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Ocean-Bottom Seismograph Observations on the Mid-Atlantic Ridge Near Lat 37°N

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Ocean-Bottom Seismograph Observations on the Mid-Atlantic Ridge Near Lat 37°N Ocean-bottom seismograph observations on the Mid-Atlantic Ridge near lat 37°N T.J.G. FRANCIS* Cooperative Institute for Research in Environmental Sciences, University of ColoradolNOAA, Boulder, Col- orado 80309 I. T. PORTER Institute of Oceanographic Sciences, Blacknest, Brimpton, Reading, RG7 4TJ England J. R. McGRATH U.S. Naval Research Laboratory, Washington, D.C. 20390 This article is one of a series appearing in the April and May issues of the Geological Society of America Bulletin on the scientific results of Project FAMOUS. These studies were undertaken in the axial area of the Mid-Atlantic Ridge between approximately 36°30' and 37°N latitudes. ABSTRACT 200 Hz. The tape speed of the recording system used by the ocean-bottom seismographs set an upper limit of about 40 Hz to Ocean-bottom seismographs were deployed twice in the FA- the frequencies recorded. It was decided to retain the original hyd- MOUS area in 1973. Few earthquakes were observed on the first rophones of the seismographs as well to be able to compare the per- occasion, but 515 were recorded on the second with magnitudes formance of the two systems. Since only four data channels can be ranging from 0 to 2.5. These occurred both in the median valley recorded in each ocean-bottom seismograph, the H-58 hydrophone and in the fracture zone adjacent to the instruments. The latter was substituted for one of the horizontal seismometers. Thus dur- events have been interpreted as activity along a single Riedel frac- ing the October 1973 deployment, each instrument recorded the ture cutting obliquely across the trend of the fracture zone. Only a outputs of the following transducers: vertical seismometer, hori- crude velocity structure could be determined on the basis of the zontal seismometer (of unknown orientation), wideband H-58 earthquake observations. A superficial surface layer with a com- hydrophone, narrowband hydrophone. Instrument problems pre- pressional velocity of 2.7 km/s overlies a main layer with a P veloc- vented the seismometer from being recorded at as wide a band- ity of 5.0 km/s and a S velocity of 2.4 km/s. The low-velocity sur- width as the H-58 hydrophone. The advantages of wideband re- face layer and the high ratio of P to S velocities for the main layer cording in seismology are now well understood, and the contrast (2.08) reflect the highly fissured, water-saturated nature of the between the seismometer channels and the H-58 hydrophone crust. The earthquake foci are thought to lie in the low-velocity sur- channel is analogous to that between the conventional long-period face layer. Finally, in spite of numerous small earthquakes, an seismometer and the broadband Kirnos instrument (Marshall and acoustical analysis of the hydrophone data indicates that distant others, 1972). The broadband instrument produces a more faithful shipping traffic is the dominant source of ambient noise on this part record of the parameter being measured. Thus, the H-58 hyd- of the Mid-Atlantic Ridge in the frequency band 5 to 32 Hz. rophone records are more faithful reproductions of the pressure fluctuations in the water than are the seismograms of the ground INTRODUCTION motion of the sea bed. Ocean-bottom seismographs (OBS) were deployed twice on the DESCRIPTION OF OBSERVATIONS Mid-Atlantic Ridge in 1973 as part of the British contribution to the FAMOUS project. In May, three ocean-bottom seismographs Although the ocean-bottom seismographs operated for similar were launched and recovered by RRS Shackleton just south of the durations and in similar situations in relation to the rift and small fracture zone offsetting the ridge at lat 36°57'N, designated fracture-zone topography, the May and October operations fracture zone A. In October, the same three instruments were showed a gross difference in seismicity. In May, OBS-2 detected 5 launched from USNS Hayes and recovered by USNS Mizar; the events, 4 of which were also detected by OBS-3. All appeared to array on this occasion straddled the next fracture zone to the north. arrive earlier at OBS-3, but only 2 were large enough for mea- The positions of the seismographs in relation to the bathymetry of surements of arrival time to be made for both instruments. In Oc- the area are shown in Figure 1; their geographic coordinates, water tober, the level of activity observed was two orders of