On March 10, 1933, one of the deadliest earthquakes in California history struck Long Beach, according to the U.S. Geological Survey. The magnitude 6.4 temblor that hit at 5:54 p.m. 85 years ago killed about 120 people. What are our chances of facing another potentially deadly disaster?

More than 120 school buildings were damaged, with 70 of them destroyed by the quake. The devastation prompted the state to enact a series of earthquake building codes, including the Field Act of 1933, which mandated that all school buildings be earthquake resistant. Since the act was passed, no school has collapsed due to a seismic event.

Images of Franklin Junior High School (above) in Long Beach from the California Geological Survey show the school before and after the 1933 earthquake and how it looks today.

Deadly quakes

The death toll from the 1906 earthquake in San Francisco is the largest of all disasters in California, but most people perished as a result of the fires that engulfed the city for days. The 1906 quake predates the Richter scale but was estimated to be a least a magnitude 7.

Earthquake-related deaths

Source: U.S. Geological Survey

Concerns for California

The U.S. Geological Survey 2014 earthquake forecast indicates that the likelihood of a moderate earthquake – from magnitude 6.5 to magnitude 7.5 – has decreased, but the chance of a stronger quake in the state has increased.

Plate movement

The rate of plate movement along the San Andreas fault is approximately 1.3 inches each year –about the same rate at which fingernails grow. A USGS report released March 2 detailed a study of the southern San Andreas fault. The study found evidence of 10 ground-rupturing earthquakes ranging from magnitude 7 to magnitude 7.5 between 800 and 1857. Predictions based on the survey forecast a 16 percent chance of a magnitude 7.5 or stronger earthquake near Kern County in the next 30 years.

Prediction models

The USGS cautions that although its most recent prediction model is vastly improved since its 2008 version, its forecasts are still approximations. The USGS uses two kinds of scientific models to predict earthquake probability:

1. The earthquake rupture forecast shows where and when the earth might slip along the state’s many faults.

2. The ground-motion prediction model estimates the subsequent shaking from a rupture.

The color-coded lines indicate the state’s forecast made in 2014:

State of shake

Monitoring movement

In the late 1980s, the USGS monitored the state’s 16 main faults. In 1994, the 6.7 magnitude Northridge earthquake occurred on an unrecognized fault line. That quake is said to be the costliest in U.S. history. Afterward, scientists began to search for and monitor many other fault lines in the state.

The San Andreas fault slips at the greatest rate in central California, but Southern California has twice the chance of being hit by a magnitude 8 earthquake along it.