Understanding Earthquakes Makes for Messy Science

by Susan Hough

Originally published in the LA Times, May 13, 1994

For most people, direct exposure to scientific inquiry comes primarily from classes taken in junior or senior high school. In those classes, the process of science appears straightforward enough: you listen to the teacher, understand what is being taught and, if you do your homework, arrive at the same answer as everybody else, the answer in the back of the teacher's book. If you do an experiment right, you get the same results as everybody else. There is none of the subjectivity that a social-studies teacher might appear to use when she or he appraises your logic in an essay. Physical science is objective stuff: black or white, right or wrong.

Except that it isn't. At the front lines of scientific research, results are often subject to interpretation, especially in fields that are inherently observational. In particular, earthquake studies--including seismology, geology, and tectonophysics--rarely dwell in unequivocal results.

Nonetheless, after centuries of endeavor, it is impressive to take stock of how much is understood about earthquake processes. We know that the Earth's crust is broken into plates; we know how they move relative to each other and at what rates. We can reconstruct the position of the plates going back hundreds of millions of years; we understand how the San Andreas fault came into existence; we can predict how the fault (and California) will look millions of years in the future.

We have a record of earthquakes on the southern San Andreas fault going back thousands of years and can estimate an average repeat time for certain segments of the fault. We understand that the Los Angeles Basin is beingly broadly compressed from north to south due to a bend in the San Andreas, and that the compression is causing the San Gabriel and Santa Monica Mountains to be pushed upward on systems of faults at their bases. We have mapped some of those faults and inferred the existence of others using sophisticated seismic techniques.

And yet, the questioning goes, why can't we predict earthquakes? One part of the answer lies in the nature of dynamic natural processes; the earthquake cycle is characterized by an inherent element of chaos that tends to confound attempts at prediction. Earthquake processes also play out, for the most part, tens of miles below the Earth's surface, away from easy observation, and they play out over geologic time. Within the context of our broad understanding of plate tectonics, it is not significant whether the next great San Andreas earthquake occurs tomorrow or 50 years from now. That is not true, of course, from a societal viewpoint.

After significant earthquakes in Southern California, there is renewed public interest in understanding exactly what happened and possible implications for the future. So the curtain hiding the wizard is thrown open, revealing to the world what may appear to be chaos and disarray. Seismologists don't seem to be able to agree on a magnitude, much less identify a fault, much less predict the next quake.

What is revealed is the scientific process, in all of its rarely unequivocal glory. Some of the apparent contradictions stem from complexities in science that are understood, but not always easily communicated. There are, for example, several different magnitude scales, of which the Richter (or, more precisely, Gutenberg-Richter) is perhaps the most antiquated. Depending on the details of earthquake rupture, any given event can have different magnitude estimates, depending on which scale is used. Also, any magnitude estimate contains an inherent level of uncertainty; estimates will often be revised as additional data are analyzed.

But other confusions, such as the identification of a fault responsible for the Northridge earthquake, result from the inherently messy nature of the scientific process. This earthquake, like scores before it, occurred on a previously unidentified fault. It is important to note that, in a broad sense, there is no real element of surprise within the scientific community: The earthquake fits into our understanding of the tectonics of the Los Angeles area, and we know that there are many buried faults that have not yet been identified. Yet it is important to understand the details of the Northridge earthquake, and the process of sorting out those details will almost inevitably involve diverse interpretations and preliminary results that prove to be wrong. That, in the end, is a reasonable microcosm for the scientific process in general: Theories are proposed and tested; some are discarded and some stand the test of rigorous inquiry. Usually, however, the process does not play out live on the nightly news.

Most Earth scientists take seriously their responsibility to provide answers in the immediate aftermath of an earthquake. We do our best to answer questions and to relate what we know. What we ask of the public is this: to accept the hard truth that sometimes the scientific community just does not have the answers; to read and listen carefully to what information is presented; to understand that we aren't in a high-school chemistry class back in Kansas, but rather, someplace that isn't always black and white.