108 EQ Lec 4

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Ivan Just when you thought it was safe to go back into the water ……… We did not go to Ivan, so ……… Ivan is coming for us! http://www.nhc.noaa.gov/archive/2004/IVAN_graphics.shtml

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The phenomenon we call a tsunami is a series of waves of extremely long wavelength and period generated in a body of water by an impulsive disturbance that displaces the water. Although tsunamis are often referred to as "tidal waves" by English-speaking people, they are not caused by the tides and are unrelated to them.

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Tsunamis are primarily associated with earthquakes in oceanic and coastal regions. When an earthquake occurs, the energy travels outward in all directions from the source. This can be illustrated by throwing a pebble into a small, still pond. The pebble represents a meteorite or some other energy source, and the pond represents the ocean. The ripples that travel out in all directions from the focus, or the point where the pebble hit the water, represent the energy that creates a sea wave. Notice how the waves become larger as they reach shore, where the water is shallower. Detecting tsunamis is a very difficult thing to do. When a wave begins in the deep ocean waters, it may only have a height of about twelve to twenty-three inches and look like nothing more than the gentle rise and fall of the sea surface. An example of how easy tsunamis are to overlook is the Sanriku tsunami, which struck Honshu, Japan, on June 15, 1896.       Fishermen twenty miles out to sea didn't notice the wave pass under their boats because it only had a height at the time of about fifteen inches. They were totally unprepared for the devastation that awaited them when they returned to the port of Sanriku. Twenty-eight thousand people were killed and 170 miles of coastline were destroyed by the wave that had passed under them.

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 Tsunamis in deep water can have a wavelength greater than 300 miles (500 kilometers) and a period of about an hour. This is very different from the normal California tube, which generally has a wavelength of about 300 feet (100 meters) and a period of about ten seconds. (The period of a wave is the time between two successive waves.)       Tsunamis are shallow-water waves, which means that the ratio between water depth and wavelength is very small. These shallow-water waves move at a speed equal to the square root of the product of the acceleration of gravity (9.8m/s/s) and the water depth. The deeper the water, the faster the wave is. For example, when the ocean is 20,000 feet deep, a tsunami travels at 550 miles per hour. At this speed, the wave can compete with a jet airplane, traveling across the ocean in less than a day.        Another important factor in considering tsunamis is the rate at which they lose energy. Because a wave loses energy at a rate inversely related to its wavelength, tsunamis can travel at high speeds for a long period of time and lose very little energy in the process.

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Panel 1--Initiation: Earthquakes are commonly associated with ground shaking that is a result of elastic waves traveling through the solid earth. However, near the source of submarine earthquakes, the seafloor is "permanently" uplifted and down-dropped, pushing the entire water column up and down. The potential energy that results from pushing water above mean sea level is then transferred to horizontal propagation of the tsunami wave (kinetic energy). For the case shown above, the earthquake rupture occurred at the base of the continental slope in relatively deep water. Situations can also arise where the earthquake rupture occurs beneath the continental shelf in much shallower water.Note: In the figure the waves are greatly exaggerated compared to water depth! In the open ocean, the waves are at most, several meters high spread over many tens to hundreds of kilometers in length.

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Panel 2--Split: Within several minutes of the earthquake, the initial tsunami (Panel 1) is split into a tsunami that travels out to the deep ocean (distant tsunami) and another tsunami that travels towards the nearby coast (local tsunami).The height above mean sea level of the two oppositely traveling tsunamis is approximately half that of the original tsunami (Panel 1). (This is somewhat modified in three dimensions, but the same idea holds.) The speed at which both tsunamis travel varies as the square root of the water depth. Therefore the deep-ocean tsunami travels faster than the local tsunami near shore.

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Panel 3--Amplification: Several things happen as the local tsunami travels over the continental slope. Most obvious is that the amplitude increases. In addition, the wavelength decreases. This results in steepening of the leading wave--an important control of wave run-up at the coast (next panel).Note also that the deep ocean tsunami has traveled much farther than the local tsunami because of the higher propagation speed. As the deep ocean tsunami approaches a distant shore, amplification and shortening of the wave will occur, just as with the local tsunami shown above.

