EARTHQUAKES

Q1: What causes EQ (2)? Earthquakes are caused by the sudden release of strain energy stored in deformed rx, resulting in fracture of those rx. Rocks on either side of the fracture shift in relation to one another-resulting in an EQ.

Q2: Describe the difference between a body wave and a surface wave (2). The energy released by an EQ is transmitted in seismic waves. Body waves transmit energy in all directions from the EQ’s focus. When body waves reach the surface of Earth, they are propagated along the top of the crust as surface waves. These waves are restricted to the top few km on Earth’s crust.

Q3: Explain why the Mercalli intensity scale doesn’t measure earthquakes objectively (4). It relies on human recollections to gauge the extent of damage. These recollections often conflict. Building type, subsurface geology, and building material type can result in different amounts of damage. The amount of damage is directly proportional to the distance from an EQ epicenter. A sparsely populated area would sustain less damage than would a highly populated area.

Q4: Describe the fundamental principle by which a seismograph works (1). Within the seismograph, there is a freely suspended weight. When the earth shakes, the body of the seismograph shakes, but the freely suspended weight remains stationary. The seismograph records the relative motion between the two parts of the machine.

Q5:List and describe 4 of the damaging effects of earthquakes (4). (Choose any 4) Ground shifts occur when blocks move vertically of horizontally. These movements destroy buildings and highways. Landslides involve the downslope movement of unstable rx, rx fragments, or soil. The movements can bury or damage structures or roadways. Liquefaction occurs when vibrating water molecules force apart unconsoliated particles, causing them to behave like a fluid. This effect causes structures to sink. Tsunami are large ocean waves caused by undersea earthquakes. Such waves can destroy coastal regions across ocean basins a great distance from the source Fires result when gas and/or electric service lines are ruptured. Seiches are large “slosh” waves in a basin. Such waves can innundate shoreline buildings and exacerbate coastal erosion.

Q6: What is a Benioff-Wadati zone?(1) These are areas within the descending slab of a subducting tectonic plate where earthqaukes occur. They happen at shallow, intermediate, and deep levels in the plate. Mapping such earthquakes provides a seismic map of a subduction zone.

Q7: Describe 3 of the theories that attempt to explain the cause of intraplate EQ. (3) (choose any 3) As erosion removes vast amounts of surface material from continents, the newly buoyant continent rises and produces stress. The weight of sed deposits causes crust to subside and reactivates faults. Excessive rainfall infiltrates faults and fractures, lubricating them Intraplate EQ are precursors to continental rifting Plate motion creates stress that is released along weakened fault areas.

Q8: What factors determine whether a building is susceptible to damage by an EQ? (3) The magnitude and duration of the EQ The ground material the structure is built on. Buildings located on unconsolidated material sustain more damage than those constructed on bedrock. The construction materials of the structure. Rigid materials such as unreinforced concrete or heavy masonry are more prone to EQ damage. Structures made of flexible material such as wood, steel, or reinforced concrete sustain less damage.

Q9: What methods are used for short-term prediction of EQ? (2) The dilatancy model of EQ is often used for quantitative prediction. Dilatancy occurs when accumulated strain energy approaches about 50% of the value needed to produce rx failure. At that point, rx begin to dilate or expand. This dilation results in detectable swarms of micro EQ, tilting or bulging of rx, changes in seismic wave velocities, variations in the electrical conductivity of rocks or soil, anomolous radio wave signals, and changes in groundwater levels and groundwater chemistry. Geologists also use qualitative data collected by observing the behavior of animals. Animals sometimes behave strangely prior to an EQ-possibly because they sense the cracking of the rocks.