The United States Geological Survey announced today that the explosive San Bruno fire caused by a ruptured PG&E gas line was recorded as a seismic event with a magnitude of 1.2.

David Oppenheimer, the project chief with the USGS in Menlo Park, said that they record events smaller than this one all the time.

“We’ll see events like a sonic boom when fighter jets fly over,” said Oppenheimer, as well as things like quarry explosions, or even when trucks drive over the sensors. So it shouldn’t be surprising that the San Bruno blast registered. Sensors pick up vibrations found in the ground surrounding them.  The USGS had two sensors very near the epicenter of the blast area, because the blast occurred so near the San Andreas fault, which runs along Highway 35.  

The seismographs picked up some interesting information from the tremor resulting from the explosion. Oppenheimer remarked that the 1.2 magnitude “grossly underestimated” the amount of energy released in the blast because the pipe was so close to the surface and much of the energy was released into the air.

Oppenheimer saidt the USGS readings may be relevant in a pending investigation of the blaze because they record the exact moment of the explosion.