones have been depicted on the same document.
The map is "like your one-stop shopping for all your news about earthquake hazards," said Don Drysdale, a spokesman for the state Geological Survey.
Earthquake hazard zones, including the Hayward one, extend at least 500 feet on each side of active faults, though at some places they are wider than 3,000 feet.
"In those zones, if someone wants to put in a housing development, before the city or county will issue a permit, the developer has to do a geological investigation and determine whether or not active faults exist on the property," Bryant said.
If an active fault is found, buildings must be set back at least 50 feet from the fault.
While 50 feet may not sound very far, "if a fault moves six inches, you can imagine what that can do to a building if it's on top of the fault. If you set it back just 50 feet, that solves a lot of problems," Drysdale said.
Those with existing houses in fault zones also are affected. "The fact that the property is in a fault zone must be disclosed when buying or selling property," Drysdale said.
The state uses topographic maps issued by the U.S. Geological Survey as the base for its fault hazard zone maps. The USGS maps break California into 553 areas, or quadrangles.
"We call a quadrangle a base map. It's typically a topo map showing rivers, mountains, valleys and streams. On those maps, we put our earthquake fault zones and our seismic hazard zones," Bryant said.
The Hayward quadrangle map, which covers 60 square miles, includes much of Hayward, Castro Valley, part of San Lorenzo and small areas in San Leandro and Oakland. The two new faults prompted the state to update the Hayward area map.
The Ashland fault was identified by putting various clues together. In Alameda County, developers of housing subdivisions or commercial buildings are required to have a geotechnical engineer or soils engineer on site during excavation, said John Rogers, engineering staff assistant in the Alameda County Public Works Agency.
In 1999, at an Ashland subdivision site between Marcella Street and Maubert Avenue a few blocks east of Bayfair Mall, a geotechnical engineer saw shifts in the soil. "He was smart enough to see what appeared to be a fault trace," Rogers said.
A geologist was called in and confirmed the fault, and the subdivision had to be redesigned so that buildings were set back from the fault.
Subsequently, at another nearby building site, there were also indications of a fault, Bryant said. Those findings prompted state geologists to look at aerial photos taken of the area in 1946, before Interstate 580 was built, in which they saw evidence of an active fault. Putting the clues together, they mapped the Ashland fault.
"It's helpful when you have earlier photographs," Bryant said.
The survey geologists then worked with Alameda County to review the information before issuing the updated map, one of 13 released this fall. The other maps include areas in Imperial, Riverside, San Diego and Ventura counties.
To see the earthquake fault hazard zone maps for the Hayward quadrangle, go to www.conservation.ca.gov/cgs/rghm/ap/Pages/official_release.aspx.