Possible submarine volcanic eruption off southern California

Deep-Sea Research, 1976, Vol. 23, pp. 265 to 267. Pergamon Press. Printed in Great Britain.

LETTER TO THE EDITORS

Possible submarine volcanic eruption off southern California (Received 19 May

1975;

accepted 12 August

1975)

Dear Editors, responding corrections to the computed locations Or~ 7 OCTOBER 1972 a sequence of approximately for the suspected volcanic explosions gave a mean one hundred explosions lasting about 40 min was source location at 31;75°N, 124.25°W. This recorded at SOFAR hydrophone stations on position, about 250 miles southwest of Point Oahu, Midway, and Wake islands (Fig. 1). These Conception, is plotted on a bathymetric chart of explosions exhibited a randomness in strength and the region, Fig. 2. time of occurrence characteristic of volcanic This position, in a general depth greater than eruptions (NORMS and JoI-rNsor~, 1969). Arrival 2200 fathoms,* is seen to lie about 4 miles north times for 37 of the explosions were measured and of a charted 1545-fathom seamount and 20 miles source locations calculated (JohNSON, 1966). As west of a charted 1385-fathom seamount. The all recording stations lay within a 20° azimuthal uncertainty in the estimate of the position could sector to the west of the source region, the com- easily reach to either seamount. Neither of these puted sources were widely scattered in longitude seamounts appears to be a member of a chain and little could be said as to a connection between such as characterizes the sites of other Pacific the explosions and a submarine topographic mid-plate volcanoes (JOHNSON and MALAI-IOFF, feature. Superimposed upon this uncertainty was 1971). The nearest such possible alignment, the uncertainty with respect to sound speed, extending from Fieberling guyot to Guadalupe which would affect all travel times systematically Island (HEY and MORGAN, 1971), passes about and perhaps significantly shift the mean of the 40 miles to the southwest, probably beyond the computed source locations away from the actual uncertainty in the estimated source location. source. The minimum recorded depths for the two However, by a happy coincidence, the seamounts which are the likeliest candidates for U.S.N.S. Silas Bent, within a few months followan active volcano are both greater than the depth ing the suspected eruption, occupied a station in at which the critical pressure of water is reached, the immediate vicinity and detonated 43 explosions about 1200 fathoms (RITTMAN, 1962). As steam is which were well recorded around the Pacific the preponderant component of volcanic gas, it (NORTHROP, 1974). By selecting the arrival times is not conceivable that volcanic explosions could for the Silas Bent shots at only those hydrophones have taken place below this critical depth. which were used in computing the suspected Indeed, MCBIRNEV (1963) speculates that it may volcanic explosions, sources could be calculated be necessary for magma to rise within tens of w h i c h w o u l d show the systematic error for the meters of the sea surface in order to obtain Oahu-Midway-Wake hydrophone array over the sufficiently rapid expansion of exsolved gases to area in question. This was done for 25 explosions produce explosions. fired on 12 and 13 May, 1973. The mean error was - - 0 ' 0 2 ° north latitude (s.d. 0.05 °) and -- 1-98° west longitude (s.d. 0.61°). Applying the cor*1 fathom = 1.8288 m. 265

266

Letter to the Editors

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Fig. 1. Sound-power-level records from SOFAR hydrophoncs at Oahu, Midway, and Wake islands showing explosion sequence suspected as volcanic in origin. Time marks are at 15-second intervals. 35" N

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Fig. 2. Bathymetric chart of region off Southern California showing adjusted mean position of the suspected volcanic explosions (marked by circled asterisk). Contours arc at 1000-fathom intervals. On the other hand, it appears that b o t h minim u m depths were f o u n d on a single traverse o f the seamounts. There has been, until now, no sufficient reason to search for shallower depths. This note, then, is written in the hope that a passing research vessel will take time to search for a shallower summit and conduct other investigations, such as dredging, that m a y help to determine the possibility o f active volcanism in this region.

Acknowledgement--I thank JOHN NORTHROP for his assistance in working with the Silas Bent shot data. Hydrophone records were provided by the Pacific Missile Range. TOM CHASEkindly provided copies of sounding collection sheets. This is Hawaii Institute of Geophysics Contribution No. 675.

Hawaii Institute of Geophysics, ROCKNE H. JOHNSON University of Hawaii, Honolulu, Hawaii 96822, U.S.A.

Letter to the Editors

REFERENCES HEY R. N. and W. J. MORGAN (1971) Parallel seamount chains in the northeast Pacific. (Abstract.)

EOS Transactions American Geophysical Union, 52, 236. JOHNSON R. H. (1966) Routine location of T-phase sources in the Pacific. Bulletin of the Seismological Society of America, 56, 109-118. JonNSOrq R. H. and A. MALAHOFF(1971) Relation of Macdonald volcano to migration of volcanism along the Austral chain. Journal of Geophysical Research, 76, 3282-3290.

267

McBIRNEY A. R. (1963) Factors governing the nature of submarine volcanism. Bulletin volcanologique, 26, 455~169. NORRIS R. A. and R. H. JOHNSON (1969) Submarine volcanic eruptions recently located in the Pacific by S O F A R hydrophones. Journal of Geophysical Research, 74, 650-664. NORTHROP J. (1974) S O F A R accuracy in the North Pacific. Journal of the Acoustical Society of America, 55, 191-193. RITTMAN A. (1962) Volcanoes and their activity, John Wiley, 305 pp.