Late Tuesday night local time, a severe 7.8 magnitude earthquake occurred 105 kilometers off the southern coast of the Alaska Peninsula, which was also felt in Anchorage and the Kenai Peninsula. Immediately afterwards, the National Tsunami Warning Center issued a tsunami warning, which was lifted about two hours later. Although the earth shook quite heavily in places like Sand Point, Cold Bay and Perryville, fortunately no people were injured and structural damage apparently limited to a damaged road and fallen things in houses.
Earthquakes are not uncommon in the Gulf of Alaska, but one of such a magnitude is not often observed, and Michael West, a seismologist at the Alaska Earthquake Center, called Tuesday night’s quake at 10:12 p.m. “very significant” and “typical of the southern coast of Alaska.” He said the quake released 15 times more energy than the 7.1-magnitude quake in Anchorage in late November 2018, which injured more than a hundred people and destroyed some infrastructure.
Still, West is surprised by the quake near the Shumagin Islands. No strong quakes have been documented in this region, which is why seismologists call them Shumagin Gap. Along the southern coast of Alaska there is a subduction zone where the Pacific Plate pushes under the North American Plate. If the pressure becomes too high, the two plates in the blocked zone detach from each other and cause an earthquake. According to West, this basically happens everywhere, just not in the Shumagin gap. Scientists have so far disagreed on whether the absence of a large quake in the region so far is due to continuous pressure reduction from small quakes, or whether a large earthquake is simply long overdue. With the latest event and the new data on this, the scientists are hoping for new insights.
By default, the National Tsunami Warning Center issued a warning, and the sirens sounded from the Aleutian Islands to the Kenai Peninsula. This type of earthquake, in particular, has a high potential to trigger a tsunami, as West explains. The evacuations in the affected areas appeared to be smooth and calm, and residents gathered in high schools, near volunteer fire brigades or sought out higher ground. At the collection points, those responsible handed out masks that people readily used.
For residents, precautionary evacuations due to a tsunami warning are almost part of everyday life. Larry LeDoux, head of the Kodiak School District, says calmly: “I’ve been experiencing this since I was a child, it’s nothing new.”
The tsunami sensors in the ocean may not trigger an alarm immediately after a quake, as the wave takes some time to reach a sensor, depending on the distance to the epicenter, according to James Gridley, director of the National Tsunami Warning Center in Palmer. Therefore, the warning was issued immediately, even if no wave has been measured yet. “We assume that there is a wave until we know better,” says Gridley. “We have to start from the worst case scenario. The safest position is to get people moving.”
At Sand Point, the warning center observed a 25-centimetre wave and maintained the warning to wait for the entire wave cycle. The warning was finally lifted at 00:30 on Wednesday.
Eight aftershocks have so far been recorded in the region, with magnitudes between 4.5 and 6.1 and about 40 smaller ones, none of which required a new tsunami warning.
Julia Hager, PolarJournal