Earthquakes associated with diffuse zones of deformation in the oceanic lithosphere: some examples

Tectonophysics,

133

166 (1989) 133-150

Elsevier Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

Earthquakes associated with diffuse zones of deformation in the oceanic lithosphere: some examples WANG-PING Department

CHEN and NINA L. GRIMISON

of Geology, University of Illinois, Urbana, IL 61801 (U.S.A.)

(Received

February

8,198s;

accepted

May 17,1988)

Abstract Chen,

W.-P.

and

lithosphere:

Grimison,

N.L.,

some examples.

Source Parameters.

1989. Earthquakes

In: D. Denham

Tectonophysics,

nature

plate boundaries, scattered regions nature

tectonics

Earthquakes associated continue

into

the continental

with the eastern

of global

characterized

tectonic

in the oceanic

lithosphere

east

of Gibraltar

and

zone) where the predominant

basin. The maximum Azores-Gibraltar earthquakes.

focal depth

plate

boundary

seems

Nubian-Somalian In regions

to be part

where a mixture

expressions

rate of displacement

is not truly intraplate

in bands

of scattered

seismicity

In two cases, the zones of diffuse

north

and

across these in

can be identified.

of the Davie

than

is quite shallow

ridge.

that are

deformation

However,

the largest

(< 15 km) near the Horizon

mode of deformation

Pacific because

nascent

plate boundary

reaches

about

50 km in the zone of ocean-ocean

where

most

of the mechanical

deformation

beneath

is apparently

of a wide zone of diffuse

is strike-slip

the rate of seismic strain

that of a proposed

lithosphere

collision must

unrelated

marks

north

(also

This zone

of the north

on the eastern broken

Fiji

end of the during

large

age at the zone of extension

to the past motion which

bank

motion.

release along it in recent

have been

ocean floors of Mesozoic

extension

of focal mechanisms

occur, the consistent

of that of the nodal planes or slip vectors.

in the oceanic

than by displacement

lithosphere

on discrete

along the Davie ridge and

the southern

termination

of the

parameter

is usually

Thus, as in the continental

seem to be better described

by a regional

the orientation

lithosphere,

compression

of the P-

zones of diffuse

or extension

field rather

faults.

Introduction Our present knowledge of global tectonics can confidently resolve plate motions on the order of 10 mm/a (e.g., DeMets et al., 1985; Minster and 0040-1951/89/%03.50

morphological

the

with typical

plate boundary.

or the T-axes, instead deformation

higher

of up to 35 km were observed

along the Davie ridge. The current this region

of earthquakes

in the southwestern

of magnitude

Focal depths

of

part of these two zones. the depth

feature

to characterize

In comparison

the deformation

boundaries

lithosphere.

fracture

tectonic

by complex

as large as 8 x 10 *’ Nm have occurred

reports,

orders

in the oceanic

the Determination

Fiji basin

lithosphere.

the average

plate reconstruction,

to previous

is an important

and

end of the Azores-Gibraltar

in the north

in the oceanic

known as the Hazel-Holme years is several

bank

width. Although

and often distinct

zones of deformation

occur in the oceanic

In contrast

are in general kilometers

models

with seismic moments

with diffuse

earthquakes

by current

strain is concentrated

of deformation

associated

and the Horizon

zones of deformation

of up to several hundred

is not resolvable because

in diffuse

diffuse zones of deformation

seismicity

zones

of Earthquakes

of earthquakes

the Davie ridge near Madagascar,

of present-day

with diffuse

Quantification

166: 133-150.

We review the results of source parameters plate boundary,

associated

(Editor),

Q 1989 Elsevier Science. Publishers

B.V.

Jordan, 1978). Although this resolution adequately accounts for displacement across most major plate boundaries, diffuse deformation occurring at less than 10 mm/a over a large region is difficult to constrain from plate kinematics.

This situation is

134

W.-P. (‘HEN

common

for

the

continental

Chase,

1978;

Molnar

and Tapponnier,

sumption

Jackson

lithosphere,

known

to take place

over relatively

1984;

1975). Although

the as-

ocean

McAdoo

slow

deformation

regions;

(e.g., Bergman

and Sandwell, boundary

In this paper,

is

the source

also

convergence

and Solomon,

1985;

Azores-Gibraltar

1985; Weissel et al., 1980;

between

upon

in the

(e.g., Ball and

Bergman,

1986; Chase, 1978; McCann

the Davie

near the

the North

plates

at rates

zon bank

1970;

Though

and Sykes,

the nature

along

the

plate

of slow deformation eastern

of

the

extension

region, faulting

in the interior the first region

end

boundary,

strike-slip

we shall

by

the zone of ocean-ocean

ridge-Madagascar

boundary,

placed

of large to moderate-sized

regions:

gion of largely

and South

Harrison,

we review constraints

in three oceanic

for instance,

American

of the lithosphere

parameters

earthquakes

lithosphere

Wiens et al., 1985) and in the Caribbean suspected

large scale deformation

N.L. GRIMISON

of millimeters/year.

works well for the

in the oceanic

broad

(e.g.,

McKenzie,

of rigid plates usually

oceanic

Indian

lithosphere

and

AND

and the re-

near

of the north lies along

the HoriFiji basin.

a major

demonstrate

near

that

plate

there

are

1984; Stein et al., 1982).

several

In both oceanic and continental lithospheres, deformation on the order of several millimeters/ year is sufficient to generate a high level of

all three regions which characterize them as diffuse zones of deformation in the oceanic lithosphere. These features include: scattered seismicity and bathymetric features indicating that de-

common

tectonic

features

associated

with

seismicity, including some very large earthquakes (e.g., Chen and Molnar, 1977; Fukao, 1973; Lee et al., 1978; Nabelek et al., 1987; Stein and Okal,

formation spreads over a broad region, a mixture of focal mechanisms where either the nodal planes

1978). Consequently, diffuse zones of slow lithospheric deformation present a potential seismic hazard to which plate tectonics is not directly

cannot delineate the plate interface and/or the slip vectors are not parallel to large-scale plate motions, and a consistent orientation of the P-or

applicable. In addition, the source mechanisms of earthquakes associated with such zones are an important source of data concerning the nature of

T-axes implying regional compression or extension. This review is not intended to give a compre-

TABLE Summary

1 of source parameters

Date

Lat.

