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Chapter 4 Tsunamis

by: Elizabeth Rubio

Chapter 4 Tsunamis GEOL 110

Elizabeth Rubio
Long Beach State
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Natural Disasters
Ewa Burchard
Class Notes




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This 3 page Class Notes was uploaded by Elizabeth Rubio on Wednesday February 17, 2016. The Class Notes belongs to GEOL 110 at California State University Long Beach taught by Ewa Burchard in Spring 2016. Since its upload, it has received 22 views. For similar materials see Natural Disasters in Geology at California State University Long Beach.

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Date Created: 02/17/16
Week 5 Day 2 2­17­2016 Chapter 4 ­ Tsunamis Japan 2011: Tsunami and Nuclear Disaster Pacific Ocean bottom vertically displaced as much as 9 m (~30 ft)  • Automated warning system alerted within seconds – Move to high ground – Nuclear plants  shut down  • Seawall did not keep water out – Nuclear plant flooded  • Pumps inundated by seawater  • Cooling systems failed ~9.1 magnitude ­Created by a subduction zone on the ocean floor ­Local tsunami comes first than a distant tsunami ­Caused by earthquakes Introduction to Tsunamis  Tsunami is Japanese for “harbor wave”  – Caused by a sudden vertical displacement of ocean water  ­Caused by physical movements of rocks and waves from gravity (tidal wave)   • Triggered by:  – Large earthquakes that cause uplift or subsidence of sea floor  – Underwater landslides – Volcano Flank Collapse – Submarine volcanic explosion –  Asteroids  • Can produce Mega­tsunami How Do Earthquakes Cause a Tsunami? • Two mechanisms  – Seafloor movement (more common)  – Triggering a landslide  • Takes an earthquake of M 7.5 or greater  – Creates enough displacement of the seafloor – Upward (crest created) or downward  movement displace the entire mass of water   – Starts a four­stage process How do earthquakes cause tsunami (cont.) 1. Earthquakes uplifts the sea floor  ­A dome forms on the surface of the water above the fault ­Dome collapses and generates the tsunami wave ­Waves radiate outward (like a pebble on a pond) 2. Tsunami moves rapidly in deep ocean ­Can travel 720 km (450mi. ) per hour ­Spacing (frequency) of crests is large and small amplitude ­Boats  in open ocean don’t notice the tsunami waves 3. Tsunami nears land, loses speed, gains height ­Depth of ocean decreases, slowing tsunami waves ­45 km (28mi) per hour (spacing between crests decreases) ­More water piles up increasing amplitudes and frequency 4. Tsunami moves inland destroying everything in its path ­Can be meters to 10’s of meters high ­Often arrives as a quick increase of sea level ­Though may arrive first, exposing seafloor ­Runup, furthest horizontal and vertical distance of the largest wave  ­Water returns to ocean in a strong, turbulent flow ­Edge waves may be generated parallel to the shore ­Second and third waves may be amplified   ­Offshore earthquakes can cause tsunami’s to go toward land and out to sea ­Uplifted dome of water splits in two waes ­Distant tsunami ­Travels out to sea and can travel long distances with little loss of energy ­examples: Oregon and Hawaii Local Tsunami ­– Travels towards land very quickly  – People have very little time to react  How Do Landslides Cause a Tsunami? Submarine landslides occur when landslides occur underneath the water  – Displaces water vertically causing landslides  • On land, rock avalanches from mountains can cause tsunami (M 7.7)  – Example: Lituya Bay, Alaska  • 30.5 million cubic meters of rock fell into ocean  • Bay water surged to 524 m (1790 ft.) above normal ­Tsunami wave 30 m height; trees stripped (horizontally 20­70m) ­1998 7.1 and tsunami Papa New Guinea 2,00 killed, and 12,000 homeless ­Tsunami wave 15m height, Submarine landslide 50 km offshore Geographic Regions at Risks from Tsunamis  All oceans and some lake shorelines have some risk  – Greater risk is for coasts near sources of tsunamis  • Greatest risk is to areas near or across from subduction zones  – Example: Cascadia zone, Chilean Trench, off Coast of Japan Case study 4.2 Indonesian Tsunami World’s largest earthquake in past four decades triggered tsunami – Was a “megathrust event ” –Most lethal tsunami in recorded history  • No warning system was set up in Indian Ocean  • Few people knew what tsunami meant prior to event, others were unfamiliar  • Education (or lack of) was a major reason for so many deaths  – Many did not know how to recognize a tsunami  – Many went to beach to watch  – Few knew what to do  • Tourists and first­generation residents ­10 Year old girl  ­Learned in school about plate tectonics just before going to Thailand ­Sounded warning to those to evacuate ­Scientists on beach in Sri Lanka ­Noticed the sea level drop ­ Sounded warning for those that went to beach to watch   Animal behavior  ­ Elephants started trumpeting about time of earthquake  ­ Ignored handlers and headed up hill  ­ Education of tsunami could have saved thousands more, especially with the distant tsunamis  Effects of Tsunamis and Linkages with Other Natural Hazards Primary effects  – Inundation of water and resulting flooding and erosion  • Shorten the coastline • Debris erodes both landscape and human structures  • Secondary effects  – Fires  ­ From ruptured gas lines or other sources  – Contaminated water supplies  • Floodwaters, wastewater treatment plants, rotting animal carcasses and plants  – Disease  • Come in contact with polluted water or soil


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