An Alaskan Perspective Of The Mexico Earthquake

Shortly after the news of the Mexico earthquake broke on the morning of September 19th, the Geophysical Institute began receiving calls requesting clarification of some of the early headlines. One of the more sensational of these was the announcement by a popular national news commentator that "California had been raised an inch" by the event.

While the seismology staff at the Geophysical Institute would not have used quite those words, they could have pointed to records showing that the ground was also rising about an eighth of an inch right here in Alaska at the very time that they were talking on the telephone. As a matter of fact, it was rising and falling, and rising and falling. The ground surface all over the world was bulging in and out, and it would continue to oscillate like that for hours.

What the commentator neglected to mention (or, more likely, failed to understand) is that he was referring to only part of a cycle in the waves that accompany every great earthquake. Viewed from the proper perspective, it is not as sensational as it at first sounds.

Land surfaces bulge many inches every day from tidal forces, and we never notice because it happens so slowly. The surface waves (contrasted with waves that travel deep inside the earth) that emanate from great earthquakes can travel around the world many times, slowly raising and lowering the ground surface. Although they travel along the surface at about 2 1/4 miles per second, the waves are fifty miles long and less than an inch high. A single oscillation lasts about 20 seconds, and the motion is discernible only to instruments. The really damaging waves are the ones that vibrate rapidly, but these are damped out at comparatively short distances from the epicenter.

In a way, these long-period surface waves are much like tsunamis, or tidal waves. When a tsunami crosses an ocean, it, too, is only a few inches high and hundreds of miles long. It will pass a ship at sea completely unnoticed, and only when it begins to pile up on itself in shallow water does it become dangerous. Surface waves from earthquakes do not pile up on themselves in this manner, but they can circumnavigate the earth more than once. This is something that tsunamis can't do because they keep bumping into continents.

There was also a report in the national news of a retired gentleman who had predicted that a major earthquake would occur off the west coast of North America to within a day or so of when the Mexico earthquake actually struck. Although details of his calculations were not given, it appears likely that his prediction was based on the anticipation of forthcoming high tidal stresses in the earth's crust. If so, it bore at least some scientific merit.

The Mexico earthquake occurred during a period of new moon, when tidal stresses are at their strongest. It is not a new idea that tidal forces may occasionally trigger major earthquakes. Seismologists believe that they may sometimes provide the last bit of stress necessary to rupture the crust in an area where stress has been otherwise quietly accumulating for many years. The Mexican epicentral area was in just such a zone.

And so was the epicentral area of the great Alaska earthquake of 1964. It may (or may not) be significant that that earthquake, the greatest ever recorded in North America, occurred at a time of extreme low tide. Although it might seem illogical, this condition places just as much strain on the earth's crust as does a period of high tide. The ground surface underlying parts of the Gulf of Alaska was permanently displaced many feet upward by the earthquake at a time when it was unweighted by the tides.

Regardless of the more tragic consequences in Mexico (you would have to hunt for many years to find a worse place than Mexico City to build a city in an earthquake-prone country) the Alaska earthquake was by far the stronger. The magnitude 8.4 Alaska earthquake of 1964 released fully eight times as much energy as the magnitude 7.8 Mexico event.