By Dr. Robert Thorson

Has the recent disaster in Japan made you jittery about large earthquakes and tsunamis? Are your family and friends waiting for the next “Big One” somewhere along the U.S. Pacific Coast?

If you’re concerned about earthquake magnitude and tsunami potential, then I suggest you focus your thoughts northward, away from central and southern California to the coasts of Washington, Oregon and northern California. In this region, coastal salt marshes contain compelling geological evidence of seven large tsunami-generating, mega-thrust earthquakes within the past 3,500 years – or one every 400 to 600 years.

The last one struck in 1700, and was so strong it was recorded in Japan. This was more than a century before the American discovery expedition of Lewis & Clark, which is why so few people are aware that megaquakes are a very real concern in this region. The next one is due at any moment.

If, however, your concern for the Big One involves the potential for death and destruction, then I suggest you keep your thoughts in central and southern California, where the earthquakes are smaller, but where the threat is higher. Here, the problem is mostly about human population and infrastructure, both of which are roughly an order of magnitude higher than in the north. Additionally, large cities are built in basins directly above faults rather than at distant removes, and a pair of nuclear reactors – Diablo Canyon and San Onofrio – bracket Los Angeles.

Within the past few weeks, dozens of people have asked me about the Fukushima disaster in Japan and its implication for the United States. Few people appear to understand the pronounced contrast in tectonic style between the northern and southern parts of the U.S. Pacific Coast. Confusion between these two regions – especially with respect to Japan – is generating undue anxiety.

To the north, the oceanic plate is being stuffed eastward beneath the edge of the North American continental plate. This is a fairly regular and predicable process in which the offshore boundary becomes locked by friction and the plates flex while strain accumulates, until finally the bond breaks. This allows the oceanic plate to lurch downward, only to be locked again.

Because the slip takes place over such a large area, the magnitudes are consistently very high and the vibrations are strong and long-lasting. Because fault slip takes place beneath the sea, a large tsunami is generated. Because the plate slides downward, it melts to produce the volcanic arc of the Cascades, the namesake for what geologists call this region, Cascadia.

To the south, in central and southern California, the oceanic plate slides sideways and northward with respect to North America along a great shear zone more than a hundred miles wide. Within it are slivers and blocks of continental crust that are being squeezed, rotated, folded and up-thrust in a complex array. Between each sliver and block is an onshore fault rising to or near the land surface.

Here, the main hazard is not magnitude, but the proximity of tens of millions of people to shallow faults capable of producing earthquakes between magnitudes 6 and 8. The tsunami threat is minimal.

In Cascadia and northern Japan, coherent crustal slabs slide against one another. There, the main unknowns involve the location and size of tearing events on a local rupture. In California, however, tearing events occur on different faults, all of which interact in complex ways. Though there has been tremendous progress in earthquake preparedness and forecasting in California, the chaotic properties of this system make short-term prediction an impossible dream.

What is your seismic psychology? Would you prefer to live with astonishing displays of power at infrequent intervals? Or would you rather take your chances with more frequent events of lower strength? But, then again, perhaps you are like me, preferring to stay out of trouble here in the land of steady tectonic habits. The geographic choice is yours.