Here, that 11-foot (3.4 meter) drop or subsidence took a line of parked cars with it. Sometimes the molten rock rises to the surface, through the continent, forming a line of volcanoes. 1). A massive tidal wave crushed the small, coastal village of Chenega four minutes after the tremors subsidedthe town lost a third of its population. There are three types of plate boundaries: spreading zones, transform faults, and subduction zones. Tsunami waves can hit with the devastating force of a powerful tornado. Other parks in the region, namely Pinnacles, Channel Islands and Joshua Tree national parks, Cabrillo National Monument and Santa Monica Mountains National Recreation Area, reveal evidence of the shearing, rotation, and uplift that occurs within the broad zone of deformation between the two plates. Along with the NOAA Pacific Tsunami Warning Center located at Ford Island, Hawaii, the National Tsunami Warning Center monitors and warns for tsunami threats 24/7 throughout the year. It includes many lesser faults in addition to the San Andreas Fault. The duration of strain accumulation in the epicentral region, as interpreted from the time interval during which the coastal submergence occurred, probably is 9301,360 years. 345 Middlefield Road, MS 977 About 200 million years ago, a large tectonic plate (called the Farallon Plate) started to subduct beneath the western edge of North America. View from the playground of the Government Hill School in Anchorage, showing damage from the 1964 earthquake. You can now see under the house to the yard beyond (center right of photo). National Park Service Sites (shown in red) What type of plate boundary causes earthquakes in Alaska? Some start and then stop, only to start again much later. Alaska's continental shelf and North American plate rose over 9 meters during the earthquake. The San Andreas Fault is responsible for most of the movement in western California, causing a sliver of the state to slide past the rest of the continent. Plafker named this event a megathrust earthquake. Hes a geophysicist with the USGS who recently co-authored a study looking back at the 1964 earthquake. On the east, the North American Plate is subducting westward, forming volcanoes of the Lesser Antilles Island Arc. This can lead to the formation of huge, high mountain ranges such as the Himalayas. Earths crust The outermost layer of Earth. Geologists call plate tectonics a unifying theory. Thats because it explains practically everything about Earths crust. To the north and northwest of the zone of uplift, subsidence forms a broad asymmetrical downwarp centered over the Kodiak-Kenai-Chugach Mountains that averages 2 feet and attains a measured maximum of 7 feet along the southwest coast of the Kenai Peninsula. Point Reyes National Seashore and Golden Gate National Recreation Area are the only two NPS sites that are right on the San Andreas Fault. Most earthquakes at divergent plate boundaries occur at mid-ocean ridges where two pieces of oceanic crust are moving away from each other. Californias San Andreas fault is a transform boundary. Together, these destructive waves swept Alaskas coastline with devastating speed and power. plate boundary (in geology) The edge of a tectonic plate, or the place where two or more tectonic plates meet. The fault responsible was the Aleutian Megathrust, a reverse fault caused by a compressional force. Thats where one tectonic plate sinks beneath another. 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! Big quake, little destruction. Science News for Students. They form due to north-south compression where the San Andreas Fault bends to an east-west orientation. Some of the plates have ocean water above them. When the plates finally give and slip due to the increased pressure, energy is released as seismic waves, causing the ground to shake. As the mid-ocean ridge separating the Farallon and Pacific Plates entered the subduction zone, the Farallon Plate separated into the Juan de Fuca and Cocos Plates. Its a bit like a rug wrinkling. Here, frozen ground holding the roots of this tree, in Anchorage, split apart during a landslide. Movement in narrow zones along plate boundaries causes most earthquakes. All rights reserved. All rights reserved. Another form of convergent boundary is a collision where two continental plates meet head-on. It explains how the continents and oceans have been shaped, and why there are earthquakes, mountains, volcanoes and tsunamis. Photos and illustrations above modified from Oregon's Island in the Sky: Geology Road Guide to Marys Peak, by Robert J. Lillie, Wells Creek Publishers, 75 pp., 2017, www.amazon.com/dp/1540611965. U.S. Geological