NR 2004-32
October 13, 2004

Contact: Anita Gore
Ed Wilson
Mark Oldfield
Don Drysdale
(916) 323-1886


SACRAMENTO – Data collected by the California Department of Conservation during the September 28 magnitude 6.0 Parkfield earthquake has yielded some surprising results that ultimately may help scientists and engineers improve public safety.

Acting State Geologist Michael Reichle, head of DOC’s California Geological Survey, said: “We’re getting some very interesting data, a change in the typical patterns we see in earthquakes. A lot of people in the scientific community are going to be excited about this information.”

Only a small portion of the data collected from 44 CGS and 10 United States Geological Survey stations scattered around the Monterey County village – instruments that have been waiting since 1982 for the San Andreas fault to rumble -- has been analyzed.

“So far, the data shows that there’s more complexity in the near-fault area than we ever could have guessed,” said Anthony Shakal, Supervising Geologist for CGS’ Strong Motion Instrumentation Program. “This means that with careful analysis, we can improve our understanding of ground shaking near the fault, which in turn could lead to changes in the building codes and engineering design practices.”

Three interesting results have been noted so far.

First, the earthquake ruptured to the northwest, stopping at Middle Mountain, rather than to the southeast, as the last two earthquakes of similar magnitude in that area had done.

“We anticipated the earthquake, which is why we had all those instruments there for all those years,” Shakal said. “We knew it would come; we just had to be patient. But the direction it took, for some reason, was different than expected, and that’s something that will have to be studied closely.”

Second, the data shows that the shaking beyond 10 kilometers (6.25 miles) from the fault was actually less than predicted by formulas currently used to develop ground motions for engineering designs. CGS also observed less-than-expected shaking at distance in the San Simeon earthquake last December and in the 1992 Landers event.

If this pattern holds up, it may mean designs can be less stringent at some distances, saving money without reducing safety. That’s the importance of actual strong shaking measurements, Shakal noted: In this case they may show that there’s a greater margin of safety in the range beyond about 20 miles from an earthquake than was previously believed.

Finally, and most significantly, there was an anomaly in the measured peak acceleration or movement.

Shaking occurred at about .3 G – about a third the force of gravity – in Parkfield, which is about six miles northwest of the epicenter and within a half-mile of the main trace of the San Andreas. The local schoolhouse moved back and forth about seven inches with the ground, but retained its structural integrity, according to Moh Huang, a civil engineer with CGS.

Meanwhile, both northwest and southeast of the village of 18 people, stations measured shaking that was three times as intense as in Parkfield.

“There seems to be no simple way to explain this in terms of the mechanics of the earthquake,” Shakal said. “Perhaps there’s some geologic effect at work here, such as the soil formation or deep geology underneath the town.”

Shakal pointed out that there was a similar but reversed phenomenon during the magnitude 6.7 Northridge earthquake in 1994: Shaking measured at a hill in the city of Tarzana was much higher than that in surrounding areas closer to the epicenter.

“Generally, there’s a huge difference in ground shaking amplitude depending on whether you’re in front or behind the direction of the rupture,” said Shakal, noting that the phenomenon is sometimes compared to the well known Doppler effect. “It’s sort of like the difference between looking down the barrel of a gun and being behind it.

“But in Parkfield, there was tremendous ground shaking both in front of and behind the direction of the rupture. And the shaking was significantly lower than in nearby areas. Obviously, we need to continue to expand our knowledge of how these things work in order to be able to plan and design for this kind of thing in future earthquakes.”

Among the other things the California Geological Survey hopes to learn from the recent Parkfield quake is the speed at which the fault rupture spread. Seismologists from the Strong Motion Instrumentation Program are digitizing and analyzing the film records taken from the Parkfield array to calculate both the ground velocity and displacement (movement) at each station. The recorded data will be used in the computer models to determine the fault rupture velocity. The rupture velocity is important in understanding and predicting strong motion.

The presence of the instrument array – a line of accelerographs along the fault and three lines extending perpendicular to the fault (picture a backwards E) – will allow CGS scientists to gather an unprecedented amount of information. Only one strong motion instrument that recorded the Loma Prieta earthquake of 1989 was near the fault, compared to the 54 at Parkfield.

The entire Parkfield area is a basically an earthquake laboratory for not only the California Geological Survey, but also the USGS and other scientific and educational entities. The self-proclaimed “Earthquake Capital of the World,” Parkfield has experienced earthquakes in the magnitude 5.5-6 range every couple of decades going back to 1857. However, last month’s temblor was a late arrival; the previous quake in that magnitude range occurred in 1966.

“Those instruments have been faithfully waiting for this earthquake,” Shakal said. “Because of careful planning and anticipation, this is the best-measured earthquake rupture process ever. We’ll be able to see details of the rupture that we’ve never seen before and begin to get an understanding of the variability of ground motions near a fault.”