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Panel 4—Runup: As the tsunami wave travels from the deep-water, continental slope region to the near-shore region, tsunami run-up occurs. Run-up is a measurement of the height of the water onshore observed above a reference sea level. Contrary to many artistic images of tsunamis, most tsunamis do not result in giant breaking waves (like normal surf waves at the beach that curl over as they approach shore). Rather, they come in much like very strong and very fast tides (i.e., a rapid, local rise in sea level). Much of the damage inflicted by tsunamis is caused by strong currents and floating debris. The small number of tsunamis that do break often form vertical walls of turbulent water called bores. Tsunamis will often travel much farther inland than normal waves.

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Do tsunamis stop once on land? After run-up, part of the tsunami energy is reflected back to the open ocean. In addition, a tsunami can generate a particular type of wave called edge waves that travel back-and forth, parallel to shore. These effects result in many arrivals of the tsunami at a particular point on the coast rather than a single wave suggested by Panel 3. Because of the complicated behavior of tsunami waves near the coast, the first run-up of a tsunami is often not the largest, emphasizing the importance of not returning to a beach several hours after a tsunami hits.

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Background The tsunami that struck New Guinea on July 17, 1998 was the most devastating tsunami since the 1976 Moro Gulf, Philippines, tsunami and may surpass that event (Lockridge and Smith, 1984; Satake and Imamura, 1995). The high reported runups and the tremendous loss of life are of great concern to all, including the international scientific community. Scientists will closely examine this event in the coming months, in the ultimate hope of mitigating such disasters in the future. New Guinea is a seismically active region, the site of an arc-continent collision, where tectonic plates are converging and sliding past each other. The tectonic boundaries and faulting in this region are very complex, as shown below.

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The Earthquake The recorded magnitude of the earthquake was 7.1, and the epicenter was located in northern New Guinea near the coast. The fault mechanism from the National Earthquake Information Center (NEIC) shown by the red-and-white ball indicates that the earthquake could have occurred as uplift on a vertical fault or sliding on a horizontal fault. The inset in the upper right corner of the figure shows the location of the epicenter.

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Bathymetry The bathymetry indicates that just offshore of the northern coast of New Guinea there is very steep and linear slope. It is possible that the relatively deep water near shore contributed to the unusual height of this tsunami. Shown below is the bathymetry in the region contoured at 200 m intervals from the 2-minute bathymetric data of Smith and Sandwell (1997). In the above figure, black circles indicate locations for the earthquake determined by the NEIC, Harvard, and the Earthquake Research Institute (ERI) at the University of Tokyo.

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The Tsunami At present, too little is known about the July 17, 1998 earthquake and about the distribution of run-up to formulate a quantitative model of the tsunami. The reported run-ups are unusually large for an earthquake of magnitude 7.1 (cf. Geist, 1998). Below, a descriptive or qualitative simulation of the tsunami is computed by making several assumptions about the source parameters of the earthquake. About the Animation The animation shows how the July 17 tsunami might have looked from a vantage point above Papua New Guinea. The coastline shown in the animation is about 550 kilometers (340 miles) long. The animation begins with the initial wave--which formed almost simultaneously with the earthquake that triggered the event--and shows how the initial wave was reflected from the island. Note other waves traveling east and west along the coast." This animation does not show run-up (56 kB)--the maximum elevation the water reached as it rose above the shoreline. Based on very preliminary data, this animation is a "descriptive model" of the tsunami. A more accurate simulation, or "quantitative model", can be developed when accurate measurements of the runup and more information about the earthquake that triggered the tsunami become available. http://walrus.wr.usgs.gov/tsunami/PNG.html#tsunami

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Survey team report: On July 17, 1998 a Mw = 7.0 earthquake struck the north central coast of Papua New Guinea. Following the earthquake a large tsunami also struck the region. Initial reports claimed that the wave was between 7 and 10 meters and that up to 3000 persons were killed or missing. This seemed to be an unusually damaging tsunami given the size of the earthquake. Members of the International Tsunami Survey Team decided that a field survey was necessary as soon as possible to try and determine the true value of the maximum run-up and to accurately map the run-up distribution along the coast. Upon arrival at the disaster relief command post in Aitape, the team was granted full access to the sealed region around Sissano Lagoon and Sissano Village, the site of the most deaths and greatest destruction. The first surveys to the Sissano region confirmed the 7 - 10 m wave reports and even found a place where the waves were larger - up to 15 m. The severe damage and extreme wave heights were confined to a relatively short (40 km) stretch of coast between Aitape and Sissano Village.

