Sensor detects quakes from the far side of the San Andreas fault
California is earthquake country. Geologists study the state's daily tremors using a network of 880 seismometers, and they constantly wonder about the next Big One. But with nearly all those sensors on dry land, they’re only getting half the story.
California earthquakes are born in the constant shoving match between the North American plate, which we live on, and the Pacific plate, which dives under the ocean and stretches westward toward Asia. Virtually all our earthquake data comes from sensing shudders in the North American plate.
To learn about the Pacific plate’s side of the story, MBARI installed a seafloor seismometer near the MARS site in 2002. (It was called the MOBB, or Monterey Ocean-Bottom Broadband seismometer.) Buried in the seafloor west of the San Gregorio fault, the sensor sits on the far western side of the entire San Andreas fault system.
How the seismometer works
A seismometer is basically a sturdy container with a suspended weight inside. When the earth shakes, the container moves but the weight stays still for an instant. The seismometer measures this relative motion to produce a graph of the shaking.
The exquisitely sensitive MOBB seismometer can measure vibrations caused by tidal currents and even feel air moving within the instrument itself. To keep these extraneous signals to a minimum, the seismometer is completely buried in the seafloor and filled with argon gas instead of air. A current meter keeps track of seafloor currents to factor them out of the data record. Finally, sophisticated data analysis sifts through the instrument noise to pinpoint movements of the earth.
The MOBB broadband seismometer records everything from very slight shaking that happens as fast as 20 times per second to shocks from large, distant earthquakes that arrive several minutes apart. Measuring such a large range of time intervals means collecting lots of data – about 3 gigabytes every three months.
By comparing waves from large earthquakes measured by seismometers hundreds of miles apart, geologists can learn about the makeup of the Earth’s interior. To stay synchronized with the rest of the world’s seismometers, MBARI’s seismometer contains a precision clock that loses less than a second per year.
Cabled observatories like MARS make seafloor seismometers incomparably more useful to earthquake scientists. As with most seafloor instruments, seismometers have to run on batteries and their data are not accessible for several months while the instrument sits on the bottom. A cabled connection to the MARS site solves these limitations.
For a seismologist, two bigger benefits are the immediate return of data and the ability to remotely alter the way the instrument samples. In the minutes and hours after an earthquake, extremely valuable data continue to arrive in the form of aftershocks. With the seafloor seismometer connected to MARS, the instrument will stay vigilant every minute of every day. And when a significant earthquake happens, scientists can instruct the seismometer to collect additional data as aftershocks arrive.