A 7.0-magnitude earthquake struck eight miles north of downtown Anchorage on Friday. Hitting in the midst of the morning commute, at 8:29 a.m. local time, the seismic slip unleashed a destructive force equaling that contained in 3.9 billion pounds of TNT.
With the underground focus of the tremor just 25 miles below the surface, damaging waves reached the Alaska city of nearly 300,000 less than 13 seconds after the initial rupture.
Seismic waves rippled out across the globe in the minutes after the quake. Compiling seismic traces from hundreds of seismometers around the world yields a “record section” (illustrated below), showing different wave phases propagating throughout the earth. P, or primary waves, arrive first, traveling several miles per second. Then come the more damaging S, or secondary waves and, finally, jarring surface waves.
The U.S. Geological Survey released a “beachball diagram” minutes after the quake, offering clues about the triggering mechanism. Indications point to “normal faulting,” a type of interface where one tectonic plate slides down the other at an incline. Most of the time, this angle is greater than 50 degrees. It’s akin to sliding down a nearly-vertical playground slide.
Tremors even shook the ground beneath the nation’s Capitol — though by then they had diminished enough that only seismometers could detect the shaking. Here’s the trace from the Global Seismographic Station closest to Washington, located in Standing Stone, Pa.
The first seismic waves got there eight and a half minutes after the quake rattled Anchorage.
Unfortunately, the trouble is not over yet. Additional quaking could be in the offing. The USGS predicts a 62 percent chance of aftershocks exceeding magnitude 5 occurring within the next day. Equally alarming is the 1 in 4 shot of a magnitude 6 happening sometime in the next week.
On its website, the USGS warned that “this earthquake could be part of a sequence.” In the two hours since the primary earthquake, nearly half a dozen aftershocks of 4.0 or greater occurred.
Anchorage is precariously perched 100 miles south of the Denali Fault. While there’s a good chance this is the fault that ruptured, it’s too early to tell for sure until more data comes in.
Alaska is no stranger to earthquakes. Located in the battle zone of the North American and Pacific tectonic plates, it sits on the northern periphery of the Ring of Fire.
These plates crunch against each other in several zones. The Aleutian Chain slid 66 feet in a 9.2-magnitude earthquake in 1964 that rattled Prince William Sound. Six years earlier, an earthquake along the Fairweather Fault led to a 40 million-cubic-yard landslide that sent waves sloshing up to 1,719 feet up a mountain overlooking Lituya Bay.