TSUNAMI ----- John Pazmino NYSkies firstname.lastname@example.org 2005 July 5
Introduction ---------- 'Earthquakes, tsunamis, and a modern journey into the center of the Earth' was presented by Dr Michael Wysession, Washington University, St Louis MO, on 30 June 2005 at American Museum of Natural History. Perhaps because it was a free public lecture, it filled its hall, Kaufmann Theater, to capacity with few empty seats left. For me this talk was paired with one I and other NYSkiers heard on June 22nd. That was 'Future eruptions of Vesuvius' at Science, Industry, Business Library. I summarized that lecture for NYSkies in file 'vesuvius.txt'. The talk was hosted by the Museum's Department of Earth and Planetary Sciences. It's agent noted that this is a taste of a new series of lectures starting in October 2005 about geophysics. He urged the audience to watch the Museum website and litterature for details when October rolls around.
Tectonics ------- Wysession first described the structure of the Earth as an iron core, blanketed by a mantle of melted rock, and skinned over by the solid rock crust. This crust extends above sea level as land but about 3/4 of it, the Earth's surface, is covered by ocean. The crust floats on the mantle and in remote eras was formed as separate tiles or plates. At one time these were gathered into one landmass, Pangea. Over the eons, the plates separated and migrated to align with the landmasses of today. This feature is continental drift or plate tectonics. The plate boundaries collide here and there. In general, they are separating in the Atlantic Ocean and converging around the Pacific Ocean. The boundaries are NOT congruent with shorelines; they may be under the sea or within the land. The crust is tens of kilometers thick. In area, they ranges from middle-size countries to major continents. The plates are named for the continent or ocean covering the greater fraction of their area.
Plate collision ------------- Around the Pacific Ocean, the plates are pressing together. As they rub, they set up stresses in the rock. When the stress builds up and the plates slip, there is a movement of the surface, an earthquake. This situation prevails all around the Pacific rim, in Japan, Alaska, west Canada, west United States, west Mexico, west South America, south Pacific islands, north Australia, Indonesia, west Pacific islands, southeast Asia, eastern China. In the US the most famous of boundaries is the San Andreas fault in California, where the North America and Pacific plates gnash. Land to the east on the fault line is on the North America plate. That to the west is on the Pacific plate. The relative motion between the two is evidenced in the offset alignments of roads and sheared structures that cross the fault. The gaps between the plates was long ago filled with loose sediment, shaping the topography of California today. Here the North American plate and the Pacific plate slide laterally and lock together. When the stress finally breaks them free for a next advance along the boundary, California suffers an earthquake. The quake is typicly several kilometers under the ground. The geographic spot on the surface above this seat is the epicenter. it has the dame lat-lon as the underground quake.
Other quakes ---------- Earthquakes can occur within a plate, like in the Mississippi valley. The cause here is the plate being buckled or wrinkled by pressure around its edges. Where the plate is weakest, the buckling can trigger earthquakes. An other cause of earthquakes, minor ones, is relaxation of the crust after depression under the Ice Ages. The quakes around new York City or mostly of this type. It did, and can, suffer the stronger ones due to plate wrinkling.
Volcanos ------Volcanos are formed along plate frontiers where one plate is shoved under an other, as is the case around the Pacific. The lower plate melts, mixes with superheated water from the ocean, and becomes magna. This magma oozes upward by the tectonic pressure and breaks thru cracks and holes on the surface; you got a volcano. Earthquakes occur at and around times of volcano eruptions by the bulk motion of the magma and shifting of mass under the ground. The onset of earthquake in volcano territory can precede an eruption, but not for sure. The magma may simply migrate around but not break the surface. The bulk of volcanos on Earth surround the Pacific Ocean as the 'Ring of fire'. Altho many of the volcanos are dormant or extinct, other nearby are quite live. In some cases, like Mt St Helens, a presumably extinct volcano can suddenly revive. The prototypical pacific island is a tall volcano with culture and development on the shoreline at its base. many in the Caribbean are like this, too, from plate collisions there.
