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Brent Wittmer
Creative Project 1- Blog Post: The 2018 Anchorage Earthquake
OCN 101
Through the Lens of Oceanography: The 2018 Anchorage Earthquake
The 2018 Anchorage Earthquake:
On 11/30/2018, at 8:29 am, just as Alaskans were starting their day, chaos ensued when a magnitude 7.1 earthquake shook Southcentral Alaska near Joint Base Elmendorf-Richardson, along the Denali Fault nearly 400km from the Aleutian Trench. Buildings began to shake violently in downtown Anchorage and across Southcentral Alaska; homes began to sink slightly into the softened ground; the Alaska railroad line was severed by landslides, cracks appeared in roads, bridges, and overpasses; goods shook off the shelves of stores; windows shattered; and utilities were lost for many. In a matter of moments, Anchorage and its surrounding communities went from business as usual to triage mode as buildings evacuated and road and airports closed. Periodic aftershocks stirred everyone’s nerves and they wondered what to do next.
Regional Earthquake Activity:
The earthquake caused widespread damage to businesses, homes, schools, and other infrastructure across the region. Some of the damage is yet to be addressed. However, earthquakes are nothing new to the Southcentral Alaska region. Each year, approximately 40,000 earthquakes are recorded in Alaska. (UC Berkley). The Southcentral area is home to the historic Great Alaskan Earthquake, which took place on Good Friday in 1964. Most of Alaska and many beyond felt the magnitude 9.2 Great Alaskan Earthquake, which was so powerful it registered in all the United States except three and resulted in a powerful and deadly tsunami that devastated Valdez and resulted in the city having to be rebuilt elsewhere(History.com; 1964 Good Friday Earthquake). The aftermath looked like something from a sci-fi movie, with streets split and shifted several dozen feet in Anchorage; cars teetering on the edge of the openings in the ground. The Great Alaskan Earthquake remains the strongest earthquake ever recorded in North America. Only one earthquake in recorded history was stronger – a 9.5 magnitude earthquake that took place in Chile in 1960. (1964 Good Friday Earthquake).
To understand better all the forces at play in these earthquakes, it is helpful to look at them through the lens of some basic geographic and oceanographic principles. Conversely, it is also helpful to view these scientific concepts through the specific lens of the earthquakes that have taken place in Southcentral Alaska, in order to bring the concepts closer to home, so to speak.
Tectonic Plates: Collisions and Boundaries:
While many elements are at play in every earthquake, the most foundational factor is that of tectonic plates. With the exception of the earth’s Continental and Oceanic crusts, the Lithosphere is the earth’s most external layer. It is approximately 60 miles thick and is made up of these massive tectonic plates which gradually drift over time. There are seven major tectonic plates, African plate, Antarctic plate, Eurasian plate, Indo-Australian plate, North American plate, Pacific plate, and South American plate. Together, these seven cover the vast majority of the sphere of the earth. Where two plates meet, a boundary exists, and boundaries are defined by the way the plates interact at the with one another. These boundaries are also where earthquakes originate.
Plate Boundaries:
Southcentral Alaska sits right on the boundary between the massive Pacific Plate and the North American plate at a unique place where two types of boundaries exist. Because the Pacific plate is gradually moving north, it slides horizontally across the eastern flank of the North American plate along much of the Pacific Northwest coast. However, as it approaches Southcentral Alaska, it shifts to a direct fontal collision between the two plates, as is illustrated in the image below. While the horizontal grinding of the two plates represents a transform boundary, the frontal collision on the north edge of the Pacific Plate represents a convergent boundary. South central Alaska sits right in the corner between the two boundaries and is subject to the extreme forces resulting primarily from the convergent boundary located at the Aleutian trench.
The variety of forces created by the convergent boundary are a recipe for significant earthquake activity. This span of the north end of the Pacific plate ramming into the Aleutian trench is associates with the “Ring of Fire”, which refers to the frequent earthquake activity along the northern portion of the Pacific plate. Along this large convergent boundary pressure is constantly building as the Pacific Plate slowly drifts at about 2.5 inches per year (UC Berkley). However, the pressure must eventually release one way or another.
