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Aftershock: Earthquake a reminder of potential danger

Earthquake Reminds Seward Residents of the Potential for Disaster

Earthquake damage near Anchorage. (Public domain photo from National Oceanic and Atmospheric Administration)

Last week as many people in the Kenai Peninsula were settling in to work, attending early classes at school or polishing off their morning coffee, a rumble swept across the landscape. Glass rattled in window panes, cups vibrated on shelves.

It was an earthquake, a relatively strong one. But no matter how exciting, frightening or jarring last Wednesday’s jolt may have been, it was a mere shadow of past disasters.

The most famous example occurred on Good Friday 1964. At 5:36 pm, a 600-mile stretch of Earth ripped loose beneath Prince William Sound. When the land finally settled after five minutes of heaving and buckling, the damage was catastrophic. In some places the land had risen as much as 38 feet, in others it sank just as much. Downtown Anchorage was in ruins.

Even more destructive, however, was the tsunami that followed.

Alaskan communities like Portage, Whittier, Valdez and Seward were inundated within minutes. Waves clawed onto shore, running up as much as 220 feet above average sea level. And the rushing water didn’t stop in Alaska. The tsunami raced across the Pacific Ocean at speeds topping 500 mph, lashing the west coast of the United States. Thirteen people were killed in Crescent City, California and another five in Oregon. Tsunami conditions were registered in at least 20 countries, including Japan, New Zealand, Peru and even Antarctica.

By the end of the day, at least 139 people were killed and some $311 million dollars in damage had occurred.

A 9.2 on the moment-magnitude scale, the Good Friday Earthquake was the second largest seismic event recorded in world history.

In Seward, much of the coast collapsed and slid into Resurrection Bay, taking with it most of the town’s train and dock infrastructure. Twelve lives were lost locally and over 300 homes were damaged or destroyed. It was one of the darkest days in the town’s history. But the Aleutian Subduction Zone, the fault responsible for the disaster, is still active. What are the chances it could happen again?

A handful of diminutive stumps along the Waterfront Trail are nearly all that’s left of Seward’s infrastructure destroyed in the 1964 earthquake/tsunami (photo: Brian Wright)

Tectonics Today

Last week’s earthquake was a fresh reminder of the very-present threat of Alaska’s seismicity. The Aleutian Subduction Zone lies at the convergence of the Pacific tectonic plate and the North American Plate. Here the denser Pacific Plate slips beneath the North American plate at a rate of roughly 6 centimeter per year. At this speed, it will take around 500 years for the fault to build enough tension to repeat the events of 1964.

In other words, the likelihood of experiencing a twin of the Good Friday disaster in our lifetime is slim. Earthquake risk, however, still exists.

Seismic Gaps: Tomorrow’s Killer?

Extending along the Aleutian Island chain to mainland Alaska, the Aleutian Subduction Zone ranges for 2,500 miles. Between 1938 and 1965 almost the entire length of this fault has ruptured in a series devastating events including the 1957 Andreanof Islands Earthquake (magnitude 8.6), the 1965 Rat Islands Earthquake (magnitude 8.7) and the Good Friday Earthquake of 1964.

There exists, however, sections of the fault that have not ruptured. These blocks, known as “seismic gaps,” have yet to release the same tension that has been vented along other stretches of the fault, making them the most likely trigger locations of major future disasters.

Perhaps the most dangerous example is the Shumagin Seismic Gap, a lengthy stretch of the Aleutian Trench near the Shumagin Islands. Recent studies of the Shumagin Gap analyzed by the University of Alaska Fairbanks concluded that the conditions in the Gap were eerily similar to those found in the Tohoku region of Japan, source of the devastating 2011 earthquake/tsunami that resulted in the deaths of nearly 16,000 people. Needless to say, the little-understood Shumagin Gap is an area of utmost concern to seismologists and coastal Alaskan residents.

Thankfully, this region is roughly 500 miles from Seward, so even a major earthquake with a magnitude 8.0 or higher would have little effect locally. However, earthquakes such as the one in 1964 have shown that far-reaching tsunamis can have major effects across the entire Pacific seaboard. A 2010 study of tsunami potential in Resurrection Bay by the Alaska Division of Geological and Geophysical Surveys catalogues the likely extent of a modern disaster in a variety of possible scenarios. The report provides vital, if a bit technical, predictions for those of us living, working, or playing near the waters.

Be Prepared

Last week’s 5.7 magnitude rumble served as a reminder that Seward is a place where global tectonics can still toss the land and sea into a frenzy of potentially devastating proportions. Inundation mapping provides citizens with a prediction of the consequences of a future event, but nobody can forecast when such a day will come, if ever.

For those in the so-dubbed “inundation zone” it is best to have a plan already in place.