San José, Costa Rica, since 1956

Country Sits on a ‘Diffuse Seismic Zone’

Five small earthquakes felt throughout the Central Valley early this week did little to steady the nerves of residents still shaky from Jan. 8’s magnitude 6.2 earthquake.

But seismologists say such aftershocks are to be expected in the wake of such a temblor.

The five miniquakes occurred between 6:45 p.m. Monday and 1:09 a.m. Tuesday.

Three registered magnitudes of more than 4.0, the biggest being the last, at 4.3. All were centered around Poás Volcano, the epicenter of the Jan. 8 quake.

Costa Rica is situated over a “diffuse seismic zone,” said Javier Pacheco, a researcher with the Volcanological and Seismological Observatory of Costa Rica (OVSICORI), based at National University (UNA) in Heredia, north of San José.

Unlike the San Andrés fault, along the West Coast of the United States, which scientists have been able to clearly demarcate, most of Costa Rican territory, Pacheco said, is situated over one microplate at the convergence of the Cocos plate, under the Pacific Ocean, the Caribbean plate, to the east, and the Nazca plate to the southeast.

But U.S. scientists are more circumspect about the microplate.

“To confirm the presence of a microplate, you really need a body of evidence,” said George Choy, a seismologist with the U.S. Geological Survey’s regional office in Denver. While the Panama microplate’s existence is “generally accepted,” he said, “it’s a little premature to say.” More detailed research is expected within a few months.

A large reason for the persistent debate, said Choy, is the difficulty of mapping fault lines once the seismic activity moves over land. “A lot of people envision very distinct boundaries (between tectonic plates), but when you talk about a plate that goes on land, all of a sudden it’s not distinct anymore (because) these forces (are) distributed over a wider area.” Fault lines that break continuously in oceans, for example, become splayed when their energy is dispersed over terrain above sea level. “Even the San Andreas fault is not a single line,” he said.

The pressures from the colliding plates surrounding Costa Rica create broad stresses within the Panama microplate, creating local faults and fissures (see map below).

Pacheco described the Jan. 8 earthquake as the result of two of these local plates sliding past each other, also called transform movement. Choy further qualified the movement as a “strike-slip” mechanism, as opposed to thrust, which would have been a more head-on collision, called subduction, when one plate is pushed under the other.

Tectonic plates usually move at a rate of 77 millimeters per year.

Both scientists said it is normal for plates to shift and readjust after a earthquake. “With high seismic activity, there will be aftershocks,” said Pacheco. “After a fault breaks, all the other faults that are strengthened by it get set off.”

The aftershocks will likely last a few months, though the rate and size of aftershocks decrease rapidly with time, said Choy.

Generally, people do not feel earthquakes with magnitudes less than 3.

It could be worse. Choy said a magnitude 8 can cause aftershocks for years, and after a magnitude 9 earthquake, they can last decades.


Know What to do in a Quake? Take This Quiz


1. If you are indoors, you should:

a) Stop, drop and roll

b) Drop, cover, and hold on

c) Get outside as soon as you can


2. Standing in a doorway is always just as good as seeking cover under a sturdy piece of furniture:

a) True

b) False


3. If you are in bed during an earthquake, you should:

a) Stay there

b) Go outside

c) Try to go back to sleep


4. You should get outside as quickly as possible during the quake.

a) True

b) False


5. If in an office, it’s OK to use the elevator to get outside when shaking stops.

a) True

b) False


6. If outdoors, the greatest danger is:

a) Cracks in the ground

b) Collapsing walls, flying glass and falling objects

c) Panic and fear


7. If you are driving a car, you should:

a) Stop no matter where you are and get outside

b) Stop as quickly as safety permits and exit vehicle

c) Stop as quickly as safety permits and stay in your car

d) Keep driving till you get home


8. If you’re trapped under debris, you should NOT:

a) Light a match

b) Move around to kick up dust

c) Shout

d) All of the above



1: B. Experts say you should drop to the ground, cover by getting under a sturdy table or piece of furniture, and hold on until the shaking stops. If you aren’t near a table or desk, you should cover your face and head with your arms and crouch in an inside corner of the building.


2: False. Use a doorway only if it is in close proximity to you and you know it is strongly supported and can bear heavy loads.


3: A. Unless you are under a heavy light fixture that could fall, stay in bed while holding your head and protecting it with a pillow. In general, if you are inside, stay away from glass, windows, doors and walls, as well as anything that could fall.


4. False. If inside, you should not leave the building until shaking stops and you can determine it is safe outdoors. Most injuries take place when people indoors attempt to move outside or to a different location inside the building.


5. False. Never use an elevator during or after any natural disaster.


6. B. If outside, move away from buildings, streetlights, utility wires and other objects that may fall. The most dangerous places are directly outside buildings, at exits and along walls.


7. C. It’s best to avoid stopping near or under buildings, signs, trees, tunnels, overpasses or overhead wires. Once the tremors stop, proceed cautiously and beware of aftershocks. Avoid roads, bridges or ramps that may have been damaged and keep an eye out for fallen signs, utility poles and wires.


8. D. There could be a gas leak so lighting matches should be avoided and battery-operated flashlights should be used. Dust is particularly dangerous, so cover your mouth with clothing or a handkerchief, and alert rescuers by tapping on a pipe or wall, or with a whistle. Shouting should only be used as a last resort, as you can inhale dangerous amounts of dust.


Compiled by Tico Times reporter Patrick Fitzgerald from information provided by the U.S. agencies Federal Emergency Management Agency, Centers for Disease Control and Prevention, and Geological Survey.

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