Survey The tidal wave had diminished by the time it hit Hawaii and Japan, causing little damage. Ground failures are an effect of seismic activity in which the ground becomes very soft and acts like liquid, causing landslides, spreading, and settling. Magnitude type: M l 2; Event type: earthquake; Tectonic Setting of Southern Alaska . Typically, a convergent plate boundarysuch as the one between the Indian Plate and the Eurasian Plateforms towering mountain ranges, like the Himalaya, as Earths crust is crumpled and pushed upward. Shallow earthquakes, less than 20 km deep, are associated with _____. Then, as Alaskas shaking ceased, things got worse much worse. Underwater landslides along the Seward, Alaska, waterfront triggered the strong waves which hit here, 2 miles (3.2 kilometers) from Seward. In Alaska, that plate dives under the North American Plate. The earthquakes that occur along these zones, called spreading centers, are relatively small. Subduction zones occur when one or both of the tectonic plates are composed of oceanic crust. The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely. Along much of the boundary, the bulk of the motion occurs along the San Andreas Fault. 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. The publication, as well as Science News magazine, are published by the Society for Science, a nonprofit 501(c)(3) membership organization dedicated to public engagement in scientific research and education. The 1964 quake woke up the dormant Hanning Bay fault on Montague Island in Alaskas Prince William Sound. Seismic waves caused the earth to "ring like a bell." As bad as the tremors were, the worst was yet to come. Photo courtesy of Robert J. Lillie. Earthquakes at divergent plate boundaries occur as new crust is created and other crust is pushed apart. The warning system doesnt monitor local tsunamis, however. Now, a full half-century after Alaskas gigantic quake, geologists are still feeling its effects. Anchorage house pulled apart as the ground on which it had been anchored opened up, creating a fissure (or graben). Parks in western California contain blocks of crust that have moved great distances north-northwestward along the San Andreas Fault. 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. U.S. Geological Survey The San Andreas Fault Zone is not the only active transform plate boundary with U. S. National Park Service sites. At 5:36 p.m., the ground began shaking violently. mantle The thick layer of the Earth beneath the crust. Plates beneath the oceans are much thinner than those under continents. Produced under a Cooperative Agreement for earth science education between the National Park Service's Geologic Resources Division and the American Geosciences Institute. Focal-mechanism studies, when considered in conjunction with the pattern of deformation and seismicity, suggest that it was a complex thrust fault (megathrust) dipping at a gentle angle beneath the continental margin from the vicinity of the Aleutian Trench. 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. It extends across the two zones from the chain of active volcanoes in the Aleutian Range and Wrangell Mountains probably to the Aleutian Trench axis. Aftershocks from the quake continued for three weeks. Notice what happens as you move your left hand away and slide your right hand toward you. Scientists next had to fit this finding to their understanding of plate tectonics. An examination of the 1964 Alaska Good Friday Earthquake from a (pre- plate tectonics theory) geological point of view. 2023, A&E Television Networks, LLC. This was the Big One. Along these boundaries, earthquakes are common and magma (molten rock) rises from the Earths mantle to the surface, solidifying to create new oceanic crust. Forty million years ago, a large tectonic plate, known as the Farallon Plate, was between the Pacific and North American plates. According to the United States Geological Survey, The 1964 earthquake was giant because of the large area of the fault that slipped during the earthquake and the large amount of slip, or relative motion, between opposite sides of the earthquake fault.. There are two kinds of plates oceanic and continental plates. Beginning about 30 million years ago, so much of the Farallon Plate was consumed by subduction that the Pacific and North American plates were in contact, forming the San Andreas transform plate boundary in western California. The convergent boundary happens when two plates slide toward each other to form a subduction zone or a continental collision. (The lithosphere is Earths outer solid layer. The 1964 earthquake in Anchorage, Alaska occurred as a result of an