Abstract
The March 25, 1998 Antarctic plate earthquake ruptured a portion of the Antarctic plate more than 200 km west of its boundary with the Australian plate. The Harvard CMT solution indicates that the earthquake was primarily a strike-slip event but the large non-double couple (NDC) component of the moment tensor suggests considerable complexity in the rupture process. We use a finite fault inversion method to determine details of the rupture process from teleseismic body waves recorded by the Global Seismic Network. The P waves are poorly fit by one or more subevents having only a strike-slip mechanism. We find that the presence of a large oblique-normal faulting subevent located to the east of the hypocenter is necessary to improve the fit. This subevent combines with a larger strike-slip subevent to the west to comprise the main moment release in the earthquake, and is the cause of the large NDC component in the long-period focal mechanism. The earthquake exhibited very high slip and high stress drop compared with most interplate strike-slip events and the rupture was largely confined to the upper 15-20 km of the lithosphere. Both constituent focal mechanisms indicate that this part of the Antarctic plate is under NW-SE oriented tension, although the origin of these stresses is unknown.
Relocated aftershocks
We relocated the aftershocks of the Antarctic earthquake in a 3-D velocity model. The top map shows bathymetry near the epicenter [Smith and Sandwell, Science, 277, 1956-1962, 1997]. Aftershocks of the Antarctic plate earthquake are shown as red dots. The mainshock location is the white star. Fossil fracture zones on this map appear as the linear, light-blue features. (Bottom) Detailed map of the source region of the mainshock. Solid circles are locations of aftershocks relocated in a 3-D velocity model. The location of the mainshock is shown by the open star. Dashed lines are the surface expressions of the faults from our preferred model of the earthquake (see "Compound Rupture" section below), and are shown with their focal mechanisms (gray shade). The Harvard CMT focal mechanism is shown plotted at the centroid location (open circle). The largest aftershock has a strike-sip mechanism and is located 100 km south of the mainshock fault trace. Also shown is a previous earthquake (9/29/81) which is located further to the east within the Antarctic Plate. The E-W orientation of the aftershocks, in addition to the better fit to the body waves achieved with this faulting geometry below, demonstrate that the earthquake occurred on a fault or set of faults oriented perpendicular to the fossil fracture zones emanating from the boundary between the Australian and Antarctic plates.
Broadband Source Inversion
The broadband P waves from the Antarctic Plate earthquake reveal the considerable complexity of the source process. The earthquake begins with a small subevent making up approximately the first 10 s, and then the records at most of the stations are dominated by one or two large pulses with a duration varying between 20 and 40 s. Most of the moment is released in this initial episode, although the rupture process continues for over 100 s and several later although smaller pulses are visible. We inverted a total of 31 P and S wave recordings, using vertical components for the P waves and horizontal components for the S waves. The fault is divided into rectangular subfaults with a dimension of 5 km on each side. Various source orientaions including both nodal planes of the Harvard CMT solution and a vertical strike-slip fault which changes its strike in the middle of the rupture were tested. As can be seen below, none of these models fit the data well.
The figure above shows variance reduction resulting from inversions of the body waves. For the CMT-mechanism inversion, the variance reduction is plotted vs. assumed rupture velocity for inversions using both nodal planes (solid and short-dashed lines). The rotated strike inversion (long-dashed line) was run at a single rupture velocity (2.2 km s-1) and involved a purely strike-slip mechanism which changes its strike at the mid-point of the rupture from the CMT strike (281o) by an amount shown along the horizontal axis. The highest variance reduction is still obtained with no rotation in strike. Also shown are the variance reductions obtained using the first-motion mechanism (diamond) and for the compound source model (solid triangle). These last two models are described below.
Compound Rupture Model
Since a rupture model having a single strike-slip mechanism does not appear to explain the body waves, and because of the large NDC component in the full CMT mechanism, we derive a model involving a compound mechanism. First, we note that the observed first-motions of the P waves do not fit the CMT mechanism, but instead demonstrate that the rupture began with a strike-slip mechanism on a fault rotated about 10o counterclockwise from the CMT mechanism. This focal mechanism is very similar to that obtained for the largest aftershock. However, using this focal mechanism to represent the entire rupture does not significantly increase the fit to the body waves (see figure above). The difference between this "first-motion" mechanism and the CMT focal mechanism involves normal faulting with varying amounts of a strike-slip component (depending upon the proportion of seismic moment allotted to each mechanism). Including an oblique-normal faulting subevent greatly increases the fit to the P waves (see above figure), especially in the first 10-30 s where the largest moment release occurs. We infer a compound rupture where the large moment release consists of simultaneous rupture of a strike-slip fault and a normal fault. The shape of the P waveforms requires the normal-faulting subevent to occur near the beginning of the rupture, and the proportion of seismic moment released on this fault must be substantial (up to 50/%) to reproduce the NDC component in the long-period focal mechanism. Its most likely location is to the east of the hypocenter.
The figure shows the compound source model for the Antarctic earthquake. Top panel shows slip for the main strike-slip fault of the earthquake while the middle panel shows slip on the normal fault. The focal mechanism for each fault is depicted to the right. The relative locations of the two faults are shown on the map of aftershocks above. The vertical axis shows actual depth for the strike-slip fault and downdip distance for the normal fault, while the horizontal axis shows increasing distance to the ESE (E for the strike-slip fault). Arrows show the direction of rupture propagation on each fault. Faulting parameters are strike=91o, dip=88o, and rake=1o for the strike-slip fault and strike=295o, dip=27o, and rake=-42o for the normal fault. Also plotted on the top fault plane are the relocated aftershocks that lie within 50 km of the fault trace projected onto the fault (dots), the hypocenter location (open star), and the CMT centroid location (black star). The solid line is the 1 m slip contour. The total seismic moment obtained for this solution is 1.65 x 1021 N-m.
Conclusions
The moment tensor of the March 25, 1998 Antarctic Plate earthquake as determined from long-period surface waves contains a very large tensional non-double couple component. Using broadband body waves, we show that the non-double component can be explained by a compound source mechanism involving simultaneous rupture of strike-slip and normal faults. The P waves, particularly those near a node for the strike-slip radiation pattern, cannot be fit without the presence of a large normal-faulting subevent to the east of the hypocenter. We are able to find a source model consistent with the Harvard CMT mechanism by constraining the proportion of moment released on the normal fault to be larger then 44 %. Although the body waves prefer a model with smaller moment release on the normal fault, our final model fits the waveforms only slightly less well while matching the magnitude of the non-double component observed in the long-period focal mechanism.
The earthquake ruptured bilaterally with a total lateral extent of about 320 km. This agrees well with the distribution of aftershocks which occurred in the following nine months. Most of the slip in the earthquake appears to be confined to very shallow depths, consistent with the age of the oceanic lithosphere in this region. The amount of displacement and inferred stress drop are very large, suggesting that the earthquake broke faults which had undergone no previous rupture. The trend of the main strike-slip fault is perpendicular to fracture zones originating at the Southeast Indian Ridge and no features in the bathymetry suggest the presence of faults with this orientation.
The earthquake initiated with a small magnitude subevent with a duration of about 10 s and having a strike-slip mechanism. Dynamic stress transfer from the seismic waves or the static stress change resulting from this strike-slip subevent probably initiated slip on the normal fault. The proximity of the rupture initiation to the Gambier Fracture Zone suggests that progression of the rupture was controlled by differences in the mechanical or thermal properties of the lithosphere resulting from the age difference across this fracture zone.
Michael Antolik, Department of Earth and Planetary Sciences, Harvard University, copyright ©2000 all rights reserved