Depending on how you count them, there are about 12 main tectonic plates, and numerous smaller ones. The 1964 Great Alaska Quake caused several downtown areas of Anchorage to collapse. In places like Hawaii and Yellowstone, a plate rides over a rising plume of hot mantle, causing earthquakes and a chain of volcanoes. Photo courtesy of Robert J. Lillie. USC Tsunami Research Group.Seismic Seiches from the 1964 Alaska Earthquake. Then, as Alaskas shaking ceased, things got worse much worse. Why do earthquakes usually occur at plate boundaries? S. Ornes. There are three main types of plate boundaries: There are four types of boundaries between tectonic plates that are defined by the movement of the plates: divergent and convergent boundaries, transform fault boundaries, and plate boundary zones. Channel Islands National Park includes five islands that are the tops of a mostly-submerged portion of the Transverse Ranges. Regional vertical deformation generated a train of destructive long-period seismic sea waves in the Gulf of Alaska as well as unique atmospheric and ionospheric disturbances that were recorded at points far distant from Alaska. The 1964 Alaska earthquake, the strongest earthquake ever recorded in North America, struck Alaskas Prince William Sound, about 74 miles southeast of Anchorage. Alaskans are familiar with earthquakes. In Alaska, more than 50 volcanoes have erupted over the last 250 years. As you slide your hands laterally past one another, a broad zone of shearing develops as several card faces slip. (The lithosphere is Earths outer solid layer. It includes many lesser faults in addition to the San Andreas Fault. No safe motoring here anymore. SSAs 2014 Annual Meeting will provide a stimulating exchange of research on a wide range of topics with colleagues from all over the world. Before sharing sensitive information, make sure youre on a federal government site. This opinion piece by USGS geologist Peter Haeussler, et al. Two earthquake faults exposed on Montague Island are subsidiary northwest-dipping reverse faults along which the northwest blocks were relatively displaced a maximum of 26 feet, and both blocks were upthrown relative to sea level. Nows a great time to start window-shopping at the. Oct. 16, 2012. The devastating 9.2 magnitude earthquake and subsequent tsunamis ravaged coastal communities and took over 139 lives. Other rocks in the San Francisco Bay Area were originally part of an accretionary wedge, similar to rocks found today in the coastal ranges of the Cascadia Subduction Zone in northern California, Oregon, and Washington. seismograph (also known as aseismometer) An instrument that detects and measures tremors (known as seismic waves) as they pass through Earth. Modified from Earth: Portrait of a Planet, by S. Marshak, 2001, W. W. Norton & Comp., New York. The earthquake was accompanied by vertical displacement over an area of about 520,000 square kilometers. . Parks in the Sierra Nevada, including Yosemite, Kings Canyon, and Sequoia national parks, contain granite-type rocks that cooled within magma chambers beneath ancient subduction zone volcanoes. This led to the establishment of the Pacific Tsunami Warning Center (originally called the West Coast/Alaska Tsunami Warning Center) to alert people when a widespread tsunami is possible. Alaska's continental shelf and North American plate rose over 9 meters during the earthquake. The land that was uplifted and sometimes dropped by the Great Alaskan Quake also fits with the plate-tectonics model. The April 1933 M6.9 earthquake, which caused considerable damage in Anchorage, appears to have occurred on such . These strike-slip faults and their associated structures are defined by linkage of mid-ocean spreading centers and subduction zones, and are exposed onshore as they traverse the continental crust between mid-ocean ridges and other plate boundary segments (Figure 2). If one plate is trying to move past the other, they will be locked until sufficient stress builds up to cause the plates to slip relative to each other. Tomales Bay is the surface expression of the San Andreas Fault, seen in the photo below. Reports of residual water sloshing (seiches) came in from the U.S. Gulf Coast and as far away as Australia. Movement in narrow zones along plate boundaries causes most earthquakes. Experiment: Are fingerprint patterns inherited? The divergent boundary occurs when two tectonic plates move away from each other. These tectonic plates move gradually, bumping and grinding against each other. Seafloor topography map source: Global sea floor topography from satellite and ship depth soundings, 1997, by W. H. F. Smith and D. T. Sandwell, Science, v. 277, p. 1956-1962. Like many of the rocks that are caught up in the zone of transform motion between the Pacific and North American plates, the rocks at Channel Islands National Park were deformed as part of the accretionary wedge during earlier subduction of