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.