Optical-Fiber Strainmeters for Earthquake Early Warning

Abstract:

Earthquake early warning (EEW) includes two tasks: detecting an earthquake and estimating its size. Detection requires low instrumental noise; estimating the size can be done most simply by estimating the moment from the static (near-field) signal, which does not saturate with magnitude. The usual approach in EEW has been to use inertial sensors (detecting acceleration) for low noise, and GPS (measuring displacement) to determine the static signal. Because a strainmeter can combine low noise and broad frequency response in a single sensor, such systems should also be considered for EEW networks. While borehole strainmeters often do not record the static offset correctly, past results from longbase laser strainmeters (LSM's) show that this can be done reliably if the strain is measured over hundreds of meters. Recent developments in optical fiber LSM's have allowed the construction of low-cost, low-power long-base LSM's with low noise in the frequency band of interest. Digital processing of the interference signal makes possible a flat frequency response from 0 to 100 Hz and a high dynamic range, with the upper limit (strain of 0.1%) set by the tensioning of the fiber. Such instruments can be flexibly sited, including in locations, such as the ocean floor, impracticable for GPS. Application of such instruments to EEW will require further, though straightforward, development of processing methods and installation techniques, as well as testing under conditions of high dynamic strain; additional modeling will be needed to determine the optimal use LSM's in an EEW seismometer/GPS network.