California, renowned for its mild climate, sunny beaches, and vibrant economy, lives under a serious geological threat, bordering the largest fault in America, the San Andreas. It represents a transform boundary between tectonic plates, separating the Pacific Plate from the North American Plate. The fault stretches approximately 1,300 kilometers along the western coast of North America from Cape Mendocino in the north to the Mexican border in the south.
The San Andreas Fault raises many questions for seismologists and natural hazard management experts, bringing closer each day the “Big One,” an earthquake that threatens the entire west coast of the United States. The fault’s formation began 30 million years ago when the Pacific and North American plates started shifting at a rate of 56 mm/year, initially forming a subduction zone (the oceanic plate diving beneath the continental plate). After 10 million years, the nature of the plate movement changed β at the boundary, they began to slide horizontally. This so-called transform plate movement led to the formation of the San Andreas Fault.
Characteristics of Transform Faults Seismologists and geophysicists distinguish three main types of faults that form at the junctions of lithospheric plates:
- Normal: when the Earth’s crust stretches, the upper plate slides down onto the lower plate.
- Reverse: occurs when the upper plate is displaced relative to the lower plate due to the compression of two converging lithospheric structures.
- Transform: two plates slide along a horizontal plane without vertical displacement.
In the San Andreas transform zone, unlike other fault types, lateral displacements occur. The Pacific and North American plates move parallel but in opposite directions. This movement causes the transformation in the zone to occur continuously, and the fault remains constantly active. Earthquakes in transform zones occur with the release of accumulated strain energy (like an elastic rebound) and can be very destructive.
The Catastrophic 1906 San Francisco Earthquake On April 18, 1906, at 5:12 a.m., the most devastating seismic event in U.S. history occurred. The earthquake, measuring 7.8 Mw on the Richter scale, consisted of two series of tremors β the first lasted for a minute, the second, more destructive, continued for 25 seconds. In the following hours, strong aftershocks shook the earth’s surface. The epicenter was 12 km from San Francisco, and ground shaking was felt over an area of 400,000 km2 and was noticeable in Nevada and Oregon. As a result of the earthquake, caused by movement along the San Andreas Fault, the Pacific Plate shifted 6 meters relative to the North American Plate in 40 seconds.
Due to the catastrophic 1906 San Francisco earthquake:
- 28,000 buildings were destroyed or severely damaged.
- The city was engulfed in flames for three days, which could not be extinguished due to damaged water mains.
- Railway lines and roads were displaced, hindering the delivery of humanitarian aid and assistance to the disaster area.
- About 5,000 people died, and 400,000 (2/3 of the population) were left homeless. After the disaster, a mass exodus occurred β residents left the city to seek refuge in the surrounding areas.
In addition to surface damage, the earthquake caused structural geological changes, which manifested as ground deformation, changes in the direction of natural water flows, soil liquefaction, and rockfalls in coastal zones.
A Major Earthquake in California is Inevitable Seismologists expect the “Big One” to strike California soon β the dormant San Andreas Fault is beginning to awaken. The section affecting the Los Angeles metropolitan area, with a population of 13 million, has remained quiet for 160 years, since the 1857 Fort Tejon earthquake. According to seismic models developed by the United States Geological Survey, the average recurrence interval for major earthquakes on the fault is 200 years. Therefore, the probability of a seismic event in the San Francisco Bay Area in the coming decades is 68%. While seismologists unfortunately cannot yet pinpoint the precise date of an earthquake, they continuously monitor the San Andreas Fault to understand its ongoing activity and assess potential hazards. The time remaining before a major seismic event presents a crucial window of opportunity to implement technologies and undertake measures that can mitigate damage and save lives. The San Andreas Fault serves as a subject of ongoing scientific investigation and a constant reminder that our planet is dynamic and poses significant risks to populations residing in seismically active zones.