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IS THE much feared “big one” about to strike? That question is back in the minds of many in California after the strongest quakes for two decades struck the state. But we don’t know if these have made an even bigger earthquake any more or less likely.

A magnitude 6.4 tremor struck southern California on 4 July, followed by a magnitude 7.1 quake on 5 July, with hundreds of smaller aftershocks in their wake. It was fortunate that the epicentres were in a sparsely populated region, near the city of Ridgecrest.

No one was killed, but the quakes were felt across the state. They left a long crack in the desert and damaged buildings and roads in the area (pictured above).

The San Andreas fault runs the length of California and lies near major cities such as Los Angeles and San Francisco. A major quake, possibly of magnitude 8 or greater, could occur on the fault at any time, potentially killing thousands of people and causing hundreds of billions of dollars in damage.

However, the latest tremors occurred on different faults far from the San Andreas. Geologists don’t know whether these latest quakes have made a major earthquake on the San Andreas fault more likely or not.

There is no way to predict big quakes hours or days in advance, and some think it may never be possible. Some regions have early warning systems that issue alerts as soon as the first tremors are detected, giving people a few seconds or more of warning before the most dangerous shaking starts. The US Geological Survey is currently developing such a “ShakeAlert” system for the west coast.

Modern buildings in California are designed to withstand sizeable earthquakes, though many could still be left unusable. However, the state has many older buildings that need retrofitting to be safe.

The vast Sacramento river delta is also protected by embankments that could fail in a big quake. If that happened and seawater floods the delta, it would cut off a lot of the fresh water supply to southern California for a year or more.