The Tasman Sea earthquake of 25 November 1983 and stress in the Australian plate

329

Tecrotioph~Gcs, 111 (1985) 329-338 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

THE TASMAN SEA EARTHQUAKE OF 25 NOVEMBER 1983 AND STRESS IN THE AUS~L~N PLATE

DAVID DENHAM Bureuu of Minetd Resources, Geologv &

Geophysics.

P.0,

Box 378, Cunherru,

ACT

2601 (A~$ru/iu)

(Received July 12, 1984; accepted September 12, 1984)

ABSTRACT Denham, D., 1985. The Tasman Sea earthquake of 25 November 1983 and stress in the Australian plate. Tectonoph.vsics, 111: 329-338. The Tasman Sea earthquake of 25 November 1983 was large enough (Me =l.l x10’” Nm) to be recorded world-wide and provide information on the state of intraplate stress in the lithosphere beneath the Tasman Sea. The earthquake occurred beneath the abyssal plain at a depth of about 25 km and was associated with almost pure dip-slip faulting. The direction of the pressure axis of the focaI mechanism is similar (139 degrees E of N) to those observed from the nearby Australian mainland. Hence both the oceanic Tasman Sea and continental Australia appear to be part of the same stress regime. However, the direction of stress in this part of the Australian plate does not coincide with the north-south direction of motion of the plate and therefore forces other than the ridge push must be invoked to generate the stresses observed.

INTRODUCTION

Knowledge of intra-plate stress is important to the understanding of intra-plate seismicity, plate tectonic models and driving mechanisms. In the last few years a considerable body of evidence has been compiled from in-situ stress measurements, earthqu~e focal mecha~sms, borehole deformations, and recent crustal movements to show that most intra-plate regions experience compressive stresses in the crust (see, for example, Sykes, 1978; Denham et al., 1979; Zoback and Zoback, 1980). The exceptions are usually in specific regions of rifting such as the Rhine Graben, the East African Rift and the Basin and Range Province of the U.S. (Sykes, 1978). There is some evidence (Zoback and Zoback, 1980; Denham et al., 1981) that within a “typical” continental intra-plate region, provinces can be identified where the stress patterns are consistent. For example, in the aid-Continent region of the U.S. the principal stress directions are mostly about 60” east of north, while in the Atlantic Coast region they are approximately 120” (Zoback and Zoback, 1980). The

330

problem

of how intra-continental

stress directions

relate

margins or to the adjacent oceanic regions has hardly because of the paucity of stress observations from offshore

to those

at continental

been examined, areas.

mainly

An opportunity to examine this problem in the Australian region occurred on 25 November 1983, when an earthquake. large enough to be recorded world-wide, took place in the Tasman

Sea. In this paper

the source mechanism

studied and the results related to the regional continent.

of this earthquake

stress field observed

is

on the Australian

TECTONIC AND SEISMOLOGICAL SETTING The 1983 Tasman Australian mainland.

Sea earthquake (see Fig. 1) took place about 500 km from the It occurred beneath about 4.5 km of water under the Tasman

Fig. 1. Regional tectonic setting of the Tasman Sea and southeastern Australia. The e~thqu~e of 25 November 1983 is indicated by a star, the three sizes of solid circle indicate earthquakes of magnitude 6.0 and greater, 5.0 to 5.9 and 4.0 to 4.9. The arrows indicate directions of pressure for well constrained focal mechanisms. The Tasman Sea formed in the go-60 Ma period. The thicker stippting represents the Paleocene Sea floor. Batbymetry is in metres; most of the bullseye bathymetry on the Tasman Sea floor is associated with the chain of Tasmanid Guyots; the contours indicate intervals of 500 m.

