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Did you feel that??? Print E-mail
Anyone on the Nicoya Peninsula on September 16, 2010 certainly felt “that”. At 9:27am, with a roar like a low-flying 747, a 5.3 earthquake rattled the area. It was centered about 5 miles offshore from Montezuma and Cabuya and only 13 kms (8 miles) deep. Several “aftershocks” between 2.5 and 4.3 occurred over the next few days, plus many hours of barely perceptible “grumbling”. Fortunately no serious damage or injuries resulted. Two much bigger 'quakes have occurred; a 7.7 in 1950, and a 7.0 in 1990, which although frightening, caused relatively little damage, because the area was much less developed with a smaller population. Today, those same earthquakes would cause widespread property damage and injury.

Richter magnitude represents the maximum strength of vibration and the distance from the epicenter of the earthquake. For each one point increase on the logarithmic scale, the magnitude increases by 10. A 6.0 is 10 times stronger than a 5.0; 7.0 is 100 times stronger than 5.0; 8.0 is 1000 times stronger than 5. Less well-known is that each one point increase releases 32 times more energy.

The Nicoya Peninsula is directly over one of the most dynamic fault intersections on earth - the “Nicoya Seismic Gap”. The Pacific Cocos Plate, moving east at about 10 cm/year (4”/year) is forcing itself under the Caribbean Plate - a classic subduction zone, creating a megathrust potential. The Cocos Plate under the Nicoya Peninsula is slowly sinking into the magma below, and taking the peninsula with it at about 1” (25mm)/year, according to Dr. Marino Protti, Director of the Observatory of Volcanology and Seismology (OVSICORI), in San José, Costa Rica.

As if it were spring-loaded, and cocked to release every 50 years, the Nicoya Peninsula is dragged down by the Cocos Plate into the Pacific Ocean, until sufficient stress builds up for it to break free and snap back, causing a large earthquake. Large earthquakes (7 or higher) have occurred in the Nicoya gap in 1853, 1900 and 1950. After 60 years, Dr. Protti believes the Nicoya gap is overdue to generate an earthquake higher than 7.5. However, researchers recently detected “slow slip events”, also known as "silent earthquakes", that involve similar motions as an earthquake, but happen so slowly that the ground doesn't shake. Slow slip detection requires special instru-mentation allied with Global Positioning Systems (GPS) to precisely measure the movements of the Earth's crust over time.

Susan Schwartz, Professor of Earth and Planetary Sciences at the University of California, Santa Cruz, leads a team that installed a network of GPS/seismic monitoring stations on the Nicoya Peninsula. "At least two slow slip events have occurred beneath the Nicoya Peninsula since 2003," Schwartz said. "We recorded the first one in 2003 with only 3 GPS stations. By 2007, we had 12 GPS stations and over 10 seismic stations, so that slow slip event was well recorded and showed that the peninsula experienced the equivalent of a 6.9 magnitude earthquake over a period of 30 days instead of the usual 10 seconds for a quake. “Previously we focused on areas of the fault that lock up, then suddenly slip, causing an earthquake” explained Schwarz. “This new discovery shows that slow slip is occurring in regions that are not strongly locked, and the big question is whether slow slip is loading the fault, leading to a big earthquake, or whether slow slip is relieving stress on the fault."

 

Schwartz feels slow slip events probably don't increase the likelihood of a major earthquake, but instead may well relieve the stresses that lead to a large earthquake. She warned that scientists still don't fully understand the implications of different kinds of fault motion.

Can we predict earthquakes?

Accurate earthquake prediction may become a reality some day, but only after much more is learned about earthquake mechanisms. For example, during the winter of 1975, Chinese officials ordered the evacuation of Haicheng in northeast China, based on a wide range of unusual observations, like changes in land elevation, ground water levels and widespread accounts of peculiar animal behavior. An increase in seismic activity (later recognized as foreshocks) triggered a low-level alert.

Later, increased foreshock activity triggered the evacuation warning and the 7.3 earthquake struck days later on February 4, 1975. Death and injury was relatively light - out of about 1 million people, 2,041 died and 27,538 were injured. Fatalities and injuries might otherwise have exceeded 150,000. The following year, on July 28, 1976, without any warning, a 7.6 earthquake struck the industrial city of Tangshan, with about 1 million inhabitants. None of the precursors observed near Haicheng were observed at Tangshan. The earthquake caused an estimated 250,000 fatalities and 164,000 injured. The 1975 Haicheng evacuation was triggered mainly by the pronounced foreshocks; large earthquakes can happen without any foreshocks.

The opposite occurred at the most heavily instrumented seismic location on earth, the San Andreas fault at Parkfield, California. Earthquakes with magnitudes of around 6.0 have occurred

on average every 22 years since 1857 through 1966, so a prediction was made for 1988 +/- 2-3 years. Today in 2010,

44 years later, it still hasn't happened.

Go to www.nicoyazoom.com for more

info on earthquakes.

Richter magnitude is only accurate for earthquakes up to 300 miles (500 kilometers) away. Scientists use other scales based on ”body waves” and surface waves, but no single measurement is perfect. Body waves, the initial shock waves emanating in all directions from the source, aren't accurate for very strong earthquakes. The body wave causes the roaring sound as the wave hits the surface and vibrates the surrounding air. Surface waves cause the damage but weaken with increasing distance. The strongest shaking occurs at the epicenter - the point on the surface directly above the focus of the earthquake. This can be a few miles or 400 miles down, but below that, rocks are too hot to store strain, so they simply deform like clay. In powerful earthquakes, ground-shaking overcomes gravity and tosses boulders into the air, as in the Assam (India) earthquake of 1897. A local witness, on Jaco beach when the 6.2 hit near Quepos in 1992, said sand and gravel shot several feet into the air.

What’s next?

Anyone on the Nicoya Peninsula on September 16, 2010 certainly felt “that”. At 9:27am, with a roar like a low-flying 747, a 5.3 earthquake rattled the area. It was centered about 5 miles offshore from Montezuma and Cabuya and only 13 kms (8 miles) deep. Several “aftershocks” between 2.5 and 4.3 occurred over the next few days, plus many hours of barely perceptible “grumbling”. Fortunately no serious damage or injuries resulted. Two much bigger 'quakes have occurred; a 7.7 in 1950, and a 7.0 in 1990, which although frightening, caused relatively little damage, because the area was much less developed with a smaller population. Today, those same earthquakes would cause widespread property damage and injury.

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