Long.

Strike

Dip

Rake

Depth

Seismic

(ON)

(“E)

(“)

(“)

(“)

(km)

moment

: s)

(second

(NW 36.01

subevent

- 10.57

‘)

b. Oct. 17, 1983

19:36:28

31.75

- 17.25

16:34:k4

39.48

- 14.44

3. May 26,1975a

09:11:52

35.98

- 17.56

4. May 26,1975b

20:19:33

36.04

5. Dec. 30, 1970

20 : 57 : 32

37.22

6. Sept. 6. 1969

14:30:43

7. May 5.1969

05:34:24 22130~26 (second

12

X2*

8

113+ -12-t

8

32 + 10

3.0 * 0.5 x 102O

6

22 + 10

4.5 + 1 x 1ozo 6.5 * 0.6 x 10’”

8Oi

5

180+

6

14+

5

80*

6

-6+

3

34*

5

290 f

5

68+

2

180+

4

--

- 17.56

58+

5

64+

4

- 148 + 10

35f

5

wlh = 5.5

- 14.93

40?

6

70+

5

25+

8

25f

5

mb = 5.0

36.96

-11.84

86*

5

86*

4

16Ok

5

41i

4

1.2* 0.3 x lo’*

35.99

- 10.34

108 k 10

8Ok

6

130 + 20

50+

5

mh = 5.5

36.23

- 7.61

5

14+

5

8.6 + 2 x 10”

122 + 20

20*

5

7.0 * 2 x 10’8

subevent

2, and Chen,

are below sea floor. Epicentral

272 + 8

44 f 10

8

Events a, b, and c (Grimison

1986). Focal depths

70+

14 f 10

c. Jan. 24,1983

Source of data:

plate boundary

Origin

02:40:33

8. Mar. 15, 1964

end of the Azores-Gibraltar

Time (h : m a. Feb. 28,1969

of events along the eastern

-lg.+

50*

3

64+

2

23k

5

64+

2

1988a);

information

No. 3 (Lynnes is that reported

74+

and Ruff,

1985); all others

by the International

(Grimison

Seismological

’ This subevent is estimated

to be 40 f 10 km and N40 o E + 30 o from the first with a time delay of about

’ This subevent

to be 12 + 5 km and N23O W + 20 o from the first with a time delay of about

is estimated

1.3 i 0.2 x 10’” 7.0 x lozO

15 s. 7 s.

and Chen,

Center

(ISC)

EARTHQUAKES

TABLE

IN OCEANIC

135

LITHOSPHERE

2

Summary

of source parameters

of events near the Davie ridge Lat.

Origin

Date

time

(“N)

(h : m

Long.

Strike

Dip

Rake

Depth

Seismic

(“E)

(“)

(“)

(“)

(km)

moment

70

- 125)

15 f 5

7.1 x 10”

40*5

9.5 f 0.3 x 10”

: s)

Pm)

1. Oct. 14,1967

23:29:31

- 3.32

38.19

(142

2. May 14,1985a

13:24:58

- 10.59

41.37

-lo*

(second

subevent

18:11:09

3. May 14,1985b

‘)

(Second

subevent

41.43

5

45*

5

-9o*

5

17+5

1.2 * 0.1 x 10’8

45f

5

-9O+

5

40*5

2.0 f 0.3 x lo’s

45+

5

-9o*

5

18 f 5

2.5 f 0.1 x lo’*

5

-10*10

-lo*

*)

-9o*

45*

-10+10

- 10.49

5

5

4. Feb. 15,1975

06:16:26

- 16.47

41.45

-10*45

20*

15

-75+30

25 f 5

1.5 f 0.3 x 10”

5. May 18,1965

01:04:17

- 17.60

49.91

-lo*45

50 f 20

-95+30

15+5

1.5 + 0.5 x 10”

6. April 4,1975

17:41:16

- 21.24

45.13

43 f 15

13*5

3.0 * 0.3 x 10”

and moment

for event No. 1 are taken from

Source parameters

are those of Grimison

Shudofsky

(1985). Focal

Epicentral

information

depth

75 f 10

and Chen (1988b) except the focal mechanism

is below sea surface.

Water

depth

is 2.5 km above

events

No. 2, 3, and 4; 1.5 km above event 5.

taken from the ISC for events prior to 1985; that of events in 1985 taken from

Epicenrers of the U.S. Geological

’ This subevent is estimated ’ The relative position

95 f 15

Preliminaty

Determination

of

Survey (USGS)

to have the same epicenter

and timing between

subevents

as the first with a time delay of about

14 s.

are the same as those for event No. 2.

hensive account of earthquakes associated with all oceanic regions where diffuse zones of deformation are suspected. Instead, we rely heavily on

neously inverts both P and SH wave data by minimizing the difference between observed and synthetic seismograms in a least squares sense

detailed studies of three such regions where large earthquakes have been recorded by standardized

(Nabelek, sequence

global seismic networks. Most of the source parameters reviewed here (Tables 1, 2 and 3) are obtained by inverting the waveform and amplitude of teleseismic P and SH waves recorded by the long-period instruments of the World-Wide Standardized Seismograph Network (WWSSN).

represented by a centroidal solution (point source) and each point source is constrained to be a pure double-couple. For each subevent, the unknown source parameters inverted for are the focal mech-

The

inversion

TABLE Summary

technique

that

we used

1984). The source is parameterized by a of subevents where each subevent is

anism (strike, dip, rake), depth, scalar seismic moment, relative amplitudes of the triangular segments comprising the far-field source time func-

simulta-

3 of source parameters

Date

of events near the Horizon

bank

Origin

Lat.

