Hi All. About 3 years ago Petra and I teamed up to do some research on earthquake prediction. It was then that she introduced me to the Rodgers Creek fault. Almost immediately I could see the potential of a major quake along that fault most particularly in the southern section of the fault.

It was about this time that she started questioning me about the potential of a tsunami along the Bay Area. From the names of some of the towns she gave me I ruled out the potential of a tsunami, but she kept after me. For about a year I couldn’t find anything that would support her vision of a tsunami in the Bay Area. It was also during this time that the name San Gregorio fault started cropping up and it wasn’t very long that we both knew where her vision was. We don’t always see eye to eye, but that is what makes our relationship work. We allow one another to have their own opinions while at the same time supporting one another’s opinion even though we don’t agree.

The following has been taking from two news groups in the Monterey and Santa Cruz area. Take Care…Don in creepy town.

One of the most widely studied faults along the Central Coast is the San Gregorio Fault. Its length is disputed, but many geologists think its southern end may be as far down the coast as San Luis Obispo. There is no controversy, however, that it is a prominent fault zone within the Monterey Bay area.

UCSC Earth Sciences Professor Karen McNally has done intensive study of the San Gregorio fault. Using state-of-the-art seismometers both on land and in the bay, she was able to study the previously unmeasureable slip-rate and small earthquakes. "Ocean surf makes noise on instruments so they have to move them back away from the water," she said. "We used digital recording techniques that make it possible to filter out the wave noise."

One of the major discoveries from this experiment is the San Gregorio might be much more of a threat than scientists expected. "It is a Class A fault, which means it has the potential to create a 7.3 earthquake and has an average slip rate of five millimeters per year," McNally said. "This puts it in the same class as the San Andreas, Hayward, and Calaveras faults."

The San Gregorio fault has not had a large earthquake in over 70 years. "The biggest quakes were thought to be 6.1 in 1926 and somewhere in the middle of the bay," she said.

Another important discovery is the direction of the fault's dipping plane. "It looks likes the San Gregorio dips down to the East," McNally said. "This could pose problems for Santa Cruz, Long Marine Lab, and UC Santa Cruz because it means the fault is closer to them. This has implications for groundshaking closer to campus."

There is a small chance the fault could create a tsunami in the event of a rupture, but it depends on the amount of displacement along the fault, not the amount of shaking.

The final step of the studies expanded the findings to a historic perspective. Gerry Simila used the results of the new velocity model, in combination with data on master (most accurately located) events captured by the offshore and onshore arrays and past seismic data from the University of California, Berkeley, and California Institute of Technology, to "relocate" the largest earthquakes in the bay since 1926. Simila’s analysis showed that all the episodes examined (ranging in magnitude from 4-6.1) occurred along either the San Gregorio or Monterey Bay fault zones.

"We suspected a high degree of uncertainty in the past estimates of event locations, so we’re not surprised to see that this analysis has shifted some locations many kilometers," Simila commented. In the case of two closely spaced, major earthquakes in 1926, the relocations moved the events from an area of no known faults to one in each of the known fault zones.

"What’s really intriguing is the revelation of recent activity, including some of the largest events, within the Monterey Bay Fault Zone, which crosscuts the Monterey Peninsula. Previously only the San Gregorio was thought to be active," Stakes noted. "This analysis also shows that although much activity occurred on the northern San Gregorio west of Santa Cruz, there were no historic events along the southern section in Monterey Bay. One could conclude that the southern half of the San Gregorio fault has been ‘locked’ in recent time, and stress on it may be building."

"This was the situation on the San Andreas system, where there were no earthquakes for the 50 years preceding the 1906 event," McNally added. "However, a future event expected on the San Gregorio would be smaller—perhaps 7.3 in magnitude." The California Division of Geology and Mines recently upgraded the San Gregorio to a "Class A" fault, indicating its potential for a major earthquake.

The results of the studies also have shed light on the nature of the earthquake mechanisms for each of the fault zones. "From initial runs of the new velocity model, it appears that motion along the Monterey Bay faults occurs as strike-slip movement on a vertical fault plane," McNally explained. "In contrast, activity on the northern San Gregorio appears to be a combination of thrust faulting and strike-slip movement; that is, the fault plane dips to the east and thrusts under the continental margin." Knowing the types of motions occurring along the faults will help the scientists to understand the magnitudes of the forces at work beneath the seafloor and to reconstruct past geological history.