Historical seismicity near Chagos: a complex deformation zone in the equatorial Indian Ocean

350

Earth and Planetary Science Letters, 76 (1985/86) 350-360 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

[61

Historical seismicity near Chagos: a complex deformation zone in the equatorial Indian Ocean Douglas A. Wiens Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130 (U.S.A.)

Received January 29, 1985; revised version received October 17, 1985 Over the last twenty years, Chagos Bank has a seismicity rate disproportionate to its supposed intraplate location. Earthquake relocation also shows a high seismicity rate in pre-WWSSN time (1912-1963), with seven events located off of the Central Indian Ridge, including large events in 1912 (M = 6.8) and 1944 (M = 7.2). This study uses the moment variance technique, a systematic search for the mechanism which best fits P, PP, SH, Love and Rayleigh amplitudes, to determine the focal mechanisms of two pre-WWSSN earthquakes. A test with a recent event of known mechanism demonstrates that accurate focal parameter determination is possible even when only a few good records are available. Moment variance analysis shows a thrust faulting mechanism for the 1944 event, northeast of Chagos Bank near the Chagos-Laccadive ridge, and a strike-slip focal mechanism for a smaller 1957 event west of Chagos Bank. The 1944 event, one of the largest oceanic "intraplate" earthquakes known (moment 1.4 X 1027 dyne-cm), indicates that the Chagos seismicity reflects not an isolated occurrence of normal faulting as previously thought, but rather regional tectonic deformation extending northeast of Chagos Bank and including thrust, normal and strike-slip events. This seismicity and previously studied seismicity near the Ninetyeast Ridge and Central Indian Basin suggest a broad zone of deformation stretching across the equatorial Indian Ocean. This zone contains all known magnitude seven oceanic "intraplate" earthquakes not associated with subduction zones or continental margins, suggesting that elsewhere such extensive deformation occurs only along plate boundaries. This study proposes that a slow, diffuse plate boundary extends east from the Central Indian Ridge to the Ninetyeast Ridge and north to the Sumatra Trench. A recent plate motion study confirms this boundary and suggests that it separates the Australian plate from a single Indo-Arabian plate.

I. Introduction Surveys of 1964-1983 oceanic intraplate seismicity [1-4] suggest that for this twenty y e a r period, the C h a g o s Bank region is the m o s t intense source of oceanic seismicity a p a r t from the conv e n t i o n a l l y d e f i n e d plate b o u n d a r i e s . T h e cause of this activity is essentially unknown. Stein [5] n o t e d the unusual c o n c e n t r a t i o n of seismicity at Chagos Bank a n d o b t a i n e d n o r m a l faulting m e c h a n i s m s for three events which i n d i c a t e d n o r t h - s o u t h extension. H e suggested the e a r t h q u a k e s r e p r e s e n t e d an isolated reactivation of a fracture r e m a i n i n g f r o m the s e p a r a t i o n of Chagos Bank f r o m the M a s c a r e n e Plateau. However, studies of n e a r - r i d g e e a r t h q u a k e s [1,4] f o u n d substantial seismicity in a region of rough t o p o g r a p h y n o r t h a n d west of C h a g o s Bank, i n d i c a t i n g d e f o r m a t i o n of s o m e w h a t larger geographical extent. The e a r t h q u a k e m e c h a nisms vary from n o r m a l to strike-slip faulting, b u t 0012-821X/86/$03.50

© 1986 Elsevier Science Publishers B.V.

all have a consistent n o r t h - s o u t h tensional axis orientation. W i e n s a n d Stein [1] also f o u n d a norm a l faulting m e c h a n i s m for the large N o v e m b e r 30, 1983 ( M s = 7.6) Chagos B a n k event, i n d i c a t i n g that the d e f o r m a t i o n involves e a r t h q u a k e s of substantial size. H o w should this unusual seismic zone be interp r e t e d ? O n e possibility is that the Chagos earthq u a k e s are p a r t of a general class of i n t r a p l a t e e a r t h q u a k e s in y o u n g oceanic lithosphere [1,4] which m a y result f r o m thermoelastic stress caused b y the cooling a n d c o n t r a c t i o n of the oceanic lithosphere. Evidence suggesting a c o n n e c t i o n between e a r t h q u a k e s in y o u n g oceanic l i t h o s p h e r e a n d thermoelastic stress includes the high rate of i n t r a p l a t e seismicity in y o u n g lithosphere [6], a n d the c o r r e s p o n d e n c e between the m e c h a n i s m s a n d d e p t h s of e a r t h q u a k e s [1,4] a n d the orientations a n d m a g n i t u d e s of stresses p r e d i c t e d b y t h e r m o elastic stress calculations [7-9]. The m a i n diffi-

