A Seismologist Sizes Things Up // A conversation with Professor Shmuel Marco, Tel Aviv University

By Shira Leibowitz Schmidt

Israel has an alarmingly small number of seismologists—scientists who study earthquakes. This is in spite of the fact that it lies along a major fault line known as the Dead Sea Fault, the most impressive tectonic feature in the entire Middle East. The word “seismology” is derived from seismos, Greek for “shaking.” Indeed, the events of last week—the earthquakes in Turkey that have killed at least 35,000 people as of this writing—have certainly shaken the world.

I had the privilege to speak to Professor Shmuel Marco, a seismologist in the Geophysics Department of Tel Aviv University. His research has focused on earthquakes and other seismic events over time, including those in the archaeological record.

When you say you study earthquakes, what exactly does that involve?

I focus on pre-seismograph earthquakes, mainly reconstructions of the past earthquake history of the Middle East using historical records, analyzing archaeological evidence for earthquake damage, and exploring the physics of lake sediments subjected to earthquake shaking.

Were you surprised by the location of the devastating earthquakes in Turkey?

No one—at least no one in the geologic community—was surprised. Earthquakes take place only in regions where they have happened in the past.

Could these earthquakes have been predicted?

There is no way to predict when or exactly where a future earthquake will happen. Nobody in the world knows. All of the precursors have proven to be anecdotal. A precursor to an earthquake can happen once or twice, but not regularly.

Can you give us an example?

In some places, a strong earthquake is preceded by a swarm of small ones, but in many other places, swarms of weak earthquakes aren’t followed by strong earthquakes. Swarms are therefore not a good sign or predictor. Another phenomenon that has turned out not to be a predictor is quiescence. An area can be perfectly quiet and inactive in terms of seismicity, and then a strong earthquake will take place, so quiet isn’t a reliable precursor either. There is no way to forecast precisely.

There is a widespread belief that animal behavior can be an indicator of an impending earthquake. Is that a myth?

There is no proof that any animal behaves differently before an earthquake. If you have a pet and I asked you how the pet behaved this morning, you might remember. If I asked you how the pet behaved on July 17, 1998, there is no chance that you’ll remember. But if there was a strong earthquake on that date, you will naturally associate it with many things and remember numerous details. This is simply a psychological phenomenon related to our memory, but it is not something that can be repeated or relied upon.

What do you say when asked where the next earthquake in Israel will take place?

My answer is that it will occur in the same region where earthquakes happened in the past. They keep occurring in the same general areas again and again, although not like clockwork, meaning that they are separated by the same interval. These are mostly known places on the edges of tectonic plates—massive, irregularly shaped slabs of solid rock that are on average 62 miles thick beneath the earth’s crust. When two plates grind against each other, it generates an earthquake. We know how to map the tectonic plates, and we know where the earthquakes are.

Israel lies to the west of the Dead Sea Fault, on the boundary between the African Plate and the Arabian Plate. The boundaries form narrow strips on the crust of the earth. Along the plate boundaries, we can be 100 percent sure that earthquakes will occur in the future. This is true not only for our neighborhood in the Middle East but for California, Turkey, Japan, Chile and many other places. By contrast, there aren’t any earthquakes in the Sahara Desert or in the jungles of the Amazon. They are also extremely rare in the middle of the European continent.

What about the likelihood of one earthquake following another soon after in the same place?

Two earthquakes don’t usually occur one after another in exactly the same place. An earthquake is a rupture in the crust of the earth. After it breaks, you cannot break it again in the same spot. It’s like smashing a glass bottle against the floor. It takes hundreds of years for the rupture to heal. The geological processes act like glue, putting the plates together. Only after they are cemented are they ready for another quake.

This is why, in twin earthquakes, there is a gap of decades or centuries. The plates move slowly, at a speed similar to the growth of our fingernails. You don’t see your nails growing. But in a month they’ll grow about five millimeters (0.2 inches), and in a year the nail will be a few centimeters longer.

In geology there is plenty of time. After a significant time span, it’s possible for another break to take place. The two strong quakes in Turkey were in two differentplaces.

So it isn’t likely that there will be another one in the same spot. But what about nearby locations?

The quakes were on two different faults. An earthquake changes the stress regime around it, so the first one triggered the second one in another fault dozens of kilometers away. A fault is a break in a weak zone in the crust. The second earthquake wasn’t in the same place as the first, but it was in the general neighborhood.

Has Israel experienced any aftereffects of the earthquakes in Turkey?



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