캘리포니아의 지진 환경: 산안드레아스 단층에서의 삶

Tectonic Setting: The World's Most Famous Fault

California's earthquake hazard is defined above all by the San Andreas Fault, a Strike-Slip FaultA fault where blocks of rock move horizontally past each other. The San Andreas Fault and North Anatolian Fault are major strike-slip faults that produce destructive earthquakes. that runs approximately 1,300 kilometers from the Salton Sea in the south to Cape Mendocino in the north, where it terminates at the Mendocino Triple Junction. The San Andreas marks the Transform BoundaryA plate boundary where two plates slide horizontally past each other. The San Andreas Fault in California is the most famous example of a transform boundary. between the Pacific Plate, moving northwest at roughly 5 centimeters per year, and the North American Plate moving generally southwest. This boundary is not a single clean fracture but rather a complex fault zone with multiple parallel and branching fault strands, including the Hayward, Calaveras, San Jacinto, and Elsinore faults that together form a broad zone of active Transform BoundaryA plate boundary where two plates slide horizontally past each other. The San Andreas Fault in California is the most famous example of a transform boundary. deformation.

The geometry of the San Andreas system creates dramatically different behavior along its length. The creeping segment between Hollister and Parkfield moves aseismically, slipping steadily without building up elastic strain for large earthquakes — Fault CreepThe slow, continuous movement along a fault without generating significant earthquakes. Some sections of the San Andreas Fault creep at 2-3 cm/year. here means the ground surface gradually offsets roads, fences, and buildings over decades. The Locked FaultA section of a fault where friction prevents movement, causing stress to accumulate. When a locked fault finally ruptures, it can produce a major earthquake. segments to the north and south, however, accumulate strain elastically, storing energy that will eventually release in large earthquakes. The southern San Andreas south of Parkfield has not produced a major rupture since the 1857 Fort Tejon earthquake (estimated magnitude 7.9), more than 165 years ago — a Seismic GapA section of an active fault that has not produced an earthquake for a long time compared to neighboring sections. Seismic gaps may indicate increased probability of a future earthquake. that many seismologists regard as overdue.

Historical Seismicity: The Great 1906 Disaster

The 1906 San Francisco Earthquake (estimated magnitude 7.9) remains the most consequential natural disaster in California history and one of the defining events of American urban history. The earthquake ruptured approximately 477 kilometers of the northern San Andreas Fault, producing ground shaking that lasted nearly a minute and caused enormous structural damage. However, the fires that broke out after the quake — fed by ruptured gas lines, driven by wind, and difficult to fight because water mains had broken — burned for three days and destroyed far more of the city than the shaking alone. Approximately 3,000 people died and 225,000 were left homeless in a city of 400,000.

The 1989 Loma Prieta Earthquake (magnitude 6.9) struck during the World Series, giving it unusual visibility as television cameras were already broadcasting from the Bay Area. Forty-two people died in the collapse of the Cypress Street Viaduct, an elevated double-deck freeway in Oakland whose lower deck pancaked onto its upper deck — a failure mode that prompted nationwide inspection of similar structures. The earthquake's epicenter on the Santa Cruz Mountains segment of the San Andreas demonstrated that segments far from major cities could still cause significant urban damage through ground shaking and Soil Amplification (Site Effect)The increase in shaking intensity caused by soft soil or sediment layers amplifying seismic waves. Structures built on soft soil can experience 2-10 times stronger shaking than those on bedrock..

The 1994 Northridge Earthquake (magnitude 6.7) struck a densely populated section of the Los Angeles metropolitan area and caused 57 deaths, 9,000 injuries, and approximately $40 billion in damage. Notably, Northridge occurred not on the San Andreas itself but on a Blind Thrust FaultA thrust fault that does not reach the surface, making it invisible at ground level and harder to detect. The 1994 Northridge earthquake occurred on a blind thrust fault. — a fault with no surface expression — buried beneath the San Fernando Valley. The discovery of numerous undocumented blind thrust faults beneath Los Angeles revealed that the region's earthquake hazard was more complex than the visible fault system suggested.

