HIDDEN EARTHQUAKE POTENTIAL IN PLATE BOUNDARY TRANSITION ZONES

Plate boundaries can exhibit spatially abrupt changes in their long-term tectonic deformation at locations of plate boundary structure changes. In those regions the superposition of short-term (earthquake cycle) kinematics and geologic-timescale plate interactions may combine to hide the earthquake cycle or long-term strain accumulation in the region. Two examples of this effect are at the southern end of the Cascadia subduction zone, in the vicinity of the Mendocino triple junction (MTJ), and at the southern end of the Hikurangi subduction zone, New Zealand. In southern Cascadia the observed signal (GPS) of crustal displacements is intermediate between Pacific and Juan de Fuca (JdF) motions. With distance north, the signal rotates to become more aligned with JdF - NAm displacements - the motions expected along a coupled subduction interface. The deviation of that subduction interface signal has been previously interpreted to reflect clock-wise rotation of a coastal, crustal block and/or reduced coupling at the southern Cascadia margin. However, the geologic record is supports the signal reflecting primarily the effects of northward crustal shortening (on geologic time scales) associated with the MTJ Crustal Conveyor. At the Hikurangi to Alpine Fault transition in New Zealand (H-AF), plate interactions switch from subduction to oblique translation along the Alpine fault system without a triple junction, but as a consequence of changes in lithospheric structure of the Pacific plate. Here the short-term, earthquake-cycle signal recorded by GPS shows a reduction in plate motion-directed displacements, which has been interpreted to reflect reduced coupling along the southernmost segment. However, this signal records both the subduction interface coupling, and the shear deformation produced by the Marlborough Fault system (MFS). This superposition of deformation signals combine to mask a strongly coupled interface, seen in the recent (November 2016) Kaikoura earthquake.