Earthquakes of Southcentral Alaska

Alaska has become synonymous with earthquakes. Earthquakes in Southcentral Alaska are indeed frequent and often powerful, although traditionally not as destructive of those in places such as California.

The frequency of earthquakes in Alaska is astounding. Covering only a small part of the Earth’s surface, Alaska accounts for fully 11 percent of the world’s earthquakes and more than half of those that originate in the United States. It experiences on average one magnitude seven earthquake every year and an earthquake greater than magnitude eight every 14 years.

Southcentral Alaska’s earthquakes are a result of the subduction of the Pacific Plate and the pressure it exerts on the terranes than form southern Alaska. Because the rocks that make up the crust of the planet are solid and elastic, they do not flow when pressure is exerted upon them. Instead, they bend as pressure builds up until a weak spot, usually along a geologic fault, gives way, unbending the bedrock with an enormous jerk, sending waves through the ground that we feel as earthquakes. This process of earthquake generation is known as elastic rebound. As the Pacific Plate moves to the northwest, even though it only moves a few inches every year, this process releases significant amounts of energy, primarily as small and moderate earthquakes, but also with the occasional large one.

Significant Earthquakes in Alaska

Most Alaskan earthquakes are associated with the subducting Pacific Plate in the Aleutian Trench. Because of Alaska’s complicated geologic structure, however, they occasionally occur just about everywhere around the state.

Because of their proximity to the Aleutian megathrust fault, the sparsely inhabited Aleutian Islands receive the brunt of Alaska’s earthquakes. But because this fault curves into the Gulf of Alaska, almost to Prince William Sound, Southcentral Alaska experiences many earthquakes as well. Two other major sources of earthquakes in Southcentral Alaska are the fault zones where the Yakutat Terrane is smashing into Alaska, in the vicinity of the St. Elias Mountains, and along the Denali fault, which arcs along the northern edge of Southcentral Alaska.

The principal reason for the relative lack of destruction in Alaskan earthquakes is that few people live in its earthquake prone regions. Another reason is that many of Alaska’s earthquakes are generated in a portion of the subducted Pacific plate that is located at a depth of over 45 miles. The intensity of an earthquake’s shaking decreases rapidly over distance. Because the energy is somewhat dissipated by the time it reaches the surface, a deep earthquake in Alaska is less hazardous on the surface than a shallow earthquake along a transform fault, such as in California.

Earthquakes travel in waves through the earth. The fastest wave generated by an earthquake, and therefore the first to be felt, is a P Wave, which is a compressional wave (like a sound wave). A slower wave, called an S Wave, shakes back and forth (imagine shaking a rope) is also generated. The slowest waves are surface waves, which travel in the outer layers of the earth. The P Wave often feels like an initial sudden jolt, followed by the somewhat smoother vibrations of the S waves which then merge into the rolling surface waves. The S and surface waves are more destructive than the P waves.

Earthquakes are measured using two scales. The first is the Richter scale, which measures the magnitude, or the amount, of energy released. The Richter scale is based on the movements of a seismograph, an instrument used to measure the amplitude of the ground motion. It is a logarithmic scale, meaning that a 6.0 earthquake produces waves that are 10 times larger than those of a 5.0. A 2.0 earthquake is hardly worth paying attention to, whereas a 9.0 is an event of worldwide importance. The second scale is the Mercalli scale, which measures the intensity of shaking at a location. The intensity of shaking tends to decrease with distance from the epicenter, but the nature of the underlying ground can affect this. Locations with thick, soft soils experience more intense shaking than locations directly on top of bedrock. The Mercalli scale uses Roman numerals I-XII, where a I is barely felt by anyone and a XII indicates complete destruction.

Southcentral Alaska has had several major earthquakes of note. Two of particular interest are the March 27, 1964 earthquake and the November 3, 2002 earthquake.

The 1964 Good Friday Earthquake

On March 27, 1964 the world’s second largest earthquake ever instrumentally recorded shook Southcentral Alaska for up to 5 minutes, an eternity for someone on the ground. At 9.2 on the Richter scale it released a million times more energy than a 5.2 earthquake, which occurs several times a year in this region.

The origin of the Earthquake was underneath the east shore of Unakwik Inlet in northern Prince William Sound, about 40 miles west of Valdez, at a depth of about 12 miles. Dozens of aftershocks shook a 400-mile long region reaching from Cordova to Kodiak.

The most apparent effect of the earthquake was to shift the elevation of Southcentral Alaska as if it were on an enormous hinge. This hinge more or less followed the border between the Chugach Terrane and the Prince William Terrane. Southcentral Alaska on both sides of this hinge zone buckled along axes of uplift and subsidence. Land to the northwest of this hinge subsided as much as 7.5 feet (Kodiak Island), while land to the southeast was uplifted as much as 33 feet (Montague Island). In addition to these vertical elevation changes, there were also horizontal ground displacements along the coastal areas. The maximum horizontal displacement occurred around Latouche Island in Prince William Sound where the ground moved about 60 feet to the southeast.

