M6.4 March 10, 1933 Long Beach, California Earthquake
The magnitude 6.4 Long Beach, California earthquake, which struck near 5pm local time on March 10, 1933, was a landmark event. It was the largest known earthquake in the Los Angeles region at the time, since the advent of written record-keeping. It occurred at a pivotal point historically, during an active debate about seismic hazard in the area.
Following the 1906 San Francisco earthquake, concern about hazard in California was focused on the San Andreas fault, which skirts the northern edge of the mountains north of the Los Angeles Basin, 50 kilometers or more from growing population centers. By 1933, the greater Los Angeles region itself had not experienced local earthquakes on a par with the M6.8 1868 Hayward, CA or M7.9 1906 San Francisco earthquake. In the decades after the 1906 earthquake, one school of thought held that local faults in the Los Angeles area were not highly active, and earthquakes on these faults would never generate severe shaking.
The 1933 Long Beach earthquake ended that debate. The earthquake caused widespread damage in the greater Los Angeles area, with about 120 fatalities. The most severe damage was concentrated in Long Beach and Compton. In some places shaking was strong enough to reportedly throw heavy machinery into the air.
The 1933 earthquake was also pivotal because it occurred soon after a local seismic monitoring network was established in southern California. As of 1933, the fledgling local seismic network comprised 7 stations designed to record local earthquakes, along with several specialized “strong-motion” instruments designed to stay on-scale during large earthquakes, which were fortuitously installed in 1932. The 1933 Long Beach earthquake was recorded by a number of these instruments. Augmenting data recorded by these instruments, local scientists and engineers including Charles Richter, Harry Oscar Wood, and Romeo Martel undertook extensive field investigations to document damage and other earthquake effects.
The Long Beach earthquake did not produce a fault break that that extended to the surface. Initial investigations (e.g., Wood, 1933; Benioff, 1938) estimated an epicenter south of Long Beach, about 5 km offshore of Huntington Beach, with propagation to the northwest along the Newport-Inglewood Fault. Hauksson and Gross (1991) presented a comprehensive analysis of available instrumental data, estimating source properties of the mainshock and larger aftershocks. Their analysis supported the conventional interpretation that the earthquake ruptured the Newport-Inglewood Fault, with rupture extending at least 13-16 km unilaterally to the northwest and a moment magnitude of 6.4 (Figure 1). A later, detailed investigation by Susan Hough and Robert Graves of the US Geological Survey confirmed this magnitude estimate, but concluded that the rupture was somewhat longer, extending about 25 km along the Newport-Inglewood Fault. The later study used modern methods to predict shaking, comparing predictions for a range of possible ruptures with the detailed distribution of damage and intensity observations. An animation based on results of this study shows how the rupture propagated to the northwest, strongly channeling energy towards the central Los Angeles Basin, including the town of Compton.
Most of the 120 deaths resulting from the earthquake were caused by collapse of unreinforced masonry buildings. Dramatic photographs captured the damage, described in front-page news stories around the country. As many of the collapsed structures were school buildings, the public immediately realized the toll could have been far worse had the earthquake struck earlier in the day, while the schools were still occupied.
The 1933 Long Beach earthquake sparked an immediate call to action from the public. 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 for the first time, California developed seismic provisions in state-wide building codes. 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. By virtue of its timing, early in the emergence of the Los Angeles region as a preeminent population center, the 1933 earthquake arguably had a greater impact on earthquake risk reduction than any other single event, setting California on a path towards improved resilience that continues to this day.
The magnitude 6.4 Long Beach, California earthquake, which struck near 5pm local time on March 10, 1933, was a landmark event. It was the largest known earthquake in the Los Angeles region at the time, since the advent of written record-keeping. It occurred at a pivotal point historically, during an active debate about seismic hazard in the area.
Following the 1906 San Francisco earthquake, concern about hazard in California was focused on the San Andreas fault, which skirts the northern edge of the mountains north of the Los Angeles Basin, 50 kilometers or more from growing population centers. By 1933, the greater Los Angeles region itself had not experienced local earthquakes on a par with the M6.8 1868 Hayward, CA or M7.9 1906 San Francisco earthquake. In the decades after the 1906 earthquake, one school of thought held that local faults in the Los Angeles area were not highly active, and earthquakes on these faults would never generate severe shaking.
The 1933 Long Beach earthquake ended that debate. The earthquake caused widespread damage in the greater Los Angeles area, with about 120 fatalities. The most severe damage was concentrated in Long Beach and Compton. In some places shaking was strong enough to reportedly throw heavy machinery into the air.
The 1933 earthquake was also pivotal because it occurred soon after a local seismic monitoring network was established in southern California. As of 1933, the fledgling local seismic network comprised 7 stations designed to record local earthquakes, along with several specialized “strong-motion” instruments designed to stay on-scale during large earthquakes, which were fortuitously installed in 1932. The 1933 Long Beach earthquake was recorded by a number of these instruments. Augmenting data recorded by these instruments, local scientists and engineers including Charles Richter, Harry Oscar Wood, and Romeo Martel undertook extensive field investigations to document damage and other earthquake effects.
The Long Beach earthquake did not produce a fault break that that extended to the surface. Initial investigations (e.g., Wood, 1933; Benioff, 1938) estimated an epicenter south of Long Beach, about 5 km offshore of Huntington Beach, with propagation to the northwest along the Newport-Inglewood Fault. Hauksson and Gross (1991) presented a comprehensive analysis of available instrumental data, estimating source properties of the mainshock and larger aftershocks. Their analysis supported the conventional interpretation that the earthquake ruptured the Newport-Inglewood Fault, with rupture extending at least 13-16 km unilaterally to the northwest and a moment magnitude of 6.4 (Figure 1). A later, detailed investigation by Susan Hough and Robert Graves of the US Geological Survey confirmed this magnitude estimate, but concluded that the rupture was somewhat longer, extending about 25 km along the Newport-Inglewood Fault. The later study used modern methods to predict shaking, comparing predictions for a range of possible ruptures with the detailed distribution of damage and intensity observations. An animation based on results of this study shows how the rupture propagated to the northwest, strongly channeling energy towards the central Los Angeles Basin, including the town of Compton.
Most of the 120 deaths resulting from the earthquake were caused by collapse of unreinforced masonry buildings. Dramatic photographs captured the damage, described in front-page news stories around the country. As many of the collapsed structures were school buildings, the public immediately realized the toll could have been far worse had the earthquake struck earlier in the day, while the schools were still occupied.
The 1933 Long Beach earthquake sparked an immediate call to action from the public. 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 for the first time, California developed seismic provisions in state-wide building codes. 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. By virtue of its timing, early in the emergence of the Los Angeles region as a preeminent population center, the 1933 earthquake arguably had a greater impact on earthquake risk reduction than any other single event, setting California on a path towards improved resilience that continues to this day.