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 when they are available [Gunst and Engdahl, 1962J . The USCGS claims an accura.cy of about 25 km in their reported depths.

    With the exception of South America, only one of the epicenters in Figure 1 was assigned a focal depth greater than 100 km in the USCGS Seismological Bulletin. Three other epicent.ers in 1960 and 1961 were reported in the same bulletins with depths between 70 and 100 km. For this region Gutenberg and Richter [1954J did not report any focal depths in excess of 70 km. However, for the earthquake listed at a depth of 107 km, only five stations were used in the detennination given on the USCGS cards. No pP readings are known to the author for t.his earthquake.

    The focal depth of the earthquake of October 7, 1960 (Table 1), was originally reported as 203 km on the USCGS cards. However, the depth was reduced to 62 km in the revision published in the USCGS Seismological Bulletin. Seismic surface waves with periods of 5 to 10 seconds have been recorded at Hallett, Antarctica; Huancayo, Peru; and Perth, Australia, for many of the earthquakes t,hat are plotted in Figure 1. The size of these waves, relative to

    St.and. Error, sec Depth, km Latitude

    No. Stations Time, h m s Longitude Source

    57 12 01 42 7SoE 37S USCGS

    12 01 30 79E 275 02 oS USCGS

    49 02 16 29 1l00W 2.03 17 20.3 oN USCGS

    22 00 20 33.6 56.7W Computer 33 06.8N USCGS

    20 14 37 56.9 123.5W Comput.er

    that of the longer-period surface waves, is nearly the same for the shock of October 7, 1960, and for other nearby epicenters of comparable magnitude that were assigned focal depths of less than 50 km. Such large short-period surface waves would not be expected for a source deeper than about 50 km and they may be indicative of an even shallower depth. Thus, on the basis of several lines of reasoning a shallow focal depth is indicated for most, and perhaps all, of the earthquakes in this region.

    DISCUSSION OF THE EPICENTER MAP

    Most of the epicenters shown in Figure 1 are located near the crests of the various oceanic ridges. The ridge systems and deeper basins are differentiated in Figure 1 on the basis of the 2000-fathom contour. The bathymetric data for this figure were taken from H.O. Chart 2562 (Antarctica) and from Fisher [1958J.

    In several areas epicenters have been located where few or none were previously known. Only three epicenters were located by Gu.tenberg and Richter [1954J in the vicinity of the oceanic ridge between 1400W and 1600E. Although the seismicity of this region seems to be considerably less tha.n that in adjacent areas, enough epicenters have been located to define a pattern that follows the crest of the oeeanic ridge. These improvements in earthquake detection, as well as recent bathymetric surveys, indicate that oceanic ridges and their associated zones of seismic activity are continuous, as has been suggested by Ewing and Heezen [1956J.

    Possible fracture zones delineated by epicenters. In the eastern Pacific the oceanic

  • 6004 LYNN R. SYKES

    ridge is intersected by a series of long straight bands of mountainous topography called fracture zenes [Menard, 1960]. Studies of magnetic anomalies in the northeast Pacific indicate large strike-slip movements along several of the fracture zones [Menard, 1962].

    In several of the oceanic areas shown in Figture 1, the epicenters are all located along narrow linear zones. One of these is the region 50 to 600S and 115 to 1500W. This region is shown in an enlarged view in Figure 2. H. W. Menard (personal communication) has recently completed a bathymetric chart of the Pacific Ocean. The 3- and 4-km contours shown in Figure 2 were taken from this map. The bathymetric data that Menard [1960J has interpreted as a fracture zone coincide with the abrupt changes in the pattern of epicenters near 558. The displacement of the line of epiccmters in this region is approximately 1000 km and is in a rightlatera.l sense . For the period 1957 to 1963 no epicenters were found on the continuation of the 55S zone beyond the crests of the oceanic ridge. Another narrow seismic zone is located between 358, 109W, and 368 , 97W. The two epicenters near 3608, 90oW, may indicate an extension of this sei"mic zone. Bathymetric data are very sparse in these two areas ; better data would be of great help in confirming the existence of fracture zones in these regions.

    A similar pattern of epicenters is observed at the equator in the Atlantic where the midoceanic ridge is offset approximately 15 in a left-lateral sense along a series of en echelon fracture zones [H eezen and Tharp, 1961]. Reported earthquake epicenters for the period January 1957 to May 1963, with bnt one exception, fall either on the crest of the mid-Atlantic ridge or on fracture zones that offset the ridge. Only one epicenter was located off the ridge crest. This epicenter may be located on the extension of a fault zone. This earthquake, on July 29, 1958, at 3.8N, 26.6W (Table 1), was located with the epicenter relocation program using 100 P-wave rea.dings.

    Shallow-focus earthquakes in Chile. Another region in which earthquakes are restricted to a fairly narrow zone is the coast of Chile. In Figure 1 the only Chilean epicenters plotted are those that were located by using 10 or more P-wave readings and for which a shallow focus (less than 70 km) was reported by the U8CGS.

    The width of the seismically active zone in Chile is about 200 km, as shown in Figure 1. Benioff [1962J has derived similar dimensions for other circum-Pacific faults. However, some of the seismically active zones associated with oceanic ridge systems appear to be considerably narrower and may be less than 50 km wide. Along the Chilean coast an abrupt decrease in seismicity occurs near 478 where the west Chile ridge intersects the continent.

    Regions of hl{;h and low seismicity. Two other oceanic are...

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