Earthquake mechanics and deformation in the Tonga‐Kermadec subduction zone from fault plane orientations of intermediate‐ and deep‐focus earthquakes

Abstract

We make use of rupture directivity to analyze 82 deep earthquakes (≥100 km depth) in the Tonga‐Kermadec subduction zone. Identifying the fault planes for 25 of them, we are able to place new constraints on both the physical mechanism of intermediate‐ and deep‐focus earthquakes and deformation within the subducting slab. We find that half of deep earthquakes with MW ≥ 6 have detectable directivity. We compare the obtained fault orientations with those expected for the reactivation of outer‐rise normal faults and with those expected for 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 subvertical fault planes consistent with a downdip‐compressional stress field. Slip along these faults causes the slab to thicken. Rupture propagation shows no systematic directional pattern. In contrast, at intermediate depths (100–300 km), all ruptures propagate subhorizontally and all identified fault planes, whether in the upper or lower region of the double seismic zone, are subhorizontal. Rupture propagation tends to be directed away from the top surface of the slab. After accounting for the angle of subduction, the subhorizontal fault plane orientation is inconsistent with the orientation of outer‐rise normal faults, allowing us to rule out mechanisms that require the reactivation of these large surface faults. Subhorizontal faults are consistent with only one of the two failure planes expected from the slab stress field, suggesting that isobaric rupture processes or preexisting slab structures may also influence the fault plane orientation. If all deformation takes place on these subhorizontal faults, it would cause the slab to thin. Assuming the slab is incompressible, this implies that the slab is also lengthening and suggests that slab pull rather than unbending is the primary force controlling slab seismicity at intermediate depths.