Paleoseismology and the Hayward Fault
A large, widely damaging earthquake will occur on the Hayward fault in the future. That much we know.
What we don't know is when.
Paleoseismology and the Hayward Fault
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. 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.
On the Hayward fault, perhaps the best and most accessible trench site is at Tule Pond, also known as Tyson's Lagoon, just south of the Fremont BART (Bay Area Rapid Transit) station in Freemont, California. At this site, the 1868 Mw6.8 earthquake is revealed in the trench by intensely-contorted, near-surface layers of silty sand and clay. A series of photos take you through digging of the trench at Tule Pond.
New Technology for Tracking Ancient Earthquakes
Recently, two new techniques have been introduced that supplement the basic trenching method. First,peeling helps preserve the information revealed in the trench by actualy removing the trench face, much the way the paint is sometimes pulled off a wall with old tape. When a peel is removed, the reading of the geologic strata in the trench walls can continue long after the trench is closed.
Next, to extend the depth of a trench study (which extends the historical period available for study), geologists utilize a new device called Geoslicer, invented by Takashi Nakat of Hiroshima University. This device can reveal faulting in the deeper, water-saturated sediments found below the water table, where geologists cannot safely trench.
On Tuesday October 2, 2001 Japanese scientists from the Geological Survey of Japan and Hiroshima University and American scientists from the U. S. Geological Survey removed a peel from one of the walls of a trench excavated the previous week. Using a rubberized epoxy compound, scientists eeled and removed the south face of the trench at Tule Pond. A series of photos take you through this peel process at the Tule Pond site.
Finally, on Wednesday, October 3, at the same location, the Japanese and American scientists demonstrated the Geoslicer, a large, crane-operated device designed and built in Japan. The Geoslicer can cut an additional section 12 feet deep (relative to the bottom of the10-foot-deep trench) into the geologic deposits, and bring it, intact, to the surface for study. The Geoslicer allows the geologists to sample below the water table. By extending the depth, the Geoslicer extends the length of the earthquake history available for study. A series of photos takes you through the work done with theGeoslicer at the Tule Pond site.
The Geoslicer travels across the country
The Japanese team next brought the Geoslicer to the New Madrid seismic zone. In conjunction with the USGS, seven samples from a liquefaction site near Blytheville, Arkansas were pulled out on October 22nd.
Tule Pond Paleoseismology Photo Gallery, September - October 2001
Digging a paleoseismic trench at Tule Pond on the Hayward fault
September 28, 2001
On September 28, 2001, paleoseismologists from the U.S. Geological Survey opened a trench on the southern Hayward fault at a site called Tule Pond, also known as Tyson's Lagoon, just south of the Fremont BART (Bay Area Rapid Transit) station in Fremont, California.
Taking a peel from the wall of the trench
October 2, 2001
On Tuesday October 2, 2001 Japanese scientists from the Geological Survey of Japan and Hiroshima University, along with USGS scientists, generated a peel of the southern trench wall at Tule Pond. By using a rubberized epoxy material, scientists removed a section of the south wall of the trench.
Using the Geoslicer
October 3, 2001
Then, on Wednesday, October 3, again at the Tule Pond location, Japanese and American scientists demonstrated the Geoslicer, a large, crane-operated device. The Geoslicer was placed in the bottom of the existing 10-foot-deep trench, where it cut an additional 12-foot-deep section of geologic deposits, and brought it, intact, to the surface for examination by the scientists.