351

culty with the idea that Chagos seismicity results from thermal stresses is that these stresses should produce a fairly uniform distribution of seismicity, as the thermal structure varies only with the age of the lithosphere, whereas the Chagos region apparently has an anomalously high seismicity level. However, the seismicity rate at Chagos need to be a serious objection if the seismicity level of the last 20 years is unrepresentative of the longer-term seismic release rate. Intraplate seismicity, due to its low occurrence rate, is particularly susceptible to misinterpretation because of a limited period of observation. For example, the seismicity of the last twenty years gives little indication that large (M = 7.5-8.0) earthquakes occur in the New Madrid, Missouri area, whereas study of a twenty-year period in the early nineteenth century would suggest a much higher seismicity rate. An alternative explanation for the Chagos Bank seismicity is that it results from tectonic deformation. In this case the high seismicity rate observed in the past twenty years is not a result of the sampling interval, but rather indicates an unusual level of intraplate deformation. This deformation may thus have important implications for studies of plate tectonics. The characteristics of the Chagos seismicity over a longer time period may hold the key to resolving this problem. Although recent studies generally consider only post-1963 events because the World Wide Standard Seismograph Network (WWSSN)

improved the quality, quantity and availability of data, modern analysis techniques allow approximate focal mechanism determination from a small number of good records, or in some cases even from records of a single station [10]. Indeed, past studies of historical seismicity have provided valuable information [11,12]. This study analyzes the historical seisrnicity of the Chagos region using earthquake relocation and an improved mechanism determination procedure in order to determine the tectonic implications of the Chagos earthquakes. 2. Earthquake relocations Attempts to find historical oceanic intraplate earthquakes quickly disclose many "ghost" events --plate boundary events which have incorrect locations due to insufficient data or inaccurate location procedures. These poorly located events obscure the fact that larger, better recorded events can be accurately located, particularly with modern computer relocation procedures. Thus, as a first step, all events in the Chagos region with intraplate locations given by Gutenberg and Richter [13] or the International Seismological Summary were relocated to ensure that they were not mislocated Central Indian Ridge earthquakes. This was done by performing the relocation procedure with various subsets of the arrival time data and examining the scatter in the epicenter lo-

TABLE 1 Earthquake relocations Date

Time

Lat. (°N)

Long. °(E)

Quality of location a

Phases used

Number of arrival times

rms residual (seconds)

Gutenberg & Richter [13] magnitude

May 11,1912 Mar 09,1930 Apr 24,1935 Aug 07,1939 Feb 29,1944 Jan 25,1951 Oct 02,1957

17 8 15 23 16 16 20

-5.5 - 3.1 0.3 4.5 0.3 - 1.8 --6.3

71.6 70.1 74.8 77.6 75.4 81.0 69.7

D C B B A B A

P,S P,S P P P P P

18 13 37 16 30 29 51

2.7 3.7 2.6 3.1 1.6 2.1 2.2

6.8 5.6 6.0 5.6 7.2 6.0 b 5.7 b

26 45 52 34 52 24 59 44 28 08 35 36 58 44

Events which were relocated to active plate boundaries or for which no accurate epicenter determination was possible have been deleted from this list. a Quality of relocation code: A = estimated uncertainty +_0.25 ° or less; B = estimated uncertainty ___0.4°; C = estimated uncertainty + 0.6 °; D = estimated uncertainty + 1°. b Surface wave magnitude determined in this study.

