Do Old Faults Ever Die? Fault-Plane Orientations of Intraslab Earthquakes 
in the Tonga-Kermadec Subduction Zone

Amanda N. Hughes, Linda M. Warren, and Paul G. Silver

One of the basic issues regarding the physical mechanism for deep 
earthquakes is the extent to which they occur on pre-existing faults: 
Do deep earthquakes reactivate faults that formed in the oceanic plate 
prior to subduction? A previous study of the Tonga subduction zone (Jiao 
et al., 2000) compared nodal plane distributions of outer-rise events to 
events as deep as 450 km depth, and found them to be very similar.  
While this comparison provides some support for the pre-existing fault 
hypothesis, a more stringent test of the hypothesis is available by 
identifying the actual rupture planes of both the outer rise and deep 
events and comparing their orientations.  Warren and Silver recently 
developed a semi-automated method to distinguish the fault plane from the 
auxiliary plane of the focal mechanism based on the directivity of the 
rupture, and we apply this method to the P waves of large earthquakes 
in the Tonga-Kermadec subduction zone.  From 1994-2004, there were 74 
earthquakes with Mw >=6.0 and depth >=100 km in the region, and we 
analyze them and the corresponding outer rise events.  The focal 
mechanisms for most of the deep earthquakes contain a subvertical and a 
subhorizontal nodal plane.  For earthquakes between 100-200 km depth, 
our preliminary results show that the ruptures tend to propagate 
subhorizontally, and that this rupture direction is often consistent
only with the subhorizontal nodal plane.  Below 450 km depth, earthquakes 
occur on both subhorizontal and subvertical planes.