Last Monday, a 6.8-magnitude earthquake rattled northern California, followed by 24 aftershocks.
One week later, a much smaller tremor, with a magnitude of 4.4, shook Los Angeles. That quake was centered about six miles north-northwest of Westwood, Calif., reports Reuters. Some aftershocks followed, with the biggest one having a magnitude of 2.7.
Are these quakes a precursor to a big one?
Well, there is “always the possibility that it’s a foreshock,” Robert Graves, seismologist with US Geological Survey, told LA Times. He added, "We’re continuing to analyze the data, but at this point, this seems to be what I would call a rather typical earthquake."
According to USGS, foreshocks are "relatively smaller earthquakes that precede the largest earthquake in a series which is termed the mainshock. Not all mainshocks have foreshocks."
But there is no way to predict foreshocks in advance, say seismologists.
Earthquake are frequent in California, but many of them happen far from population centers, out in the desert or in the ocean, says Erin Burkett, geophysicist with the USGS Science Application for Risk Reduction (SAFRR) Project.
But not always. The epicenter of the 7.8-magnitude earthquake that struck San Francisco in 1906 was just two miles offshore. That quake killed 700 people and caused more than $500 million in damage (about $13 billion in today's dollars).
The Pacific Plate, the world's largest tectonic plate, is sliding northwest past the American plate. But the movements of these plates, however, are about the rate of growth of fingernails, says Lisa Grant Ludwig, professor at UC Irvine.
For the plates to move, certain adjustments have to be made in the alignment around them. This is what is causing these earthquakes, says Egill Hauksson, seismologist at Caltech Seismological Laboratory.
Kate Hutton, Staff Seismologist at the California Institute of Technology Seismological Laboratory says that such earthquakes could help researchers learn about fault structures within that particular area.
Were these two latest quakes connected? Probably not, say scientists. But seismologists predict that a 6.5 California quake or slightly bigger is imminent. But when and where it will hit is anybody's guess, says Peggy Hellweg, Operations Manager at the Berkeley Seismological Laboratory.