352 cations, thus minimizing problems caused by clock errors or misidentified phases at individual stations. Recent study suggests that nearly all oceanic intraplate seismicity occurs in the upper 30 km of oceanic lithosphere [6]; thus, the relocations were performed with the depth fixed at 10 km to stabilize the inversion. (For several of the events a variety of assumed depths from 5 to 50 km were used; the effect of these differences in the assumed depth were insignificant.) Solutions for the two earliest events were also improved by including S-wave arrival times. In the final relocations stations with residuals greater than 5-10 seconds were deleted. The relocations show that several events occurred on the Central Indian Ridge; for several others the epicentral uncertainty was too large to ensure an intraplate location or the intraplate solution relied on only one or two arrival times. These events were not considered further. Table 1 shows the results of the relocations for the intraplate events. Table 1 also lists the estimated uncertainty of the solution; these estimates were taken to be somewhat larger than the scatter of epicenters produced by inverting various subsets of the arrival time data. The most interesting relocation is that of the May 11, 1912 event. This M 6.8 earthquake can be definitively located as an intraplate earthquake in the vicinity of Chagos Bank. Its location uncertainties are about + 1 ° ; thus its exact position at Chagos Bank cannot be determined but a location on the Central Indian Ridge can be ruled out. 3. Mechanism determination from sparse data

Pre-WWSSN earthquakes pose several difficulties for standard mechanism determination techniques, including a small number of good records, poor station distribution, low-gain instruments, and occasional problems with instrument reliability (i.e. wrong instrument constants or reversed or unknown polarity). These problems plagued many early attempts to determine focal mechanisms. Modern analysis techniques, including synthetic body waves [14,15] and surface waves [16], present a possible solution to this problem. Unfortunately, often data of a single wave type (i.e. P, SH, or surface waves) are insufficient to constrain the focal mechanism because of the small number of good records. Thus past mechanism

determinations for historical events generally placed considerable reliance on P-wave first motion reports in the ISS, which are notoriously unreliable when not confirmed by viewing the actual records. The moment variance technique used in this study overcomes this problem by using several types of data in a systematic analysis. This technique involves determining a preliminary focal depth and rupture duration by modeling P and SH waveforms. Estimates of these parameters can generally be found independently of estimates of the focal mechanism [10]. Then a grid-search technique is used to determine the best focal mechanism. First, possible solutions which violate more than 20-25% of impulsive P, SH, and SV polarities and pronounced S polarizations read from actual records are eliminated from consideration. Then the remaining regions of the model space are searched for the mechanism providing the best fit to the amplitudes of the P, SH, Rayleigh, and Love waves (unlike the wave shapes, the amplitudes of the P and SH phases are a strong constraint on mechanism type). P and SH waveforms are not used in this analysis unless they show a reasonable match to the synthetics. The amplitudes are matched by computing the seismic moment for each datum (a wave type at a given station) and then choosing the mechanism which minimizes the variance in the logs of the moments. PP amplitudes from stations with delta greater than 70 ° are also used in the moment variance procedure to help constrain the mechanism type for events which show a definite azimuthal variation in PP amplitude. Because the take off angle of the PP ray is that of a station located at one-half the distance of the recording station, PP amplitudes allow a more complete sampling of the Pwave radiation pattern that direct P waves alone. PP phases are processed by taking the Hilbert transform of the raw data to remove the effects of the PP caustic [17-19]. The transformed data is then compared with a P-wave synthetic computed for a station distance of one-half the actual distance, with the appropriate corrections for increased attenuation and geometrical spreading. Previous studies [19] show that the seismic moment can be accurately retrieved from PP data using this procedure. The moment variance method has several ad-

353 vantages. It uses only first motions which are confirmed by viewing actual seismograms. It minimizes the reliance placed on any single datum (i.e. an individual P polarity or the Love/Rayleigh ratio at one station), thus making it less susceptible to error caused by instrument problems such as reversed polarities or incorrect magnifications. It also takes advantage of a maximum amount of independent data by using up to five wave types at each station, an important consideration when the number of stations is limited. Finally, the gridsearch technique minimizes the possibility of selecting a local error minimum, a significant problem with sparse data sets. The width and magnitude of the variance minimum also allows estimation of the uncertainty of the solution. The moment variance method was tested with several records from the September 12, 1965

180

V~tA~C~ SLp

~P2s

270

s~

MOMENT VARIANCE 2 STATIONS

Eiill

270 3~eO 270 ~,0

270 360 TSO

1o¢r0

STRIKE 2 5 0

SLIp 240

P A N D SH INVERSION

STRIKE 2 6 3

2to i

DIP 5S

OIP 4 4

SLiP 2 4 6

SURFACE W A V E S

[:i:ii:i

leo

27

3

STRIKE 2 7 0

DiP 6 0

SL}p 2 e o

Fig. 1. Test of the moment variance procedure for the September 12, 1965 Chagos event using limited data (records from two stations and polarities and polarizations from two others). At left are slices through the model space at various constant dip values; contours of moment variance are 25% (solid line) and 50% (dotted line) above the minimum value. Subsequent trials with a finer grid found a variance minimum for the mechanism shown (top right). Right side also shows mechanisms found in previous studies using the whole suite of WWSSN data [4,5]. This test demonstrates that the moment variance procedure can accurately determine mechanisms from the amount of data available for many historic events in the Indian Ocean.

