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.
6:25 a.m., 37 seconds. When I talked to Douglas Given, a geophysicist at the United States Geological Survey (USGS), he estimated it took Monday’s quake 1 to 1.5 seconds to travel to reach the epicenter. After it hit the epicenter, it expanded nearly three miles before a piece of government equipment noticed.
6:25 a.m., 38.24 seconds. In the Santa Monica mountains, the first USGS seismograph registers the quake.
The USGS operates 360 sensors stations through southern California. They’re spread out across the land, in mountains and cities, farmland and fields, listening. Each sensor station has two kinds of seismographs: An extremely sensitive “weak motion sensor” that registers tiny shakes, vibrations that humans can’t feel; and a “strong motion sensor” that registers big movements.
The weak motion sensors can register “negative magnitude-value” quakes, Given tells me. While we often think of the magnitude scales used for earthquakes as “starting” at zero, that’s not true: The degrees of earthquake magnitude are somewhat arbitrary. While humans can rarely sense quakes below magnitude-2, quakes can get much subtler.
In southern California, the USGS doesn’t operate these double sensors alone. They do so in conjunction with the California Institute of Technology in Pasadena. This kind of university-agency partnership is common for the USGS: There are about 20 of these regional offices throughout the country, each running a network of seismographs. In northern California, the USGS partners with the University of California Berkeley. Together, the two seismic networks employ about 25 people.
In the south, the USGS also gathers data from about a hundred other seismographs. The state’s Department of Water Resources runs seismic sensors, as does the local utility company, Pacific Gas and Electric. Some of these only activate when an earthquake triggers them.
Each seismograph has three components, which can each measure movement along one vector. The three combined lets them track all three dimensions: vibrations along the axes north-south, east-west, and up-down. The weak sensors “start clipping”—in other words, top out—around magnitude 3 or 3.5 quakes.
On Monday, then, the USGS first relied on its big sensors, its strong motion seismographs.
Three seismographs recognized the quake shortly after the Santa Monica mountain sensors. They measured it at:
6:25 a.m., 38.94 seconds, in Northridge, CA;
6:25 a.m., 39.53 seconds, in Burbank, CA; and
6:25 a.m., 39.78 seconds, in Santa Montica proper.
All four sensors are continuously linked to a data center in California. (The agency wants to keep its data local, to increase security and reduce latency.) The USGS has two different systems for identifying quakes: an early warning system, and a “routine” system. Once those four sensors triggered, the USGS early warning system knew it had an earthquake on its hands.
The USGS didn’t alert the public, though. The early warning system is insufficiently funded—it hasn’t been vetted well enough for public use—so the USGS doesn’t share those results. Nearly three seconds elapsed between when the fault slipped and when the U.S. government knew what had happened—but once it knew, it couldn’t do anything.
The USGS computer system waited, instead, for its routine system to account for the quake. The routine system is slower and more exact. It doesn’t issue an alert until 90 seconds after the first tremor. So the USGS system, after receiving data from many, many more of its seismic sensors across the region, published its first standard alert about the earthquake at
The USGS only promises to publish information this fast about California quakes—“in a few minutes” is the language it uses. Worldwide quakes may take a couple hours to process. But once it publishes an earthquake report, it distributes that information over many channels: Twitter, a Google Earth file, a news feed like RSS, and email.
== PRELIMINARY EARTHQUAKE REPORT ==Region: GREATER LOS ANGELES AREA, CALIFORNIA
Geographic coordinates: 34.133N, 118.487W
Depth: 8 km
Universal Time (UTC): 17 Mar 2014 13:25:36
Time near the Epicenter: 17 Mar 2014 06:25:37
Local standard time in your area: 17 Mar 2014 05:25:36
Location with respect to nearby cities:
9 km (5 mi) NNW of Westwood, California
10 km (6 mi) NW of Beverly Hills, California
12 km (7 mi) W of Universal City, California
12 km (7 mi) N of Santa Monica, California
562 km (348 mi) SSE of Sacramento, California
That email arrived in a special inbox which Los Angeles Times developer Ken Schwencke set up three years ago.