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The survey was conducted by a multinational team with representatives from Japan, the United States, Australia, and New Zealand. The team was broken up into two groups, the Japanese and everyone else. The Japanese team traveled overland from Wewak to Aitape measuring run-up along the way. Japanese team members also installed seismographs in the region to measure aftershock activity. The rest of the team traveled by ship from Wewak to the west stopping at some of the offshore islands. The two groups reunited in Aitape before a survey of the Sissano area was conducted. The boat continued west as far as Serai Village where run-up values were seen to diminish considerably. Images of the aftermath of the Papua 1998 Tsunami http://www.usc.edu/dept/tsunamis/PNG/random1.html http://www.usc.edu/dept/tsunamis/PNG/random2.html

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Summary chart presenting observed (top) and computed (bottom) time series for nine tide gage stations along the coasts of California and Oregon. Vertical axes are deviation from background sea level in cm; horizontal axes are minutes of elapsed time after the main shock, with a vertical line indicating the computed arrival time at each station. Bathymetric contour interval is one kilometer. The dotted line rectangle denotes the numerical model computational region. Cape Mendocino is just south of North Spit, and the solid star marks the earthquake epicenter. Note that, at Crescent City (CC), the first wave packet arrives within minutes of the computed travel time (vertical line); this is the non-trapped tsunami energy, which traverses deep offshore water. Three hours after this first arrival, the largest observed tsunami wave is recorded at this station; this second tsunami wave packet corresponds to the trapped edge wave mode.

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1992 Earthquake Computed vertical crustal deformation for a single fault plane model. Heavy, solid contours represent uplift, dashed subsidence; the first solid contour is 0.2 cm, and all other contours are at 20 cm intervals. The solid line rectangle delineates the surface projection of the fault plane (see Generation Mechanism section in text for fault plane parameters [Gonzalez, et al., 1995]), and the location of the epicenter is indicated by a star . Note that the uplift region is located in shallow water near the coast, an optimal location for generating coastally trapped edge wave modes.

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Distant Tsunami Animation http://www.pmel.noaa.gov/tsunami/Mov/andr1.mov Inundation of Aonae during Hokkaido-Nansei-Oki Tsunami http://www.pmel.noaa.gov/tsunami-hazard/vasily.mpg http://www.pmel.noaa.gov/animations/Aonae.all.mpg

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As part of the U.S.National Tsunami Hazard Mitigation Program (NTHMP), the DART Project is an ongoing effort to develop and implement a capability for the early detection and real-time reporting of tsunamis in the open ocean. DART is essential to fulfilling NOAA's national responsibility for tsunami hazard mitigation and warnings. Project goals are: 1) Reduce the loss of life and property in U.S. coastal communities. 2) Eliminate false alarms and the high economic cost of unnecessary evacuations. DART stations are sited in regions with a history of generating destructive tsunamis to ensure early detection of tsunamis and to acquire data critical to real-time forecasts. Buoys shown on the accompanying map represent an operational array scheduled for completion in 2003.

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A DART system consists of a seafloor bottom pressure recording (BPR) system capable of detecting tsunamis as small as 1 cm, and a moored surface buoy for real-time communications. An acoustic link is used to transmit data from the BPR on the seafloor to the surface buoy. The data are then relayed via a GOES satellite link to ground stations, which demodulate the signals for immediate dissemination to NOAA's Tsunami Warning Centers and PMEL. http://www.pmel.noaa.gov/tsunami/Dart/Flash/CODEframe4DART.html

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Important Facts to Know about Tsunamis Tsunamis that strike coastal locations in the Pacific Ocean Basin are most always caused by earthquakes. These earthquakes might occur far away or near where you live. Some tsunamis can be very large. In coastal areas their height can be as great as 30 feet or more (100 feet in extreme cases), and they can move inland several hundred feet. All low-lying coastal areas can be struck by tsunamis. A tsunami consists of a series of waves. Often the first wave may not be the largest. The danger from a tsunami can last for several hours after the arrival of the first wave. Tsunamis can move faster than a person can run. Sometimes a tsunami causes the water near the shore to recede, exposing the ocean floor. The force of some tsunamis is enormous. Large rocks weighing several tons along with boats and other debris can be moved inland hundreds of feet by tsunami wave activity. Homes and other buildings are destroyed. All this material and water move with great force and can kill or injure people. Tsunamis can occur at any time, day or night. Tsunamis can travel up rivers and streams that lead to the ocean.