Chagos Bank event to determine whether accurate mechanism determination can be expected using the amount of data typically available for historic earthquakes in the Indian Ocean. The data consisted of P, S, Love and Rayleigh wave records for two stations (Stuttgart and Mundaring) and polarity and polarization data from two other stations in Australia and Europe (Toledo and Charters Towers). Fig. 1 (left) shows contour plots of moment variance as a function of strike and slip for various values of dip. The region of low moment variance indicates a normal faulting mechanism with east-west striking nodal planes. Further trials with smaller increments in strike, dip and slip produced a variance minimum for a mechanism of strike 250 °, dip 55 °, slip 240 °. Fig. 1 (right) shows this mechanism along with mechanisms determined in previous studies from the whole suite of WWSSN data using body wave inversion [4] and surface wave analysis [5]. The mechanism determined using moment variance analysis of the limited dataset is in excellent agreement with those determined from the complete WWSSN dataset. The seismic moment obtained, 4.9 × 10 25 dyne-cm, compares well with moments determined in the previous studies from surface waves (6.8 × 102S) [5] and body waves (3.7 × 10 25) [4]. Additionally, the approximate depth was easily determined from the P and SH waveforms. Attempts to analyse the mechanism using only data from Mundaring yielded a moment variance minimum at strike 260 °, dip 45 °, slip 230 °, but another local minim u m had a similarly low moment variance suggesting uniqueness might be a problem. Thus, this test of the moment variance technique indicates with reasonable amounts of data (2 stations or more) accurate focal mechanism determination could be expected. Data from a single station provide useful constraints but may be unable to distinguish a unique solution. These results are in accord with previous attempts to determine focal mechanisms from limited data [10].

4. Earthquake analysis Two events yielded enough good records for detailed analysis--the February 29, 1944 M 7.2 event located at the base of the Chagos-Laccadive Ridge about 800 km northeast of Chagos Bank, and the October 2, 1957 M s 5.7 event located

354

FEB. 29, 1 9 4 4 f

d

STRIKE 115

DIP 50

SLIP 60

STRIKE 70

DIP 55

SLIP 8 0

MOMENT VARIANCE SLIP 90

0 DiP 25

DIP 45

90

180

SLIP 90

180 0

iiiiiiiiiiiii

SLIP 90

180 0

DIP 70

180

i......i

illllli~ ......

STRIKE 270

iiiiii~

Fig. 2. Mechanism determination for the February 29, 1944 event. All first motions read from actual records indicate compression. Plots (bottom) show moment variance contours of 30% (solid line) and 50% (dotted line) above the minimum value. The moment variance analysis indicates a thrust faulting mechanism; the only possible strike slip mechanisms violate the P-wave polarities at Riverview and Christchurch. Mechanisms at the top correspond to two local variance minima as determined using a finer grid; the mechanism on the left shows slightly lower variance.

between Chagos Bank and the Central Indian Ridge. Fig. 2 shows the mechanism determination for the 1944 event. All impulsive P-wave first motions obtained from copies of actual records are strong compressions, including a very pronounced PKiKP arrival at Tucson. The first motion data are basically consistent with either a thrust or strike-slip faulting mechanism, but neither of the nodal planes are constrained. The moment misfit grid search was performed using four P records (De Bilt--radial, Uccle--radial, Tashkent, and Christchurch), two SH records (De Bilt and Uccle), three PP records (De Bilt, Uccle and Christchurch) and surface wave records from De Bilt, Uccle, Christchurch, and Riverview (unfortunately, surface waves at Tashkent were off scale). Fig. 2 (lower) shows slices through the model space at dips of 25 ° , 45 ° , and 70 ° together with contours of minimum moment misfit, which generally favor a thrust mechanism. The one possible strike-slip mechanism indicated, strike 330 ° , dip 25 ° , slip 170 °, can be ruled out because it violates compres-