The Locked Segments: Seismic Gaps and Recurrence

The concept of a Seismic GapA section of an active fault that has not produced an earthquake for a long time compared to neighboring sections. Seismic gaps may indicate increased probability of a future earthquake. — a fault segment that has not ruptured recently and is therefore considered overdue for a large earthquake — is central to California earthquake hazard assessment. The Earthquake Recurrence IntervalThe average time between major earthquakes on a particular fault. Estimated from paleoseismology and historical records. The Cascadia subduction zone has a recurrence interval of ~500 years. of major earthquakes on locked segments of the San Andreas can be estimated through PaleoseismologyThe study of prehistoric earthquakes through geological evidence such as fault trenches, uplifted terraces, and tsunami deposits. Extends the earthquake record back thousands of years., the study of past earthquakes preserved in geological features such as offset stream channels, fault scarps, and distorted sedimentary layers in trenches dug across the fault.

PaleoseismologyThe study of prehistoric earthquakes through geological evidence such as fault trenches, uplifted terraces, and tsunami deposits. Extends the earthquake record back thousands of years. studies of the southern San Andreas suggest that the Coachella Valley segment ruptures approximately every 200 to 300 years in earthquakes of magnitude 7.5 to 8.0, and that the last rupture occurred around 1690 — suggesting roughly 335 years of accumulated strain. The Carrizo Plain segment, site of dramatic Fault ScarpA cliff or steep slope formed by vertical displacement along a fault during an earthquake. Fault scarps can be meters high and provide visible evidence of past earthquake activity. features and offset stream channels, has a mean Earthquake Recurrence IntervalThe average time between major earthquakes on a particular fault. Estimated from paleoseismology and historical records. The Cascadia subduction zone has a recurrence interval of ~500 years. of approximately 175 years and last ruptured in 1857. The famous "Big One" scenario — a magnitude 7.8 rupture of the southern San Andreas — is not a question of whether but when, and official probability estimates place the likelihood at approximately 60 percent within 30 years.

The Hayward Fault: Urban Exposure

While the San Andreas receives the most attention, the Hayward Fault running through the densely populated East Bay cities of Oakland, Berkeley, Fremont, and Hayward may pose the greatest immediate threat to the San Francisco Bay Area. The Hayward Fault's last major rupture was the 1868 earthquake (estimated magnitude 6.8), which damaged much of the East Bay when that area was sparsely populated. Today, the fault runs directly beneath or close to University of California Berkeley's Memorial Stadium, several hospitals, major highways, and the BART rapid transit system. A repeat of an 1868-scale event would cause far greater damage.

Use Seismic Risk Checker to assess the seismic hazard at specific California locations and compare ground shaking probabilities across the state.

California's Preparedness Culture: Ongoing Work

California has among the most sophisticated earthquake preparedness infrastructure in the United States, including the ShakeAlert Seismic Alert SystemMexico's SASMEX, one of the world's first public earthquake early warning systems, operational since 1991. Provides up to 60 seconds of warning for Mexico City from coastal earthquakes. — a West Coast Earthquake Early Warning (EEW)A system that detects an earthquake and sends alerts to people and systems before strong shaking arrives. Can provide seconds to tens of seconds of warning, enough to take protective action. network — mandatory retrofit programs for soft-story and unreinforced masonry buildings in many jurisdictions, and the Alquist-Priolo Earthquake Fault Zone Act prohibiting new habitable structures directly on active fault traces. Yet significant vulnerabilities remain: hundreds of thousands of older soft-story apartment buildings in Los Angeles have been retrofitted only partially, Unreinforced Masonry (URM)Brick or block construction without steel reinforcement, which is extremely vulnerable to earthquake shaking. URM buildings account for the majority of earthquake fatalities worldwide. buildings still exist in smaller California cities, and the state's critical lifelines — water systems, bridges, natural gas networks — remain partially vulnerable to the scenario earthquakes that official models anticipate.