For all but the earliest of risers, at 4:31 a.m. in the morning of May 1, Skagwegians were provided with a rocky awakening. A magnitude 6.2 earthquake originated at a point roughly 54 miles away from Skagway. A barrage of follow-up shakes followed, ranging from a milder magnitude 3.2 just a few minutes later, to a magnitude 5.2 aftershock at 4:49 a.m. and a quake of magnitude 6.3 after 6 a.m., according to the United States Geological Survey [USGS].
Borough Manager Scott Hahn reported that no major damage occurred in the municipality.
“I got out and drove around town looking for some, I inspected some buildings, nobody is reporting any problems here,” Hahn said.
Nearby in the Yukon, some structures did not escape without a bit of damage. According to a press release, superficial cracks appeared in St. Elias School in Haines Junction, and the library at Elijah Smith Elementary suffered broken windows.
Whether they rouse people from sleep or not, strong earthquakes are nothing new for the region or state. In 2016, Alaska saw a magnitude 7.1 earthquake occur southwest of Anchorage, and in 2014, a magnitude 7.9 quake rattled the ground near the tip of the Aleutian Islands.
Many years ago, Alaska played host to the second-largest earthquake ever recorded, a devastating magnitude 9.2 that lasted approximately 4.5 minutes, left buildings in crumbles and knocked bridges into the water. Also known as the Great Alaskan Earthquake, in 1964 this disaster took the lives of over 100 people, and left thousands without shelter and sustenance. It even caused an underwater landslide in Valdez, which generated a catastrophic local tsunami.
Six years earlier in Lituya Bay, a magnitude 7.7 earthquake cracked free a massive rockslide, which slammed into the bay’s waters and stirred up a mega-tsunami. Several trolling boats were in the bay at the time. One was swamped and sunk by the wave. Another was wrecked and a third managed to ride out the wave across the bay.
Even as far back as the late 19th Century, large earthquakes have made the ground rattle in the Last Frontier, such as the 1899 Yakutat earthquake (magnitude 8.0) and the 1904 Lake Minchumina earthquake in central Alaska (magnitude 7.3).
“Alaska is the most seismically active state in the United States (in 2015, it had almost twice as many earthquakes as the second-ranked state, which is Oklahoma),” Simon Kattenhorn, a professor and director with the Department of Geological Sciences at the University of Alaska Anchorage, said in an email.
Alaska’s dubious honor in U.S. seismic standings is thanks to its location, according to Kattenhorn. The state sits along the infamous Pacific Ring of Fire, a volcanically active zone around the margin of the Pacific Ocean associated with the boundary of the Pacific plate – the largest tectonic plate on the planet.
These plate boundaries are well known for producing large earthquakes, and the two largest earthquakes in history – the 1964 quake outside Anchorage and a magnitude 9.5 earthquake in Chile – both occurred along the Pacific plate boundary. Other countries along the Ring of Fire, such as Japan, Russia and Peru, have also suffered large and damaging earthquakes.
“The Pacific plate that’s under the Pacific Ocean, is kind of moving to the north, northwest, it’s sliding past southeastern Alaska and it’s sliding back into the mantle underneath southern Alaska and the Aleutians,” said Peter Haeussler, research geologist with the USGS.
In Alaska, many of the largest earthquakes occur along the Aleutian Islands and in the Gulf of Alaska, as a result of the subduction – where one tectonic plate slides beneath another – of the Pacific plate beneath the North American plate. In the 20th Century alone, this region saw six earthquakes of magnitude 8 or higher.
“That kind of setting makes the biggest earthquakes in the world and historically the biggest ones in the U.S. and the biggest ones in Alaska,” Haeussler said.
Haeussler said the area has been in the same tectonic setting for the last 200 million years.
“That’s not going to change any day,” Haeussler said. “It just means that we’re living in earthquake country and we need to be prepared.”
Kattenhorn said that Alaska will “undoubtedly” continue to experience large earthquakes in the future, particularly along the Aleutian Islands, but most of Alaska has the potential to be impacted by earthquakes.
The great majority of these earthquakes will be too small to do much damage, or may not even be felt by most people at all, Kattenhorn said, but it is not unusual for a damaging earthquake to occur in Alaska at least every few years.
Beneath the soil and waters near Skagway, two major players stand out in terms of earthquake-producing faults.
One is the Queen Charlotte-Fairweather Fault System, according to State Seismologist with the University of Alaska Fairbanks Michael West.
This is the major fault line that runs along Southeast Alaska, West said. It comes in off the coast, and dips inland at Glacier Bay National Park.
West said the Queen Charlotte-Fairweather Fault was the one that ruptured in 1958, which resulted in the Lituya Bay incident.
“In Skagway, you are blessed with at least a bit of distance from that fault,” West said. “You’re about a hundred miles or so from the outer coast, and from that fault system.”
The second nearby fault is the culprit for the May 1 earthquakes.
“Those occurred close to the Denali Fault, which is really well-known up here in the interior and south central [Alaska],” West said.
The Denali Fault has a history of high-powered earthquakes farther north, but West said typically that fault tends to be quieter near Skagway.