sional P-wave polarities at Christchurch and Riverview. Subsequent trials with smaller increments in strike, slip, and dip found two local minima in the moment misfit. The solution with lowest moment variance, strike 110 °, dip 50 °, slip 60 °, (Fig. 2, top left) has a moment variance approximately 20% lower than the other solution (Fig. 2, top right). The contours in the moment variance plots (Fig. 2, bottom), denoting levels of moment variance 30% and 50% greater than that of the preferred solution, provide good estimates of the uncertainties in this mechanism. These contours indicate that a number of different mechanisms are permitted, but all show predominently thrust faulting (except the one strike-slip solution which violates the polarities at Riverview and Christchurch). The key constraint in obtaining this result is the high amplitude of the P waves relative to SH, Love and Rayleigh waves at all the stations. These high amplitudes produce large moment misfits for any mechanism which is largely strike-slip. The fault strike is not well constrained, as strikes between about 70 ° and 140 ° provide good fits to the amplitude data. The strike and dip of the fault planes are constrained by the Love and Rayleigh amplitudes and by the relatively low amplitude of the SH waveforms at De Bilt and Uccle. The PP waves, which are very prominent at the European stations but small or missing at Christchurch and Riverview, also provide useful constraints. Fig. 3 shows the body waveform modeling resuits for the 1944 event, which provide a good fit to the data for a depth of 10 ( + 5 ) km. The waveforms are also quite sensitive to time function duration; the synthetics shown were produced for a duration of 12 seconds. The waveforms from De Bilt and Uccle favor a somewhat longer time function than the waveforms at Tashkent and Christchurch, perhaps suggesting southeastward rupture propagation. The synthetic waveforms were calculated for the focal mechanism with the lowest moment variance (Fig. 2, top left); synthetics calculated for the alternative mechanism (Fig. 2, top right) actually provide a marginally better fit to the waveforms (particularly the PP waveforms). Although there are numerous uncertainties involved in calculating stress drop and rupture length (resulting from assumptions about rupture mechanics and, in this case, from the poor distribution of recording stations), this duration sug-

355

OCT

BODY W A V E MODELING - FEB 2 9 , 1 9 4 4 S WAVES UCCLE

MOMENT

PP WAVES

DE mLT

VARIANCE

SLIP 180

90 0 DATA 10

HILBERT TRANSFORMED DATA

22 DEPTH

SYNTHETIC

S,=

~

DIP 3 5

90

180 STRIKE

P WAVES OE StLT RADIAL

2, 1957 27o ~

UCCLERADIAL

DE BILT V•RTICAL

27C STRIKE

35

DIP

90.

~IP

~81).

3E

22 DEPTH

SLP 180

90

DIP 8 5 DIP 6 0

90

180

180

S'mIKE 220EPTH 270

270

!'""'~

!Z~i.... ~ ........

90

TASHKENT

SLIP 180

90 0

a'l

CHRISTCHURCH UCCLEVERTICAL

270

0

STRIKE gI:SSI]

~iiiii~ ~"""i ".......

270.

a4

STRIKE

115

DIP 50

SLIP 60

Fig. 3. Body wave modeling results for the February 29, 1944 event. The P and SH waveforms are well modeled at a depth of 10 km and a time function duration of 12 seconds for a variety of stations and instruments. Hilbert transformed PP waves are also modeled.

gests a fault length of approximately 4 0 - 8 0 km. The median seismic moment obtained from the combined body wave and surface wave analysis is 1.4 × 10 27 dyne-cm for the preferred mechanism, and 9 × 1 0 26 dyne-cm for the alternative mechanism. This large moment suggests that if this event is truly intraplate it is one of the largest oceanic intraplate earthquakes known. Fig. 4 shows the mechanism analysis for the October 2, 1957 event. Because of the earthquake's size ( M s = 5.7), only a small number of good records are available for this event. The moment variance analysis was performed using SH, Love, and Rayleigh wave data from Matsushiro and P, Love and Rayleigh wave data from Tashkent. Although the P wave at Matsushiro and the SH wave at Tashkent were not observed, the amplitude of the noise in the data at these stations provided upper limits on the wave amplitudes. The moment variance analysis (Fig. 4) strongly indicates strikeslip faulting along nearly vertical northwest and