oceanic plate sinking under a continental or land plate. Hes a research geologist with USGS and its Alaska coordinator of earthquake hazards research. The earthquake triggered a swell of devastating tsunamis, landslides and submarine. No Alaskan who survived would ever forget this day. The April 1933 M6.9 earthquake, which caused considerable damage in Anchorage, appears to have occurred on such . Subduction zones usually have a deep trench along the top. HS-ESS2-1, HS-ESS2-2, HS-PS4-1, MS-ESS2-1, MS-ESS2-2, MS-ESS2-3, MS-PS4-1, MS-PS4-2. lithosphereThe upper layer of Earth, which includes its thin brittle crust and upper mantle. The San Andreas Fault is just one of several faults that accommodate the transform motion between the Pacific and North American plates. Comparable horizontal movements are presumed to have affected those parts of the major zones of uplift and subsidence for which retriangulation data are unavailable. Movement on the megathrust was accompanied by subsidiary reverse faulting, and perhaps wrench faulting, within the upper plate. Point Reyes National Seashore, California. Another form of convergent boundary is a collision where two continental plates meet head-on. Towns such as Whittier, Alaska, were inundated by tsunami waves before the earthquake had even subsided. Question: What type of tectonic plate boundary exists along the edge of the North American plate near the coast of Northern California, Oregon, and Washington? The sedimentary and metamorphic rocks across the fault line are similar to those found in Redwood National and State Parks on the North Coast of California. But 50 years ago, that now infamous Alaska quake shook steadily and terrifyingly for almost five solid minutes. Finally, the transform boundary, that the San Andreas . At still others, known as transform boundaries, plates slide past each other. Its a well-documented certainty. W.R. Hansen/USGS, 1964 Alaska quake files. Abridged from (Click on arrows and slide left and right to see labels.). epicenter The underground location along a fault where an earthquake starts. 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. , Divergent boundaries where two plates are moving apart. Remnants of the ancient volcanic mountain chain remain. These spruce trees had been growing along the shores of Resurrection Bay in southern Alaska. Map showing plate boundaries that were active during the 1964 Great Quake. the earthquake and tsunami caused the deaths of over . The damage was caused entirely by the shifting of ground along the fracture, USGS concluded. The accretionary wedge rocks are found in Channel Islands National Park, Golden Gate and Santa Monica Mountains national recreation areas and Cabrillo National Monument. For example, rocks found today in Point Reyes National Seashore north of San Francisco were originally part of the line of granite rocks formed beneath ancient subduction zone volcanoes. Sign up for event updates and exciting announcements. According to the postulated model, the observed and inferred tectonic displacements that accompanied the earthquake resulted primarily from (1) relative seaward displacement and uplift of the seaward part of the block by movement along the dipping megathrust and subsidiary faults that break through the upper plate to the surface, and (2) simultaneous elastic horizontal extension and vertical attenuation (subsidence) of the crustal slab behind the upper plate. Retriangulation over roughly 25,000 square miles of the deformed region in and around Prince William Sound shows that vertical movements there were accompanied by horizontal distortion, involving systematic shifts of about 64 feet in a relative seaward direction. You'll be happy you did. The long mountain ridges and narrow bays in the region surrounding U. S. Virgin Islands National Park are a product of compression due to the convergence, in addition to lateral motion due to shearing along the transform plate boundary. Both plates are capped by oceanic crust. Earthquakes occur along fault lines, cracks in Earths crust where tectonic plates meet. HISTORY reviews and updates its content regularly to ensure it is complete and accurate. Earthquakes and tsunamis can happen along any coastline, at any time of the year, but Alaska is particularly prone to them because it sits on the convergence of two tectonic platesthe Pacific Plate and the North American Plate. The March 27, 1964, earthquake was accomp anied by crustal deformation-including warping, horizontal distortion, and faulting-over probably more than 110,000 square miles of land and sea bottom in south-central Alaska. NRA = National Recreation Area 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. subsidence A downward shift in some section of Earths surface. National Park Service sites along the transform plate boundary in California contain rocks formed during the earlier subduction that occurred in western North America. Quake leaves destruction, fear. Science News for Students. S. Ornes. 