Case Study · Earthquake · 1906
The deadliest earthquake in U.S. history didn't kill most of its victims directly. A three-day fire — fed by broken gas lines and fought without water — did. The lesson from 1906 is still the most important thing to understand about earthquakes.
1906 San Francisco Earthquake · April 18, 1906
San Francisco in April 1906 was one of the most prosperous cities in the American West. Its population had grown to nearly 400,000. The wharves along the Bay were piled with goods. The Palace Hotel — the largest in the world at its opening — anchored the downtown. The city's wooden neighborhoods climbed the hills in dense rows, separated by narrow streets, connected by a network of water mains that engineers had spent decades building beneath the city.
At 5:12 in the morning of April 18, 1906, the San Andreas Fault ruptured without warning. The shaking lasted 45 to 60 seconds. Buildings collapsed. Chimneys fell. Gas lines broke open underground. And in those first moments, something else happened — something no one could see: the water mains fractured along 300 miles of pipeline throughout the city. The ground stopped shaking. The fires started. And there was nothing to put them out with.
April 18, 1906
Date
3,000+
Lives Lost
$400M
1906 Dollars
San Francisco
Location
Earthquake
Disaster Type
The earthquake registered at magnitude 7.9 and ruptured 296 miles of the San Andreas Fault. It was felt from Oregon to Los Angeles and as far east as Nevada. But the shaking, as violent as it was, killed relatively few people directly. The three-day fire that followed — burning 4.7 square miles, destroying 28,188 buildings, and leaving 225,000 people homeless — killed most of the more than 3,000 who died. It remains the deadliest earthquake in U.S. history, and its central lesson — that secondary disasters from broken infrastructure can be more deadly than the initial event — is as relevant today as it was in 1906.
The Science
Think of a fault not as a crack in the ground, but as two enormous slabs of rock pressed together under immense pressure — like two hands pressed palm-to-palm and slowly sliding in opposite directions. As long as friction holds them together, no earthquake occurs. But stress accumulates over years and decades. When the friction finally fails, the slabs snap past each other, releasing that stored energy as seismic waves in all directions. The 1906 earthquake released energy equivalent to 15 megatons of TNT — accumulated over roughly 150 years of stress on the San Andreas Fault since its last major rupture in 1838.
The San Andreas Fault runs directly along the San Francisco Peninsula — the fault trace passes through the city itself. In 1906, much of the city was built on "made land" — marshes and tidal flats that had been filled with rubble and debris to create buildable ground as the city expanded toward the Bay. This type of soil, called fill, behaves very differently during an earthquake than solid rock. Think of it like Jell-O compared to granite: fill can amplify seismic waves dramatically, increasing shaking by a factor of 10 or more. The worst-damaged neighborhoods in 1906 were those built on fill. The same soil behavior contributed to the Marina District collapse in the 1989 Loma Prieta earthquake.
San Francisco's water distribution system in 1906 relied on a single primary reservoir and a network of large iron pipes that ran beneath the city streets — the same streets that buckled and heaved during the earthquake. The pipes were rigid, joined at intervals, and buried in the ground that was moving. When the fault slipped, the pipe network fractured in hundreds of places simultaneously. Firefighters arrived at hydrants and opened them. Nothing came out. The city burned for three days because there was no water to fight it.
Timeline
01
1838–1906: The San Andreas Fault accumulates 68 years of tectonic stress following its last significant rupture. Smaller earthquakes hint at the building pressure. No serious earthquake preparedness exists. San Francisco's building stock — dense wooden Victorian neighborhoods — is highly combustible.
02
April 18, 5:12 AM: A foreshock is felt, then 20–25 seconds later the main rupture begins. Forty-five to sixty seconds of violent shaking. Buildings collapse, chimneys fall, foundations crack. 52 fires are reported within 30 minutes of the main shock. Fire Chief Dennis Sullivan is mortally wounded by falling debris at the California Hotel.
03
April 18–20: With no water pressure, firefighters attempt to create firebreaks by dynamiting buildings. Some efforts succeed; others ignite new fires. The Army is called in under General Frederick Funston. Martial law is effectively declared. The fire burns 4.7 square miles over three days, destroying 28,000 buildings.
04
April 18 – weeks following: A major aftershock at 8:14 AM on April 18 strikes already-compromised structures. Dozens of significant aftershocks follow over weeks. 225,000 people are homeless. Tent cities form in Golden Gate Park and along the beaches west of the city. Rebuilding begins almost immediately.
Human Decisions
What went right
General Frederick Funston, commanding the Presidio, ordered troops into the city without waiting for orders from Washington. The Army provided organization, labor for firebreaks, supply distribution, and a degree of order when civilian infrastructure had completely collapsed. The Army relief report remains one of the primary historical sources for casualty counts.
The Ferry Building at the foot of Market Street — built on pilings over the Bay — survived the earthquake and fire largely intact. It became the primary evacuation point for hundreds of thousands of residents, and ferries carried people across the Bay continuously for days. The Bay itself provided the escape route the destroyed roads could not.