331

Basin. This basin formed separated from Australia. Basin became main tectonic hot-spot Seamount

from about 80 to 60 Ma B.P. when the Lord Howe Rise At 60 Ma B.P. spreading activity ceased and the Tasman

part of the Australian Plate (Hayes & Bingis, 1973). Since then the activity in the region appears to have been associated with the

volcanism

which

Chain (Wellman,

produced

the Tasmanid

Guyots

1983). Some of these volcanoes

and

the Lord

Howe

are shown as bathymet-

ric features rising from the abyssal plain in Fig. 1. They are poorly dated except for Lord Howe Island (31.53’S, 159.07”E) which has been assigned an age of 7 Ma B.P. (McDougall et al., 1981). Evidence from the free-air gravity anomalies (Haxby et al., 1983) implies that these seamounts extend as far south as 44’S on a linear trend with an azimuth of about 20” east of north. The earthquake distribution beneath the Tasman Sea does not appear to correlate with the volcanic centres and it is not certain, from the earthquakes plotted in Fig. 1, whether there is a single east-west trending zone at about 40”s or whether there are two groups of epicentres: one clustered near the continental shelf and the other at about 156”E east beneath the abyssal plain. The tectonic significance of these earthquakes is therefore somewhat enigmatic. During the period from April 1883 to December 1886 a remarkable series of over 2500 earthquakes, presumably originating from the western part of the zone, were reported as being felt in Tasmania, Flinders Island and southeastern Australia (Shortt, 1885, 1886; Biggs, 1886; Burke-Gaffney, 1952). The largest of this sequence appeared to be that of 12 May 1885, which is plotted on Fig. 1 as having a Richter magnitude of M > 6. Since 1886 the level of seismic activity has been comparatively low although several small (M, < 5.0) earthquakes have been located, along the complete length of the zone at 4O”S, since 1960 when sensitive installed in Tasmania and in southeastern Australia.

seismographs

were

On the mainland of Australia the stress regime is better known than for the offshore region and most of the earthquakes appear to be associated with compressive stresses in the crust. Figure

1 shows the principal

axes of compression

for most

of the earthquakes for which reliable mechanisms have been obtained. These data are taken from published results and data in press. (Fitch, 1976; Denham, 1980; Denham et al., 1981; Denham et al., 1982; Bock and Denham, 1983; Denham et al., 1984, 1985). All the earthquakes

for which reliable

mechanisms

are available

were

associated with compressive stresses. They mostly occur within the Lachlan Fold Belt (Crook and Powell, 1976). This was formed in the Palaeozoic (570-225 Ma) by a process of continental accretion and cratonization. It currently consists of a series of volcanics, granites and deformed (and in several areas, partly metamorphosed) sediments. The deformations are associated with four main orogenies starting in the Ordovician (450 Ma) and continuing until the Carboniferous (300 Ma). The main structural trends are approximately north-south. The cratonized fold belt provided the source of sediments for the surrounding Sydney, Gippsland, Otway and Murray Basins, which have evolved since Permian

332

times.

During

the Cainozoic,

the region

has been tectonically

quiet,

although

the

opening of the Tasman Sea (- 80 Ma) and the Southern Ocean ( - 55 Ma) affected the margin of the continent and were probably associated with widespread Tertiary volcanism

in eastern

(Wellman,

1979). The azimuth

Australia

and uplift

in the southeastern

of the regional

compressive

part of the continent stress regime associated

with the recent earthquakes is approximately 120’. The stress is presumed associated with current tectonic activity rather than a palaeo stress, because consistency of the stress direction throughout fact that the average direction is not parallel

HYPOCENTRAL

PARAMETERS

The USGS solution follows: Origin time: Latitude: Longitude:

(USGS,

the southeast of the continent and the to any of the major geological trends.