Mountains ------- Mountains can be raised up by plate motions. The Rockies and Andes are the result of plates pressing together with the overburden pushed upwards. The Himalayans are the result of the India plate migrating north into the Asia plate. These mountains are still today building up several centimeters every year. Slips in the mountains here cause the earthquakes in west Chine, Nepal, north India, Pakistan. Mountains can be formed thru the buckling process, like the Appalachians Apparently the rock here is soft enough to yield under pressure and prevent strong buildup of stress. There are few strong quakes along the Appalachians. Along the flanks of the India plate is the situation similar to California. The plate rubs sideways against the next one (forget the name) containing Indonesia. Both severe quakes and strong volcanos are frequent in and around Indonesia.
Shock waves --------- An earthquake is the sudden release of energy that sends shock waves thru the surrounding crust and mantle. These propagate the wave long distances, around the world. By tracking these waves, we can suss out the thickness and density of the mantle and core. The speed and deflection of the waves gives clue for the composition of these regions. Because the waves can bounce off of the core or deflect thru the mantle, there is a ''skip' effect. There are 'dead' zones around the quake center where there is little or no effect, yet farther away on the same radial line, strong motion is felt. Eventually the waves die out by frictional losses, but they can endure, circulating around the world several times, for many days or weeks. Waves from small quakes are spent within minutes.
Atom bombs -------- Until the Cold War, earthquake stations to monitor the waves were far and few between. Most were at what ever convenient place in a college laboratory. One I remember is the long-gone seismograph in the lobby of Shepard Hall, City College of New York. Data were collected by drum recorders, with the paper bands stored for later inspection. The Cold War made it essential to monitor underground shocks from atom bomb testing. The US set up dozens of bases, mostly circumscribing the Soviet Union. Because time was critical to evaluate any bomb explosion, electronic and automatic recording methods were developed, along with suitable alarms. As it turned out, the Soviet Union occupied so vast an area that the stations were deployed adequately on the globe to look after natural earthquakes. The signature of an earthquake is quite different from that of a bomb blast. The bomb explodes at one point for an instant. A quake usually is spread over a large area and lasts many seconds. A bomb is set off tens of meters underground while true quakes are seated kilometers deep. Except for truly minor quakes, like the relaxation ones of the City, a quake releases orders more energy than any atom bomb. Hence, while some real bomb tests were caught, the bulk of the records was a new comprehensive chronicle of earthquakes. The methods and techniques of the Cold War monitoring bases are now mainstreamed into modern seismologic stations around the world, They give realtime, short leadtime, notice of quakes any where on the planet. Data, in addition to being sent to geophysicists, are often posted on public websites for anyone to follow.
P and S waves ----------- Quakes raise up two kinds of wave, the same two in mechanical systems. The P, pressure, wave moves the material of its medium to and fro in line with its own direction of motion. This is similar to a sound wave, where the air molecules are squeezed or stretched away from the source. The spacing of the squeezed and stretched zones is the wavelength. This motion causes the pressure on an obstruction in the way, like the ear for hearing. The S, shear, wave moves the medium laterally across its own direction, like a mild water wave. Little pressure is put on an obstruction. The water bobs up and down but otherwise stays put. The P and S waves are sometimes called primary and secondary because typicly the P waves arrive at a remote place first, followed by the S wave. Both do move the ground, so both can inflict damage. The mantle, core, and crust transmit P and S waves differently. By mapping the two waves for a quake over the globe, additional intelligence about the Earth's interior can be captured.
Tsunamis ------ 'Tsunami' is a Japanese word. We use it probably because in areas of the world affected by tidal waves, Japan was the farthest advanced culture to study them first. A tsunami is a mass motion of water outward from a quake epicenter that eventually crashes onto land. It rises from a vertical motion of the sea bottom of hundreds of meters. Such motion can be a direct upward thrust of the sea floor when the colliding plates heave up the crust. Or it could be a drop of the floor, collapsing into a chasm between the plates. In either case, a slug of water many kilometers around and displaced verticly a hundred meters is then suddenly let go as the quake peters out. It is important to understand that tsunamis are NOT a standard feature of quakes or volcanos. Most quakes and volcanos agitate the ground, causing turbulence in surrounding waters. Cartoons and lousy movies make like tsunamis are part of any strong quake. It is only those events that bodily displace the sea floor verticly that produce tsunamis. As examples, the June 2005 quake off of northern California and the July 2005 undersea volcano near Okinawa did not cause tsunamis.
