Ch. 8 Focus Questions

How does seismicity on mid ocean ridges compared with the seismicity at convergent or transform boundaries? Do all earthquakes occur at plate boundaries?

At a divergent plate boundary (mid-ocean ridges) two oceanic plates form and move apart. Divergent boundaries are segmented and spreading segments are linked by transform faults. Therefore, two kinds of faults develop at divergent boundaries. Along spreading segments , stretching generates normal faults, whereas along the transform fault that link spreading segments, strike slip faulting occurs. Seismicity along mid ocean ridges takes place at shallow depths.

What is the Wadati-Benioff zone, and why was it important in understanding plate tectonics?

It is the sloping band of seismicity. It explained the reason why there were not earthquakes from layers so far down because it was brittle and break apart.

Explain how the vertical and horizontal components of an earthquake are detected on a seismograph.

Vertical-motion seismograph has a heavy weight (like a pendulum) suspended from a spring. The spring connects to a sturdy frame that has been bolted to the ground. A pen extends sideways from the weight and touches a vertical revolving cylinder of paper that has been connected to the seismograph frame. When an earthquake happens and causes the earth to shake the weight remains fixed, and the paper moves around under the pen. Horizontal is the same except it records shakes in the other direction.

What is a tsunami, and why does it form?

A tsunami is a giant wave traveling at speeds of about 800 km/hr (500mph). They are formed by the rise of the sea floor shoving up overlying water. Then the water from above the upthrust sea floor moves outward from above the fault zone.

Describe elastic-rebound theory and the concept of stick-slip behavior.

Elastic-rebound theory is the concept that earthquakes happen because stresses build up, causing rock to bend elastically until slip on a fault occurs. Stick-slip behavior describes when earthquakes occur due to slip on a preexisting fault. Faults act like scars, they remain weaker than the surrounding crust. When stress builds up the fault slips before the stress becomes great enough to fracture surrounding intact rock.

Explain how liquefaction occurs in an earthquake, and how it can cause damage.

Liquefaction occurs in beds of wet sand or silt where the ground is shaking and causes the sediment grains to try to settle together. But because of the spaces(pores) between grains are filled with water, water pressure in the pores increases and pushes the grains apart, and the wet silt or sand because a fluid-like slurry. It can cause damage because buildings whose foundations lie in liquefaction material may sink or even tip over.

Explain the differences among the scales used to describe the size of an earthquake

Mercalli intensity scale defines the intensity of an earthquake by the amount of damage it causes. Richter Scale allows you to determine the magnitude using the amplitude of the largest deflection on a seismograph and the distance from the epicenter. Moment-magnitude scale allows you to measure the moment magnitude by the amplitude of several different seismic waves, the area of a fault slip, and the amount of slip that occured.

Describe the motions of the four types of seismic waves. Which are body waves, and which are surface waves?

P-waves: compressional waves can be generated by pushing and pulling on the end of a spring. The vibration direction is parallel to the direction of wave movement. Body waves S-waves: shear waves can be produced by moving the end of a rope up and down. As the waves pass through rock, the vibration direction is perpendicular to the direction of the wave movement. Body waves L-waves: as it passes, the ground surface moves back and forth like a slithering snake. Die out with increasing depth. Surface waves. R-waves: make the ground surface go up and down. Die out with increasing depth. Surface waves.

How are long-term and short-term earthquake predictions made? What is the basis for determining a recurrence interval, and what does a recurrence interval mean?

Seismologists cannot make accurate short-term predictions.(on the time scale of hours to weeks) However some short term clues are the detection of foreshocks. Also they can predict with a precise laser surveying the ground. Long term predictions are based on the identification of the seismic zones and the recurrence interval (the average time between successive events).

Compare normal. reverse, and strike-slip faults.

Strike-slip faults the fault surface is usually near vertical and the footwall moves either left or right or laterally with very little vertical motion. A special class of strike-slip faults is the transform faults which are a plate tectonics feature related to spreading centers such as mid-ocean ridges. A reverse fault is the opposite of a normal fault — the hanging wall moves up relative to the footwall. Reverse faults are indicative of shortening of the crust. The dip of a reverse fault is relatively steep, greater than 45°. A thrust fault has the same sense of motion as a reverse fault, but with the dip of the fault plane at less than 45°. Thrust faults typically form ramps, flats and fault-bend (hanging wall and foot wall) folds. Thrust faults are responsible for forming nappes and klippen in the large thrust belts. Faults may be reactivated at a later time with the movement in the opposite direction to the original movement (fault inversion). A normal fault may therefore become a reverse fault and vice versa.

What types of structures are most prone to collapse in an earthquake? What types are most resistant to collapse?

Structures built on land underlain by weak mud that could liquefy are prone to collapse in an earthquake. Also structures built on top of, on, or at the base of steep escarpments could fail and produce landslides. Also, buildings downstream of dams are dangerous because they could crack and collapse causing a flood. Buildings that are resistant to collapse include ones that are wider at the base and that have cross beams added for strength. Wrapping a bridge's support columns in cable and bolting the span to the columns will prevent the bridge from collapsing so easily. Placing buildings on rollers or shock absorbers lessens the severity of the vibrations.

Describe the types of damage caused by earthquakes.

The modified mercalli intensity scale has a 1-12 rating of the damage caused by the earthquake. Types of damage: ground shaking and displacement, landslides, sediment liquefaction, fire, tsunami, and disease.