1906 샌프란시스코 지진: 현대 지진학의 탄생

The Setting: San Francisco in 1906

By April 1906, San Francisco was the largest and most prosperous city on the American West Coast, with a population of approximately 400,000. The city had grown explosively since the Gold Rush of 1848, and its rapid development paid little attention to the geological hazards underfoot. The city sat astride the San Andreas Fault, the great 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. system running nearly the full length of California. The northern section of this fault runs directly beneath the San Francisco Bay Area. Geologists of the era had little understanding of plate tectonics — that framework would not emerge for another six decades — but some scientists had noted the pattern of Fault LineThe trace of a fault on the Earth's surface, visible as a line or zone of broken rock. Active fault lines are mapped by geologists to assess earthquake hazard for nearby communities. features crossing the California landscape and associated them with seismic activity. Buildings in San Francisco ranged from well-constructed stone and brick commercial structures to hastily built wooden frame houses on the steep hillsides, and to fill-land neighborhoods constructed atop the former bay margins where LiquefactionA phenomenon where saturated, loose soil temporarily loses strength and behaves like a liquid during strong shaking. Can cause buildings to sink, tilt, or collapse into the ground. risk was extreme.

The Earthquake: April 18, 1906

At 5:12 AM on April 18, 1906, the San Andreas Fault ruptured along a segment stretching approximately 477 kilometers from near San Juan Bautista in the south to Cape Mendocino in the north. The Fault RuptureThe breakage of rock along a fault during an earthquake, releasing stored elastic energy as seismic waves. Rupture length can range from meters (small quakes) to 1,000+ km (great earthquakes). propagated at about 3 kilometers per second, completing the rupture in roughly three minutes. The MagnitudeA single number that quantifies the total energy released by an earthquake. Each whole number increase represents roughly 31.6 times more energy released. of the earthquake has been estimated at M7.9 by modern methods, with an EpicenterThe point on the Earth's surface directly above the hypocenter (focus) where an earthquake originates underground. Often reported as the earthquake's location in news reports. near the Golden Gate. The 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. visible after the earthquake in Marin County displaced features by as much as 6 meters horizontally — direct evidence of the massive 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. motion. The shaking was felt from Oregon to Los Angeles and as far east as Nevada. In San Francisco, the initial shaking lasted approximately 45 to 60 seconds. Much of the city was built on land reclaimed from the bay using rubble, sand, and garbage fill — material highly susceptible to LiquefactionA phenomenon where saturated, loose soil temporarily loses strength and behaves like a liquid during strong shaking. Can cause buildings to sink, tilt, or collapse into the ground. 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.. In the Marina District and along the waterfront, the ground simply gave way. Brick and masonry buildings, designed only for gravity loads with no consideration of lateral seismic forces, crumbled. Wooden-framed buildings on more competent soil performed remarkably better.

The Science: The Birth of Seismology

The 1906 San Francisco earthquake was pivotal in the history of seismology precisely because it occurred in a scientifically literate society capable of studying it systematically. The California State Earthquake Investigation Commission, led by geologist Andrew Lawson, spent two years compiling an exhaustive scientific report published in 1908 — the famous Lawson Report — that remains one of the foundational documents in earthquake science. The report identified the San Andreas Fault as the source of the earthquake, mapped the Fault RuptureThe breakage of rock along a fault during an earthquake, releasing stored elastic energy as seismic waves. Rupture length can range from meters (small quakes) to 1,000+ km (great earthquakes). zone in detail, and documented the patterns of Ground Rupture (Surface Faulting)Visible displacement of the ground surface along a fault during an earthquake. Structures built across a surface rupture zone can be torn apart regardless of their structural strength. across the California landscape. It was the Lawson Report that introduced the concept of the Fault LineThe trace of a fault on the Earth's surface, visible as a line or zone of broken rock. Active fault lines are mapped by geologists to assess earthquake hazard for nearby communities. as a persistent geological feature along which repeated earthquakes occur. SeismographAn instrument that detects and records ground motion caused by seismic waves. Modern digital seismographs can detect movements smaller than a nanometer. records from around the world were collected and analyzed, helping scientists understand how Seismic WaveAn elastic wave generated by an earthquake or explosion that propagates through the Earth. Seismic waves carry the energy released at the earthquake source to distant locations. energy propagates globally. Harry Fielding Reid of Johns Hopkins University developed the elastic rebound theory based on measurements of the pre-earthquake and post-earthquake survey markers, establishing the now-fundamental understanding that earthquakes occur when accumulated elastic strain energy along a fault is suddenly released. This theory remains the basis of modern seismology. The Distance from Epicenter tool illustrates how shaking intensity decays with distance from the EpicenterThe point on the Earth's surface directly above the hypocenter (focus) where an earthquake originates underground. Often reported as the earthquake's location in news reports. — a pattern rigorously documented for the first time after the 1906 earthquake.