Force of water ------------ Water is not compressible like air. It's volume under force remains the same. As the slug of water is let go, it is blocked by underlying water and must divert horizontally. There begins a real mass migration of water outward from the source. The volume can be immense on human scale, hundreds of cubic kilometers. On the high sea away from land and over deep bottom the water can spread out and reduce in thickness. The wave may be only a few meters high. Ships may not associate such a wave with a tsunami, but treat it as a random fluctuation from a distant storm. As the wave reaches shallow bottom, approaching land, the volume is conserved by piling the mass higher. The wave can reach many tens of meters. Unlike a regular wave, the material is actually traveling with the wave and is piled up indefinitely far behind it. Thus, the land is waved over by a solid block, not a thin wall, of water. Damage is from the mechanical collision of this water against land structures. Recall that each cubic meter of water is one ton of mass. A chunk of water 5 meters on a side is the equivalent of a train engine or military tank! At slow speed, such a collision is messy enough, like that of a train and car or tank and fort. The tsunami hits land at hundreds of kilometers per hour, way too fast to outrun or elude with no warning. Since this mass has a vastly greater kinetic energy transferred to the shore obstructions, the damage is all the more violent. Kinetic energy for a given mass increases with the square of the speed.
Hawaii ---- Hawaii is a unique case on Earth. It is under the greatest threat from tsunamis. Being in the center of the Pacific Ocean, nowhere near a plate border, a tsunami anywhere in that ocean can attach Hawaii from any direction. Hawaii, from plausible threat and an actual destruction of Hilo in the 1940s, built an elaborate early warning and civil defense program for future tsunamis. Buoys, satellites, quake stations, airplane reports, keep close watch on the ocean movement. The population is drilled regularly on responding to alarms and taking certain escape roads to the high interior of the islands. Its volcanos, briefly, are not tectonic features. They are built from a hole in the underlying crust, perhaps torn out by buckling of the Pacific plate, that lets magma ooze to the surface. Hawaii's volcanos don't erupt in the conventional volcano sense. They release magma continuously. They are the world's most active volcanos, by its myriads of cubic meters of lava sent out every day. Despite their vigor, the Hawaii volcanos are tame enough to visit, up to the very craters, under National Park Service supervision. You see the lava, steam, noxious fumes close up on paths laid out across the volcanos. You also see towns blanketed by lava, some of it still too hot to walk on. Hawaii's quakes come from the internal shoving around of magma under the volcanos. Because the pressure and stress is continuously released, there is little chance for monster quakes like those at the Pacific rim. Hawaii, the eternal target of tsunamis will likely never generate any of its own.
Afterwaves -------- Most people think of the initial water mass approaching the land, which is what they see in news films about tsunamis. Tsunamis can come in several waves. The original massive block of water may divide into several along the way to the target. What happened in past strikes is that rescue teams went to the shore after the first wave and were totaled by the second one following a couple minutes later. Without remote sensing and realtime knowledge of its data, you can not tell on the scene what lies over the horizon rushing toward you at jet plane speed. An other cause of afterwaves is diffraction. Water 'bends' around obstructions, like islands, much like sound does around furniture in a room. You can hear a person while in the 'shadow' of a large cabinet, yes, a little weaker than if in the direct line of hearing. Thus, an island is vulnerable not only on the side facing the oncoming wave, but on all coasts. A third cause is reflection off of distant shorelines, specially long ones with cliffs. The tsunami bounces off in an other direction. It is a second or higher number wave attacking the target from an other direction. When the valley of one wave crosses the peak of an other, they net out to yield momentarily a weaker wave. Peak-peak or valley-valley intercepts make an amplified wave at that point. The result can be a series of waves of assorted, but dangerously large, height striking the target over several hours or even a full day.