the Farallon Plate. The invention of new tools and faster data processing have drastically improved NOAAs ability to warn the public of tsunamis and to forecast their wave heights. It is also the second largest earthquake ever recorded, next to the M9.5 earthquake in Chile in 1960. This is an earthquake. Most seismic activity occurs at three types of plate boundariesdivergent, convergent, and transform. Porosity increases, probably related to horizontal displacements in the zone of subsidence, were reflected in lowered well-water levels and in losses of surface water. The Sierra Nevada are the eroded remnants of the volcanic arc developed when the Farallon Plate subducted beneath the continent. Today, almost 60 years since the Great Alaska Earthquake, the Tsunami Warning Centers issue tsunami warnings in minutes, not hours, after a major earthquake occurs. . in that one quake, it was the first time that you could actually see plate motion in action. . (Click on arrows and slide left and right to see labels.). subsidence A downward shift in some section of Earths surface. See whats revealedevery Thursday at NightLife. Systematic horizontal movements of the land relative to bodies of confined or semiconfined water may have caused unexplained short-period wavessome of which were highly destructiveobserved during or immediately after the earthquake at certain coastal localities and in Kenai Lake. Both plates are capped by oceanic crust. Thrust earthquakes (as opposed to strike slip) are far more likely to generate tsunamis, but small tsunamis have occurred in a few cases from large (i.e., > M8) strike-slip earthquakes. Scientists are still working out many details of plate tectonics. Heres evidence of the impacts of those waves that surged into the deep-water port of Whittier after the earthquake. March 21, 2011. Parks in western California contain blocks of crust that have moved great distances north-northwestward along the San Andreas Fault. The large white star represents the zone where plates lock together for centuries then suddenly let go, causing the largest earthquakes. Together, these destructive waves swept Alaskas coastline with devastating speed and power. USGS published the results of investigations of the Alaska earthquake of March 27, 1964 in a series of six Professional Papers. Exhibits along the trail include the reconstruction of a fence that was offset 16 feet (5 meters) during the 1906 earthquake. Our plate boundary comprises ~2,500 miles of the circum-Pacific "Ring of Fire" where ongoing subduction gives rise to arcs of active volcanoes and the largest earthquakes in the world. Cabrillo National Monument south of San Diego also lies within the broad zone of deformation between the two plates. Wide zones of deformation are usually characteristic of plate boundaries because of the interaction between two plates. The foreground shows a ditch (or graben) that opened up roughly 12 feet (or around 3.7 meters) deep. Map showing ground motion and shaking intensity based on instrumental measurements of shaking along with information about local geology and the earthquakes location and magnitude. In that one quake, it was the first time you could actually see plate motion in action. Afterwards, geologists realized subduction zonesareas where two tectonic plates (huge slabs of rock made of the earths crust and upper mantle) meet and one bends under the otherplayed a major role in creating the immense Alaskan quake. Often it is the older and colder plate that is denser and subducts beneath the younger and warmer plate. Tomales Bay. A massive tidal wave crushed the small, coastal village of Chenega four minutes after the tremors subsidedthe town lost a third of its population. convergent plate boundaries divergent plate boundaries transform plate . Transform boundaries occur on the north and south sides of the Caribbean Plate. Credit: Photo Courtesy of Robert J. Lillie. This photo shows a small snow slip in the center foreground. Along convergent boundaries, neighboring plates either collide head-on or a denser ocean plate dives beneath a lighter continental plate. April 30, 202323:15:06 AKDT (May 1, 202307:15:06 UTC)62.0155N148.8858W Depth 13.6 miles (22 km), This event has not been reviewed by a seismologist, 2156 Koyukuk Drive, PO Box 757320, Fairbanks, AK 99775. Maximum indicated uplift in the Alaska and Aleutian Ranges to the north of the zone of subsidence was l feet. Farther east, the continent began to rift apart in the Basin and Range Province. Policies and Notices, U.S. Department of the Interior | There are three types of plate boundaries, convergent, divergent, and transform. This sudden displacement of the ocean floor, along with earthquake-induced landslides, generated massive local tsunamis that resulted in 70 percent of the fatalities in southern Alaska. The movement releases stored-up 'elastic strain' energy in the form of seismic waves, which propagate through the Earth and cause the ground surface to shake. 