Rocks break, earth quakes

By Jennifer Barrera - Staff Reporter



The impact of twin earthquakes in Mexico last month were felt in last Friday's Science Seminar.

On Sept. 8, Chiapas got hit by an earthquake the magnitude of 8.1. Then, on Sept. 19 Mexico City was rocked by a magnitude 7.1 quake. Then the Mexico City quake hit — about 45 buildings collapsed, hundreds lost their live and some people were under as buildings collapsed.

Jacob Selander, a Highline geology professor, focus on how earthquakes work.

"I won't be focusing on the damage and result in Mexico," Selander said. "I will focus on more the earthquake [mechanics.]"

Looking at an earthquake map shows that the location where the earthquake hit was near of the surface. Colors represent how intense the earthquake was or how old is each one is and the size represents how big each one was.

"How does the crust or earthquake behave during an earthquake?" Selander asked.

"Pressure from fault lines going over each other on intersecting each other is released all at once," a student responded.

"I can skip my next 10 slides. Thank you," Selander said.

When a rock being bent breaks, it causes an earthquake. After the rock breaks, the seismic waves of energy released by the earthquake travel through the earth's crust. This produces a shock wave. When the waves are traveling, and intersect near the surface, there is violent shaking.

"A good sample is clapping your hands that creates a shock wave that travels through air and hits your eardrum," Selander said.

One theory used to explain how the rocks break down and make an earthquake is called "Elastic Rebound Theory" when stress is applied to a rock to its breaking point, it causes them to move around. But, the rebound part makes the rock go back to its initial shape and go into a new position. That continues to apply stress and the cycle is repeated over and over.

"Applying more strain and elastic could cause a bigger earthquake," Selander said.

A second theory that explains the movement of the plates is called "Plate Tectonics Theory" where there are three different boundaries. First, convergent boundary is when two plates move towards each other and collide each other.

Second is a divergent boundary when two plates move away from each other. The last is a transform where two plates move past each other horizontally.

"When a plate has moved we can measure the plate with geologic record or satellite," Selander said.

Mexico City is built on a lakebed that has loose wet sediments to liquify the soil. When systemic waves vibrate, it causes the crust to move farther away the soil cracks and is pushed down.

The Pacific Northwest earthquake are not parallel with Mexico. Seattle hasn't had a megathrust earthquake in more than 250 to 600 years — this is where one plate tectonic is forced underneath another.

"A major quake is expected here in the Pacific Northwest, but no one can predict when or where," Selander said.

The next Science Seminar will be Oct. 27. The topic is "Skeletons and Specimens" presented by Briana Gabbel, at Building 3 room 102 at 1:30 to 2:35 p.m.

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