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If you are on land: Be aware of tsunami facts. This knowledge could save your life! Share this knowledge with your relatives and friends. It could save their lives! If you are in school and you hear there is a tsunami warning, you should follow the advice of teachers and other school personnel. If you are at home and hear there is a tsunami warning, you should make sure your entire family is aware of the warning. Your family should evacuate your house if you live in a tsunami evacuation zone. Move in an orderly, calm and safe manner to the evacuation site or to any safe place outside your evacuation zone. Follow the advice of local emergency and law enforcement authorities. If you are at the beach or near the ocean and you feel the earth shake, move immediately to higher ground, DO NOT wait for a tsunami warning to be announced. Stay away from rivers and streams that lead to the ocean as you would stay away from the beach and ocean if there is a tsunami. A regional tsunami from a local earthquake could strike some areas before a tsunami warning could be announced.

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If you are on land: Tsunamis generated in distant locations will generally give people enough time to move to higher ground. For locally-generated tsunamis, where you might feel the ground shake, you may only have a few minutes to move to higher ground. High, multi-story, reinforced concrete hotels are located in many low-lying coastal areas. The upper floors of these hotels can provide a safe place to find refuge should there be a tsunami warning and you cannot move quickly inland to higher ground. Local Civil Defense procedures may, however, not allow this type of evacuation in your area. Homes and small buildings located in low-lying coastal areas are not designed to withstand tsunami impacts. Do not stay in these structures should there be a tsunami warning. Offshore reefs and shallow areas may help break the force of tsunami waves, but large and dangerous wave can still be a threat to coastal residents in these areas. Staying away from all low-lying areas is the safest advice when there is a tsunami warning.

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If you are on a boat: Since tsunami wave activity is imperceptible in the open ocean, do not return to port if you are at sea and a tsunami warning has been issued for your area. Tsunamis can cause rapid changes in water level and unpredictable dangerous currents in harbors and ports. If there is time to move your boat or ship from port to deep water (after a tsunami warning has been issued), you should weigh the following considerations: Most large harbors and ports are under the control of a harbor authority and/or a vessel traffic system. These authorities direct operations during periods of increased readiness (should a tsunami be expected), including the forced movement of vessels if deemed necessary. Keep in contact with the authorities should a forced movement of vessel be directed. Smaller ports may not be under the control of a harbor authority. If you are aware there is a tsunami warning and you have time to move your vessel to deep water, then you may want to do so in an orderly manner, in consideration of other vessels. Owners of small boats may find it safest to leave their boat at the pier and physically move to higher ground, particularly in the event of a locally-generated tsunami. Concurrent severe weather conditions (rough seas outside of safe harbor) could present a greater hazardous situation to small boats, so physically moving yourself to higher ground may be the only option. Damaging wave activity and unpredictable currents can effect harbors for a period of time following the initial tsunami impact on the coast. Contact the harbor authority before returning to port making sure to verify that conditions in the harbor are safe for navigation and berthing.

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Earthquake Assignment (Due next Thursday) Do the “Travel Time” exercise (print map and journal to turn in) Do the “Epicenter & Magnitude” exercise (print map and journal to turn in) Do the Assessment (print Certificate and assessment scores) Registration number is 476231 http://www.sciencecourseware.com/eec/Earthquake/ Turn off Pop-up Blocker!

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Earthquake Prediction What constitutes a useful earthquake prediction? Has there ever been a useful prediction?

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One well-known successful earthquake prediction was for the Haicheng, China earthquake of 1975, when an evacuation warning was issued the day before a M 7.3 earthquake. In the preceding months changes in land elevation and in ground water levels, widespread reports of peculiar animal behavior, and many foreshocks had led to a lower-level warning. An increase in foreshock activity triggered the evacuation warning. Unfortunately, most earthquakes do not have such obvious precursors. In spite of their success in 1975, there was no warning of the 1976 Tangshan earthquake, magnitude 7.6, which caused an estimated 250,000 fatalities.

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Methods of Earthquake Prediction Animal behavior Seismic Gaps Monitoring precursory phenomena

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Seismic gap A seismic gap is a section of a fault that has produced earthquakes in the past but is now quiet. For some seismic gaps, no earthquakes have been observed historically, but it is believed that the fault segment is capable of producing earthquakes on some other basis, such as plate-motion information or strain measurements.

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It has been shown that humans can trigger earthquakes by pumping water into fault zones. Should we use this method to trigger small earthquakes in a controlled fashion so as to prevent a large earthquake in a populated area?