The Telltale Waves

By detecting slight shifts in the tilt of the ground, or leakage of underground gases, or local changes in the natural magnetic field, scientists can determine that dangerous stresses and strains are building up in the earth. Yet they are still unable to predict reliably when or even where earthquakes will strike. Now, as a result of Russian findings in a remote region of Central Asia and a parallel discovery in New York State, seismologists may well have moved a little closer to a long-sought goal: developing an accurate early warning system for major upheavals of the earth.

That possibility is based upon studies of the two basic types of seismic waves that are given off by all earthquakes: 1) P (or pressure) waves, which alternately compress and expand the earth in the direction of their travel; and 2) S (or shear) waves, which cause motion of the earth in a direction perpendicular to their path. Because a quake's P waves travel through the earth slightly faster than its S waves, they arrive at seismic listening posts ahead of the S waves. While investigating the small tremors that often occur in the Garm region south of the Central Asian city of Tashkent, Russian seismologists were surprised to discover that in the days or weeks before a serious jolt, the relative velocities of the two types of waves changed. The interval between arrival times decreased significantly. Then, just before a big quake, the velocity relationship reverted to normal.

At first, Western seismologists suspected that the change in velocity was peculiar to the geology of Central Asia: it seemed unlikely that the phenomenon could be used as a predicting tool in other quake-prone areas. Yash Aggarwal, a 30-year-old graduate student of Indian descent at Columbia University's Lamont-Doherty Geological Observatory, did not share the skepticism. As part of his doctoral work, he decided to study the seismic records of the swarms of microquakes that had occurred during 1971 in the Blue Mountain lake region of New York's Adirondack Mountains. Aggarwal's hunch paid off. Writing in Nature, he and his associates report that they also found large and significant changes in the relative velocity of P and S waves prior to more serious tremors. Furthermore, they note, the duration and intensity of the effect--which changes the relative velocity of the waves by as much as 13% --was directly proportional to the magnitude of the eventual jolt.

Aggarwal, as well as his mentor. Seismologist Lynn Sykes, thinks the change in wave velocity may be caused by the rapid opening of small cracks in water-saturated underground rock of the fault zone. Because P waves travel swiftly through water, they probably slow down when the voids appear. The S waves seem less affected by the fissuring. Then, as ground water seeps into the cracks, the P waves speed up again. Seismologists do not know how widespread the newly discovered phenomenon is, but if it is indeed common to all seismically active areas, it may eventually be used to predict the earth's upheavals--including such disasters as the quake last December that destroyed much of Managua, Nicaragua.

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