1906 Earthquake A Reminder to Be Prepared

CALIFORNIA GEOLOGICAL SURVEY: REMEMBER ‘06
BUT DON’T FORGET TO LOOK AHEAD AND BE PREPARED

SACRAMENTO – April 18 marks the 100th anniversary of the Great San Francisco Earthquake. The quake is a landmark event in California history -- noteworthy for the scope of its destruction and the resilience of San Francisco’s residents in rebuilding their decimated city. The upcoming anniversary has become a rallying point for evaluating and improving California’s earthquake awareness and preparation.

“The centennial anniversary of the San Francisco earthquake will be remembered with a great deal of pomp and circumstance, and it should be, because it’s the most significant earthquake we’ve experienced,” said State Geologist Dr. John Parrish, head of the Department of Conservation’s California Geological Survey. “But people need to realize that there have been dozens of other large earthquakes that have caused damage or fatalities in the last two centuries. So preparedness is an everyday thing, not something we only need to think about every 100 years.”

Since 1800, according to CGS, there have been 63 “significant” earthquakes centered in or offshore of California. These quakes have been either magnitude 6.5 or larger, or caused loss of life or more than $200,000 in damage (not adjusting for inflation). There have been several other such quakes centered in Mexico, Arizona or Nevada that have caused damage in California.

“California is one of the most seismically active locations in the world,” said Dr. Tousson Toppozada, a seismologist specializing in the history and hazards of earthquakes. “Of all the states, Alaska has had larger earthquakes and has more frequent seismicity than California. However, in California, the size of the population and economy, and the amount of vulnerable infrastructure, make the large earthquakes here stand out more than those in Alaska.”

The 1906 San Francisco event occurred before instrumentation to measure an earthquake’s size was available. The earthquake, listed by CGS as having a magnitude of 7.8, and resulting fire killed an estimated 3,000 people, displaced nearly a quarter-million people, and did $524 million in property damage (unadjusted for inflation).

“We’ve come a long way since 1906, both in terms of what we know about earthquakes and how we prepare for them through building codes, engineering practices, and other public safety considerations,” Parrish said. “While it’s unlikely that a repeat of the 1906 earthquake would cause nearly as many fatalities, there still would be millions, if not billions, of dollars in damage simply because there are more – and more expensive -- things to be damaged now than there were in 1906.”

In 1700, 69 years before the first Spanish mission was established in California, a rupture on the Cascadia subduction zone somewhere between Cape Mendocino and Canada produced an earthquake similar in scale to the December 2004 quake off the coast of Indonesia. On January 26, 1700, this earthquake shook the entire West Coast and, like the magnitude 9.0 Indonesian quake, spawned a cataclysmic tsunami – one that damaged villages from North America to Japan.

“While the Cascadia subduction zone is capable of producing the largest earthquakes in California history, the majority of the state’s population is a considerable distance away from the area that would be most dramatically affected,” said CGS Chief Seismologist Dr. Michael Reichle, who was part of a Department of Conservation/Office of Emergency Services effort in the 1980s to create eight earthquake planning scenarios for active faults in urban areas.

“The worst-case scenario for the state most likely would be a magnitude 7 or greater quake centered on one of the faults beneath Los Angeles or the Bay Area. We don’t believe any of those faults are capable of generating an earthquake as large as the Cascadia subduction zone, but there are millions of people and many billions of dollars of infrastructure close to those faults.”

About 700 miles long, the Cascadia subduction zone is where the Juan de Fuca Plate is being pushed toward and under the North American Plate at a rate of about 40 millimeters per year. Built-up stress along this subduction zone is periodically released in massive earthquakes.

“Geological evidence suggests that there have been large earthquakes along the subduction zone every 400 to 600 years,” Toppozada said. “It has been just over 300 years since the last really big one. So, while there’s no need to panic, we need to be aware and prepare.”

The first fatal earthquake recorded in California occurred December 8, 1812. Centered on the San Andreas fault in Wrightwood, northeast of Mt. Baldy in Los Angeles County, the temblor was responsible for 40 deaths at San Juan Capistrano. A church tower – the tallest structure west of the

Mississippi at the time – collapsed onto the church’s roof.

The largest earthquake in California following the arrival of Europeans was the Great Fort Tejon earthquake of January 9, 1857, along the southern part of the San Andreas fault. Estimated to be a magnitude 7.9 event, it killed only one person but caused damage from Monterey to San Bernardino County and left a 220-mile surface scar in the ground.

“The majority of the population in the state at that time was either in the gold country or in the port city of San Francisco,” Toppozada said. “The area that was affected was very sparsely populated.”

A much smaller earthquake, the magnitude 6.4 temblor of March 1933, killed 115 people (only the ’06 quake caused more fatalities) and did $40 million in property damage in Long Beach. It was particularly devastating to buildings made of unreinforced masonry. As a result of that temblor, the California Legislature passed the Field Act, which authorized the Division of the State Architect to review and approve all public school plans and specifications. Today, CGS reviews the geology of many of the proposed sites for new schools around the state at the behest of the State Architect.

“The Long Beach earthquake was the first to result in seismic safety legislation,” Reichle said. “The Field Act undoubtedly has saved many lives, but many communities still have unreinforced masonry buildings. The Long Beach earthquake also led to the inclusion of earthquake provisions in many building codes. Building codes are evolving continuously. Some building types, such as non-ductile concrete buildings built in the 1950s and ‘60s, have been shown by subsequent earthquakes to pose a collapse hazard and could not be built today. Still, a significant number of these buildings exist.”

Other earthquakes smaller than magnitude 7.0 have had devastating results. The magnitude 6.6 San Fernando quake of February 9, 1971 killed 65 people and injured more than 2,000, and caused $505 million in damage. The magnitude 6.0 Whittier Narrows earthquake of October 1, 1987 left eight dead and did $358 million in property damage to 10,500 homes and businesses in the Los Angeles area. The magnitude 6.9 October 17, 1989 Loma Prieta earthquake killed 63 people and injured more than 3,700, and did about $6 billion in damage. The magnitude 6.7 Northridge earthquake of January 17, 1994 caused 57 deaths, more than 9,000 injuries and did about $40 billion in property damage.

“We’ve tried to learn something to help us better protect public safety from every earthquake,” Reichle said. “For example, the San Fernando earthquake had a great deal to teach us about construction

practices. The Loma Prieta earthquake alerted us to the damage liquefaction and landslides can cause event at great distance from the epicenter. The Northridge earthquake taught us how damaging blind thrust earthquakes can be.”

For all the seismic activity California has experienced over the years, Parrish said the state has been “lucky” in some respects.

“Many of our largest earthquakes – such as our most recent magnitude 7.0, the 1999 Hector Mine event -- have occurred in areas that are sparsely populated, or were centered far offshore, so in that sense we’ve been lucky,” he said. “And we’re better prepared for earthquakes from the standpoint of modern construction and emergency response than most places in the world. That’s a legacy of not only the 1906 San Francisco quake, but of many others that aren’t as well known or remembered.

“Earthquakes can strike at any time, without warning, and while government agencies such as ours, the academic world and private industry are working to protect public safety, people must be prepared at the individual and family level, too.”

Department of Conservation

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1906 Earthquake A Reminder to Be Prepared