Open AccessP wave detection thresholds, Pn velocity estimates, and T wave location uncertainty from oceanic hydrophones
Author(s) -
Slack Philip D.,
Fox Christopher G.,
Dziak Robert P.
Publication year - 1999
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/1999jb900112
Subject(s) - seismology , geology , amplitude , hydrophone , geodesy , arrival time , waveform , coda , magnitude (astronomy) , p wave , physics , medicine , oceanography , quantum mechanics , voltage , astronomy , transport engineering , engineering , cardiology , atrial fibrillation
P wave arrivals recorded by the U.S. Navy's SOund SUrveillance System (SOSUS) hydrophone arrays were used to estimate earthquake detection thresholds and P n velocities in the northeast Pacific Ocean. The Navy hydrophones have been used successfully to detect and locate oceanic earthquakes using their waterborne acoustic tertiary ( T ) waves; however, use of these hydrophones for seismic body wave detection allows regional seismic analyses to be extended to the oceanic environment. The P wave detection threshold of the SOSUS hydrophones was quantified using the epicentral distance and magnitude of 250 northeast Pacific Ocean earthquakes. Earthquakes with body wave magnitudes as low as 2 have detectable P wave arrivals at epicentral distances of ≤500 km. Earthquakes with m b between 3.5 and 5 were detected ∼50% of the time at distances of 100–1500 km, while events with m b > 5 were all detected, even out to distances of 1000–1500 km. Both P and T wave hydrophone arrival times were used to estimate the epicenters of 100 earthquakes. The peak amplitude of the T wave coda and the onset of the P wave were used as the earthquake arrival times to estimate event locations. T wave arrival time residuals have a Gaussian distribution with zero mean, which implies that using T wave peak amplitude is consistent with using the P wave onset as the arrival time. There are typically ≤6 stations used to derive a T wave based location, hence location error ellipses are not well constrained. A Monte Carlo technique was employed to estimate T wave event location uncertainty. T wave locations have error bars of ∼1 km in latitude and longitude when >3 hydrophones are used for a location estimate. The detected P wave arrivals and earthquake locations were used to measure Pn velocities. Pn velocity values of 7.9 ± 0.1 and 8.0 ± 0.1 km/s were found for the Pacific and Juan de Fuca plates, respectively. A Pn velocity of 7.5 ± 0.1 km/s was measured for rays traveling northward from the Mendocino Triple Junction along the Cascadia subduction zone. A Pn velocity of 7.7 ± 0.3 km/s was estimated for ray paths originating onshore western North America and traveling to the offshore hydrophones.