MECHANISM

1984) for the 25 November

19h 56m 7.8s 40.451”s 155.507”E

mb =

1983 earthquake

was as

6.0

MS = 5.8

18 km

Depth: Stations

AND FOCAL

to be of the

used:

107

The seismograms from stations in the Australian region confirmed the USGS magnitudes. The directions of motion from the P-wave first arrivals were read from all available long and short period seismograms and also from recordings made at SRO, ASRO and DWWSS stations. The focal mechanism solution for the short period data is shown in Fig. 2 using Jeffreys-Bullen travel-times. Essentially all the stations in south-east Australia give a downward first motion with the exception

of the Tasmanian

stations

SAV and TAU, and two of the stations

in a microearthquake network near Melbourne. The first arrivals at ADE, and TAU indicate that these stations must be close to a nodal plane because, on the photographic records the short period first motion is up, while on the long period records

the first motion

is down.

Figure

3 shows copies of the seismograms

from

ADE and TAU as well as the output from the digital station at TAU. Whereas the times of the first arrivals at ADE are similar on both the short and long period seismograms, it is clear on the TAU photographic records that the first arrival is too small to be seen on the long period seismograms. However, on the digital records a small upward first motion is clear on the high gain recording (Fig. 3b). Although there are several inconsistent readings, the short period solution indicates an almost pure dip-slip mechanism with a shallow dipping plane, defined by the regional Australian stations and a steeply dipping plane defined by the distant stations. There is some uncertainty in the definition of the steeply dipping plane because the first motions from the stations KOU, MSV, PVC, PEL, and SPA were

333

TABLE

1

Focal parameters

of the 1983 Tasman

Long-period

Short-period

Solution:

Sea earthquake U.S.G.S.

*

Harvard dip

*

azimuth

dip

azimuth

dip

azimuth

P-axis

106

29

319

03

118

06

azimuth 334

dip 15

T-axis

276

61

208

82

215

50

185

13

B-axis

014

04

049

07

023

39

066

08

Double couples Strike

(A) 208

(B) 13

(A) 236

(B) 41

(A) 243

(B) 357

(A) 251

(B) 52

Dip

17

14

48

41

52

62

60

31

Slip

105

86

100

79

144

44

100

74

Seismic moment (Nmxlo’s)

1.1

* Data taken from EDR No. 11-83 (USGS,

Fig. 2. Focal earthquake.

mechanism

on an equal area projection the pressure,

1.1

1984)

for the short-period

Solid and open circles represent

2.3

first motions compressive

for the 25 November

and dilative

of the lower focal sphere. The circumscribed

tension and null axes (the adjacent

numbers

first motions

1983 Tasman respectively,

Sea

plotted

crosses show the nodal poles and

show the azimuth

and dip angles respectively).

334

extracted from bulletins and the seismograms were not examined. However. although there is some uncertainty in the orientation of the planes, the type of faulting involved There

is unambiguous

and undoubtedly

caused by compressive forces in the crust. but these are mainly in the main “up” quadrant,

are some inconsistencies

associated

with stations

Hz) and therefore

having a maximum

unreliable

magnification

for weak first motions

at high frequencies

at teleseismic

distances.

( - 10

Table

1

TAU

wwss SPZ

LPZ

owwss (TAu) LPZ

(0)

(bf

1

1

3200 2400

200 “E (:

I00

800

.%

0

0

-800

-100

E ==t -1600

- 200

t~

-2400

-

-3200

19 54

Y ,

, 19.56

I 19,58

I 20

00

> 20.02

1

1 20.04

I

I 20.06

I

t 20.08

-300

Time

L 19.54

19.56

19.58

20.00

Time

Fig. 3. Seismograms recorded at Adelaide (ADE) WWSS, and Hobart (TAU) WWSS and DWWSS. Notice that at ADE the direction of first motion for the short period P-waves is different from that recorded on the long period seismograms. The digital data displayed in (a) and (b) are the same except for differences in magnification. Notice &hesmall upward first motion in (b).