360

360

MOMENT

1.4 x 1025dyne-era

Fig. 4. Mechanism determination for the October 2, 1957 event. Plots show moment variance contours of 75% (solid line) and 125% (dotted line) above the minimum value. Subsequent analyses with finer grids determined the strike-slip mechanism shown.

northeast striking nodal planes. Moment variance analysis using a finer grid size than shown in Fig. 4 produces a variance minimum for the mechanism shown. The most important constraints on this mechanism were the pronounced S V / S H

BODY WAVE MODELING - OCT MA TSUSHIRO - SH

2, 1957

TASHKENT - P

I

DATA

4

14

DEPTH

24 50 SECONDS

50

Fig. 5. Body wave modeling results for the October 2, 1957 event. The waveforms are best modeled at a depth of 14 km and a time function duration of 3.5 seconds.

356

polarizations at Soviet stations and the opposite polarization at Matsushiro, the P, SH, and SV first motions, the Love to Rayleigh ratios at Matsushiro and Tashkent, and the low amplitude of the Tashkent P wave relative to the surface wave amplitudes. Modeling of the Tashkent P and Matsushiro SH waveforms shows excellent agreement with the data for a depth of 14 ( + 5) km and a time function duration of 3.5 seconds (Fig. 5). 5. Discussion

Fig. 6 shows the results of this study of pre-1964 seisrnicity, together with the locations and representative focal mechanisms for 1964-1983 seismicity. These results show that the Chagos-Laccadive region is the site of recurrent seismicity. The 1912 M 6.8 event in the vicinity of Chagos Bank indicates that the recent high seismicity rate is not an artifact of the short time period of observation; Chagos Bank has an unusually high rate of seismicity even when a much longer time period is considered. The 1957 M 2 5.7 and 1930 M 5.6 events and recent moderate sized (M~= 5-6) events indicate frequent lower magnitude intraCHAGOS SEISMICITY 1912-1983 '

I

m~,~

tt

¢"

CHAGOSLACGADI b VEI, RIDGE

:r-A.

..0r,d,_.00__

•

i 0~ N

........

S BANK

..... ; o o \

- . . . .

-.-

.......

- I l l

7~' E

8b E

le-1912-1963 SEISMICITY •

1964-1983 SEISMICJTY

Fig. 6. Off-ridge seismicity of the Chagos region together with 500- and 3000-m bathymetric contours [20,21]. Stars denote pre-WWSSN events relocated in this study. Representative mechanisms from previous studies [1,5] suggest regional northsouth tensional stress, whereas the 1944 mechanism found in this study shows thrust faulting.

plate events in the region west and north of Chagos Bank (Fig. 6). The April 25, 1970 ( M S= 5.1) event occurred at the epicenter of the 1957 event, suggesting that it is a recurrence of the larger 1957 earthquake. The mechanisms of the events are similar and the depths are identical to within the resolution of the modeling technique [1], but the moment of the 1957 event is about four times larger. The pattern of faulting north and west of Chagos Bank may represent repeated reactivation of weak zones (presumably fracture zones) by a regional stress regime characterized by north-south extension and approximately equal values of horizontal (east-west) and vertical compression, resulting in variable components of strike slip and normal faulting. The pre-WWSSN results show substantial seismicity northeast of Chagos Bank where there is little recent seismicity. The 1944 M 7.2 and 1935 M 6.0 events indicate that the northern parts of the Chagos-Laccadive Ridge are not aseismic, as previously assumed[5], and the 1951 M s 6.0 event indicates seismicity in the Central Indian Basin east of Chagos Bank. The mechanism of the 1944 event (Figs. 2,:6) is particularly interesting because it shows thrust faulting with a NE-SW or N-S compressional axes orientation, whereas previously studied events in the Chagos region are consistent with N-S tensional stress (Fig. 6). One possible explanation for'the focal mechanism of the 1944 event is that it reflects stresses unrelated to those found at Chagos Bank, such as ridge push stresses or bending stresses associated with the ChagosLaccadive Ridge. The orientation of the compressional axis of the 1944 event is approximately consistent with that expected from ridge push stresses. However, ridge push stresses are a simple function of the age of the oceanic lithosphere [22-24] and these stresses are not known to cause earthquakes of this magnitude ( M = 7.2, moment 1.4 × 1027 dyne-cm) elsewhere. It also seems unlikely that the event reflects bending stresses due to the Chagos-Laccadive Ridge; small gravity anomalies indicate the ridge is mostly compensated at depth [25,26] and the strike of the fault planes is oblique to the orientation of the ridge. The most likely interpretation is that the. 1944 event is related to the same deformational process causing the other Chagos earthquakes. Outside of

