Paroscientific, which manufactures sensors used to measure pressure at the bottom of the ocean with high precision, are already used by the National Oceanographic and Atmospheric Administration for its tsunami sensors. But an engineering quirk prevents the sensors from measuring the gradual ground motions that build up pressure along earthquake faults.
The instruments can measure seafloor pressure, or the weight of water above the sensor, to an extremely precise fraction of a millimetre. But the readings lose accuracy over time, and the error is proportional to the quantity measured. On the ocean floor, where pressures are tens to hundreds of times that on the surface, the readings can change by 10cm per year. In between major earthquakes, this is much more than the seafloor might shift up or down due to tectonic forces.
“If you want to measure how the seafloor is moving, you don’t want your reading to change by a larger value than the thing that you’re measuring,” said Dana Manalang, an engineer at the UW’s Applied Physics Laboratory who is working on the project.
However, by calibrating the pressure sensor against the air pressure inside the metal case that houses the instrument, which is roughly one atmosphere, the pressure sensors can autonomously track small bulges and slumps on the seafloor.
The sensitive quartz crystal that measures the seafloor pressure can now be connected to measure pressure inside its titanium instrument case, with a known pressure and another barometer to check the value. The prototype instrument was attached in mid-June to the Monterey Accelerated Research System, a cabled seafloor observatory that lets researchers communicate directly with the instrument.
“That chunk of seafloor actually does not move much. We’re looking for a null result,” Manalang said. “If successful, the next step would be to deploy similar instruments in some more geologically active areas.”