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MIDTOWN MANHATTAN — While the Caribbean is no stranger to earthquakes, the seismic activity that’s rattling the southern coast of Puerto Rico right now is not only stronger than the area is used to, it’s also further unique in its persistence.

Its tremors have shaken the island at magnitudes ranging from 4.5 to 6.4 for three days in the last week. The effect that can have, in real terms, was explained by Troy Morgan, the principal engineer of Exponent, an engineering and scientific consulting firm.

“The concrete itself is very brittle, kind of like chalk,” he said, describing the effect that a strong quake can have on building materials, like concrete. “It just snaps,” he said, “when you bend it.”

Morgan has a PhD in civil and environmental engineering and is a recognized expert in the fields of seismic isolation and passive energy dissipation systems. In other words, he knows very well the damage earthquakes can cause, and how to prevent damage.

Regarding something as simple as concrete, Morgan said, “We embed rebar, or reinforcing steel, in that concrete. That really helps it bend and absorb energy during an earthquake.”

In the case of the southern Puerto Rico quake, even some reinforced concrete couldn’t hold up to the temblor.

Morgan explained what was specifically happening underground.

“Two tectonic plates, two parts of the Earth’s crust, basically, meet one another,” he said.

The North American Plate and the Caribbean Plate, he added, are causing the Puerto Rican seismic activity.

“As they slide,” Morgan said, “that movement releases energy, and that energy causes ground shaking, and that ground shaking causes damage.”

He also said that it’s common for quakes to occur 30 kilometers, or about 19 miles, beneath the surface. The ones this week in Puerto Rico were much more shallow.

The 6.4 magnitude temblor felt on the island on Monday, Morgan said, “was only 10 kilometers deep, which is relatively shallow. That’s where you start to see a lot of damage.”

According to the U.S. Geological Survey, the island is one of the most active locations in the U.S. for earthquakes. However, Puerto Rico’s worst quakes are usually in the northwest of the island, or off its coast.

In fact, on Aug. 4, 1946, an earthquake with a magnitude of 8.1 was centered in Mona Passage, the stretch of sea between Puerto Rico and the Dominican Republic, northwest of Puerto Rico. The quake, and its subsequent tsunami, resulted in more than 2,550 deaths.

Also, in 1787, a quake centered north of Puerto Rico was estimated to have registered a magnitude of up to 8.25 (some experts say that it was more likely 6.9). Even though the temblor was felt across the entire island of Puerto Rico, there were no reported deaths.

They were considerably less intense than those episodes is the current one. The quakes experienced in the last week are the latest ones, and they are strong, relative to past seismic activity on the south shore of the island.
The quakes have occurred exactly ten years this coming Sunday from the Haitian earthquake, in which at least 100,000 people died.

Whether its mass destruction, as seen in the Haitian quake, or significant property damage, and few people lost or injured, as in the recent Puerto Rican quake, they’re reminders that any potential natural disaster is more bearable, according to Morgan, if people will “be prepared,” he told PIX11 News.

“Have your 72-hour emergency kit with water and food and flashlights,” he suggested.