Chatelain, Jean-Luc

Grasso, J.R.

Prévot, Richard

Roecker, S.W.

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Horizon / Pleins textes

GEOPHYSICAL RESEARCH LETTERS, VOL. 2 1, NO. 19, PAGES 2 159-2 162, SEPTEMBER 15, I994 
A shallow double seismic zone beneath the central New Hebrides 
(Vanuatu): evidence for fragmentation and accretion of the des- 
cending plate? 
Richard Prévot' and J-L. Chatelaid 
ORSTOM, Nouméa. Nouvelle Calédonie 
S.W. Roecker 
Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, New York 
J-R. Grasso 
Laboratoire de Géophysique Inteme et de Tectonophysique, CNRS, Univ J. Fourier, Grenoble, France 
Abstract. A shallow double seismic zone (SDSZ) has been 
found in the descending Australian plate beneath the central 
part of the New Hebrides island arc, directly above a large gap 
in intermediate depth seismicity and between two seismic 
boundaries. Ambient seismicity occurs mostly in the upper part 
of the SDSZ, while earthquakes in the lower part occur in clus- 
ters (swarms or aftershocks of large earthquakes). The distance 
between the upper and lower levels of the SDSZ is 50-70 km, 
and they are joined at 80 km depth by a near-horizontal band of 
seismicity. Thrust-faulting mechanisms predominate for ear- 
thquakes in the upper level of the SDSZ. Those in the lower le- 
vel, however, appear to be normal faulting, despite their being 
aftershocks of large thrust events. We suggest that with the ab- 
sence of a pull from the detached lithosphere the upper part of 
the Australian plate in the region of the SDSZ is resistant to 
subduction, and thus the downward displacements caused by 
large earthquakes in the adjoining regions result in a localized 
rebound. The location of the aftershocks within the plate sug- 
gests that a new plate boundary is forming, which will even- 
tually replace that outlined by the residual seismicity in the up- 
per level. Thus the leading edge is decoupling , and the boun- 
dary will eventually. shift back to the lower level of the SDSZ. 
Introduction 
On a regional scale, the New Hebrides Wadati-Benioff zone ap- 
pears uniform down to a depth of about 300 km pubois, 1971; 
Isacks and Molnar, 1971; Pascal et al., 19781. However, the 
central part of the arc between about 16.5"S-17SoS contains 
several peculiar features. First, earthquake locations from both 
worldwide [Pascal et al., 19781 and local @Whelot et al., 
1985; Prévot et al: 19911 networks show a sizable gap in the in- 
termediate depth seismi& of the Wadati-Benioff zone. Pre- 
vious authors have interpreted this feature as evidence of a de- 
tachment of the lower part of the downgoing slab [Chatelain et 
'Now at Laboratoire de Géophysique Inteme et de Tectonophysi- 
que, CNRS, Univ J. Fourier, Grenoble, France. 
Copyright I994 by the American Geophysical Union. 
Paper number 94GLO 196 1 
0094-8534194194GL-0 196 1$03.00 
al., 1992,1993; Choudhury et al., 1975; Louat et al., 1982; Pas- 
cal, 1973, Pascal et al., 19781. Second, there appear to be 
strong lateral variations in seismicity, characterized by two 
barriers to aftershock development by Chatelain et al. [1986] 
(Figure 1). Third, there are a number of anomalous surface fea- 
tures in this region: a seamount is heading toward the trench 
near 17.2"S, and the strike of the trench changes from nearly N- 
S to NNW-SSE. The trench itself becomes less well defined 
north of this region. 
An additional anomalous feature discussed in this paper is the 
clustered seismic activity (i.e., swarms and aftershocks of main 
events) which, along with the ambient seismicity of the region, 
makes up a shallow double seismic zone (SDSZ) in the upper 
80 km of the lithosphere. We suggest that this SDSZ is a result 
of the fragmentation of a portion of the downgoing Australian 
plate which, because of detachment, has lost the pull of the li- 
thosphere that preceded it. The upper level of the SDSZ is thus 
the residual activity of an old, nearly defùnct plate boundary, 
while the lower level outlines a new boundary where the lea- 
ding edge of the Australian plate is presently breaking off and 
rebounding to the surface. The decoupled fragment will perhaps 
be accreted onto the Pacific plate as an ophiolite. 
Data and Observations 
A network of 19 telemetered seismograph stations (Figure 1) 
was operated from 1978-1990 in the New Hebrides as a joint 
project between the Institut de Recherche pour le Développe- 
