Hi All. About 2 years ago Petra told me about a dream she had and I told her I didn’t think this would be possible. In this dream she saw a tsunami hitting the West Coast somewhere near Monterey to San Francisco and that the tsunami had been caused by a nearby fault. This fault is most likely the San Gregorio fault. At that time there wasn’t any information on the web that covered the San Gregorio fault to any extent. That has now changed.
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.
One of the major discoveries 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. 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.
Another important discovery is the direction of the fault's dipping plane. It looks likes the San Gregorio dips down to the East. This could pose problems for Santa Cruz, Long Marine Lab, and UC Santa Cruz because it means the fault is closer to them.
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. There is also the chance that a tsunami could occur from an undersea landslide caused by a small earthquake. There is some evidence that this may have happened in 1859 24 September at Half Moon Bay when the waters of the bay receded 15 feet and returned suddenly. The earthquake was intensity IV Modified Mercalli scale. According to the scale this is the description of an IV earthquake. (IV. During the day felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, and doors disturbed; walls make creaking sound. Sensation like heavy truck striking building. Standing motorcars rock noticeably.) On the Richter Magnitude scale this would be between a 2.8M and 3.4M. As you can see the quake itself was not strong enough to cause a tsunami, but could have caused a landslide, which in turn caused the tsunami.
The Monterey Bay Canyon, less than a mile off the shore of Moss Landing between Santa Cruz and Monterey, has sides sloping sharply down nearly two miles. Such steep drop-offs create ample opportunities for underwater landslides. Additionally, Monterey Bay itself is bisected by two faults, the Monterey Bay fault and the San Gregorio fault.
Now I’ll say it so Petra doesn’t have to “I told you so”. Take Care…Don in creepy town.