Montclair MUTH 463 - chapter5KEY (12 pages)

CHAPTER 5 TSUNAMI THE GREAT WAVE Correct answers are indicated by an asterisk both in short answer and multiple choice questions True or False questions can be easily prepared from multiple choice questions Web Sites http www asiantsunamivideos com Video Clips of the Asian Tsunami http www ngdc noaa gov seg hazard tsu shtml http www pbs org wnet savageearth tsunami index html http www pmel noaa gov tsunami http www tsunami noaa gov http www tsunami org http www1 tpgi com au users tps seti spacegd7 html Videos Geologic Hazards Slide Sets Tsunami National Geophysical Data Center NOAA Dept 930 325 Broadway Boulder CO 80303 CHAPTER 5 End of Chapter Answers 1 What are three of the main causes of tsunami sub sea earthquakes landslides into a body of water volcano flank collapse into a water body sub sea volcanic eruptions asteroid impact into an ocean 2 Of the three main types of fault movements strike slip faults normal faults and thrust faults which can and which cannot cause tsunami Why strike slip cannot normal and thrust faults can strike slip faults do not displace water normal and thrust do displace water 3 About how high are the largest earthquake caused tsunami waves in the open ocean about 15 meters 4 How does the height of a tsunami wave change as it enters a bay Why wave height increases Since more water of each wave is forced into a narrower area the height must rise to accommodate the volume of water 5 How many tsunami waves are generated by one earthquake ten or more 6 How fast do tsunami waves tend to move in the deep ocean about 760 km hour or 475 mi hour 7 Do tsunami speed up or slow down at the coast Why They slow down because the waves drag on and are held back by the shallower bottom 8 Why is even the side of an island away from the source earthquake not safe from a tsunami because the waves can refract around the coast of an island to strike the opposite side 9 For a subduction zone earthquake off the coast of Oregon or Washington how long would it take for a tsunami wave to first reach the coast less than 20 minutes 10 Because the Atlantic coast of North America experiences few large earthquakes what specific other event could generate a large tsunami wave that could cause catastrophic damage there flank collapse of a volcano in the Canary Islands or landslide of a large section of the continental shelf 11 What specific evidence is there for tsunami events having struck bays on the coasts of Washington and Oregon flattened and drowned forests massive sand sheets that cover coastal peat soils 12 What is a seiche be specific a wave that sloshes back and forth from end to end of an enclosed body of water CHAPTER 5 Short answer questions 1 In December 2004 a pair of closely related natural disasters killed tens of thousands of people a What was the initial cause of the event not the one that apparently killed most of the people the general name for the type of event a major earthquake b Where specifically was that initial event off the southwest coast of Sumatra c What was the specific kind of that initial event the variety of the general event noted in part a and the exact nature of it earthquake on a subduction zone thrust fault movement d What secondary event the one that killed most of the people was caused by the initial event noted above the general name for that type of event tsunami e Explain concisely and in detail exactly how the initial event was related to the secondary event the earthquake caused the tsunami when fault movement on the ocean floor suddenly pushed up a large mass of water 2 Along the San Andreas Fault in California how many magnitude 7 earthquakes would it take to relieve the same stress in the rocks as one magnitude 8 earthquake about 32 3 Why are tsunami waves in the open ocean limited in height 15 meters is about the largest offset on a fault on the ocean floor so that is also the largest wave height generated 4 About how high are the largest earthquake caused tsunami waves in bays about 30 meters 5 In December 2004 a giant earthquake struck southeastern Asia a Exactly where was that earthquake just off the southwest coast of Sumatra b why was it there Explain clearly collision between two tectonic plates c What was the magnitude give the number of that earthquake 9 0 d What type of fault movement caused the earthquake thrust fault movement or subduction zone movement e What was the nature of the boundary marked by that fault a subduction zone f How long was the break along that fault approximately 1200 km g Which country or countries would have been severely damaged by the earthquake Sumatra 6 Which is the more dangerous location for a tsunami hazard a straight stretch of open coast a rocky point or a bay or what is the most hazardous area along a coast a bay a Why a bay focuses the wave forcing the water into a narrower area and causing the wave to rise 7 What are the approximate tsunami wave velocities over the continental shelf or nearshore about 100 km hour 8 What are the approximate wavelengths of tsunami waves 200 km 9 What are the approximate times between tsunami wave crests 10 to 35 minutes 10 On low lying coastal flats near Anchorage Alaska the foundations of buildings dropped below sea level during the giant 1964 earthquake Decades later their foundations are again above sea level a What type of tectonic boundary caused the earthquake and what plate motions are involved Subduction zone boundary the Pacific Ocean floor is descending under the continental margin b Explain exactly what tectonic forces led to the ground sinking and later rising and why Be brief and to the point While the subduction zone fault was locked descent of the ocean floor pulled down the edge of the continental margin causing it to slowly buckle up Release of the boundary during the earthquake permitted the ground to drop suddenly and the sea move in Decades later the edge of the continent is again rising since the subduction zone fault is again locked 11 What type of event has repeatedly generated high tsunami waves in coastal fjords of southeastern Alaska Be specific large rockfall or landslide triggered by a nearby earthquake 12 Why do ships in the open ocean not notice passage of a tsunami wave the time between wave crests is 10 to 30 minutes and the wave rises and falls less than 15 meters in that time 13 How dangerous are tsunami in the open ocean and why not dangerous at all The wave height is so much smaller than the wave length and the time