Early Monday morning, Los Angeles suffered the strongest earthquake it had seen in 20 years. The 4.4-magnitude seismic event woke up sleeping Angelenos, but thankfully did little additional damage.
The quake came at 6:25 a.m. Eight minutes later, the Los Angeles Times had posted a story about the quake. It had an odd final paragraph:
This information comes from the USGS Earthquake Notification Service and this post was created by an algorithm written by the author.
Indeed, the a story was written by a program, but read like normal newspaper prose. In the eight minutes that passed—from tremor to coverage—what happened?
6:25 a.m., 36.8 seconds. On Monday, six miles underground—a fault line slipped.
The vibration from that slip traveled upward. It took about a second to travel the 9.9 km until it hit the ground near Meadowcrest Road in Westwood, Los Angeles, a few blocks away from the Bel-Air Presbyterian Church.
It kept traveling, outward, rippling.
When we imagine earthquakes, we often imagine concentric rings of vibrations traveling outward from the center. While that’s a fine description of what the quake feels like on the ground, it belies that earthquakes are 3D. After a fault slips at a quake’s hypocenter, it emits energy outward in an expanding sphere. The first place that sphere touches the ground becomes the epicenter. An earthquake seems to ring outward from that spot, but those concentric rings are really just where that expanding sphere of vibrations meets the ground.