Long.

Strike

Dip

Rake

Depth

time

(ON)

( o E)

(“)

(“)

(“)

(km)

(h : m

: s)

Seismic

PW

1. Oct. 3,197l

13:24:38

- 14.60

171.69

8

85 f 13

12 + 5

6.5 * 1.2

2. Aug. 22,1976

21:09:42

- 14.03

170.95

284 f 10

57 f 11

-27+13

12*3

1.2 f 0.2 x 10’8

-62&25

9*3 15 f 4

9

4.5 + 0.8

x

10”

8*4 13 f 5 28

1.6 + 0.2

x

1018

16k

182 f 12

3. May 23,1983

06:54:38

- 13.82

171.29

265 + 20

63*13

4. Nov. 21,1984a

14:33:22

- 14.49

171.14

112*

8

95 f 10

6*13

5. Nov. 21,1984b

18 : 17 : 53

- 14.54

171.07

110*

9

86 f 23

2 f 20

6. Nov. 23,1984

04:46:10

- 14.36

171.41

126 it 13

82 f 12

16 + 15

25. Dec. 25,1985

02:35:51

- 13.91

169.91

282

67

All source parameters

taken

from Yu and Chen (in prep.,

below sea surface with an average

1988) except

water depth of 3 km. Epicentral

the event in 1985 taken from Preliminary

Defermination

the last event (Dziewonski

information

of Epicenters

-7

x

10”

*1x10”

9.3 + 1.3 x 10’8 5.5 x 10”

et al., 1986b).

Focal

depths

are

taken from the I.S.C. for events prior to 1985; that of

of the U.S.G.S.

136

W.-P.

tion, and location

and time delay of the centroid

of the particular

subevent

relative

gent, divergent,

To obtain

realistic

estimates

the best-fitting

solution

of the uncertain-

we perturbed

tematically

searched

it by forward increase 55lo%,

that

or by visually

some

sys-

squared

range

of

the observed

detected

source

parame-

characteristics

The Jordan,

by the rms

of diffuse

in the oceanic

near

1. A

sketch

1986),

where

Gibraltar.

map

illustrating

southwest

a synthesis

The stippled

100 km to a maximum

convergence

along the east-

plate boundary

slow (- 2-3 mm/a; 1978) rate of relative

e.g., Minster and plate motion along

1931; six had a magnitude greater than 7 (Chung and Kanamori, 1976; Fukao, 1973; Hadley and Kanamori, 1975: Udias et al., 1976). Moreover, the sense of plate motion along this boundary is rather unique, changing from near the Azores to NW-SE

of the

present-day

deformation

is taking place

area indicates

width of about

five large (M > 6) historical

$0

the diffuse

along

near the Azores

(at 25.23O N, -21.19OE)

the

at the RM2 pole (Minster was drawn by passing

Azores-Gibraltar

Islands

zone of ocean-ocean

km near the continental

events since 1931 (solid circles) are plotted.

the trace of a small circle centered and Africa

the

extension

of

lithosphere.

SW-NE extension convergence near

IBERIA

Fig.

zones

the western boundary between the Eurasian and African plates has generated a band of unusually high seismic activity extending from the Azores Islands to Gibraltar (Fig. 1). Ten events with M > 6 have occurred along this boundary since

(Grimison and Chen, 1986, 1988a, 1988b; Yu et al., 1987; also Yu and Chen, in prep., 1988). In

Chen,

to constrain

and

in the good-

The details of data analysis and discussion for each geographic region are published elsewhere

only

settings,

ought

ern end of the Azores-Gibraltar

moderate-sized events, estimates of the source parameters are precise enough for tectonic interpretations on a regional scale.

we present

tectonic

from reliable

in this report

Zone of ocean-ocean

error but were judged to be important. Given the wide coverage of stations and the pass-band of the instruments which is favorable for the analysis of

this paper,

made

general

deformation

(rms) error of

when changes

ness of fit were not readily

N.L. GRIMISON

space around

by either a notable

comparing

waveforms

and transcurrent

ters summarized

Sub-

and

The allowable

is determined

in the root mean

synthesized

inversion.

solution

the parameter

modeling.

each parameter

the synthesis

we first established

by formal

sequently,

AND

common features observed in those regions. Since the three regions under review encompass conver-

to that of the

first subevent. ties for the source parameters,

CHEN

margin.

convergence

1978)

the circle through

Gloria fault was measured.

boundary

and a regional.NNW-SSE

Epicenters

Bathymetric

and Jordan,

plate

40

which

increases

of 15 recent

contours

(Grimison

compression in width

events (m,

from

plate motion

the point where the direction

about

> 5) and those of

are 3000 m (Searle et al., 1982).

of the instantaneous

and occurs

between

of the tangent

Part of Eurasia of the

EARTHQUAKES

IN OCEANIC

137

LITHOSPHERE

12.30.70

1.24.83

9.6.69

2.28.69

5.5.69

3.15.64

Fig. 2. Source mechanisms of earthquakes and a detailed bathymetric map of the region of convergence at the eastern end of the Azores-Gibraltar

plate boundary (Grimison and Chen, 1988a). Fault plane solutions for the events studied by Grimison and

Chen (1986, 1988a; Table l), together with that of the event of Nov. 25, 1941 (DiFilipo, 1949; Udias et al., 1976) are plotted in equal area projections of the lower hemisphere of the focal spheres

with the darkened quadrants showing compressional first

motions of P-waves. A mixture of strike-slip and thrust faulting characterizes the area. Bathymetric contours are in loo0 m intervals (Searle et al., 1982).