357

the equatorial Indian Ocean magnitude 7 oceanic intraplate earthquakes are completely unknown, as the only known magnitude 7 earthquakes not directly associated with active plate boundaries are found along passive continental margins [27] or sites of active volcanism such as Hawaii. This suggests that magnitude 7 oceanic intraplate earthquakes are very rare occurrences; the fact that the 1944 (M s =7.2) and 1983 (M~ = 7.6) Chagos-Laccadive events are found within the same region suggests they are part of the same deformational process. Thus, the 1944 event and several smaller events indicate that the intraplate deformation in the Chagos region extends over a larger area and involves a more complicated pattern of stresses than was previously thought [5]. The level of seismicity found near Chagos in both the historical and more recent time periods is unusual for intraplate regions, and is difficult to explain in terms of processes operating on normal oceanic lithosphere, such as thermoelastic stress. The 1944 earthquake, as it occurs in older lithosphere where thermoelastic stress should be reduced, supports this observation. Thus, it appears the Chagos seismicity represents an anomalous level of tectonic stress in the equatorial region of the Indian Ocean which involves both compressional and tensional stress fields. 6. Implications for Indian Ocean tectonics In view of the unusual intensity of the Chagos Bank seismicity, it is important to ask whether a relationship exists between the Chagos-Laccadive Ridge seismicity and intraplate deformation previously noted farther east in the equatorial Indian Ocean [13,28-35]. Fig. 7 shows seismicity of the Indian Ocean located off the conventional plate boundaries; the concentration of magnitude seven "intraplate" earthquakes in the equatorial region is particularly impressive. Using the previous argument on the rarity of large intraplate earthquakes, the fact that the Chagos-Laccadive and Ninetyeast Ridge seismicity zones are located in adjacent areas of the same plate is strong evidence that they are related. Several additional arguments also favor such a connection. First, the seismic zones seem nearly continuous, connecting south of Sri Lanka. This is suggested by Fig. 7; such a conclusion is

INTRAPLATE SEISMICITYOF THE INDIAN OCEAN

-O'N

-20" S

t

40" S

6()" E

80" E }i~ M ) 6,9 • M>5.4

100" E

Fig. 7. Seisrnicity of the Indian Ocean 1910-1983 located off the conventionally accepted plate boundaries. Note the concentration of seismicity (particularly magnitude 7 events) in the equatorial region.

further strengthened by the presence of several small intraplate earthquakes near 80°E which have magnitude less than 5.5 and are not shown in Fig. 7. Additionally, the crustal deformation studied in the Central Indian Basin using marine seismic profiling [31,32] extends as far west as 79°E, only a few hundred kilometers from the location of the 1944 earthquake. (This western limit on the region of deformed sediments may simply reflect the western edge of Bengal Fan deposits--C.A. Stein, personal communication, 1985). Finally, the February 29, 1944 thrust faulting mechanism can be considered the westward continuation of the north-south compressional stress field found in the Central Indian Basin [30-34]. Thus, the 1944 earthquake, along with the magnitude and distribution of seismicity and the marine geophysical data are consistent with a single large region of intraplate deformation in the equatorial Indian Ocean stretching from the Central Indian Ridge to the Sumatra Trench (Fig. 8). What sort of plate tectonic process does this unique zone of deformation indicate? Stein and Okal [30] note that the level of left-lateral strike-slip seismicity rate along the Ninetyeast Ridge is corn-

358

Fig. 8. Schematic diagram of the Indian Ocean deformation. This diffuse plate boundary extends east from the Central Indian Ridge to the Ninetyeast Ridge, then north to the Sumatra Trench and includes zones of extensional,convergent, and strike-slipmotion. A recent plate motion study by Wiens et al. [39] suggests this boundary separates the Australian plate from a single Indo-Arabian plate.

