San Francisco: earthquake overdue

At four sections below the city, the tension in the rock is dangerously high

The Bay of San Francisco: Four sections of fault are here under enormous tension. © Rich Niewiroski Jr./ CC-by-sa 2.5 us
Read out

Megacity in danger: The subsoil of San Francisco is in four places under enormous tension, as US seismologists warn. Along these sections of the San Andreas system, the rock is hooked, it threatens a discharge in the form of a strong quake. Also in other places in the region, the return period for strong quakes has already been reached or exceeded.

California lies on a plate boundary: Here, the Pacific plate moves past the North American plate. This transform disorder is branched in places and divided into several sub-distortions. So also in the region of San Francisco, where east of the San Andreas Fault run the major disturbances Hayward and Calaveras along with some neighboring branches.

When the creeping stops, danger threatens

As long as the plates slide past each other at these cracks in the earth's crust, the risk of earthquakes is low. But if the rock gets stuck and can not move, tension builds up, which jerkily discharges when the rock suddenly breaks under pressure. "The extent of creep or hooking controls the strength and timing of large earthquakes in the northern San Andreas system, " said James Lienkaemper of the US Geological Survey (USGS).

To better estimate the danger, the subsoil movement in the San Francisco area is monitored by seismometers and GPS stations. With the recent development of the monitoring network, researchers have now pinpointed the acute threat to the San Francisco Bay area - with unsatisfactory results.

Dislocations around San Francisco © USGS

Alert for four sections under San Francisco

Most of the faults, they found, provide sufficient tension by moving slowly a few millimeters per year. But in 28 percent of the cases, the rock is entangled underground - and has been for a long time. Researchers have estimated that four sections of the fault system under the city of San Francisco have already stored enough energy to produce larger earthquakes. display

Three more sections - Hayward, Rodgers Creek and Green Valley have already reached or exceeded their typical quake recovery period. Even there, a quake is therefore overcrowded. "The San Andreas Fault and its two major sides, Hayward and Northern Calaveras, have been quiet for decades, " warns Lienkaemper. However, that could change in the near future. "Our study urges us to prepare for the next big earthquake today." (Bulletin of the Seismological Society of America, 2014)

(Seismological Society of America, 14.10.2014 - NPO)