Geology Test 2

Describe some large North American earthquakes and how they were generated.

Alaska 1964- 9.2 earthquake, killed 128 people; triggered landslides and collapsed part of downtown, event was caused by thrust faults associates with the Aleutian island subduction zone. caused a tsunami; seafloor uplifted. san francisco-1906 (290 mi from san andres fault) 7.8 magnitude northridge 1994- 6.7, 60 people dead, thrust fault Mexico city 1985- 8.0, subduction zone, 9,500 people died yellowstone- 7.3. slip along a normal fault, buried campers and formed a new lake new madrid- fault zone in the crust 1811 Charleston- 1886 high risk area along the east coast; 7.3

• Describe how to use arrival times of P- and S-waves to locate an epicenter.

P-S intervals are measured from the seismograms

earthquakes associated with subduction zones

a subduction zone where an oceanic plate underthrusts beneath another oceanic plate or continental plate, undergoes compression and shearing along the plate boundary, it produces very large earthquakes.

Explain what a Modified Mercalli intensity rating indicates.

a value of 1 reflects a barely felt earthquake. a value of XII indicates complete destruction of buildings with visible surface waves throwing objects into the air. III- felt strongly by people V- felt by nearly everyone; somethings broken VI- felt by all; heavy furniture moved, damage slight X and XI- some well built structures destroyed; bridges destroyed

Summarize why certain areas of the United States experience earthquakes, while others do not.

because of tectonically active areas

Briefly describe how tsunamis are monitored to provide an early-warning system.

buoys relay small changes in sea level detected by ocean bottom sensors as a tsunami passes overhead.

Summarize the kinds of damage tsunamis have caused.

death and destruction along coastlines.

Describe how earthquakes can cause destruction, both during and after the main earthquake.

direct damage from an earthquake results when the ground is shaking because of seismic waves, especially surface waves near the epicenter of the earthquake secondary effects-such as fires and flooding that are trigged by the earthquake

How Do Earthquake Waves Travel?

earthquakes generate vibrations that travel through rocks as seismic waves.

Describe some characteristics of fault scarps and ruptures

entire fault does not begin to slip at once, the earthquake rupture starts in a small area(hypocenter) and expands overtime along the preexisting fault plane. as the edge of the rupture migrates outward it may eventually reach earth's surface causing a break called a fault scarp. it continues to grow along the fault plane and the scarp fault lengthens. it relieves some stress and the rupturing will stop when the remaining stress can no longer overcome friction along the surface.

How Does Faulting Cause Earthquakes?

faults slip because the stress applied to them exceeds the ability of the rock to withstand the stress. rocks respond to the stress in one or two ways they either flex or bend or they break and slip. breaking and slipping causes earthquakes

Summarize the kinds of field and remote measurements geologists use to investigate recent earthquakes.

geologists carfully measure its location, dimensions and orientations, drawings and photographs. when a fault moved it can offset natural and human made features. road and fences; geologists can measure how much and in what direction the fault has offset these features. shallows trenches a dug accross the faults expose what is bellow the surface; faulting commonly causes changes in the topography of the land surface.

Summarize ways geologists do long-range forecasting and short-range prediction

long range- based mainly on knowledge of earthquakes in the past. studies aim to determine the locations and recurrence intervals of past earthquakes shot term- monitoring the activity along an earthquake prone fault

Summarize how continental collisions cause earthquakes

one continental plate underthrusts beneath another. collisions can be extremely complex, as different parts collide at diff times and rates. it cause large tectonic stresses that shear and fault a broad zone within the overriding and underthrusting plates, as a result earthquakes are widely distributed.

Observe different seismic records of an earthquake and tell which one was closer to the epicenter.

p wave

the order in which seismic waves arrive at a seismometer.

p waves arrive first, marked by the larger squiggles. the s waves arrive later, surface waves arrive last and cause intense ground shaking

Where Do Most Earthquakes? Occur

plate boundaries and narrow belts that coincide

Describe how earthquakes and seismic waves are caused by volcanoes, landslides, meteoroid impacts, and humans.

seismic waves, once generated they spread in all directions. volcanoes cause the ground to shake through several processes. it causes compression, transmitting energy as seismic waves. landslides (catastrophic) cause ground shaking meteoroid impacts accompanies the ground shaking humans cause earthquakes by making reservoirs to store water fill rapidly and loading the crust which responds by flexing and faulting; humans also caused earthquakes by injecting waste water underground into a deep well at the rocky mountain in denver.

Describe how the build up of stress can strain and flex rocks, leading to an earthquake.

slippage along a fault generally happens in a sudden, discrete jump. faulting reduces the stress on rocks allowing some of the strained rocks to return back to their original shaped. (elastic behavior)

Describe areas of the world that experience a high risk of earthquake activity.

south america middle east US including Hawaii and Alaska

• Explain or sketch how we calculate local magnitude

the amplitude of S Waves decreases as a wave propagates. we plot the relationship between distance, earthquake magnitude and s waves amplitude on a graph called nomograph connect the distance and amplitude of the S wave

Explain what a hypocenter and epicenter each represent.

the place where the earthquake is generated is called the hypocenter or focus. the epicenter is the point on the earth's surface directly above where the earthquake occurs. epicenter and hypocenter are the same if the seismic happens on the surface.

how seismic waves are recorded, and the order in which they arrive at a seismometer.

the recorded seismic data are uploaded to computers that processes signals from hundreds of instruments registering the same earthquake. this computers calculate the location of the hypocenter and the magnitude or strength of the earthquake. front the data we get gain insight about how and where the earthquakes occur

Describe why the eastern United States has earthquake risks.

the region is subjected to stress generated at far off plate boundaries; plate driving force- ridge push.

• Summarize some generalizations about the distribution of earthquakes, especially the relationship to plate boundaries.

the seismicity is more widespread reaching far away from the plate boundaries and into the middles of the continents