What happens in the first 20 minutes after an earthquake?
April 06, 2020

Beth Grassi

“As far as earthquakes go, for everyone in Alaska, there is someone looking out for you 24/7,” said Ken Macpherson, who manages the Alaska Earthquake Center’s data analyst team. Most of the center’s staff take turns being the “on-duty” seismologist, which means being on call around the clock for a week. 

A dedicated smartphone sounds an alarm when an earthquake over a threshold magnitude occurs, and the clock starts ticking. The center’s protocol requires the on-duty person to review and report on the earthquake within 20 minutes, which includes travel time if that person is away from a computer or wifi. When asked to pick one word to describe being on duty, center staff responded: important, exciting, nonstop, stressful, triage, and responsibility. 

Minutes 0–4: Auto-Alert 

The center has seismic stations across Alaska that continuously send data back. When a seismic wave reaches a station, a computer program at the center records the wave’s arrival time. By comparing when the wave arrives at various stations, the program determines the location and depth of the earthquake’s origin. 

Seismic waves, like sound waves, take time to travel. On average, waves travel about 5 miles/second, which varies based on depth and the type of rock the wave travels through. For an earthquake located 25 miles outside of Fairbanks, people in town feel shaking a couple of seconds later. It may take the wave longer to reach enough stations for the program to calculate a location, usually about 1–2 minutes. Once the program has a location, it uses the amplitude of the seismic wave at each station, adjusting for distance from the earthquake’s origin, to determine magnitude. The whole process takes only a couple minutes.


If the program determines the earthquake magnitude is over a certain threshold, it triggers the alarm and the seismologist responds. The thresholds are tailored by region, with lower thresholds in highly-populated areas where it’s more likely someone is close enough to feel smaller earthquakes. For Anchorage, Fairbanks, and Juneau the threshold is magnitude 3.5; for Cook Inlet and the Mat-Su areas it’s 3.8; for the rest of mainland Alaska it’s 4.0; and for the sparsely populated Aleutians it’s magnitude 5.0. 

“For the week you’re on duty, it impacts your life. You can’t go to the movies or go camping. You have to watch out for the dead spot in the grocery store,” said Macpherson. Everyone in the duty rotation had a story about leaving groceries in the cart and hurrying to the office or home computer to confirm critical earthquake information. Listening for the alarm can become a little too ingrained. “When you hear that klaxon sound in a movie, there’s a moment of panic...until you realize you’re not on duty,” said Miriam Braun, one of the seismic data analysts. 

The devices for the alarms have changed over time. Natalia Ruppert, a long-time seismologist at the center, recalls during her student days in the 1990s it was an audible alarm from the “sensaphone” in the middle of the seismology lab, with loud beeping and then an electronic voice announcing when an earthquake happened. These messages were forwarded to the duty person’s home phone in the evenings and on weekends. The alarms had to be turned off manually by someone entering a code and pushing a button on the sensaphone. 

The next alarm version was a pager the on-duty person carried, which gave a little more information about the earthquake. This was accompanied by a flip-phone cell for calling to confirm someone was responding. “Snow, rain, middle of the night, it didn’t matter, you had to drive to the office to turn off the alarm,” said Ruppert. At that time there was also a computer with the earthquake map in the middle of the lab that played a rooster sound followed by an electronic voice announcing each new earthquake magnitude and location. 

For a long time, the main computers the seismologist could use to review data were in the lab, so the on-duty person had to drop everything to head to the office. If the on-duty person was already there, pushing the big red button in the lab halted the alarm. About 15 years ago a duty laptop entered the scene, but slow internet was a problem off-site, so often the duty seismologist still had to dash to the office. 

Minutes 5–20: Race to Confirm Data

Why do we need a seismologist on duty? The computer program sifts through data much faster than a person, but is limited to working with station arrival times. The program is set to find the source of an earthquake in Alaska, but seismic waves can be detected from across the globe. On the other end of the spectrum, earthquakes that occur close together in time or space are sometimes difficult for the program to separate. A trained seismologist, simply by looking at the waves on a screen, can easily distinguish a teleseismic (long-distance) wave originating in Indonesia from one originating in Iniskin, Alaska, or decipher whether two events were erroneously combined into one. A seismologist knows the geological setting of Alaska and can determine if the depth and location the program assigns an earthquake make sense for that region. 

The program calculates an earthquake’s magnitude by averaging the magnitudes at each station. Sometimes the seismologist adjusts the initial magnitude as data arrive from more stations or removes stations whose magnitudes are significantly different from the average. 

These are all aspects of the data the on-duty seismologist must review for quality control in just minutes. This reviewed information is the “solution” the seismologist releases to the public.

Some weeks are quiet, but some keep the duty seismologist hopping. Ruppert recalls her busiest stretch on duty. “In June of 2014 we had a magnitude 7.9 in the Aleutian Islands, so it was remote, but still had a very active aftershock sequence—that was my busiest week. I had about 20 alarms, and there were other parts of the state active too. That number was the most alarms I’ve ever had.” The Rat Island earthquake generated 2,500 aftershocks by the end of that year.

Minutes 20+: Sharing the Solution

Sometimes an earthquake alarm triggers coordination with partner organizations. Lea Gardine, a center seismologist, has been on the duty rotation since she was a graduate student. On August 6, 2008, she was on duty when a swarm of earthquakes in the Aleutians triggered the alarm. At first, they seemed like routine Aleutian events, but then her husband, a graduate student working for the Alaska Volcano Observatory at the time, also received an alarm. 

The joint alarms generated several hours of intense observation and discussion. “There were biologists on Kasatochi Island that called the Coast Guard to pick them up because they felt earthquakes. So we had to coordinate with the USGS National Earthquake Information Center and AVO,” Lea Gardine said. The sequence appeared to be tectonic but marched toward Kasatochi Volcano, indicating volcanic activity. The largest earthquake was a magnitude 5.8 on August 7, just three and a half hours before the first ash emissions were seen at the volcano.

Today, the process of being on duty is streamlined. A smartphone replaced the pager. The biggest advance is that the on-duty seismologist, if they have high-speed internet at home, can answer the alarm without dashing to the office. (A recent video from Macpherson shows how a seismologist can review data remotely.)


The flip side of this streamlining, according to Matt Gardine, the operations seismologist, is that, “The basics of locating an earthquake haven’t changed over time, but the expectation of the speed of getting the solutions out has increased. It’s no longer ok to wait an hour to release it, it’s more like 15 minutes. The quicker you can get information to emergency response services, the better.” 

If the final solution for the earthquake is more than a magnitude 4.0 or if people report a smaller magnitude event in the USGS Did You Feel It website, the seismologist announces the final description of the earthquake via email, social media, and the center’s website.

Then the on-duty seismologist resets the clock and waits for the next alarm.


Ken Macpherson shows the duty phone, which is kept in arm's reach of the on-duty seismologist during a week-long shift.