The results will be presented in the Friday, Nov. 16, 2001 issue of Science in the article, "Steep-Dip Seismic Imaging of the Shallow San Andreas Fault near Parkfield," by Virginia Tech professor John Hole, Rufus Catchings of the U.S. Geological Survey (USGS), Virginia Tech student K. C. St. Clair, Michael Rymer of USGS, David Okaya of the University of Southern California, and Virginia Tech graduate B.J. Carney, now at Columbia Natural Resources, Inc. of West Virginia.
The researchers learned more about the properties of the rocks near the surface in the fault area, and that the fault line deep underground is not directly under surface trace line, says Hole, geological scientist at Virginia Tech.
The study site -- halfway between Los Angeles and San Francisco and a 45-minute drive plus a short hike from the nearest highway -- is where a future Big Science project is planned. Scientists have proposed drilling a hole through the SAF. "We are looking down about 1 kilometer (km) or .625 miles -- not very deep. "Our primary survey goal is to look at near-surface geologic structures to help plan the drilling. We also want to image and better understand the fault," says Hole.
Seisometers record echoes, such as from a blast, to reveal how seismic waves, similar to sound, move through rock. Seismologists can interpret such attributes as wave speed and positions of rock boundaries. Being used as a basic research tool for the first time at an active fault site was a computer-based technique for collecting and processing energy reflections from underground vertical structures. The reflection imaging system was developed by the petroleum industry to locate oil pooled along the steep, vertical sides of salt domes. Hole worked with the U.S. Geological Survey to collect the field data and received funding from the National Science Foundation (NSF) to process the reflection image at the San Andreas Fault.
The reflection images Hole captured, published in Science, are the first such captured and published of an active vertical fault. The pictures are helping geoscientists understand the subsurface.
In the seismic image, the most important geology is immediately to the west of the fault, an area previously discovered to have high electrical conductivity. The new seismic image revealed that the same volume of rock has low seismic velocities. "Most likely, the rock has a lot of spaces -- 10 to 30 percent porosity -- filled with salt water."
The porous rock continues below the area of measurement. "The high conductivity, low velocity of the nearby rock tells us something is weak, broken. It's too bad we couldn't see deeper, where the earthquakes are," says Hole.
The reflection image included high-resolution vertical lines corresponding perfectly with the SAF, Hole says. "Because of the high resolution result, we are learning something geologically interesting. The reflector is vertical in the upper half kilometer (0.5 km), then slopes steeply to the southwest. This is interesting because the location of earthquake faults in the subsurface are not precisely known. The earthquakes define a vertical plane that does not quite line up with the surface trace but, by drawing a thick line representing errors, seismologists define the fault as still being somewhat under the surface trace. The reflector image suggests the fault has enough of a bend to the southwest that it is several hundred meters southwest of the surface trace," Hole says.
"Actual misalignment of the surface with the deeper fault seems unlikely as the surface rock is too weak to generate an earthquake and should break vertically above the stronger, deep fault. However, the reflection image of the actual deep fault line makes a difference if you are trying to drill into it," Hole said.
Scientists know where the fault is because it is actively moving. It is slipping an inch per year. A decades-old, well-photographed fence line that is cut by the fault has a zag of four feet.
A 1966 earthquake emboldened scientists to predict another major earthquake in 1988, give or take five years. The prediction was based on six previous major quakes that were about 22 years apart. As a result, a great deal of equipment has been placed at the site. The predicted earthquake hasn't happened, but the site is well studied. (The major Colinga earthquake in 1985 -- only 30 miles away along a previously unknown fault -- may have released the pressure.)
Because the site has been so well instrumented, there is excellent documentation of several small, very shallow earthquakes -- magnitude 1-2, 3 km down -- that repeat every one or two years.
"That is why this site was selected for a proposal to drill through a fault and record what happens in a bore hole. The plan is to drill 2 km down from 1.5 km away then angle toward the fault through a small, repeating quake site. The researchers working on that project will measure samples along the path of the drill to understand the physics of the site," says Hole.
Hole has received NSF funding to repeat his measurements to a depth of 5 km (more than three miles) -- below the depth of the drilling target (by the time they hit the fault, they’ll be 3.5 to 4km down). That will require a 50 km or 31 mile line on the surface to give the sound waves elbow room to echo and reflect off a vertical surface 5 km underground.
"The easiest surfaces to image are those boundaries that face the source of energy -- in this cases, horizontal surfaces under the ground," Hole says. "However, some faults are vertical. To measure a seismic echo or reflection off a vertical fault, energy waves have to turn, bounce off the surface, and turn again on the way back to the seismic receiver. Energy waves will bend as they pass through different materials, such as when light passes through a prism. The geometry of refraction seismology requires great distances, knowledge of the energy speed to image reflections , and understanding of the geology."
The San Andreas Fault Observatory at Depth (SAFOD) is part of Earthscope, a project that has received NSF and National Research Council approval. SAFOD is phase one, but is not yet funded.
Contact Dr. Hole at 540-231-3858 or hole@vt.edu
He will be out of town and unavailable on Thursday, November 15, 2001.
He will check his phone mail and e-mail regularly.
Rufus Catchings and Michael Rymer of USGS
U. S. Geological Survey, MS 977, 345 Middlefield Road, Menlo Park, CA 94025
catching@usgs.gov, 1- 650-329-4749, fax 1- 650-329-5163
mrymer@usgs.gov, 1- 650-329-5649, fax 1- 650-329-5163
David Okaya of the University of Southern California
Department of Earth Sciences, Los Angeles, CA 90089
okaya@earth.usc.edu, 1- 213-740-7452, fax 1- 213-740-0011
Virginia Tech graduate B.J. Carney, now at Columbia Natural Resources Inc. of West Virginia
Columbia Natural Resources Inc., 900 Pennsylvania Ave., Charleston, WV
bjcarney@nisource.com, 1-304-353-5000, fax 1-304-353-5217
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Science