Seismicity of the South Pacific ocean

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  • JOURNAL OF GEOPHYSICAL RESEARCH VOL. 68, No. 21 NOVEMBER I, 1963

    Seismicity of the South Pacific Ocean 1

    LYNN R. SYKES

    Lamont Geological Obsen)atory, Columbia Univendty Palisades, New Y01k

    Abstract. A map of earthquake epicenters is presented for the South Pacific Ocean for the period 1957 to 1963. For much of this region the accuracy in locating epicenters has been improved by nearly an order of magnitude in comparison with that of previous studies. In several areas of the South Pacific the seismic activity is confined to narrow zones that are less than 50 km wide . Near 55S and between 122 and 135W the pattern of epicenters is offset approximately 1000 km. Seismic and bathymetric data indicate that a fracture zone intersects the Easter Island ridge in this region . Epicentral dn.ta suggest that several other fracture zones may be present in the South Pacific and eastern Indian oceans. Regions of relatively high seismicity on the oceanic ridges may be related to the presence of fracture zones that

    intersect the crest of the ridge. Seismic activity along these fracture zones is restricted aJmost exclusively to the region between the displaced crests of the oceanic ridge. A number of topographic profiles across the oceanic ridges in the South Pacific and eastern Indian oceans are compared with the epicentral determinations.

    INTRODUCTION

    Until recently the determination of earthquake epicenters in the South Pacific was subject to considerable uncertainty. In the southern part of this region, epicentral determinations were in doubt by as much as 5 [Gutenberg and Richter, 1954]. Topographic information was incomplete for a large part of the South Pacific. Hence, neither the seismic belts nor the large-scale geological features were known with precision.

    Several recent developments led to an improvement in the determination of epicenters in the southern hemisphere. The establishment of several first-class seismic stations in Antarctica during the IGY (1957-1958) made the greatest contribution to improved epicentral determinations in this region. In addition, electronic computers made it possible to utilize a very large number of P or P KP readings in determining a least-squares solution to the residuals of the travel-time data. Electronic computers enabled the U. S. Coast and Geodetic Survey to report an increased number of epicenters. Also, it is now possible to recompute the locations of older epicenters for which no least-squares solution was available. Likewise, improvements in the design of echo sounders and an increase in the

    1 Lamont Geological Observatory contribution 657.

    number of scientific cruises led to a better understanding of the bathymetry.

    It is apparent that greater accuracy can be achieved in the determination of earthquake epicenters in many areas of the world as more seismic stations are installed and better data become available. With this greater accuracy it should be possible to obtain a much clearer understanding of the relationships between seismicity and geological structure.

    LOCATION OF EPICENTERS

    Epicentral data were derived from the epicenter cards and seismological bulletins of the U. S. Coast and Geodetic Survey (USCGS) and were supplemented by da,ta from the bulletins of the Bureau Central International de Seismologie (BCIS). In Figures 1 and 2 epicenters located by computer with data from 10 or more stations are denoted by black dots. Those computed with data from fewer than 10 stations are indicated by open circles; those derived without the aid of a computer are denoted by crosses. These epicenters were computed either by the USCGS using a program developed by Gunst and Reece [Gunst and Engdahl, 1962J, or by ilie author using a program written by Bolt [1960J and modified by J. Kuo and others at the Lamont Geological Observatory. Epicenters have been determined by the USCGS with the aid of a computer since August 1960. When these US

    5999

  • 6000 LYNN R. SYKES

    Fig. 1. Earthquake epicenters in the South Pacific for the period January 1957 to May 1963. n = number of stations used in locating the epicenter. Bathymetry from H. O. Chart 2562 (Antarctica) and the Preliminary Report on Expedition Downwind [Fisher, 1958]. 2000-fathom contour is shown.

  • SEISMICITY OF PACIFIC OCEAN 6001

    135

    140

    145

    150

    160

    165

    55S

    50

    ..... .,.... . 55

    65

    70

    75 S

    Fig. 2. Enlarged view of seismically active region in the vicinity of 50 to 60S and 115 to 150W. Symbols and dates are the same as in Figure 1. The track for R. V. Vema cruise 16 and approximate ships' tracks for Downwind [Fisher, 1958] and for R. V. Ob [Zhivago, 1962] are also shown. Bathymetric data for 3- and 4-km depths taken from Menard [1960] and an unpublished bathymetric chart of the Pacific.

    CGS epicenters were plotted with other epicenters for the period January 1957 to August 1960, most of the latter coincided with the trend of the USCGS computer epicenters. Nevertheless, a few of the 1957-1960 events deviated from the pattern shown in Figures 1 and 2. These, as well as several other epicenters, were recomputed using the Bolt program, and they are listed in Table 1. Several other earthquakes that are discussed in this paper are also listed in the table. These recomputations reduced the scatter in the epicentral map considerably and also helped to fill in several gaps in the seismic data.

