We’re used to thinking about the consequences of coastal erosion, and maybe even of sea level rise: things that make the shoreline slowly recede or the water slowly get higher, resulting in problems either way. What we’re not necessarily accustomed to—but perhaps should be thinking about now—is a sudden, dramatic change in altitude.
The culprit in this case is not erosion but earthquakes. A new report from Cal State Fullerton shows that three earthquakes, ranging from a couple of thousand years to 700 years ago, caused portions of the California coastline to drop as much 3 feet, instantly. The study looked specifically at the Seal Beach wetlands in Orange County, but scientists say further quakes along the same Newport-Inglewood Fault could cause the same phenomenon in other areas. That’s bad news for the western portion of Los Angeles, Long Beach, and San Diego.
The quakes would have to be large. In 1933, a magnitude 6.4 quake in Long Beach did cause rapid subsidence, though not as dramatic as the earlier quakes the report describes; the three earlier earthquakes would have been even larger than that. Because of the orientation of the fault, a major quake is likely to cause land to drop rather than move horizontally, which is more typical. It was previously believed that this particular fault would produce a major earthquake only once every 2,300 years, on average, but the new report says that estimate was wrong, and the average time between them is more like 700 years. The last major earthquake to cause a dramatic sinking of the coast was in roughly 1450—about 567 years ago.
The authors of the report reached their conclusions after finding buried organic material resulting from the sudden death of surface vegetation, likely caused by inundation with saltwater. You can read more about the process of collecting and analyzing sediment samples here. Nearly two years ago, this article in the New Yorker reported on a similar phenomenon in the Pacific Northwest; the article predicted that another huge earthquake could hit the region at any time—in fact might be overdue. The article went viral and won its author, Kathryn Schulz, a Pulitzer Prize; prompted the White House to host an Earthquake Resilience Summit; and reportedly inspired some Oregon and Washington residents to pack up and move east. (Schulz offers some clarification of the original article and commentary on the reaction to it here, including a detailed explanation of what constitutes “overdue” to seismologists.)
For many of us living near an ocean, it’s one more thing—along with all the causes of slow long-term coastal changes—to think about at three o’clock in the morning, if we’re so inclined. In any case, the process of sampling and reading the centuries-old sediment deposits is pretty darned interesting in itself.