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. But it took some geological sleuthing to determine how and why. Railroad tracks warped. This resulted in a line of volcanoes stretching all the way from what is now Alaska to Central America. The mantle is semi-solid and generally divided into the upper mantle and the lower mantle. SSAs 2014 Annual Meeting will provide a stimulating exchange of research on a wide range of topics with colleagues from all over the world. Divergent boundaries - where two plates are moving apart. The plate boundary is a broad zone of deformation with a width of about 60 miles (100 kilometers). Why do earthquakes usually occur at plate boundaries? The 1964 Great Alaska Quake caused several downtown areas of Anchorage to collapse. Molten rock from the mantle erupts along the opening, forming new crust. They also sought to locate the quakes epicenter. This motion causes earthquakes. The San Andreas Fault that runs through much of California is an enormous transform plate boundary. March 21, 2011. These plates constantly move. They occur where plates are subducting, spreading, slipping, or colliding. Active volcanoes of the Lesser Antilles Island Arc form as the North American Plate subducts beneath the Caribbean Plate. For more information on how you can prepare for a tsunami, visit the National Tsunami Hazard Mitigation Program. First posted September 10, 2012 Revised August 8, 2013, For additional information: 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. These cracks generally ran at high angles to the faults and could run a few hundred feet and span 6 inches (15 centimeters). 1719 N Street, N.W., Washington, D.C. 20036, People and animals sometimes team up to hunt for food, Pokmon evolution looks more like metamorphosis, These beetles drink water using their butts. They recalled seeing asphalt roads rise and fall like waves and the ground opening and closing before them, water shooting up through the ensuing cracks. Alaskan tectonics are dominated by the Pacific-North American plates. At this boundary, the Pacific Plate slides beneath the North American Plate, causing the majority of Alaskas earthquakes, including the 1964 earthquake. Tectonic plates constantly push against each other. In fact, West notes, four out of every five U.S. quakes occurs in this northernmost state. uplift A upward shift in some section of Earths surface due to tectonic activity along a fault. Learn about the great leaps in research over the past 50 years. Fifty years later, it continues to shape Alaska, its people, and the science of earthquakes. When oceanic and continental plates meet, the denser oceanic plate moves under the less dense continental plate. This pattern has since been seen at other subduction zones. Learn more about UAs notice of web accessibility. The April 1933 M6.9 earthquake, which caused considerable damage in Anchorage, appears to have occurred on such . She now estimates that it takes at least 500 years of plate motion to set the stage for such big earthquakes. The earthquake hit at 3:11 pm approximately 100 miles (160 km) off the coast of Chile . A map of some dozen major tectonic plates and several minor ones. Look at your fingernails and watch them grow. Plate Tectonics and Our National ParksSite Index, Plate TectonicsThe Unifying Theory of Geology, Tectonic Settings of NPS SitesMaster List. Afterward, the roots daily bathing for hours each day in salt water killed these trees. Channel Islands National Park, Santa Monica Mountains National Recreation Area and Joshua Tree National Park are within the Transverse Ranges, a block of crust that rotated as a result of the shearing motion. Cabrillo National Monument south of San Diego also lies within the broad zone of deformation between the two plates. Excerpt from the TV show The Big Picture produced by the US Army in 1966 about the Alaska Earthquake and its tragic effects. Alaskas continental shelf and North American plate rose over 9 meters during the earthquake. Modified from Beauty from the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest, by Robert J. Lillie, Wells Creek Publishers, 92 pp., 2015, www.amazon.com/dp/1512211893. Page Last Modified: Thursday, December 01, 2016, 04:21:41 PM, Download the latest version of Adobe Reader, free of charge.
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