There’s a Whole Lot of Shaking Going On

We are clearly in the midst of unsettling times, unsure of when, or if, the next shoe might drop. And it may be the last thing Angelenos need right now, but the earth itself has not been particularly settled in recent weeks either.

As many area residents have undoubtedly noticed, L.A. has had more than its share of felt earthquakes since September, including last month’s widely felt 4.0 temblor near Compton. Then there was Tuesday night’s 5.1 temblor near Anza--not within the L.A. Basin but close enough and large enough to have been felt here.

Unfortunately, the evaluation of earthquake patterns is oddly similar to the evaluation of fragmented intelligence information on national security matters: Disparate and ambiguous information almost always makes more sense after the disaster strikes than before. To the enormous frustration of experts, our ability to make definitive predictions is often extremely limited.

However, a few observations can be made. Small earthquakes occur all the time in Southern California; our seismic network records an average of 15,000 earthquakes every year. In the years following the 1992 7.3 Landers and 1994 6.7 Northridge earthquakes, however, the number of smaller earthquakes (not counting aftershocks) decreased. Many seismologists now believe that these decreases were the result of a reduction in stress caused by the big events. If this is the case, at some point the region should emerge from the “stress shadow” and return to a higher rate of earthquakes.

Indeed, over the last two months the level of small earthquakes in the L.A. Basin has risen to a rate last seen in the late 1980s and very early 1990s. During this time, the region experienced not only an increased rate of small (magnitude 2 to 4) earthquakes but also several moderate, damaging earthquakes, including the 1987 Whittier Narrows, 1988 Pasadena and 1991 Sierra Madre earthquakes.

It may not be clear for some time--months or possibly years--whether we have emerged from the stress shadow of the Landers earthquake. Even the concept of a stress shadow after large earthquakes is best characterized as a working hypothesis, rather than fact. It is further unclear that a period of normal seismic activity necessarily will include bigger earthquakes soon, or how big such earthquakes might be. The next “big” earthquake still might be years away; it also could be a relatively modest (magnitude 5.5-6.0) event.

But for the public to be able to trust its earthquake experts, we have to trust the public to accept our assessments even when we can’t offer easy reassurances or unambiguous answers. At the moment we find ourselves in just such a position, delivering messages much like the ones we’ve heard in recent days from political leaders. First, that we have experienced a period of unrest and have some signs, albeit ambiguous, that it may continue. And second, that vigilance, not panic, is in order.

The good news is that there are a number of easy, inexpensive steps that can make a difference: Check your supply of bottled water and nonperishable food, have flashlights with fresh batteries, fasten shelves to the walls, make sure heavy objects are not hung or perched above beds, store breakable objects in low cabinets with latches, and, if you haven’t done it already, now would be a good time to get that water heater strapped securely to wall studs.

Over the longer term, consider the structure you live in. Relatively minor investments can often greatly increase the earthquake resistance of a home. If there are no other seismic shoes left to drop, your modest investments will have helped prepare you for other types of disasters, while buying you peace of mind.