Within days of the earthquake, neighborhood committees had formed to distribute food, manage the tent cities in Golden Gate Park, and coordinate the early stages of rebuilding. The disaster predated any formal federal emergency management; the immediate response was almost entirely civilian and community-organized.
What went wrong
San Francisco's water supply in 1906 depended on one reservoir connected to one distribution network with no seismically isolated backup. This was not an oversight unique to San Francisco — it reflected the standard engineering practice of the era. But the result was that a single event destroyed the city's entire firefighting capacity simultaneously.
The decision to create firebreaks by dynamiting buildings — executed by soldiers who were not demolition experts — produced inconsistent results. In several documented cases, improperly placed charges ignited new fires on the windward side of the intended break, accelerating the spread of the conflagration rather than containing it.
Contemporary official reports cited approximately 700 deaths. Researchers have since established that the true figure was likely over 3,000. The suppression of the true death toll is well-documented; San Francisco's civic leadership feared that an accurate casualty count would deter investment and migration to the rebuilding city. The discrepancy was not resolved in official records for decades.
The compound effect
The earthquake lasted less than a minute. The fire burned for three days. According to NOAA's analysis of the event, the three-day conflagration caused substantially more damage than the earthquake itself — and it burned because the earthquake had destroyed the one system that could have stopped it. The broken water mains were not an unfortunate coincidence. They were the predictable result of rigid underground infrastructure meeting a major seismic event. The lesson: prepare not just for the initial disaster, but for what it disables.
What Changed
Following the earthquake, San Francisco constructed the Auxiliary Water Supply System (AWSS) — a separate, dedicated high-pressure water system for firefighting, entirely independent of the domestic water supply. The AWSS draws from the Bay and from cisterns distributed across the city, so a single rupture cannot disable the entire firefighting network. The system, expanded and upgraded multiple times since 1908, remains in service today and proved its value during the 1989 Loma Prieta earthquake.
The 1906 earthquake exposed the catastrophic inadequacy of building standards that gave no consideration to seismic forces. While San Francisco's post-quake rebuilding was initially rapid and not as seismically sound as it should have been, the event accelerated the development of earthquake engineering as a discipline. California's building codes today are among the most seismically stringent in the world, tracing their intellectual origin to the questions raised by 1906.
The Earthquake Engineering Research Institute (EERI), established in 1949, cites the 1906 earthquake as the foundational event of the modern earthquake engineering profession. The ShakeAlert Early Warning System now deployed across California, Oregon, and Washington — which sends warnings to cell phones seconds before shaking arrives — exists because of a century of research that began with 1906. The earthquake that struck San Francisco on a spring morning 120 years ago is still shaping how the West Coast builds, plans, and prepares.
If It Happened Today
What You Can Do Now
You don't have to live near the San Andreas Fault for these lessons to apply. The cascade failure principle — that earthquakes disable the systems needed to respond to them — is relevant wherever seismic risk exists.
In 1906, broken gas lines fed the fires that destroyed the city. Today, post-earthquake fires remain the most common secondary disaster. Locate your gas meter shutoff valve now, before any earthquake occurs. Keep a wrench nearby. Know the procedure. After significant shaking, if you smell gas, shut it off and leave. Don't reenter until utility personnel have inspected the line.
Earthquake preparedness guideAfter the 1906 earthquake, San Francisco had no municipal water service for weeks. Modern systems are more resilient but a major earthquake can still fracture water mains across an entire city. The minimum is one gallon per person per day for three days — stored now, before any shaking. After an earthquake, tap water may be contaminated even if it flows.
Water storage guideIf you live in California, Oregon, or Washington, the ShakeAlert Earthquake Early Warning System can send an alert to your phone seconds before shaking arrives. Go to your phone's settings and enable Earthquake Alerts. Those seconds matter — they're enough time to move away from windows, get under a table, or stop a vehicle safely.
Set up local alertsThe majority of earthquake injuries in modern U.S. buildings are caused not by structural collapse, but by falling objects — toppled bookcases, sliding refrigerators, shattered glass. Secure tall furniture to wall studs with L-brackets, move heavy items to lower shelves, and identify safe drop-cover-hold positions in every room. These take an afternoon and cost under $30.
Home safety guideAfter 1906, people reentered collapsed and fire-damaged structures looking for belongings, and some were killed by subsequent aftershocks and structural failures. After a major earthquake, do not reenter a building until it has been inspected and posted as safe by a building official. Aftershocks will follow. The structure that looked intact at 9am may not be after the 8:14am aftershock of 1906.
First 72 hours guideNext step
The 1906 earthquake happened in seconds. The preparation that saves lives happens long before any shaking starts. The earthquake preparedness guide covers drop-cover-hold technique, gas shutoffs, supply storage, structural risk, and what to do in the days after a major event.
Earthquake preparedness guideSources