Gibraltar (Fig. 1; McKenzie, 1972; Minster and Jordan, 1978). Plate divergence at the western end of the boundary is separated from a broad zone of convergence to the east by a presently aseismic segment, the Gloria fault. Ocean-ocean convergence at the eastern end of this plate boundary is occurring at a rate of several millimeters/year (Minster and Jordan, 1978) with no clear evidence of a Benioff zone (e.g., Purdy, 1975). A simple clear plate boundary does not seem to be present in the area. Instead, this zone is characterized by scattered seismicity, complex bathymetry, and

large positive gravity and geoid anomalies (e.g., Souriau, 1984; Karner et al., 1985) (Fig. 2). To date, the source parameters for all moderate to large earthquakes along the eastern end of the Azores-Gibraltar plate boundary, since records from standardized global seismic networks became available, have been studied using teleseismic body waves (Grim&on and Chen, 1986, 1988a; Lynnes and Ruff, 1985). These results are summarized in Fig. 2 and Table 1. The scattered seismicity generally overlaps with a broad zone of complex bathymetry which increases in width from about 100 km to a maxi-

138

W.-P

mum width of about

CHI-IN

AND

N L. GRIMISON

300 km near the continental

margin (Fig. 2). Two moderate-sized,

recent events

(Jan. 24 and Oct. 17, 1983) and the largest instrumentally

recorded

earthquake

(Nov.

25, 1941) in

this region took place near the northern

margin

this zone. The depth of the large historical unknown

while one of the recent

curred

at a depth

Grim&on

earthquakes

35 km

oc-

(Table

1;

and Chen, 1988a).

The southern marked

of about

of

event is

part

of the convergence

by more frequent

large-gravity al., 1985;

seismicity

and geoid anomalies Souriau,

1984).

The

zone is

(Fig. 2) and

(e.g., Karner maximum

et

focal

depths

are again quite large. While the centroidal

depth km, a depth seems

of the large earthquake in 1969 is about 30 large aftershock of this event is found at a of about 50 km (Table 1). Therefore, it likely that rupture zones during the largest

earthquakes extend downward through most of the mechanical lithosphere on both sides of this zone of deformation.

Fig. 3. Equal-area

projection

focal sphere showing of the fault plane

of the lower hemisphere

the consistent

solutions

NNW

summarized

of the

trend of the P-axes in Table 1 (Grirnison

and Chen, 1986). Event No. 4 (Table 1) which is an aftershock of No. 3 is excluded. point

source,

centroidal

For events consisting solutions

bered according

of more than one

are used. Events

are num-

to Table 1.

The focal mechanisms of all the events in the Atlantic to the east of 20” W (Fig. 2) indicate a combination of strike-slip and thrust faulting without an apparent pattern to them. The slip vectors do not show any consistency either. In fact, Minster and Jordan (1978) noted that the slip vectors show significant deviations from the sense of plate motion predicted by the RM2 pole for Eurasia and Africa. On the other hand, the orientation of the P-axes is a remarkably consistent parameter (Fig. 3) indicating a regional NNW compression. Since none of these observations simple simple

convergent conceptual

is typical

of a

plate boundary, we propose a model for this diffuse plate

boundary (Fig. 4a). To take into account the large geoid and gravity anomalies as well as the occasionally large depths of earthquakes, a standard flexure model of an elastic thin plate is modified to include at least two mechanical discontinuities, one on each edge of the ocean-ocean convergence zone (Grimison, 1987; Karner et al., 1985). In principle, shear stress along the discontinuities and horizontal compression can be included in the governing equations of plate flexure. Unfortunately there are no reliable data on the magnitudes of these stresses. Nonetheless, our ignorance

of these stresses does not change the basic physics of this model: part of the load (i.e., bathymetry resulted from crustal thickening in response to horizontal compression) is regionally supported by the flexure of semi-infinite plates while the rest is taken up locally by the finite plate whose flexure is small due to its short length. Figure 4b shows a comparison of the observed (SEASAT) and theoretical geoid anomalies based on the model in Fig. 4a over the Gorringe Seamount. The observed anomalies are quite precise because the profile is approximately parallel to the tracks of the SEASAT near the Gorringe Seamount. The discontinuity at about 200 km north of the Gorringe Seamount appears to be required by the geoid data while that immediately south of the Gorringe is implied by the depth of large earthquakes (Table 1). Although the model proposed here offers a simple explanation of the gravity field (Grimison, 1987; Karner et al., 1985) and it appears to be a sensible first approximation of the present-day tectonics in the area reflected by the earthquake data, the local isostatic models proposed by Purdy (1975) and Souriau (1984)

EARTHQUAKES

IN OCEANIC

139

LITHOSPHERE

Conceptual

a

Model

1

Geoid Anomalies

me,erf ,,,-0 / Tore Model

Bathymetryk!] ._ Fig. 4. a. A conceptual two semi-infinite (bathymetry)

(regions

is regionally

1 and

compensated

finite plate. The mechanical

3). This

cross

represented

section

are inferred

by a plate of finite length (region

is taken

by flexure of the semi-infinite

discontinuities

the crust may or may not have the same density.

the Gorringe approximately deflections

of the observed

Seamount.