magnitude depths, and operating times are given in Table 1. Unfortunately, greater: 515 events were detected by OBS-2, 184 by OBS-3. Of the tape recorder of OBS-1 failed to operate, for different reasons, these, 106 were detected by both instruments, and depending on on both deployments so that on each occasion only two records the velocity model employed, it was possible to locate between 50 were available for interpretation. and 60. The use of the wider-band H-58 hydrophone improved the The instruments themselves have already been described in the detection threshold for the October operation but did not alter the literature (Francis and others, 1975). In May 1973, the specifica- overall picture. Of the 515 events detected by OBS-2 in October, tions for the ocean-bottom seismographs were essentially un- 342 would have been detected by the seismometer channels alone, modified from those during their deployment on the Mid-Atlantic operating at the same gain settings as in May. A plot of the cumula- Ridge at lat 45°N the previous year (Francis and Porter, 1973). In tive number of seismic events detected by each of the two instru- October 1973, however, the seismographs were improved by the ments as a function of time is shown in Figure 2. OBS-2 was a more addition of U.S. Naval Research Laboratory type H-58 hyd- efficient detector than OBS-3 because the former was both instru- rophones that have a flat response to pressure in the range 0.5 to mentally quieter and closer to most of the activity. The detection rate at OBS-2 varied between about 50 and 200 events per day, but over periods of a few days at a time the rate appeared relatively * Present address: Institute of Oceanographic Sciences, Blacknest, Brimpton, Read- ing, RG7 4RS England. constant. Geological Society of America Bulletin, v. 88, p. 664-677, 18 figs., May 1977, Doc. no. 70505. 664 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/5/664/3418474/i0016-7606-88-5-664.pdf by guest on 03 October 2021 32°30'W 37° 20^- - 37°20'N 36° ¿tfN - 36°40'N 32 30 W Figure 1. Bathymétrie sketch map of the northeastern part of the FAMOUS area showing positions of ocean-bottom seismographs. Bathymetry from unpublished chart by J. D. Phillips, Woods Hole Oceanographic Institution. Depths greater than 1,500 fm shown by vertical lines; those less than 1,000 fm have cross pattern. Positions of three seismographs in May 1973 shown by Roman numerals; the positions in October 1973 shown by Arabic numerals. Solid circles indicate teleseismically determined earthquake epicenters for the period January 1963 through October 1974. TABLE 1. PARAMETERS OF OCEAN-BOTTOM SEISMOGRAPH STATIONS Instru- Lat Long Water Recording on Recording off ment (N) (W) depth (Greenwich (Greenwich (m) mean time, mean time, date) date) OBS-I* 36°54.0' 33-13.2' 2,896 (Tape recorder failed to 25 EVENTS / DAY unclamp) in 2 obs-ii* 36-49.3 ' 33-16.0' 2,640 16:00, 18:00, LU > May 11 May 19 iLl li. OBS-III* 36°48.7' 33-10.1' 2,022 16:00 18:00 o May 13 May 20 Iloci CD OBS-lf 37°04.2' 32°53.8' 3,079 (Tape recorder jammed 12 min z 3 after start) Z OBS-2f 37-00.1' 32-53.25' 2,480 21:00 05:09, LU Oct. 13 Oct. 21 < OBS-3f 37-08.3' 32°51.8' 2,424 16:00, 06:00 Z Oct. 14 Oct. 22 <_> * From RRS Shackleton deployment, May 1973. fFrom USNS Hayes and Mizar deployment, October 1973. Figure 2. Cumulative number of earthquakes detected by OBS-2 and OBS-3 in October 1973 as a function of time. The number is plotted at 2-h 286 287 288 289 290 291 292 293 294 295 intervals. JULIAN DAY Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/5/664/3418474/i0016-7606-88-5-664.pdf by guest on 03 October 2021 666 FRANCIS AND OTHERS r +5 Pa | H . i ill - - 5 Pa tj +0.5 Aim r P t ' ^ III!. * ^ ' nnnrTTnminrTirT^ B I is Figure 3. Set of four earthquakes detected by OBS-2 in October 1973. In each case H is the wideband hydrophone trace filtered from 2 to 40 Hz. Z and Y are the vertical and horizontal seismometer traces, respectively, filtered from 1 to 12.5 Hz. Equivalent ground displacement on the seismometer channels is shown for the peak magnification at about 8 Hz. The response of the hydrophone channel to pressure is effectively flat within the filtered band. Each event is identified as follows by the time to the nearest minute and the Julian day: (A) 03:18/290, (S - P) = 1.42 s; (B) 18:30/293, (S - P) = 2.49 s; (C) 18:08/293, (5 - P) = 2.68 s; (D) 15:42/292, (5 - P) = 3.03 s. A small selection of the records obtained by OBS-2 and OBS-3 events [(S — P) > 2.5 s], T phases occur that have propagated are shown in Figures 3 and 4.
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