Nanoearthquakes at H2O 2

Nanoearthquakes at H2O 2 Hydrophone and seismic observations of the first nanoearthquake are shown from the compressional arrivals through the shear wave reverberations in the sediments. The seismic channels are converted to a Wood-Anderson response for calculation of ML magnitude ~ -0.7. The hydrophone channel is dominated by converted compressional wave energy, and effectively filters out shear waves. A small precursor is consistent with a refracted head wave from a Layer 2B basalt velocity, assuming a Layer 2A thickness of about 700 m. Strong conversion of P-to-S and S-to-P is evident at the sediment-basalt interface. Horizontal components are oriented into radial and transverse polarizations, as determined from the polarization of the compressional waves. Note that Ps is substantially larger on the radial component. Multiple reverberations in the sediment layer and shear wave birefringence constrains the shear velocity structure. Arrows indicate predicted arrival times from a simple 3-layered velocity model. The other nanoearthquakes show essentially the same pattern of arrivals.

Nanoearthquakes (ML < 0) have been observed seismoacoustically at the Hawaii-2 Observatory GSN station, H2O, on the seafloor midway between Hawaii and California (Butler, 2003). Out of a larger data set, the analysis of a sequence of 5 events (ML -0.7 to -2) within an hour yields a number of interesting scientific insights for the uppermost oceanic crust and sediments at the H2O site. The events are consistent with fluid-saturated cracks/faults with very low stress drops (< 1 bar) and fault dimension < 150 m. Using P, and successive water reverberations, PwP & PwwP, the distance can be constrained to about 3.7 km from H2O. The polarization of P waves for the two main events and largest foreshock are all at the same azimuthal bearing of 220° ± 180°. The extremely short durations (< 0.1 s) of the nanoearthquakes illuminate the uppermost oceanic crust internally like a “seismic strobe light” that creates distinct seismic arrivals that can be used to investigate the seafloor structure at the H2O site. The observed shear wave reverberations coupled with an ODP determination of layer thickness of 29 m near H2O constrains the average shear wave velocity of the sediments to be about 97 m/s. Using differential timing of PwP-P and S-P, the uppermost crust has a compressional velocity of about 3.0 km/s and shear velocity of 1.5 m/s, consistent with Layer 2A pillow-like basalts observed by ODP drilling nearby. Surprisingly, a clear shear wave birefringence is evident for the radial/transverse orientation determined solely from the P-wave polarization. The radial SV component is 0.085 seconds faster than the transverse SH, indicating 3% anisotropy in the basalt, which is consistent with vertically oriented cracks in the basalt. The fidelity of the signals, particularly on the horizontal components, is so good that one can use the particle motion to identify phases at high frequencies, which is only possible with well-coupled, buried sensors.

Butler, R., The Hawaii-2 Observatory: Observation of Nanoearthquakes, Seismol. Res. Lett.,74(10), 290-297, 2003.

Butler, R., The littlest earthquakes in the GSN?, Seismol. Res. Lett., 73, 273, 2002.
Supported by NSF Cooperative Agreement EAR-0004370


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