Intraplate Stresses Inferred From the Mw 8.1 1998 Antarctic Plate Earthquake

S. Das
Department of Earth Sciences,  University of Oxford, Oxford, England, United 
Kingdom OX1 3PR


The occurrence of this left-lateral strike-slip earthquake on an east-west fault  
provides evidence on the nature of intraplate stresses in this region.  The 
Antarctic plate is entirely surrounded by ridges and transform faults and has no 
subducting portion, so the principal source of intraplate stress is ridge push 
and the low plate velocity relative to the hotspot frame of reference of 12 mm 
yr-1 implies that there is little or no drag force at the base of the plate.  
The stress regime in the main shock region is rotated by 90¡ from the plate 
boundary stress direction, the plate boundary being located only 200 km away.  
Thus the earthquake is not due to the stresses from this plate boundary.

The earthquake consisted of two ruptures, with the first event (Mw 8.0) jumping 
over a 70-100 km barrier with a 40 s delay, and continuing to propagate to 
result in the second Mw 7.7 earthquake.  The stress drops of the two subevents 
were 24 MPa and 21 MPa, respectively (Henry et al., 2000).

If the 1998 earthquake had occurred in rock with no preexisting planes of 
weakness, then the P and T axes of this earthquake would have indicated the 
maximum and minimum components of the stress, respectively.  Equally important 
evidence is provided by the fact that no intraplate earthquake has ever been 
detected on a nearly north-south trending fracture zone passing through the main 
rupture (T3a), or on the north-south fracture zones to the east of the 
earthquake.  North-south planes of weakness are revealed by the southern cluster 
of aftershocks, which are 5¡ from the strike of the auxiliary plane of the first 
subevent.  Stresses in virgin rock aligned with the P and T axes would not have 
created a new fault plane for the 1998 earthquake but would have preferentially 
caused earthquakes on T3a.  We know that this has not occurred, both because no 
such earthquakes have been detected and also because a large intraplate stress, 
as evidenced by stress drops of 24 MPa and 21 MPa for the two subevents, has 
been able to accumulate.  We therefore conclude that the main shock occurred on 
a pre-existing east-west plane of weakness.  With this conclusion a range of 
stress orientations are possible, subject to the constraint that they do not 
preferentially induce earthquakes on T3a.  If the strength of the plane on which 
this earthquake occurred is approximately equal to the strength of T3a, then 
this would require that the principal stress axes are either the same as the P 
and T axes or are rotated clockwise from them.  Note that this argument would be 
equally valid if the earthquakes of the southern cluster had, in fact, occurred 
on east-west fault planes. 

Henry, C., S. Das and J. H. Woodhouse, 2000.  The great March 25, 1998 Antarctic 
Plate earthquake: Moment tensor and rupture history, J. Geophys. Res., 105, 
16,097-16,119.