345 Middlefield Road, MS 977 There are two kinds of plates oceanic and continental plates. Hes a research geologist with USGS and its Alaska coordinator of earthquake hazards research. Magnitude type: M l 2; Event type: earthquake; Tectonic Setting of Southern Alaska . The Earthquake Trail at Point Reyes weaves back and forth across the fault line. Alaskas continental shelf and North American plate rose over 9 meters during the earthquake. By midnight, 131 people were dead. Feb. 13, 2013. This is known as subduction. A convergent plate boundary is a location where two tectonic plates are moving toward each other, often causing one plate to slide below the other (in a process known as subduction). Prior to the 1964 Alaskan earthquake, scientists had limited knowledge of what happens far beneath the earth. When this happens, some part of the surface is said to subside. Quake leaves destruction, fear. Science News for Students. Seismicity of the United States, 1568-1989 (Revised), In general, the deepest and the most powerful earthquakes occur at plate collision (or subduction) zones at convergent plate boundaries. The Alaska earthquake of March 27, 1964, magnitude approximately 8.3-8.4, began at 6:36 p.m. Its epicenter was in the northern part of the Prince William Sound area; focal depth was 20-50 km. They were lifted out of the ocean as part of the accretionary wedge of an ancient subduction zone. Most earthquakes rattle the ground for just seconds. The cumulative movement within the broad San Andreas transform plate boundary has had dramatic effects on a landscape that initially developed as part of an ocean/continent subduction zone. By definition, intraplate earthquakes do not occur near plate boundaries, but along faults in the normally stable interior of plates. Each giant slab was expected to be moving slowly. Due to the long duration of the earthquake, catastrophic ground failures occurred. The belt of seismic activity and major zones of tectonic deformation associated with the 1964 earthquake, to a large extent, lie between and parallel to the Aleutian Volcanic Arc and the Aleutian Trench, and are probably genetically related to the arc. . At an ocean-ocean convergent boundary, one of the plates (oceanic crust and lithospheric mantle) is pushed, or subducted, under the other (Figure 4.6. In fact, megathrust events are the largest type of earthquake on the planet, notes Peter Haeussler. April 30, 202323:37:54 AKDT (May 1, 202307:37:54 UTC)61.7528N146.4391W Depth 19.4 miles (31 km), This event has not been reviewed by a seismologist, 2156 Koyukuk Drive, PO Box 757320, Fairbanks, AK 99775. Modified from Parks and Plates: The Geology of our National Parks, Monuments and Seashores, by Robert J. Lillie, New York, W. W. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172. The San Andreas Fault is just one of many active earthquake faults in a broad zone of shearing along the transform plate boundary in the San Francisco Bay Area. (en Espaol). When the plates finally give and slip due to the increased pressure, energy is released as seismic waves, causing the ground to shake. That will give you an idea of how fast the plates move relative to one anotherabout a fraction of an inch to a few inches per year! Continental transform faults play a critical role in accommodating strain along major tectonic plate boundaries. epicenter The underground location along a fault where an earthquake starts. In Alaska, that plate dives under the North American Plate. Seiches, a sloshing of water back and forth in a small body of water, were observed as far away as Louisiana, where a number of fishing boats sank in a harbor. They have been transported about 300 miles (500 kilometers) in a north-northwestward direction along the transform plate boundary. As the plates collide, the oceanic plate is forced beneath the continental plate. Sign up for the Academys monthly newsletter and get a promo code for 10% off at our online retail store. Where plates crash together, one dives (subducts) beneath the other, causing volcanoes (red triangles) to erupt on the overriding plate and earthquakes (black stars) at a variety of depths. They also forecasthow large any resulting tsunami will be as it crosses the ocean. The major area of uplift trended northeast from southern Kodiak Island to Price William Sound and trended east-west to . Movement on the megathrust was accompanied by subsidiary reverse faulting, and perhaps wrench faulting, within the upper plate. Since neither plate is stronger than the other, they crumple and are pushed up. First posted September 10, 2012 Revised August 8, 2013, For additional information: Download the latest version of Adobe Reader, free of charge. Scientists now classify the bumping and grinding between plates in three different ways. What are the three types of convergent boundaries?
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