Finding Fault with Alaska's Largest City

Not all parts of Anchorage shake the same during an earthquake. Downtown Anchorage receives twice the jolt of the Chugach Mountain foothills, according to Geophysical Institute scientists who studied the reactions of different parts of town to the same earthquakes.

Niren Biswas is a seismologist who leads an ongoing project with scientists from around the world. Together, they are finding what areas of Anchorage are more vulnerable to shake, rattle, and roll during earthquakes. Biswas and his coworkers scattered 20 seismometers in and around the Anchorage Bowl, and installed one each in Chugiak and Palmer. Unlike seismometers used to detect earthquakes all over the state, the ones in Anchorage only read earthquakes with a magnitude of 3.5 or greater.

All 22 seismometers operate around the clock, feeding their information via telephone line to the Municipality of Anchorage Data Center. From there, institute scientists in Fairbanks check the information every day on the Internet. During the four years the seismometers have been working in Anchorage, 70 earthquakes of 3.5 or greater have triggered the system. Using that data, Biswas and colleagues Artak Martirosyan and Utpal Dutta found that sections of the city react quite differently to earthquakes.

Downtown Anchorage, the nose of the city that juts out into Cook Inlet, shakes with more than two times the motion of the Chugach Mountain foothills. Since downtown is where most of Anchorage's 258,000 people congregate during the week, it's not an ideal situation. "This section of Anchorage will be very dangerous during a large earthquake," Biswas said. Seismologists know the problem with downtown Anchorage; it's called the Bootlegger Cove Formation, soils rich in clay and sand that act as a liquid when shaken. These soils lie beneath most of the city from C Street westward to the ocean.

Their fragile nature was on display March 27, 1964, when a magnitude 9.2 earthquake with its epicenter in Prince William Sound ripped through Alaska. Liquefaction, the scientific name for what happens to wet, sandy soils during a large earthquake, caused entire West Anchorage neighborhoods to slide toward the ocean in an area now known as Earthquake Park. Though people never rebuilt homes in West Anchorage, the city is still quite vulnerable to a large earthquake.

Following the 1964 earthquake, residents of Valdez moved the entire city from a site on the Lowe River plain to its present location on the bedrock of Mineral Creek. Moving the city is not an option in Anchorage, home to 41 percent of Alaska's people. Realizing this, Biswas said his goal with the seismic project was to make maps of shaking hazards that can be used by engineers and architects designing new buildings in Anchorage.

Even though downtown Anchorage is a bad place to be during an earthquake, Biswas said the technology exists to build earthquake-proof buildings and highway bridges. The only problem is in convincing builders to spend the extra money to make structures a bit stronger. "If people are going to live in Anchorage a long time, it will save them money in the long run," Biswas said.