335

lists the adopted parameters for the short period solution. Because of the inconsistencies between the short and long period seismograms, a solution based only on long-period first motions and P-wave synthetics was also obtained. This is shown in Fig. 4. The northwesterly dipping nodal plane is well constrained by the arrivals at TAU, ASP and CTA but the other is very poorly constrained. However, the quality of the fit with the synthetics means that the adopted solution is consistent with all the available P-wave data. The synthetics were generated using the methods described in Kanamori and Stewart (1976), thus only P, pP and SP were used and the pwP and SWP components involving the water layer were not included. However, the contributions from these components are usually small (Chinn and Isacks, 1983) and would not significantly perturb the gross features of the modelling. The adopted solution gives a depth of 22 km (plus the water depth of 4.5 km), a rise time of 2 set, a rupture time of 4 set and a seismic moment of (1.1 rt 0.2) X 1018 Nm. The latter was obtained by averaging the waveform amplitudes from the 11 stations shown in Fig. 4. A crustal velocity of 5.9 km/s was used in the modelling process. The synthetics are very sensitive to the depth of focus and the uncertainty in the depth is estimated to be less than 2 km. Using the relations from Nuttli (1983) for intra-plate earthquakes we obtain the following

Fig. 4. Focal mechanism notations

of the 25 November

1983 Tasman

are the same as for Fig. 2. Long period P-arrivals

shown together and synthetic

with synthetic seismograms

seismograms

Sea earthquake at different

for the same faulting

are the same but the amplitude

from long period

stations

data. The

of the global network

are

model. The time scales for the recorded

scales do not correspond.

336

source parameters: Fault length L = 13 km Fault width W =lOkm Average fault displacement D = 32 cm Average stress drop = 2.2 MPa The differences between the focal mechanisms shown in Figs. 2 and 4 probably relate to the mechanics of the faulting associated with the earthquake. Thus, the first arrivals used to obtain the mechanism in Fig. 2 were monitored on the high-gain, short-period, regional stations. These arrivals probably correspond to the initial movements which are associated with a shallow dipping thrust fault, whereas the main rupture is associated with a steeper dipping fault and the Iarge amplitude P-waves recorded on the long period seismographs and modelled in Fig. 4. The evidence therefore suggests that the causative fault plane dips northwest. The solution shown in Fig. 4 is similar to those published by U.S.G.S. (1984) and Table 1 lists all four solutions.

DISCUSSION

Both the short and long-period mechanisms are very similar and involve almost pure dip-slip faulting as a result of compressive forces acting ESE in the crust. The directions of the pressure axes are consistent with those obtained for most of the earthquakes in southeastern Australia (120°, Denham et al., 1983). Thus it appears that the stress regime in this part of the Tasman Sea is similar to that in the adjacent continent. It has been argued that, in eastern North America and South America east of the Andes, the orientations of maximum compressive stress are controlled by ridge push and resistive forces at the plate boundaries (see Mendiguren and Richter, 1978; Richardson et al., 1976). However, in the Tasman Sea region of the Australian plate the spreading direction is almost north-south (see Minster and Jordan, 1978). Thus if ridge push is an important factor in the generation of intraplate stress, the forces at the other boundaries of the plate must provide even more significant effects to perturb the direction of maximum compressive stress to an almost NW-SE direction. Perhaps the boundary conditions around the eastern edge of the Australian plate are sufficient to generate stresses within the plate by simple deformation processes resulting from changes in the resistive forces. In this instance the Pacific Plate, which is moving approximately SE-NW in this region may play a leading role. Whatever the cause of the stress, it appears that both the oceanic and continental parts of the Australian Plate in the southeast Austr~ia/Tasm~ Sea region, are in compression and are probably part of the same stress regime.

331

ACKNOWLEDGEMENTS

I thank Hiroo Kanamori for the program to calculate the body-wave synthetic seismograms, Marion Leiba, Peter McGregor and Cedric Wright for critically reviewing the manuscrip, Pauline Greig for typing the manuscript, Rex Bates and Mike Steele for preparing the diagrams and the Director, Bureau of Mineral Resources, Geology and Geophysics, for permission to publish. REFERENCES Biggs, A.B., 1886. The Tasmanian Bock, G. and Denham, relationship Burke-Gaffney, Chinn,

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