He also refined some calculations of recurrence intervals to consider whether past earthquakes were correctly interpreted as unique earthquakes. McPhillips calculated recurrence intervals for sites on the San Andreas, Hayward, and San Jacinto faults. Those numbers help them determine a recurrence interval, which is the average time between earthquakes at the same site along a fault. They tally how many past earthquakes they see and determine the number of years separating each earthquake. Inside a trench, they can see when and how frequently an earthquake hit that spot. To expose what happened, researchers can dig trenches along a fault. For instance, fault scarps (when a side of a fault has moved vertically with respect to the other), offset or folded layers of sediment and soil, and parts of the landscape that have been tilted, uplifted, or torn apart are all signs of an ancient earthquake.įor the most part, researchers only see evidence for earthquakes above magnitude 6, because smaller quakes don’t typically leave behind evidence. Whenever an earthquake happens, it leaves telltale signs that eventually get buried or imprinted into outcrops and other geologic formations. Using past clues to understand future earthquakes

Notably, the last six quakes (from 1134 to 1868) occurred at intervals of 95 to 183 years, with an average interval of about 150 years. USGS scientists have found evidence for 12 quakes on the southern Hayward Fault during the past 1,900 years. The Hayward Fault’s most recent damaging earthquake was more than 150 years ago. In addition to the San Andreas Fault, McPhillips also studied two additional sites on the Hayward and San Jacinto faults.

Geologists find that the total accumulated displacement from earthquakes (sudden fault movement) and creep (steady fault movement) is at least 350 miles (560 km) along the San Andreas fault since it came into being about 15-20 million years ago. In southern California, the last rupture of the San Andreas Fault occurred in 1857. The relative movement along the fault varied from 2 to 32 feet (0.5 m to 9.7 m), which means if two people were standing on opposite sides of the fault, in some places one would slide away from each other by up to 32 feet from the other. The San Andreas Fault first captured the world’s attention on that fateful April day 116 years ago when it ruptured along the northernmost 296 miles (477 km) of the 800 mile- (1300 km) long fault that marks where the Pacific plate slides by the North American plate. “If some of these older earthquakes are misidentified, then the long wait we are currently experiencing might be more typical.” It's California's fault “The wait between earthquakes might be longer than we previously thought because the evidence for older earthquakes isn’t always 100% accurate,” McPhillips said. In a new publication, USGS geologist Devin McPhillips studied sites along three faults in California with long histories of earthquakes to better understand what’s happening. Although no one can predict when an earthquake will occur, scientists can better assess seismic hazards by calculating the probability of a ground-rupturing earthquake occurring in a specific area within a certain number of years.Ĭonsidering the historical average rate of large earthquakes on some faults in California, including the San Andreas, the odds that there hasn’t been a major earthquake in the last 116 years are very low. Researchers study older significant earthquakes like this one to better understand how frequently they strike on a fault. The earthquake and subsequent fires killed an estimated 3,000 people and left half of the city’s 400,000 residents homeless. The magnitude 7.9 quake caused shaking that lasted up to 60 seconds and sent vibrations all the way to southern Oregon, Los Angeles, and central Nevada. The Great San Francisco Earthquake struck at 5:12 a.m. The San Andreas may be the most well-known fault in California, but no one alive today experienced its most recent significant earthquake that ruptured 116 years ago.

Earthquakes in california