How a few seconds of shifting deep below the Earth’s surface caused the biggest earthquake in Anchorage since 1964

Second in a series

At 8:29 a.m. on Nov. 30, 2018, around 35 miles underground and roughly 7 miles north of downtown Anchorage, part of the earth was breaking.

Within seconds, the energy from that break spread up and out, through Anchorage, Eagle River and the Matanuska-Susitna Borough.

Just about everyone who was in Anchorage that day remembers what happened above ground. But the science of what occurred underground reveals how a few seconds of movement, miles below us, caused widespread damage in Alaska’s most populated area.

Understanding how the magnitude 7.1 earthquake happened starts with understanding what makes up the earth below. If the tectonic plates that make up the Earth’s crust are like a jigsaw puzzle, it’s as if they are being jammed together by a frustrated toddler. They’re rubbing against one another and slipping beneath each other, according to state seismologist Michael West.

The two tectonic plates that connect Alaska and the rest of North America with the Pacific Ocean collide 200 miles off the coast of Alaska. There, a deep trench outlines the state’s southern coast from the Gulf of Alaska past the tip of the Aleutians. That’s where the Pacific Plate begins its descent below Alaska, at a rate of about 2.5 inches every year, West said.

The more it sinks, according to West, the further beneath the North American plate it goes — sort of like an escalator going down.

[ABOVE: An animation by the Alaska Earthquake Center shows the Pacific Plate descending beneath the North American Plate in what’s known as a subduction zone.]

“The rock that is underneath Anchorage was actually out in the Pacific Ocean with little crabs and critters crawling all over it about 5 million years ago,” West said.

That part of the old ocean floor was stuffed under Prince William Sound and the Kenai Peninsula before reaching Anchorage. As it moves, the plate is subject to heat, pressure and chemical reactions. Eventually, it will be “consumed back into the earth’s mantle," West said.

As the Pacific Plate is pulled farther into the earth, it bends and stretches.

Last November, that stretching and bending reached a breaking point.

[ABOVE: This animation from the Alaska Earthquake Center shows the Pacific Plate being stretched and bent as it was pulled into the earth beneath Alaska during the November 2018 earthquake.]

“That earthquake occurred because that Pacific Plate was being stretched almost like taffy, except it wasn’t stretching very well, so it broke,” West said.

Where that plate broke, along a fault, or fracture in the Earth’s crust, is where the earthquake began.

The movement along that fault lasted about 12 seconds, according to Alaska Earthquake Center seismologist Natalia Ruppert. Within seven seconds of the rupture below, shaking was felt above.

The shaking felt across Southcentral Alaska — all of the ground moving, chimneys wavering and china plates flying — was seismic waves produced by the huge amount of energy released during the shifting.

“As that rock breaks and moves, there’s an energy that’s released in that movement that then vibrates and radiates through the earth," said Lea Gardine, a seismologist at the Alaska Earthquake Center. "So it’s comparable to a sound wave. If you break a stick, that snap sound resonates through the air. It’s a similar process in the earth.”

[Last year’s 7.1 earthquake woke Alaska up. Experts say it wasn’t a true test of our readiness.]

As is often the case, the shaking above ground lasted longer than the initial movement along the fault, Ruppert said. Roads buckled, foundations cracked, cabinets whipped open, and the region saw what scientists called the most significant earthquake since the Great Alaska Earthquake in 1964.

The movement started registering around the state on different seismometers, which are the devices that track earthquakes and send information to the Alaska Earthquake Center.

“They essentially sit out and wait to be shaken,” Gardine said. “They actually just measure vibration and then they transmit that data to us here in the lab."

As that information was heading to the lab, in real time, computers and seismologists tried to determine an initial location and magnitude for the earthquake, Gardine

said. After a few minutes, they spoke with researchers at the U.S. Geological Survey to determine the magnitude as a group. That number was 7.0, but the magnitude was later revised to 7.1 following a monthslong scientific review.

Within minutes of the initial earthquake, the series of aftershocks began, including one that rippled through Anchorage at magnitude 5.7 just six minutes after the first shake. To date, at least 11,000 aftershocks have occurred. Because of the earthquake’s depth and type, researchers did not expect so many aftershocks, Ruppert said.

“The intensity of the aftershock sequence was unexpected,” Ruppert said. And they’re supposed to keep rolling through Anchorage over the next 18 months.

A day after the earthquake, USGS research geologist Rob Witter hopped into a helicopter and got to work charting how the ground had changed. Armed with cameras, Witter and his colleagues documented landslides and other ground failures, flying over populated and wilderness areas near Anchorage. They collected information to help determine where risks might exist in preparation for the next big earthquake.

To that end, the researchers were interested in seeing if the ground had failed in the same places as the 1964 earthquake. They went to Sunset Park in Government Hill, where the neighborhood’s elementary school used to stand. The 1964 earthquake caused a huge landslide that essentially split the school in two.

The researchers used photos from 1964 and matched it up with images they took at the site days after the earthquake. They found small cracks, less than a quarter of an inch in width, running tens of feet in the same spots where the ground had failed in 1964.

“But the shaking was not long enough, or perhaps not the right frequency to reactivate the slides,” Witter said. “So, we were lucky. Had the earthquake shaking continued, we’re not sure what would have happened.”