1964 Alaska Earthquake Areas of Uplift and Subsidence

The Earthquake shifted Southcentral Alaska as if it were on a giant hinge. Those areas to the north and west of the Contact Fault (the tectonic hinge) subsided. Areas to the south and east were uplifted. Other parts of Alaska felt the shock waves from the earthquake, but the ground itself did not permanently shift.

The earthquake damaged the region in several ways. Some of the most severe damage was from large ocean waves. These waves originated from two sources. One was the uplift of the sea floor along the coast. This uplift generated tsunami waves that struck at Kodiak, Seward, Valdez, Chenega and Cordova and rose more than 30 feet high in some places, wiping out docks, boats and houses. The tsunami traveled throughout the Pacific Ocean, even killing 12 people in California and 4 in Oregon. Undersea landslides generated other, more localized, waves. Waves of this origin completely destroyed the waterfronts of towns like Seward and Valdez. The wave in Port Valdez reached a height of 170 feet while another 90-foot wave obliterated the Prince William Sound community of Chenega Bay, killing one-third of its residents.

Most destruction along Cook Inlet was caused by down warp of the region combined with settling of unconsolidated sediments such as silt, sand and gravel. The town of Portage sunk at least 6 feet, placing it within the tidal zone and it therefore had to be abandoned. Some ruined houses can still be seen today along the Seward Highway. The Homer Spit subsided 4 to 6 feet. The subsidence of the coast along Cook Inlet raised the high tide level in relation to the land and has caused increased bluff erosion over the years.

Anchorage was severely damaged due to the fact that much of its coastal land consists of gravel (of glacial and riverine origin) underlain by a saturated layer of fine sand and clay (deposited in a former marine or estuarine environment). The shaking of the earthquake caused the finer materials to liquefy. This created a relatively fluid layer that slid downhill, carrying the more stable above-lying layers with it. This destroyed many buildings along the Ship Creek bluffs, Knik Arm and carried off a significant part of the Turnagain Heights neighborhood down coastal bluffs toward the sea. Tilted trees and mounds of disrupted clay, which are remnants of this huge landslide, can still be observed in Earthquake Park today.

All told, this earthquake killed 131 people along the Pacific Rim, and the estimated damage was 300-400 million dollars (1964 dollars). Geologists have examined the past earthquake record in Turnagain Arm and on Middleton Island in the Gulf of Alaska and have tentatively concluded that an earthquake of this scale occurs in Southcentral Alaska on average about every 600 to 950 years.

The 2002 Denali Fault Earthquake

In the afternoon of November 3, 2002 I was sitting at my office desk when I sensed the nearly subsonic rumble of an earthquake (yes, they are audible, but barely). The computer started to creak on its stand and I settled in for a nice ride. The ride, however, lasted much longer than normal - enough to make me a bit nervous. If you live in Anchorage long enough, it’s easy to tell when a “real” earthquake has hit, rather than the standard temblor that is felt a number of times a year. Indeed, an unusually large earthquake had occurred, although at a much more distant location than I would have thought. It did not originate in Prince William Sound or across Cook Inlet, as I would have expected, but 175 miles away, on the Denali Fault between Cantwell and Paxson.

The Denali Fault earthquake registered at 7.9 on the Richter scale, but luckily, it occurred in a sparsely inhabited area causing only a few injuries and no deaths. Significant damaged did to the man-made objects that were in the region. Roads were torn up, runways were damaged by liquefaction of the soil and cabins were heaved around. Luckily, the Trans-Alaska Pipeline performed exactly as it was designed. It slid back and forth atop horizontal beams, rather than rupturing and creating an environmental and economic catastrophe. Massive landslides poured down from the mountains of the Alaska Range, burying glaciers in clearly visible layers of dirt and rock. The earthquake was powerful enough to generate surface waves that sloshed water in Louisiana harbors.

2002 Denali Fault Earthquake

The 7.9 magnitude Denali Fault Earthquake on November 3, 2002 shook most of Interior and Southcentral Alaska. A likely related 6.7 magnitude earthquake just 10 days prior preceded it. The maximum ground displacement of 29 feet occurred near Mentasta.

From the epicenter, the ground ruptured eastward for 185 miles at a speed of about 2 miles per second, several times faster than a speeding bullet. The maximum horizontal ground displacement was 29 feet, near the fault between Mentasta Lake and Paxson.

The Denali Fault is a slip-strike, or transform fault, where one section of plate moves laterally compared to another. In this case, Southcentral Alaska is slowly rotating in a counterclockwise direction relative to Interior Alaska, which lies to the north of the Denali fault. Earthquakes along this type of fault are generally smaller than those created in subduction faults, but they occur at shallower depths and may therefore wreak more havoc on the surface. This one occurred a little more that 3 miles down, as opposed to the 1964 earthquake, which was more than 12 miles down. This was the largest earthquake to strike on land in North American since a 7.9 magnitude event occurred in southern California in 1857. A 7.2 earthquake struck along this fault in 1912. Evidence suggests that an event of this magnitude occurs after a build-up of strain for 600 years.

Home