Sections of the San Andreas Fault in California are Overdue for an Earthquake

California is known for its earthquakes. Located along the San Andreas Fault, it's had some catastrophic events in the past. Now, though, scientists have found that four urban sections of the Sand Andreas Fault system in Northern California have stored enough energy to produce major earthquakes.

Earthquakes occur partially from strain and its release as slip. The earthquake cycle in particular reflects this accumulation of strain and then slip. Fault creep is the slip and slow release of strain in the uppermost part of the Earth's crust. During large earthquakes, much greater stress is released in only seconds.

"The extent of fault creep, and therefore locking, controls the size and timing of large earthquakes on the Northern San Andreas Fault system," said James Lienkaemper, co-author of the new study, in a news release. "The extent of creep on some fault sections is not yet well determined, making our first priority to study the urban sections of the Sand Andreas fault, which is directly beneath millions of Bay Area residents."

Understanding the amount and extent of fault creep directly impacts seismic hazard assessment of the region. In fact, monitoring of creep on Bay Area faults has expanded in recent years as more residents have moved to the area. In this case, the alignment array measurements made by the San Francisco State University Creep Project and recently expanded GPS station networks provide the primary data on surface creep. Researchers then used this data to estimate the average depth of creep for each fault segment.

So what did they find? It turns out that four faults have accumulated sufficient strain to produce a major earthquake. Three creeping faults have large locked areas that have not ruptured in a major earthquake of at least magnitude 6.7 since the reporting of earthquakes by local inhabitants. In fact, the three faults appear to be nearing or have exceeded their mean recurring time; this means that they're due for an earthquake.

"The San Andreas Fault and its two other large branches, the Hayward and Northern Calaveras, have been quiet for decades," said Lienkaemper. "This study offers a good reminder to prepare today for the next major earthquake."

The findings are published in the journal BSSA.