consider the renovation of the GSN in future years including the requirements for W phase deconvolution.
A new seismometer has been developed and tested, which senses mass position through interferometry using fiber optics for light transmission. Unlike the STS-1 above, a force balance feedback is not used to reduce mass movement to maintain linearity in the displacement sensor. The resultant dynamic range is much greater than a conventional seismometer and is achieved by counting interference rings with a Mickelson interferometer. The output can be shaped computationally as needed and could be used to provide data with high fidelity at low frequencies for measuring the W phase.8 The optical seismometer has a response that is flat to ground acceleration between DC and about 1 Hz. There is, thus, no need to deconvolve the instrument response for W phase band measurements. Another of the benefits of the optical approach is that with good response at tidal frequencies, absolute calibration against earth tides on a continuous basis is straightforward. The optical seismometer remains under development for horizontal component testing and reducing noise levels at low frequencies—a borehole version is being tested.
To increase the longevity of the STS-1 seismometer, replacement feedback circuitry has been developed to replace the aging electronics (http://www.metrozet.com/). The corner frequency of the STS-1 remains at 1/360 Hz for the new electronics.
PTWC staff indicated that they are in the process of implementing a W phase algorithm, but a careful vetting of the algorithm before it can be reliably applied will be required.
Recommendation: Before implementing the W phase algorithm in TWC operations, the NOAA Tsunami Program should validate the W algorithm to both a sufficient dataset of synthetic seismograms and to waveforms from past great earthquakes, paying particular attention to its performance in “tsunami earthquakes” and to the assessment of a lower-magnitude bound for its domain of applicability.
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