    Accuracy of epicentral determinations. The epicentral locations are relative to the travel-

    time tables used (in this case, the JeffreysBullen tables). The suitabil ity of these tables has been demonstrated for SQveral of the wellrecorded nuclear explosions for which the differences between the computed ep icenters and the officially st ated positions are less than 0.1 0 [Bolt, 1960; Gunst and Engdahl, 1962]. Table 1 shows that epicenters determined by means of the Bolt program agree to within 10 or 20 km with the locat ions derived by the USCGS using a computer. However, differences of several seconds in the time of origin were obtained from the two programs. These differences seem to be related to uncertainties in the depth of focus and do not significantly affect the location of

  • 6002 LYNN R. SYKES

    TABLE 1. Computations of Earthquake Epicenters

    Computation Using Program of Bolt [1960] Other Solutions

    Date Time, h m 8

    Oct. 7, 1960 20 01 11.4 Aug. 30, 1960 06 45 12.7 Nov. 22, 1955 03 24 03.2

    July 28, 1958 18 33 46.2 Aug. 22, 1958 23 18 34.5 April 15. 1960 03 25 39.3 Nov. 14, 1958 05 04 24.9 Dec. 17, 1959 16 48 54.7 June 24, 1958 06 36 15.2

    Feb. 19, HJ58 01 20 21.2 Dec. 4, 1956

    July 30, 1058 15 10 19.6 Nov. 9, 1959 04 18 55.0 Nov. 29, 1959 I(l 17 38.7 Oct. 17, 1959 08 35 00.2 July 26, 1958 08 35 03.5 April 21, 1959

    Dec. 31, 1957 14 28 15.1

    July 29, 1958 21 37 22.6 Aug. 27, 1958 02 25 35.4 July 11, 1959

    Latitude Longitude

    20.3S 114.1 oW

    21.00S 1I3.9W

    24.58 122.5W

    26.5S 115.9W

    26.7S lI5.5W

    26.9S 113.4W

    36. 00S 102.8 oW

    36.2S 102.8W

    46.08 75. 8W

    37.08 111.9 oW

    49.7S 115.2W

    54.5 oS 136.5W

    57.2S 146.9 oW

    57.3 oS 161.2W

    150.G oS 166.1W

    Stand. Error, sec

    Magnitude No. Stations

    Easter Island to Chile

    5U-5Yz (PAL)

    6;Y.i'-7 (PAS)

    51 (MAT)

    6Yz (PAS)

    6 (PAS)

    1.05 12

    2.17 20

    1.60 47

    1. 70 24

    0.83 21)

    2.45 45

    2.18 23

    2.37 27

    2.17 14

    Depth, km Time, h m s

    62 20 01 20.1

    40 06 45 16.4

    03 24 04

    03 24 05

    03 24 00

    18 33 45

    23 18 33

    03 25 38

    05 04 25

    16 48 51)

    06 36.'1

    Easter [sland to Rallenll I sland. 1. 70

    24 01 20 20 6;Y.i' (PAS)

    10 07 54 2.30

    8 15 10.2 3.07

    9 04 18 53 5;Y.i' l.(J6

    9 10 17 40 2.03

    16 08 35 00 1. 72

    7 08 35 12 P and PKP data insufficient

    for location. Not. plotted in Figure 1. 15 24.7

    New Zealand to Macquarie Island

    45.00S 6Yz 1.84 166.00E 39 14 28 15

    Epicenters outside the Area of Figure 1 03.SoN 5;Y.i' 1.30 26.6W 100 21 37 25 04.0 oS 5;Y.i'-6 (MAT) 2.94

    104.3 oW 6U-6YZ (STRA8B) 24 02 25 32 36.9S 6U-6Yz (PAS) 2.15

    Latitude Longitude Source

    20.3S USCGS 114.1 oW Computer

    21.00S USCGS lI3.7W Computer

    24.4 oS ISS 122.6W

    25.00S BCIS 122.5W

    24YzS USCGS 123W

    26YzS USCGS II 5Y:;i oW

    26YzS USCGS 115W

    278 USCGS and 113 oW BCIS

    36S USCGS 102 oW

    36YzS USCGS 101YzW

    ,....,47 oS BCIS "-'80 oW

    37 P-1loS USCGS 111 oW

    45Yz8 USCGS 106 oW

    "-'48 oS BCIS ",1200W

    157 oS BCIS 13GoW

    57S USCGS 147YzW

    57Yz8 U8CGS 161W

    60YzS U8CG8 168YzW

    "-'62 Yz 8 BCIS l72YzW

    45S USCGS 165YzE

    04N USCGS 26YzW 04YzS USCGS

    104YzW 37YzS BCIS

  • SEISMICITY OF PACIFIC OCEAN 6003 TABLE 1. (Continued)

    Compu tation Using Program of Bolt [1960] Other Solutions

    Date Time, h m s

    12 01 41.1

    April 21, 1960 02 16 31.7 Sept. 23, 1960 00 20 32.5 July 23, 1961

    Latitude Longitude Magnitude

    7S.6E

    02. 5S 5%; (BERK) lO9.3W 20A oN 4 (PAL) 56.7W

    5 %;-6 (PAS), (BERK)

    the epicenters. Thus, in the detennination of epicentrallocations, an accuracy of 10 to 20 km "hould be possible for most well-recorded earthquakes.

    Depth of focus. In the absence of pP data a surface focus is assumed for the Bolt [1960J program. In the USCGS program the location, origin time, and depth are computed from Pwave readings; the depth is checked against other data such as pP-P times