perpendicular

of the Moho. The theoretical

the same density (2.8 Mg/m3) density elastic

to the trend

3.3 Mg/m3. plate thickness

of the Gorringe

geoid anomalies

as the underlying

of the equivalent

(dashed

The load (shaded),

elastic plate,

50 km assuming

a Young’s

are calculated

Redrawn

The solid

between

of the load of the

in-fill (stippled),

T, the crustal

thickness.

and Taken

based on the model in Figure 4a over below

to the tracks of the SEASAT the bathymetry

that the bathymetric

show

and

calculated

highs and the in-fill have

away from the load is taken to be 10 km over a mantle of

is found to be about

1031 f 10z9 dyn cm (1O24 * 1022Nm).

of 1Or2 dyne/cm2

from fig. 30 of Grimison

cannot be ruled out due to the non-uniqueness intrinsic to the modeling of gravity data. In short, the consistent orientation of NNW trending P-axes for all earthquakes in the eastern end of the Azores-Gibraltar plate boundary clearly indicates that this region is a zone of ocean-ocean convergence between Eurasia and Africa. Eastward continuation of this zone connects with the continental collision between southem Spain and northern Africa (Fig. 1). However,

curves

by assuming

thickness

modulus

Part

(1987).

curve) geoid anomalies

Seamount.

crust. The crustal

2) sandwiched

of compression.

at (36.5 o N, - 11.5 o E) and trends N30 o E, parallel

By trial and error, the flexure rigidity is about

the direction

plates while the rest is taken up locally by the buoyancy

7”’ is the thickness

(solid curve) and theoretical

The profile is centered

along

from the large depth of earthquakes.

from fig. 26 of Grimison b. A comparison

3

km

model for a diffuse zone of compression

plates

\

\

dlbo

b

ro

(10 ” N/m2)

and a Poisson’s

The equivalent ratio of 0.25.

(1987).

typical features associated with a subduction zone are not observed. Instead, deformation spreads out over a diffuse belt of 100 to 300 km wide in the oceanic lithosphere. Thus the controversy of whether Africa is being subducted beneath Eurasia (McKenzie, 1972) or the other way around (Udias et al., 1976) is no longer a critical issue. The deformation across this zone seems better described as a regional compressional strain field without a through-going subduction zone.

140

W-P.

Extension in the Davie ridge-Madagascar

region

recordings East

The East African extensive

system

maximum about

rift zone is the Earth’s

of active continental

rate of divergence

7 mm/a

is estimated

based on global

Africa is in general

The recent seismicity

The

is seismically

except near its southern trast,

and

the western

erate-sized

quite diffuse (Fig. 5).

determined

rift.

Afar

N.L

GRIMISON

length

region

of the

of northern

active, but the eastern

south of Afar is almost devoid of teleseismic

to be

in eastern

does not cover the entire

African

Ethiopia

Its

plate reconstruc-

tions (e.g., Chase, 1978). Seismicity southern

most

rifting.

C HEN AND

length.

from teleseismic

nisms

end in Tanzania. activity

by mod-

throughout

In both rifts, the earthquake for virtually

In con-

arm is characterized

earthquake

arm events

focal

all the larger events

its

mecha-

show nor-

2c )t

. 1CI

-

\

0

-10

-20

-30

Fig. 5. Seismicity

of the East African

rift system (Grimison

by the ISC between

1964 and 1979 are shown

m,, 2 5.0. Numbered

circles represent

events

which

occurred earthquake

occurred

on the Davie faulting

in the Davie

same as in Fig. 2. Shaded

events whose mechanisms ridge-Madagascar

ridge in 1985. Fault

(Fairhead

and Chen. 1988b).

as filled circles.

and Stuart,

plane

region solutions

1982; Shudofsky,

Large

are reviewed (also

Epicenters

filled circles

1985; Dziewonski

the epicenters

in this paper.

see Fig. 6)

of representative

of all earthquakes

mark

and

events

four

Also included large

are plotted

et al.. 1986a). Layout

regions mark the two arms of the rift with regions south of about and Chen (1988b).

with rnh t 4.7 reported of larger

earthquakes

of

are five large historical

to moderate-sized

events

which

to show the local directions of the fault plane solutions

5 o S inferred

of

is the

from the results of Grimison

EARTHQUAKES

IN OCEANIC

141

LITHOSPHERE

In addition, scattered seismicity occurs in regions where there is no topographic expression of rifting. A broad belt of activity extends southwestward from Lake Tanganyika in the western rift into Zambia where events with mb from 5.3 to 5.9

ma1 faulting with horizontally oriented T-axes that are perpendicular to the strike of the rift valley (e.g., Fairhead and Girdler, 1971; Maasha and Molnar, 1972; Fairhead and Stuart, 1982; Shudofsky, 1985).

0

50”

40”

,O"

loo Lake

Fig. 6. Bathymetric with the shaded

map of the Mozambique

area marking

Chen, 1988b). Fault plane lower hemisphere

Channel

region

between

the East African

the trace of the Davie ridge along which large normal

solutions

of the six events (circles)

of the focal spheres.

DSDP site 242 (Simpson,

Also plotted

reviewed

are the epicenters

coast

faulting

and Madagascar

earthquakes

here (Table 2) are plotted

et al., 1982)

(Grimison

with equal area projections

of five large (M z= 6) historical

Schlich et al., 1974) on the Davie ridge is marked

(Fisher

are observed

events (stars within

by a special open circle symbol.

and of the

circles).

