Deep Earthquake Mechanics and Dynamics in the Tonga-Kermadec Subduction Zone

Linda Warren, Department of Terrestrial Magnetism, Carnegie Institution of Washington
Amanda Hughes, Department of Geology, Washington and Lee University
Paul Silver, Department of Terrestrial Magnetism, Carnegie Institution of Washington

By identifying the fault-plane orientations of 25 earthquakes >100 km depth in the 
Tonga-Kermadec subduction zone, we have determined new constraints on the physical 
mechanism of intermediate- and deep-focus earthquakes and deformation in the subducting 
slab.  After identifying the fault planes with rupture directivity, we compare the 
fault-plane orientations with the orientations expected for the reactivation of 
outer-rise normal faults and the creation of new faults in response to the ambient 
stress field.  Earthquakes >300 km depth match the patterns expected for the creation 
of a new system of faults: we observe both subhorizontal and near-vertical fault 
planes, and the ruptures show no systematic patterns in the directions of rupture 
propagation.  In contrast, at intermediate depths (100-300 km), ruptures tend to 
propagate subhorizontally away from the slab surface and all identified fault planes, 
in both the upper and lower seismic zones, are subhorizontal.  After accounting for the 
angle of subduction, this orientation is inconsistent with the orientation of outer-rise 
normal faults, allowing us to rule out mechanisms that require the reactivation of these 
surface faults.  Subhorizontal faults are consistent with only one of the two failure 
planes expected from the slab stress field, suggesting that the ambient stress field is 
not the only factor controlling fault-plane orientations.  Pre-existing weak zones of 
other orientations or isobaric rupture processes may be important.  If all deformation 
takes place on these subhorizontal faults, it would cause the slab to thin, indicating 
that slab pull rather than unbending is the primary force controlling slab seismicity at
intermediate depths.