ment en Coopération (ORSTOM) and Come11 University. For 
this study, 2422 of the best locations were selected from a 
network catalog of about 30,000 local events. Many of these 
events were located previously in a variety of one- and three- 
dimensional structures [e.g., Roecker et al., 1988, Prévot et al., 
19911 and the patterns they reveal are robust. Therefore we are 
confident that the trends discussed here are not simply artifacts 
of the location procedure or of poor data quality. 
To characterize seismicity outside of the network, the data set 
of locally recorded hypocenters was supplemented by 383 well- 
recorded earthquakes reported in the Intemational Seismologi- 
cal Center (ISC) catalog. Clusters and swarms were distin- 
guished from ambient seismic activity using the second order 
moment technique [Reasenberg, 19851. 
Ambient activity in the central New Hebrides occurs largely wi- 
thin a single contiguous zone (Figure 1). Clustered activity oc- 
? (  2159 
2160 
15 
S 
16 
'5 d 
PRÉVOT ET. AL.: SDSZ BENEATH THE CENTRAL NEW HEBRIDES 
166 E I67 168 169 
Figure 1. Map view of the central New Hebrides island arc. Solid triangles denote seismic stations of 
the local network. The two heavy dashed lines are the seismic boundaries (B1 and B2) identified by Cha- 
telain et al. [1986]. The ORSTOM seamount is denoted by O.S. South of the O. s., the trench is shown 
by a solid line, while to the north its inferred position (after Collot et al., 1985) is shown by a dashed line. 
The ORSTOM seamount is denoted by O.S. Numbers on focal mechanisms denote the following: (1) 
composite mechanism from lower level clustered events, (2) best constrained individual mechanism from 
an upper level event, (3) best constrained individual mechanism from an event located at the eastern edge 
of the clustered activity, (4) CMT solution for the large 81/07/15 thrust and (5) CMT solution for the 
81/07/19 aftershock. Data used to construct mechanisms 1-3 are show in Figure 4. Open squares are 
events taken from ISC catalog (1968-1988). The activity located by the ORSTOM-Come11 network 
(1978-1991) is divided into clustered seismicity (solid circles) and background seismicity (open circles). 
Letters A-G locate the cross sections shown in figures 2 and 3. 
curs within this zone as well in the regions north and south of 
the seismic boundaries identified by Chatelain et al. [1986] 
(Figures 1-3), but between these boundaries it occurs in distinct 
zones separated from the ambient seismicity by about 50 km, 
thus defining a SDSZ (sections CC' and DD' in Figure 2). The- 
se two zones are connected at a depth of about 80 km by a 
nearly horizontal zone of seismicity that outlines the top of the 
seismic gap in the deeper Wadati-Benioff zone (section CC' in 
Figure 2 and Figure 3). We note that nearly all of the clustered 
activity defining the lower part of the SDSZ are either after- 
shocks of the 26 August 1979 (Mw = 6.2) and 15 July 1981 
(Mw = 7.1) earthquakes or are part of a swarm that occurred in 
April 1986. Both of the mainshocks were thrusts with the pro- 
bable rupture planes dipping about 30 degrees ENE (Figure l). 
Focal mechanisms for events in the upper level of the SDSZ ty- 
pically show thrust faulting (e.g., mechanism #2 figures 1 and 
2), while those in the eastern lower part of the SDSZ suggest 
nearly horizontal slip (e.g., mechanism #3 in figures 1 and 2) . 
Mechanisms of events in the lower level, however, appear to be 
the reverse of those in the upper. A centroid-moment tensor 
(CMT) solution (Dziewonski et al., 1983-1990) for one of the 
larger aftershocks in this area (mechanism #4 in figures 1 and 
2) shows normal faulting, and while the station spacing is not 
amenable to constraining individual mechanisms, a composite 
PmVOT ET. AL.: SDSZ BENEATH THE CENTRAL NEW HEBRIDES 
200 
2161 
- 0 1 1  m o  - 
- 0  D 
I I a i  I 0 1  I I I 
A' 
! 
:Gof- 
j 1 
I 
233 I I 
ZmD D' 
I 
I 
I - 
I 
i 
i 
d 7- E' 
T c' 
Y v  . 
I I , 9 :  
Figure 2. Cross sections of the Wadati-Benioff zone beneath the central part of the New Hebrides island 
arc. Meaning of symbols same as in figure 1. T denotes the location of the trench . Note that the steeply 
dipping zone in A-A' is not present in the nexq cross section to the south (B-BI). The shallow double 
seismic zone is prominent in section C-C' and appears to a lesser extent in sections D-D' and E-E'. Back- 
