NEW_Accomplishments.indd - IRIS

MONITORING OTHER SEISMIC SOURCES
2006 IRIS 5-YEAR PROPOSAL
Mid-Ocean-Bottom and Land-based Microseism Correlations
Peter D. Bromirski • Scripps Institution of Oceanography, UCSD
Fred K. Duennebier • University of Hawaii, Honolulu
Ralph A. Stephen • Woods Hole Oceanographic Institution
Inversion of the double-frequency (DF) microseism band for ocean wave heights from archived seismic data can provide an
important diagnostic of climate change (Bromirski et al., 1999; Bromirski et al., 2005a). The usefulness of such inversions requires
knowledge of the source areas and propagation attributes of microseisms. The Hawaii-2 Observatory (H2O) is an excellent site for
studying microseism characteristics since it is located far from shorelines and shallow water. The DF microseism band can be divided
into short period and long period bands, SPDF and LPDF, respectively. A strong correlation of seismic amplitude with wind speed
and direction is observed at H2O in the SPDF band from about 0.20 to 0.45 Hz (Bromirski et al., 2005b), implying that the energy
reaching the ocean floor is generated locally by ocean gravity waves. Near-shore land seismic stations see similar SPDF spectra, also
generated locally by wind seas. Correlation of swell height above
H2O with the LPDF band from 0.085 to 0.20 Hz is often poor,
implying that a significant portion of this energy originates at distant
locations. The LPDF microseism signals recorded at the H2O
correlate with signals at other seismic stations around the North
Pacific, clearly shown by comparing difference spectrograms
(Figure 1). Comparison of relative amplitudes gives an indication
of the source region, e.g. higher relative amplitudes at COLA
and LLLB compared with JCC, H2O, and KIP implies a Pacific
Northwest coastal source region. The times of low relative microseism
energy correlate across the stations, indicating that there is
little energy being coupled into LPDF microseisms anywhere in
the North Pacific during these times. Most of the LPDF energy
at H2O appears to be generated by high amplitude storm waves
impacting long stretches of coastline nearly simultaneously, and
the Hawaiian Islands appear to be a significant source of LPDF
energy in the North Pacific when waves arrive from particular
directions. The highest DF levels observed at mid-ocean site
H2O occur in the SPDF band when two coincident nearby storm
systems develop. This extreme event at H2O is not observed at
continental sites, indicating high attenuation of these signals. At
near-coastal seismic land stations, both SPDF and LPDF microseism
levels are generally dominated by local generation at nearby
shorelines (Bromirski and Duennebier, 2002). High relative SPDF
levels are generally not observed concurrently at JCC and H2O
(Figure 1), indicating that DF microseisms generated near H2O
do not propagate well, consistent with low effective Q, and also
indicating that open-ocean generated DF microseisms will not
significantly affect the statistics of ocean wave parameters determined
from inversion of land microseismic spectra.
Frequency (Hz)
0.3
0.2
0.1
0.3
0.2
0.1
0.3
0.2
0.1
0.3
0.2
0.1
0.3
0.2
0.1
D 10 20 30J 10 20 30F
December 2001 − January 2002
−10 −5 0 5 10 15
Spectral Difference (dB)
Difference spectrograms of vertical component data from
eastern North Pacific seismic stations located in Hawaii (KIP),
central Alaska (COLA), southwestern Canada (LLLB), northwestern
California (JCC, NCEDC), and mid-ocean H2O during
December 2001 and January, 2002, showing the similarity
of energy in the LPDF microseism band. Dark vertical strips
indicate times when data are missing.
COLA
LLLB
JCC
H2O
KIP
Bromirski, P.D., R.E. Flick, and N. Graham, Ocean wave height determined from inland seismometer data: Implications for investigating wave climate changes in the NE
Pacific, J. Geophys. Res., 104 (C9), 20,753-20,766, 1999.
Bromirski, P.D., Vibrations from the “Perfect Storm”, Geochem. Geophys. Geosys., 2(7), doi:10.1029/2000GC000119, 2001.
Bromirski, P.D., and F.K. Duennebier, The near-coastal microseism spectrum: Spatial and temporal wave climate relationships, J. Geophys. Res., 107 (B8), 2166,
10.1029/2001JB000265, 2002.
Bromirski, P.D., D.R. Cayan, and R.E. Flick, Wave spectral energy variability in the northeast Pacific, J. Geophys. Res., 110(C3), C03005 10.1029/2004JC002398, 2005a.
Bromirski, P.D., F.K. Duennebier, and R.A. Stephen, Mid-ocean microseisms, Geochem. Geophys. Geosys., 6, doi:10.1029/2004GC000768, 2005b.
Stephen, R.A., F.N. Spiess, J.A. Collins, J.A. Hildebrand, J.A. Orcutt, K.R. Peal, F.L. Vernon, and F.B. Wooding, 4 (10), 1092, Ocean seismic network pilot experiment,
Geochem. Geophys. Geosys., 4 (10), 1092, doi:10.1029/2002GC000485, 2003.
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