142

W -P (‘HEN

have occurred

in recent years (Maasha

1972; Fairhead Wagner

and Langston,

Seismicity bique

and Molnar,

and Stuart, 1982; Shudofsky, 1986) (Fig. 5).

is also concentrated

channel

between

and Madagascar,

1985;

the coast

nent of strike-slip

motion.

current

seems to be concentrated

seismicity

of east Africa

an area quite remote

from surfi-

cial features

that are usually

associated

with the

East African

rift zone (Figs. 5 and 6). In May and

hypocenter

zambique

cluster

Davie

ridge,

the northern

a prominent

bathymetric

that area. This earthquake

sequence

recorded

the

zone

since

anywhere 1928.

part

along Moreover,

of the

feature

in

is the largest

East at least

African

rift

five other

events with M 2 6 have occurred along the Davie ridge since the turn of the century (Gutenberg and Richter, 1949; Fig. 5) Thus the source parameters of this earthquake

sequence

and other

moderate-

sized events nearby (Grimison and Chen, 1988b) provide new constraints on the nature of deformation in the southern part of the Nubian-Somalian plate boundary which apparently extends into the oceanic lithosphere. The N-S trending Davie ridge (or the Davie fracture zone) (Fig. 6) is outlined by bathymetry and gravity anomalies which are characteristic of an oceanic transform fault (Rabinowitz, 1971; Bunce and Molnar, 1977; Scrutton, 1978; Scrutton et al., 1981). Recently, Coffin and Rabinowitz (1987) traced this feature as far north as 2O S where it joins the Kenyan coast. This relic transform fault has been interpreted by many investigators as the trajectory of Madagascar’s southward movement starting in the middle Jurassic from an original northerly position adjacent to Kenya and Tanzania (McElhinny et al., 1976; Bunce and Molnar, 1977; Norton and Sclater, 1978; Scrutton et al., 1981; Segoufin and Patriat, 1981; Rabinowitz et al., 1983; Coffin and Rabinowitz, 1987). Along this ridge, from about 2”s to as far south as about 18” S, all the large to moderate-sized earthquakes (Table 2) have focal mechanisms of pure normal faulting with NNW trending nodal planes (Fig. 6; Grimison and Chen, 198813). The strikes of the nodal planes are parallel to that of the Davie ridge, but there is no significant compo-

the along

seismic deformathat of the

More importantly, a major rift system does not seem to exist along the Davie ridge either. A joint epicenters

near

although

tion along the ridge does not resemble Mesozoic.

June of 1985 a sequence of large events ( mh up to 6.4) occurred off the coast of the Tanzania-Moborder

Therefore,

the Davie ridge, the present-day

in the Mozam-

AND N.L. GRIMISON

(e.g.,

Dewey,

1971)

of the

of the fourteen

location

largest

events

in the

large earthquake

sequence

of a few tens

with no apparent 1988b). erated

lineation

Furthermore,

(Grimison

the P and

by the two largest

have nearly identical

of 1985 appears

of kilometers

events

waveforms

as a

in diameter and Chen,

SH waves

gen-

of this sequence on the long-period

records of the WWSSN at teleseismic distances (Grimison and Chen, 1988b). These observations suggest that the largest earthquakes along the Davie ridge seem to cluster about a small volume. Thus while the significant level of scattered seismicity implies that extensional deformation must be taking place at different parts of this ridge throughout much of its length, the limited extent of the source region of the large sequence in 1985 seems to suggest that a continuous rift or a system of large-scale normal faults has not developed throughout the length the ridge. At the site of DSDP hole 242 (Fig. 6) the top of the Davie ridge showed material but instead

no indication of volcanic was largely composed of

compacted chalk (Simpson, Schlich et al., 1974). However, the fracture zone’s magnetic signature generally consists of positive anomalies with some high values occurring on occasional transects (Segoufin, 1981) suggesting possible magmatic activity in the area (Coffin and Rabinowitz, 1984; Mougenot et al., 1986). On a larger scale, other features that are characteristic of an extensional environment do seem to occur in the general offshore area between eastern Africa and Madagascar. At the northern end of the Mozambique channel, the Comores Islands to the east of the Davie ridge are volcanic in origin (Fig. 6). A general trend of decreasing age towards the northwest was reported there with the youngest rocks found in active volcanoes on the island of Gran Comoro (Esson et al., 1970; Hajash and Armstrong, 1972). The seamounts in

EARTHQUAKES

the

IN OCEANIC

southern

143

LITHOSPHERE

Mozambique

land, Basas da India)

channel

are probably

.origin.

A large number

curred

in this area

(Europa

earthquakes

in 1970 and

were

with a recent phase of volcanic

(Fairhead

and Girdler,

can

the

from

characterized

and

to Zambia.

by scattered

the deepest earthquakes

of water

depth

Fairhead

magnitude

2 3.5 reported

Preliminary

Determination

the large number

addition along

the Both

seismicity

other regions

zones.

to

with some of

( 2 30 km) found in Africa.

3000 m and 5000 m. Contour by the International

minor

them

and aseismic)

the two seismically since

are about of Nubia

(e.g.,

is also conmost

active

the two seismically

ac-

2000 km apart, the southern and

Somalia

between

10 o S

a diffuse zone of extension the oceanic

regions

straddles

between

both

and

the con-

lithosphere.

deeper

than 5000 m, and stippled

is 1000 m. Solid dots

Center

relatively

between

of

recent

and 20 o S is apparently

areas indicate

Seismological

south

1982). Thus it is not clear if

(seismic

along

place

concentrated

which tinental

interval

the

occurred

Nonetheless,

boundary.

are

Rift and a noticea-

the two zones,

also

tive regions

from

to that of the most

takes

to

and Stuart,

centrated

plate boundary

is similar

of extension

the total strain

of recent

map of the north Fiji basin. Shaded

between

In

seismicity

activity

Although no extensive volcanism and rift morphology are present in either region, the mecha-

Fig. 7. A bathymetric

amount

10”s.

concentration

the seismicity

channel,

Tanganyika

ble

probably

10 “S and 20 “S: one from Tanzania

Mozambique

Lake

active part of the East African

seismicity

years along the Nubian-Somalian between

in oc-

1971).

two zones of strain

be identified

nism of earthquakes

also volcanic

of small

associated

In summary,

Is-

the years

show epicenters 1964 and

regions show areas

of shallow

earthquakes

1979 and those reported

of

in the

of Epicenters from 1980 to 1985. Events outside the region outlined by dashed lines are excluded to avoid of earthquakes

near subduction

zones around

the basin. Taken

from Yu and Cben (in prep., 1988).