“Which is why the earthquakes that occurred on Monday morning [May 1] kind of stood out,” West said. “So we are keenly interested in what happened on Monday.”
A daisy chain of dangers
Having the ground roil beneath your feet can be unsettling, but an earthquake itself is often only a trigger for deadlier events.
“There’s often this phrase, that earthquakes don’t kill people, buildings do,” Haeussler said. Collapsing buildings aren’t the only threat an earthquake can pose. For coastal Alaska and areas like Skagway, the water can be especially hazardous.
West said when most people think of what a tsunami is, they picture an earthquake setting up a massive wave, that “races across the ocean, and people on distance shores have just hours to evacuate and get out of the way before the tsunami arrives.”
“In coastal Alaska, really our threats are something quite different,” West said. “Yes, an earthquake in Kamchatka, Russia, or in Japan can in fact produce a modest tsunami in Alaska, but the real fear are tsunamis that are generated locally from an earthquake.”
The Lituya Bay mega-tsunami, while an extreme example, is representative of the hazard that exists, West said.
“There are a number of different sources, this is the reason that coastal Alaska is so vulnerable to these, it’s the same reason that you get tourists showing up all summer long, it’s the same reason that Skagway is an unbelievably beautiful place,” West said.
A landslide from the mountains into the water is not the only danger, however. Submarine landslides could also be stirred into life by a strong earthquake.
Thanks to glacial runoff, the bottoms of local seafloors are coated with sediments.
This “muck” presents a large, perhaps underappreciated threat, according to West.
“Increasingly we are recognizing the role of so-called submarine landslides,” West said.
Such an event is not unprecedented in Skagway. While not caused by an earthquake, the municipality did experience a submarine landslide in 1994. A submarine landslide occurred at south end of the railroad dock, which collapsed. The landslide generated a tidal wave that uprooted the ferry dock and sent it spinning into the Broadway dock. The disaster took the life of dock worker Paul Wallin and caused damage exceeding $1 million.
The Nov. 11, 1994 issue of The Skagway News reported waves as high as 12 feet after 800 feet of the railroad dock slid underwater.
A submarine landslide gives little warning; there are few cues one is occurring.
“Just all of the sudden a wave materializes,” West said. “Anytime the sea floor moves rapidly you generate a wave.”
Put frankly, West said the makeup of the nearby seafloors are prone to causing submarine landslides.
“You [Skagway] build up muck on the seafloor much, much faster than most places would,” West said. “Because of that, you have muck – to use the technical term – that breaks free more often. So combine that with an area that experiences, and regularly experiences, significant earthquakes, and you’ve got yourself a recipe for what we refer to as local tsunamis.”
Unfortunately, there is no neat and easy threshold of earthquake magnitude that would cause one of these underwater landslides.
The subject has undergone much debate, West said, but the simple answer is that “no,” there’s not a set number that would put the ground into motion.
“Obviously the bigger the earthquake, the more potential there is,” West said. “We certainly have plenty of evidence for even small earthquakes, call it magnitude 4 or 5, triggering modest landslides, we know that’s possible.”
As if that weren’t enough, these “mucky soils” can also lead to an effect called liquefaction. When wet soils are subjected to vibration, the soil can temporarily behave like a liquid – essentially turning large parts of hard ground into quicksand.
“In which case anything sitting on top of them lets say misbehaves,” West said.
This “daisy chain of hazards” is what leads to damage and loss of life, according to West. The cascading effects that stem from a strong shaking of the ground can potentially trigger events with a deadly impact on humans.
“We have very strong systems set up for warning people of tsunamis that come from a distance,” West said. “If 1964 happens again, the tsunami sirens are going to go off, people are going to have at least some time in your areas to potentially evacuate.
“In Valdez in 1964, in Seward in 1964, these tsunamis struck before the ground had stopped shaking. Honestly, in those cases, you barely have minutes.”
The Lituya Bay mega-tsunami also struck without much warning. Reportedly, the wave was sighted within three minutes after the earthquake was first felt, and tore across the bay at estimated speeds of 80-plus miles per hour, according to the USGS professional paper “Great Waves in Lituya Bay Alaska,” published in 1960.
“Education is really the key here,” West said. “The solution to living with these is not necessarily fancy technology, it’s good education. There are in fact places where we should think twice before building. We do need to think in our communities about where are the inundation zones, where are the safe zones, where are the unsafe zones.
“And we need to train our citizens, we need people to understand that if the ground shakes for a significant period of time…if you’ve got something that shakes the ground for 10, 20 seconds, you should be evacuating if you are in the inundation zone.”
Skagway has a tsunami evacuation route marked throughout the town with circular blue signs. The route heads up State Street, and up the Klondike Highway. In the event of a tsunami, it is advised to get at least 50 feet above sea level. If that is impossible, go to the highest level of a multi-story building. Keeping a disaster supply kit on-hand is also advisable.
For more information, including a complete disaster kit list, visit www.ready.alaska.gov.
For the tsunami evacuation map, visit www.ready.alaska.gov/plans/mitigation/tsunamis and click the link for Skagway.