Earthquake in Southern California 90 Years Ago Changed the Way We Build
When large, damaging earthquakes happen, it’s natural for people in the United States to ask, could it happen here? Large earthquakes have struck the U.S., many of them now beyond living memory. Some of these earthquakes provided impetus for risk-reduction efforts. The1933 Long Beach, California, earthquake was one such event.
On March 10, 1933, the greater Los Angeles region, along with the rest of the country, struggled economically during the Great Depression. But not all local news was grim. Pleasant spring weather conditions prevailed across the region, and, at noon that day, the newly restored frigate Constitution sailed into Long Beach Harbor with pomp and circumstance. Buoyed by the discovery of oil fields shortly after the turn of the century, the Los Angeles area was on its way to being a major economic hub.
Earthquakes had always been felt in the area, but the region had not experienced damaging events on par with those that struck the San Francisco Bay Area in 1906. A large earthquake struck the San Andreas Fault closer to Los Angeles in 1857, but few people lived in the area at the time. Scientists had, however, realized that earthquakes posed a hazard in the region. The San Andreas Fault had been mapped, in Southern as well as Northern California. By 1933, earthquake professionals led by Harry Oscar Wood, had established a fledgling network of Wood-Anderson seismometers across Southern California, the beginnings of today’s California Integrated Seismic Network. Additionally, the first specialized instruments designed to record large earthquakes on scale, so-called strong motion instruments, had been installed in Southern California in 1932.
As of 1933, however, the severity of hazard across Southern California remained the topic of active debate. Business leaders were aware of earthquakes and their hazards, conferring regularly with Wood and other experts, and taking steps towards risk mitigation. At the same time, they actively worked to tamp down public talk of a fearful bogeyman that could scare away Eastern capital needed for continuing economic development.
Meanwhile, among the community of California earthquake professionals, concern focused on large earthquakes like the great 1906 San Francisco earthquake. Local faults had been recognized and mapped in the Los Angeles area, but experiences with local earthquakes, for example, a 1920 event near Inglewood (now estimated to have been close to magnitude 5,) suggested that moderate earthquakes on local faults would cause, at most, minor, localized damage.
Scientists’ understanding of potential shaking from large earthquakes was also limited in 1933. Based largely on observations from Japan, engineers had estimated expected levels of shaking in strong earthquakes. “Early instruments designed to stay on scale in large earthquakes assumed the strongest shaking would be no greater than about half the acceleration of gravity, that is, one ‘g-force’,” said seismologist Susan Hough.
At 5:54pm, March 10, the earth weighed in on these matters with an earthquake that jolted the region. Using data from the early seismic network, seismologists were able to pinpoint the epicenter, or nucleation point, near Huntington Beach. Detailed investigations from 1933 onward showed the earthquake broke the Newport-Inglewood fault, continuing some 25 km (15 miles) towards the north, packing an especially strong punch in the city of Long Beach. The city of Compton also sustained severe damage, including catastrophic collapse of some buildings. Using available data, the magnitude was later estimated to be 6.4.
Even though earthquake monitoring networks were in their infancy in 1933, a marriage of detailed observations, available instrumental data, and modern science has led to a good understanding of the Long Beach earthquake and the shaking it produced. The reverberations from this earthquake also had a more far-reaching impact on risk reduction efforts in California.
The earthquake’s precise timing was fortunate for the local population. In Southern California, schools had been built over the decade or so before 1933 to accommodate the rapidly growing population. Public school buildings were reasonably well-built, but they were masonry structures built without special resistance to earthquake shaking. The Long Beach earthquake claimed 120 lives, almost all of which were caused by collapse of unreinforced masonry buildings.
Some of the most dramatic photographs of damage caused by the 1933 earthquake showed catastrophic collapse of school buildings, described in front-page news stories around the country. Shortly before 5 p.m., mercifully, school buildings were overwhelmingly empty; otherwise, the tragedy of that day could have been far worse. Tony Gugliemo was reportedly the only child killed in a southland school building that day. A popular and athletic boy from San Pedro, he had returned to Woodrow Wilson High School to take a shower following an after-school track meet. For his sister Selma, who lived into her late 90s, the memories of that day never faded. With no awareness of earthquakes, she clung to her mother, “thinking it was the end of the world.”
Shaking was recorded by several early strong motion instruments, including one in Long Beach. The recording at Long Beach was blown off-scale, it’s upper limit not high enough. Astonishingly, some observations showed that the earthquake had generated accelerations upwards of 100% gravity (e.g., one ‘g-force’), strong enough to throw heavy objects such as machinery into the air in some locations. “But,” said Hough, “it wasn’t until the 1971 Sylmar earthquake, recorded by many more instruments, that earthquake professionals appreciated the potential for such extreme shaking levels.”
In 2020, the Long Beach earthquake was investigated in detail using modern methods by USGS scientists Susan Hough and Robert Graves, leading to a computer simulation showing how waves from the earthquake rocked the Los Angeles region. “These simulations show predicted shaking effects,” said Robert Graves, who has pioneered the methods to generate simulations, “not recorded observations, but they show how subsurface structure influenced seismic waves radiated by the fault.”
The 1933 earthquake demonstrated two important points. First, that moderately large earthquakes, like this M 6.4, can cause serious damage in a metropolitan region. And secondly, “Long Beach demonstrated the need for appropriate engineering to mitigate risk,” said structural engineer Robert Reitherman, “and the continued collection of strong motion data has been critical to this effort.”
With the earthquake bogeyman no longer hypothetical, followed by the public’s immediate call to action, earthquake professionals united with policymakers and business leaders to launch the modern era of earthquake risk reduction in California. The Field Act, spearheaded by local Assemblyman Donald Field, was passed barely a month later, laying out strict guidelines for construction of K-12 public schools in California. And via the Riley Act, California required for the first time that all cities and counties establish departments to regulate building construction. The vulnerability of unreinforced masonry structures had been illustrated before, as far back as the 1872 Owen’s Valley earthquake, but the Long Beach earthquake hammered the lesson home.
At an estimated magnitude 6.4, the lessons of the 1933 Long Beach earthquake were all about earthquake impact. The earthquake that rocked Southern California 90 years ago took a grim toll on lives and property. But the earthquake also knocked the region out of its complacency. By virtue of its timing, early in the emergence of the Los Angeles region as a preeminent population center, the 1933 earthquake had an outsized impact on earthquake risk reduction, setting California on a path towards improved resilience that continues to this day.
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