hemisphere projection of the focal mechanisms are shown in section C-Cl, with numbers corresponding 
to those in figures 1. 
mechanism (mechanism #1 in figures 1 and 2) suggests normal 
faulting as well. 
Discussion and Conclusions 
Double seismic zones have been observed in several Wadati- 
Benioff zones [e.g.. Fujita and Kanamori, 1981; Kawakatsu, 
1986; Smith et al., 19931 but the SDSZ in the central New He- 
brides appears to be different from all of these. For example, 
most double seismic zones occur at greater depths (65-85 km), 
the distance between the upper and lower zones is generally 
less than about 20 km, and the characteristic pattern of upper 
level mechanisms with P axes parallel to the dip of the zone 
and lower level mechanisms with T axes parallel to the dip of 
the zone appears to reflect changes in the internal stress field of 
the subducted lithosphere ("sagging" or "unbending"). Like the 
New Hebrides, the DSZ in the Mendicino subduction zone re- 
ported by Smith et al. [1993] is shallow (1040 km depth), but 
has thrust mechanisms on both the upper and lower levels with 
fault planes parallel to the seismicity, and they interpret this 
SDSZ as the boundaries of a sliver of lithosphere trapped when 
the trench there stepped back. The SDSZ in the central New 
Hebrides is distinguished by being shallow, by having the two 
zones separated by a large distance (at least 50 km), and by 
fault plane solutions indicating shallow thrust faulting and dee- 
per normal faulting. The New Hebrides SDSZ is thus unlike 
any other DSZ in most respects, and therefore is likely to be 
caused by a different mechanism. 
Several lines of evidence, including elastic wave attenuation 
marthelot et al., 19851, elastic wave velocity structure prévot 
et al., 19911, and uplift rates on the islands of Malekula and 
Santo [Mitchell and Warden, 1971; Taylor et al., 1985; Chate- 
lain et al., 19921, suggest that the deeper part of the subducted 
Australian plate has detached at about 80 km depth, removing a 
substantial pulling force from the upper part of the plate and 
' 
. 
Figure 4. Cartoon illustrating the proposed mechanism for the 
generation of the New Hebrides SDSZ. (a) The shallow slab 
above the detachment is pulled down by longer slabs on either 
side. (b) The overextension of the shallow slab by large thrust 
event (black star) near the boundary B1 causes a readjustment 
in the form of normal faulting within the plate. 
causing at least part of it to rebound [Chatelain et al, 19921. We 
appeal to this detachment to explain the occurrence of the 
SDSZ. Clearly, subduction continues in a normal fashion along 
most of the arc, but in the region of shallow detachment there 
are no large thrust events and those large thrusts (such as the 
81/07/15 event) that occur near the boundaries of the region 
trigger swarms of normal faulting events within the lower level 
of the SDSZ. This reversal of motion suggests that this part of 
the plate is resistant to subduction, in that it attempts to re- 
bound after being displaced downward (Figure 4). 
Interestingly, this rebound occurs not along the original plate 
boundary (as outlined by the upper level events of the SDSZ) 
but along a new boundary (the lower level of the SDSZ) some 
50 km further back into the descending plate. It therefore ap- 
pears that a consequence of slab detachment at depth is a frag- 
mentation of the downgoing plate at the surface. We suggest 
that this portion of the leading edge of the Australian plate, as 
outlined to the east and west by the SDSZ and to the north and 
south by the seismic boundary zones of Chatelain et al. [1986] 
eventually will completely decouple and accrete onto the Paci- 
fic plate, perhaps as an ophiolite. 
Achowledgnients. We thank C. Baldassari, F. Bondoux, R Cam- 
pillo, M. Chauvin (deceased), C. Douglas, R Foy, L. Mollard, D. 
Nikedau, J.C. Willy, E. Yakeoula, end all others involved in the da- 
ta collection and analysis operation. This work was supported by the 
Institut de Recherche pour le Développement en Coopération 
(ORSTOM) and the Laboratoire de Gophysique Interne et Tectono- 
physique of J. Fourier University, Grenoble. 
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A shallow double seismic zone beneath the central New Hebrides (Vanuatu) : evidence for fragmentation and accretion of the descending plate ?

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