144

W-l’

Fig. 8. A map summarizing compilation

the focal mechanisms

of known magnetic

the position

of the youngest

the central

part

earthquakes

anomalies

anomalies

of the basin,

near the Horizon

of earthquakes

in the north

(Yu and Chen,

Fiji basin (Malahoff

and J for that of the Jaramillo

bank.

faulting near the Horizon

Fault

of sea-floor

plane

the Horizon

N.L. GRIMISON

bank

and a

1981. 1982) with R representing

solutions

spreading

is evident

for the seven largest

in the same layout as in Figure 2. Bathymetric

of the basin were removed

Strike-slip

198X) near

et al.. 1982a; Weissel,

event. Clear patterns

but not near the Horizon-Hazel-Holme bank (Table 3) were plotted

in prep..

C‘HEN AND

contours

only in shallow

in the central

part

for claritv.

hank, north

fracture zone (Chase, 1971) a feature which was identified primarily from a set of observations showing that it seems to separate a northern re-

The Horizon-Hazel-Holme bank is an ENE trending bathymetric high in the northwestern part of the north Fiji basin (Fig. 7). This bank was

gion of relatively low heat flow (Luyendyk et al., 1974) thicker sediments (e.g., Chase, 1971; Luyendyk et al., 1974) and high seismic velocity and low attenuation (Dubois et al., 1973) from

Fiji basin

often

associated

Fig. 9. Results equal-area indicated

with the so-called

of inversion

projection

of teleseismic

curves) seismograms

body waves for the earthquake

of the nodal planes for P and SH waves and locations

by open circles for dilatation

used in the inversion,

Hazel-Holme

are plotted

and filled circles for compression,

with the same absolute

a. Synthetic

seismograms

focal depth of 12 km below sea level. b. Synthetic

amplitude

calculated

of October of stations

scale. Vertical

calculated

used are shown.

Both the observed

for the best-fitting

seismograms

3. 1971 (No. 1 in Fig. 8). Lower hemisphere,

bars on the seismograms

centroidal

for a solution

by Chinn and Isacks (1983).

Polarities

(solid curves)

solution

(Table

of first motions

and the synthetic indicate

are

(dash

the time window

3). which shows a shallow

with a large focal depth of 48 km as reported

EARTHQUAKES

IN OCEANIC

145

LITHOSPHERE

P WAVES

SNM

SNG+-y&&X’ .. p.. ,,

EVENT 1 OCT. 3, 197 1 SX WAVES

146

younger

oceanic crust to the south where scattered

magnetic found

anomalies

of Holocene

(e.g., Malahoff

1982).

Luyendyk

observation lineations

that

age have been

et al., 1982a; Weissel,

et al. (1974) the

overall

is approximately

1981,

emphasized

trend N-S

the

of magnetic in the central

part of the basin (Fig. 8) while vague lineations unknown trend present

age north

E-W rate

of the Horizon

(Kurentsova

models

(e.g., Minster

across

However, judging

that no plate boundary

bank

and Shreider,

of deformation

bank is unknown. by

AND

W -P (‘HEN

is required

of present-day and Jordan,

to be less than the resolving of these models.

global 1978)

of

seem to

1971). The the Horizon

faulting

(Fig.

between

the observed

8). Figure

waves generated

9 shows and

(48 km) focal depth

a comparison

synthetic

at a shallow

varying

the later part

of the P waves cannot

with flat-lying

velocity

structures

at stations

SH

of October

3,

amplitudes

of

be matched

near the source,

a shallow focal depth significantly of the P waves

P and

(12 km) and a deep

for the event

1971. While the azimuthally

N.L. GRIMISON

improves

such as TAU,

and SHK. More importantly,

the fit GUA,

the SH waves which

from the fact

were

in this region

cannot

plate

Notice that most nodal planes for earthquakes near the Horizon bank trend west-northwest or

motion

the rate is likely

power (-

10 mm/a)

The shallow seismicity of the north Fiji basin generally follows the trend of major bathymetric features. In particular, background seismicity seems to delineate much of the Horizon bank, and the Fiji fracture zone near the Fiji Islands (Hamburger and Everingham, 1986; Fig. 7). Since the installation of standardized global seismograph network, the largest earthquakes that occurred in the interior of the basin took place near the Horizon bank, including a large earthquake sequence in November of 1984. Published results of the mechanisms of two earthquakes, however, do not give a consistent view of the active deformation in the region. Assuming a strike-slip mechanism determined from P wave first motions, Chinn and Isacks (1983) modeled the P waves of the event of October 3, 1971 recorded at four stations and estimated a large focal depth of about 48 km. This result is unusual considering the young (I 10 Ma) age of the source region (e.g., Chen and Molnar, 1983; Wiens and Stein, 1983). Eguchi (1984) determined a poorly constrained mechanism of predominantly thrust faulting for an event nearby (Aug. 22,1976) based on the polarity of first motions. Recently, a study of the source parameter of the six largest earthquakes (Table 3), including a detailed reanalysis of the two events mentioned above (Yu et al., 1987; also Yu and Chen, in prep. 1988), shows that the Horizon bank region is characterized by shallow (lo-20 km) oblique strike-slip faulting with a component of normal

not

analyzed

be matched

north-northeast,

by Chinn

and

Isacks

(1983)

by a deep source (Fig. 9).

