Potentially active faults in the rapidly eroding landscape adjacent to the Alpine Fault, central Southern Alps, New-Zealand

摘要

Potentially active faults are exposed in the steep glaciated topography of the central Southern Alps, New Zealand, immediately adjacent to the Alpine Fault plate boundary. Four major faults exposed along the flanks of three of the highest mountain ranges strike 10-23 km (potentially 40 km) NNE oblique to the Alpine Fault, dipping 57° ± 12° NW in the opposite direction. Youngest discernable motions were reverse dip-slip, accommodating both margin-perpendicular shortening and dextral margin-parallel components of plate motion. Kinematic analysis yields a compression axis (295/10° ± 9° trend or plunge) equivalent to the contemporary shortening determined from seismological and geodetic studies, suggesting the faults may be active, although definitive evidence for recent movement or single event displacements is lacking. There are 106 other potentially active faults mapped in central Southern Alps with strike lengths 4-73 km. Earthquake parameters were assigned from fault trace lengths and historical earthquake statistics, indicating potential for Mw 5.5-7.4 earthquakes at recurrence intervals of 1000-10,000 years. Such long recurrence intervals are consistent with the faults having little surface expression, with rapid erosion of these seismically agitated mountains erasing any evidence of surface rupture during periods between earthquakes. The central Southern Alps faults exemplify the difficulty in fully deciphering long-term (e.g., Holocene or Quaternary) records of seismicity in tectonically active regions with rapidly evolving landscapes. Although there may be little evidence of surface ruptures remaining in the landscape, the faults are still an important potential source of earthquakes and seismic hazard.