parable to that along many Conventional plate boundaries and suggest the earthquakes reflect motion between the western and eastern parts of the Indian plate caused by the resistance encountered by the western segment at the Himalayan boundary. Studies of current plate motions [36,37] indicate that splitting the Indian plate approximately along the Ninetyeast Ridge improves the fit to the plate motion data, and that the new boundary is statistically justified. However, substantial seismicity along the Ninetyeast Ridge does not extend south of 10°S. Thus the earthquake data do not support a boundary extending south along the Ninetyeast Ridge to the Southeast Indian Ridge. Instead, the results of this study indicate that such a plate boundary would trend westward from the Ninetyeast Ridge to the Central Indian Ridge (Fig. 8). Thus the magnitude and unique nature of the Indian Ocean deformation suggests it may represent a plate boundary extending from the Central Indian Ridge to the Sumatra trench along which small amounts of relative motion between southeastern and northwestern parts of the conventionally defined Indian plate are taken up [38]. The distribution of seismicity and deformation and the

absence of morphologic expression indicate this unusual boundary is slow and diffuse over much of its length. The earthquake mechanisms indicate left lateral strike-slip motion along the northern Ninetyeast Ridge, north-south convergence in the Central Indian Basin, and a complex pattern of motions, including both convergence and northsouth extension near Chagos Bank. These motions suggest a counter-clockwise rotation of the southern part of the Indian plate relative to the northern part, with the pole of rotation located near Chagos Bank. This interpretation of Indian Ocean tectonics based on study of historical seismicity near Chagos Bank can be tested using an independent dataset - - p l a t e motion data (spreading rates, transform azimuths, and earthquake slip vectors) from along the Indian Ocean ridge system. A recent study of current Indian Ocean plate motions by Wiens et al. [39], conducted since this paper was first submitted, found the proposed boundary can be resolved by the plate motion data. Surprisingly, they also found that the Central Indian boundary is more significant than the presently accepted plate boundary between Arabia and India along the Owen Fracture Zone; given the Central Indian boundary the plate motion data cannot resolve any motion along the Owen fracture zone. Thus the diffuse boundary described in this paper apparently separates the Australian plate from a single Indo-Arabian plate (Fig. 8). The pole of rotation found by the plate motion inversion is located near Chagos Bank, as first suggested in this study on the basis of earthquake focal mechanisms. The proximity of this pole to Chagos Bank helps explain the complex pattern of deformation found in this region. Further implications of this new boundary are discussed in Wiens et al. [39].

Acknowledgements I thank Seth Stein and Carol Stein for interesting discussions of Indian Ocean tectonics, Rodey Batiza and Joe Engeln for comments on an earlier draft, Emile Okal and several anonymous reviewers for helpful reviews, and Steven Bratt, Eric Bergman, Christopher Lynnes, David McAdoo, Larry Ruff, David Sandwell, and Scan Solomon for copies of their papers prior to publication. I also thank Emile Okal for helpful advice about

359 obtaining historical seismograms, and Kristine H e n r i c k for h e l p w i t h m a n u s c r i p t p r e p a r a t i o n . I a m e s p e c i a l l y g r a t e f u l to n u m e r o u s s e i s m i c o b s e r v a t o r i e s a r o u n d the w o r l d for p r o v i d i n g s e i s m o g r a m s for this study, a n d also to the U . S . G e o l o g i cal S u r v e y (in p a r t i c u l a r W i l l i e L e e a n d J a m e s N e w b e r r y ) for access to t h e h i s t o r i c a l s e i s m o g r a m c o l l e c t i o n at M e n l o Park. T h i s r e s e a r c h w a s b e g u n w h i l e the a u t h o r w a s at N o r t h w e s t e r n U n i v e r s i t y . S u p p o r t w a s p r o v i d e d b y N S F g r a n t E A R 8206381 and NASA Crustal Dynamics Contract NAS527238 at N o r t h w e s t e r n U n i v e r s i t y a n d b y r e s e a r c h initiation f u n d s at W a s h i n g t o n University. A c k n o w l e d g e m e n t is m a d e to t h e D o n o r s o f T h e Petroleum Research Fund, administered by the A m e r i c a n C h e m i c a l Society, for p a r t i a l s u p p o r t o f this research.

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