Magnitudes from C. Richter to M and the W phase - Tsunami Warning and Preparedness - page 235

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 iber optics for light transmission. Unlike the STS-1 above, a force bal-
ance 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 idelity at low
frequencies for measuring the W phase. 8 The optical seismometer has a response that is lat to
ground acceleration between DC and about 1 Hz. There is, thus, no need to deconvolve the in-
strument response for W phase band measurements. Another of the beneits of the optical ap-
proach 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 fre-
quency 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 suficient 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.
REFERENCES
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3. Hanks, T.C. and H. Kanamori. 1979. A moment magnitude scale. Journal of Geophysical Research 84(B5):2348-2350.
4. Tsuboi, S., K. Abe, K. Takano, and Y. Yamanaka. 1995. Rapid determination of M wp from broadband P waveforms. Bulletin
of the Seismology Society of America 85(2):606-613.
5. Ishii, M., P.M. Shearer, H. Houston, and J.E. Vidale. 2006. Teleseismic P wave imaging of the 26 December 2004 Sumatra-
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7. Kanamori, H. and L. Rivera. 2008. Source inversion of W phase: Speeding up seismic tsunami warning. Geophysical Journal
International 175(1):222-238.
8. Zumberge, M., J. Berger, J. Otero, and E. Weilandt. 2010. An optical seismometer without force feedback. Bulletin of the
Seismological Society of America 100(2):598-605.