Subduction Zone: Rising Mountains and a Diving Plate:
While much of the force created at the convergent boundary is absorbed into the earth’s crust, causing the Chugach mountain range an others to continue to grow upward, the Pacific Plate actually dives under the North American Plate deep under the continental crust creating a “subduction zone”. A Subduction Zone takes place at a convergent plate boundary and describes this interaction whereby one plate dives beneath another. The Pacific-North American plates convergent boundary and subduction zone along the Aleutian trench is illustrated in the image below.
Building Pressure and Energy Release:
Because the plate surfaces are not smooth, they tend to catch on one another and much of the force of the interacting plates builds slowly over time until it can no longer be stored and the plates slip along one another. As the plates become unsnagged from one another, all the stored-up energy is translated into motion and the plates grinding along one another sends shockwaves throughout the earths continental and oceanic crust. Interestingly, the forces of a subduction zone can be translated to vertical forces, whereby the diving plate presses upward on the upper plate. According to the UC Berkley Seismology Lab, in the case of the Great Alaska Earthquake of 1964, this type of force pushed the Pacific Seabed upward several dozen meters and caused, which resulted in significant water displacement and led to a massive and deadly tsunami (UC Berkley). However, the same lever effect of the subduction zone had a different result in the 2018 Anchorage Earthquake, for which the epicenter was located far inland from the coast, near the Denali Fault. Because the epicenter was inland, the earthquake did not result in a significant tsunami like the Great Alaskan Earthquake.
The 2018 Anchorage Earthquake:
In review, on the 30thof November last year, the forces of Alaska’s massive subduction zone met a point where they could no longer be stored up. As unbelievable forces pushed up and down and side to side, rocks crumbles and the two plate shifted along one another initiating the the 2018 Anchorage Earthquake, which sent shockwaves throughout lithosphere, into the continental crust, and transferred them into all of Southcentral Alaska’s homes, workplaces, schools, and roads. Structures unable to withstand the violent shake were damaged, while others sunk because of liquefaction, the process of sandy moisture-rich earth softening during strong earthquakes. In just a few moments, millions of dollars of damage was done and Alaska was hurtled into emergence response.
Although great advances in knowledge and technological progress has been made relating to the science of seismology, the fact remains that there is still no mechanism to predict earthquakes. In an area like Southcentral Alaska, this is not the most comforting fact because earthquakes are relatively frequent and can be very large. However, the scientific progress made to this point has been the result of scientific observations of earthquakes. In theory, more earthquakes provide more observation, which will hopefully result in technological advances in earthquake-resistant structures, the ability to avoid liquefaction-prone building sites, and even perhaps one day the ability to estimate earthquake activity.
Works Cited
(n.d.). Retrieved October 3, 2019, from https://pubs.usgs.gov/fs/2003/fs014-03/alaska.html.
1964 Good Friday Earthquake. (n.d.). Retrieved October 7, 2019, from http://www.valdezalaska.org/discover-valdez-history/valdez-history-1964-good-friday-earthquake.
1964 Good Friday Earthquake. (n.d.). Retrieved October 7, 2019, from http://www.valdezalaska.org/discover-valdez-history/valdez-history-1964-good-friday-earthquake.
History.com Editors. (2018, March 6). 1964 Alaska Earthquake. Retrieved October 1, 2019, from https://www.history.com/topics/natural-disasters-and-environment/1964-alaska-earthquake.
Plate tectonics. (2007, July 21). Retrieved October 4, 2019, from https://www.sciencelearn.org.nz/resources/339-plate-tectonics.
UC Berkeley. (2018, December 3). Seismo Blog. Retrieved October 7, 2019, from http://seismo.berkeley.edu/blog/2018/12/03/a-quake-inside-a-diving-plate.html.
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