The Impact: Fire and Ruin

The earthquake's immediate toll was severe, but the fires that followed were catastrophic. Gas mains ruptured across the city. Chimneys collapsed, scattering burning coals. Within 30 minutes of the earthquake, dozens of fires had started, and the water mains that might have fought them had been broken by the earthquake. Over three days, 52 separate fires burned across San Francisco, eventually merging into an inferno that destroyed approximately 490 city blocks and 28,000 buildings. The official death toll was originally reported as 478, a figure deliberately minimized by city authorities eager to protect San Francisco's commercial reputation. Modern historical scholarship puts the actual death toll at approximately 3,000, making it the deadliest natural disaster in California history. Approximately 225,000 of San Francisco's 400,000 residents were left homeless. The city's downtown, south of market, and Chinatown districts were nearly completely destroyed. Ironically, the earthquake — a natural disaster — killed far fewer people than the fires — a human-made consequence — highlighting the importance of Secondary Earthquake HazardsHazards triggered by earthquake shaking rather than the shaking itself — including tsunamis, landslides, liquefaction, fires, dam failures, and chemical releases. Often cause more damage than shaking. in earthquake planning.

The Response: Rebuilding a City

San Francisco's response to the 1906 earthquake was remarkable for its speed if not always its wisdom. Army troops under General Frederick Funston were deployed within hours, with orders to shoot looters on sight — an action later disputed as legally dubious. Over 20,000 people were housed in temporary camps in city parks. The Red Cross coordinated relief operations. Reconstruction began almost immediately, with city officials determined to rebuild quickly and on the same footprint, largely ignoring the scientific findings of the Lawson Commission regarding Fault LineThe trace of a fault on the Earth's surface, visible as a line or zone of broken rock. Active fault lines are mapped by geologists to assess earthquake hazard for nearby communities. locations and hazardous soil conditions. Within three years, much of San Francisco had been rebuilt — but without meaningful improvements to earthquake resistance. The political and economic pressure to restore confidence in the city outweighed the scientific lessons. This pattern of rapid rebuilding without incorporating hazard knowledge would be repeated many times in earthquake disasters around the world throughout the 20th century.

The Legacy: Seismology and Building Science

The 1906 San Francisco earthquake is rightly called the birth of modern seismology. The elastic rebound theory, the recognition of the San Andreas as a major 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., the first systematic study of 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. and LiquefactionA phenomenon where saturated, loose soil temporarily loses strength and behaves like a liquid during strong shaking. Can cause buildings to sink, tilt, or collapse into the ground., and the beginnings of strong-motion SeismographAn instrument that detects and records ground motion caused by seismic waves. Modern digital seismographs can detect movements smaller than a nanometer. networks all trace their origins to the scientific work inspired by this disaster. The earthquake led to the first serious efforts to develop building design standards that account for horizontal seismic forces, eventually evolving into the Building Code (Seismic)A set of legal requirements governing the design and construction of buildings to ensure minimum levels of earthquake safety. Updated after major earthquakes reveal new vulnerabilities. systems that protect lives in California today. It inspired the founding of the Seismological Society of America in 1906 and motivated investment in SeismographAn instrument that detects and records ground motion caused by seismic waves. Modern digital seismographs can detect movements smaller than a nanometer. networks across the western United States. The lessons of 1906 about LiquefactionA phenomenon where saturated, loose soil temporarily loses strength and behaves like a liquid during strong shaking. Can cause buildings to sink, tilt, or collapse into the ground. in fill areas were tragically relearned in the 1989 Loma Prieta earthquake when the same districts — now rebuilt — again suffered disproportionate damage, demonstrating how slowly institutional memory translates into physical risk reduction.