Hi All. Almost all quakes occur in the following manner. There are some exceptions, but for the most part this is what takes place before, during and after a quake.

These five stages arise from the Elastic Rebound Theory. This section will describe each of the five stages of an earthquake. The information in this section was taken from the books by Rikitake [1976], Meyer [1977], Eiby [1980], Bolt [1988], and Bolt [1993].

Stage I of an earthquake is the buildup of elastic strain. As the two sides of a fault move, elastic strain slowly builds up in the rocks, and the rock particles become compressed together.

Stage II is dilatancy and development of cracks. The rocks are now packed as tightly as possible, and the only way the rocks can change shape is to expand and occupy a larger volume. This increase in volume is called dilatancy. The volume increase is caused by the formation of microcracks. As microcracks form, the water that normally fills the pores and cracks in the rocks is forced out, much like when you step on wet beach sand. Air now fills the pores and cracks in the rocks. During this process, the rocks become stronger and can store more elastic strain. This process can be detected on the surface by uplift and tilting of the ground.

Stage III is the influx of water and unstable deformation in the fault zone. During this stage, water is forced back into the pores and cracks in the rocks by the surrounding water pressures, much like when water fills the footprint in the sand. As the water returns, the dilatant rock loses its increased strength. The rocks are already strained beyond their normal capacity, and the rate at which the rocks fall in strength determines the instant of failure. The inflow of water also prevents further generation of microcracks; thus, the rocks stop expanding. In addition, the water in the rocks provides lubrication for the eventual release of the built-up strain.

Stage IV is fault rupture, or the earthquake. Eventually, the rocks can no longer resist the strain. The fault suddenly ruptures, producing an earthquake. When the fault ruptures, the elastic energy stored in the rocks is released as heat and seismic waves. It is these waves that constitute an earthquake.

Stage V is the sudden drop in stress followed by aftershocks. Most of the elastic strain energy is released by the principal earthquake; however, additional smaller ruptures occur producing aftershocks. The aftershocks release the remaining strain energy, and eventually the strain in the region decreases and stable conditions return.

The whole process then starts all over again. As a rule it’s not in the same area, but in the area near the ends of the rupture.

Looking at the Bay Area in regards to the San Andreas Fault we can see where the next logical location for a major quake to occur. By knowing the past history of quakes in the immediate area we can get an idea as to where the next major quake could occur.

The Santa Cruz Mountain earthquake (October 8tht 1865) struck the Bay Area including the Santa Cruz County at 12:45pm PST. The 6.6 magnitude quake centered in the Santa Cruz Mountains along the San Andreas Fault caused more damaged to the San Francisco area and destroyed about 0.5 million dollars in structural property damage in the Santa Cruz county. Fortunately there was no personal injury that resulted in deaths.

The Great Earthquake of 1906: in the San Francisco Bay Area goes down in history as one of the most significant earthquakes in the history of California. With a magnitude of 8.3, which lasted for more that one minute, this quake was felt from southern Oregon to south of Los Angeles and inland as far as central Nevada. The total length of the ruptured extended about 430 kilometers and affected many towns along the San Andreas Fault line. It is estimated that 3,000 people died as a result of the quake and caused 524million in property damage. One of the many counties affected was Santa Cruz County; the quake set off landslides in the Santa Cruz Mountains claiming the lives of the lives of 8 mill workers and several other lives totaling about a dozen deaths. Two cities in Santa Cruz County, (Watsonville and Santa Cruz city) just 16-32 km from the epicenter had the highest percentage of damages in the county. The life of the county of interrupted for weeks, roads had collapsed, buildings torn down, power failure reached many areas, and fires started by the quakes ripped through many areas including Santa Cruz destroying many building.

Earthquake in the Santa Cruz Mountain November 8, 1914: This quake occurred at 6:31pm, and lasted 4minutes located near the crest of the Santa Cruz range in the small town Laurel. The quake was felt in Santa Rosa in Sonoma County to the southeast in Soledad in Monterey, and as far east as the San Joaquin valley. One of the highest intensity was in Watsonville area of Santa Cruz County, which measured IV on the Rossi-Forel scale. Fortunately this quake did not cause much damage or resulted in deaths or injuries.

The Loma Prieta earthquake occurred on Tuesday, October 17th, 1989 at 5:04 pm PST. With a magnitude of 7.1 it was the largest earthquake to shake Santa Cruz County since the great earthquake of 1906. The rupture occurred on a 30mile length of the San Andreas Fault called the Santa Cruz Segment in the Santa Cruz Mountains, near Loma Peak at a depth of 11.5 miles underground.

When we see where past quakes have occurred we can see that the next location for a major quake would be the area between Portola Valley and the San Francisco Peninsula. At present time there is very little movement being seen in this area giving an indication that this segment of the fault is locked. This may seem like a contradiction in terms, but as long as this segment remains locked there won’t be a major quake in this area. However as with anything else it can’t stay locked forever.

There are other faults in the Bay Area that fall into the same category. These are the Rodgers Creek fault and the southern segment of the Hayward fault just to name a few. The Bay Area is an interesting place to live in. It could be even more interesting in years to come. Take Care…Don in creepy town
Reference:

http://homepage.smc.edu/morris_pete/california/atlas2/cgatlassantacruz2.html

http://elvis.engr.wisc.edu/uer/uer96/author3/