oblique

to

the

east-northeast

trending bathymetry and background seismicity (Figs. 7 and 8). Consequently. the earthquakes cannot represent right-lateral slip along the socalled Hazel-Holme fracture zone implied in several tectonic models of the north Fiji basin (cf. Luyendyk et al., 1974; Falvey, 1975, 1978; Hamburger and Isacks, 1987). Although the northern end (near 16 “S) of the Plio-Pleistocene anomalies along 173” E (Fig. 8) might be the site of a ridge-ridge-ridge triple junction (e.g.,, Kroenke et al., 1987) clear patterns of recent sea-floor spreading have not been documented either to the immediate north (Gill et al., 1984) or south of this site (Malahoff et al., 1982a; Weissel, 1981, 1982; Fig. 8). Thus one cannot identify features that are clearly associated with a ridge-transform-ridge configuration which might explain the occurrence of these events of strike-slip faulting. While this possibility cannot be ruled out, we note that the sub-horizontally oriented T-axes, trending north-northwest and perpendicular to the trend of the bathymetry and seismicity (Figs. 7, 8 and lo), are the only consistent parameters among the focal mechanisms of different earthquakes. We interpret that the present-day deformation near the Horizon bank is characterized by a maximum regional extension (strain) parallel to the north-northwest trend of the T-axes. During individual earthquakes, oblique strike-slip faulting took place along planes striking oblique to the overall trend of the bathymetry and seismicity. Therefore, despite the fact that the zone of strain concentration is marked by the seismicity

EARTHQUAKES

IN OCEANIC

141

LITHOSPHERE

magnitude

higher

than

that

along

a proposed

nascent plate boundary immediately north of the north Fiji basin (Kroenke and Walker, 1986).

Conclusion A review of the mechanisms earthquakes

+

along

the

eastern

and depths of end

of

the

Azores-Gibraltar

plate boundary, near the Davie

ridge-Madagascar

region, and the Horizon bank

in the north Fiji basin suggests that the deformation in those largely oceanic regions is diffuse in nature. The zone of deformation

Fig. 10. Equal area projection

of the lower hemisphere

focal sphere showing the consistent of the fault plane solutions

NNW

summarized

of the

trend of the T-axes in Fig. 8. Events are

numbered according to Table 3.

can spread over

hundreds of kilometers in width and is accompanied by complex bathymetry and scattered seismicity. Typically earthquake faulting shows a mixture of mechanisms with no indication of major through-going faults and the nodal planes have no fixed relationship to the overall trends of the zone of deformation. Instead, the P- or the T-axes of individual events show a consistent orientation, indicating

and bathymetry, at present the Horizon bank does not seem to represent a through-going tectonic

regional compression or extension, reminiscent of observations in zones of recent deformation in the

fault. Given the complicated tectonic history (e.g., Carney and Macfarlane, 1978; James and Falvey, 1978; Malahoff et al., 1982b; Gill et al., 1984) and the possible interaction of several microplates in

continental lithosphere (e.g., Molnar and Tapponnier, 1975; Tapponnier and Mohuu, 1977, 1979). Moreover, these general features of diffuse zones of oceanic deformation seem to be present regardless of the type of tectonics involved. Some of the diffuse zones reviewed here extend into the con-

the north Fiji basin (e.g., Chase, 1971; Falvey, 1978; Brother, 1985; Hamburger and Isacks, 1987), how the inferred regional extension fits in with the clear pattern of young sea-floor spreading in the central part of the basin and the deformation along the Fiji fracture zone remains an open

the oceanic lithosphere in their nature of seismic deformation at such slow rates.

question whose answer probably cannot be found without a detailed analysis of all the earthquake source mechanisms and marine geophysical data on a basin-wide scale. Whether the region near the Horizon bank is an

Finally, the mere fact that several large earthquakes have occurred in each tectonically distinct region over the past 25 years points to the curiously high level of seismicity associated with zones of diffuse deformation in the oceanic litho-

active fracture zone in the conventional sense of plate tectonics or not, it is clear that this region is an important tectonic feature in the southwestern Pacific. Despite the diffuse nature of earthquake faulting, the total scalar seismic moment release of the six events between 1971 and 1984 reached about 2.4 X 1019 Nm, which is several orders of

sphere. The occurrence of the 1755 Lisbon earth-

tinental lithosphere with no apparent change in the style of deformation, thus suggesting that there is no clear distinction between the continental and

quake (e.g., Machado, 1966) and the 1969 Portugal earthquake (e.g., Fukao, 1973) are reminders of the potential seismic hazard associated with diffuse zones of oceanic deformation whose slip rate is too low to be confidently resolved by our current knowledge of plate tectonics.

148

W.-P

Acknowledgments

Chen.

W.-P.

and

Molnar.

tracontinental

results from several studies

that at the time of this writing

are not yet pub-

lished. We would like to encourage out these relevant

but unpublished

near future. In particular, amount

marine

gathered

from

Doherty

data

Geological

helpful

comments

This research

bank.

results

bank

Observatory.

was supported

were

Lamont-

We also thank

from two anonymous

America Chung,

the

used

of the

reviewers.

by the U.S. National

Science Foundation under grants EAR83-19095, and EAR86-07128. partially Chevron

With

D.S. and lsacks,

EAR81-20497, Grimison was

supported by a graduate fellowship from Oil Co. during the course of this re-

search.

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