Indian Ocean tsunami ------------------ The tidal wave that striked in the Indian Ocean in December 2004 emanated from an earthquake in Indonesia. It seems that a large slab of ocean floor lapsed into a void between the India and other plate during the quake. This set off the immense up-down heave of water that radiated away as the tidal wave. This was a singular event for the Indian Ocean, there not being any major tsunami before in recorded history. It suffers routine quakes, some originating in Indonesia. The nations in this ocean regarded tidal waves as a Pacific Ocean factor and never established any scheme for detecting, tracking, and then coping with tsunamis. There was no organized warning of any sort. The only effort was by haphazard phone calls or emails by geophysicists to colleagues in the various target countries. After these colleagues got word, there was no way to disseminate alerts to the public and then mount an evacuation. The result is now well known from the global news channels.
Defense ----- Most people on Earth live along the oceans, simply because the water provides transport, drinking, industrial fluid, recreation, fishing, and other desirable factors of human existence. The people on the ocean are concentrated into towns and cities, some of colossal size. New York is not the largest city on Earth, but a very large one with about 9-1/2 million residents. This count includes the folk who opted out of the census. Around New York is the cosmopolitan zone of some 53 million residents. By quirk of geography this region runs parallel to and close by the ocean. Virtually all traffic, commerce, business is carried out along this zone, with only limited escape routes at right angles to it. Hence, for the City, regardless of warning, there is little hope of effectively moving its population to safer ground to sit out the tidal wave. After the tsunami passes, and assuming there was a substantial evacuation, there will be little left to return to. Just about every structure will be toppled, crushed, soaked, filled with mud and gook. Roads and rails will be washed out. Electric, water, and other utilities will be cut. Supplies of food, medicine, other necessities will be hampered by lack of transport to the target area. For a target like New York, recovery may take decades or even a full century because so much of the American civilization will be wiped out. The rest of the US, allowing that it was not hit by the wave, will be too weak to deliver timely sufficient relief. The situation in almost any other country is far worse. On the whole, the US, and parts of Europe, are the highest level of cultural advancement on Earth. If it can not, and by all simulations will not, survive and sustain a tsunami attack, no one else will.
Warnings ------ In earlier times, a tsunami striked without warning, instilling dread and fear of the ocean. Because a tidal wave can spring from a remote, unknown, quake, there was no casual association of the wave with a local event, by which in the future to foretell them. Today, we not only have ground bases for watch for the initiating quake, but satellite monitoring of ocean movement. If a wave is associated with a definite quake, the internal quake waves travel to the target within minutes, offering warning if properly interpreted. The water wave comes many minutes to hours later, depending on distance to the target. Targets may be widely dispersed over the ocean around the quake, like they were for the December 2004 event in the Indian Ocean. Warnings must be sent to each target in time for defense or escape. Such action presumes a developed scheme of alarm and response, plus close cooperation among the targets. More locally, buoys are set up in the ocean, as they are along the Pacific coast of the US, to watch the water level. Changes indicative of a tsunami trigger alarms on shore to initiate defense plans. US NOAA in spring 2005 began a project to install such buoys along the Atlantic and Gulf coast. These are in anticipation of a plausible tsunami rising from the Canary Islands.
Conclusion -------- Only a week earlier I heard the Vesuvius talk, which emphasized the futility of moving people from the volcano zone for a predicted eruption. Briefly, Vesuvius is surrounding by conurbation, including the town of Naples, to within a few kilometers of its crater. Altho there were two massive blowups in history, in 79 and 1631, few, if any, lessons were learned about living with the volcano. For tsunamis, the only one on the East Coast reliably recorded was that associated with the quake in Lisbon, Portugal, in the 1700s. Few enough people lived on the coast back then to suffer unrecoverable damage. Because no tidal waves happened since then, it was in time forgotten. The other threat of tsunami comes from the crash of an asteroid. The instantaneous conversion of kinetic energy to mechanical motion of water could raise global tidal waves that would inundate just about all of human civilization. Everywhere, not just in certain countries. With the damage so great and remaining resources so little, human existence on Earth after an asteroid collision could well be thrown back to the stone age level.