Evaluating the long-term history of surface-rupturing faults in a low strain-rate region; questions for incorporating active fault data into seismic hazard assessments

Abstract:

Eleven historic surface-rupturing earthquakes (M_W 4.7 – 6.6) have occurred in arid, low-relief, bedrock-dominated areas in the central and western parts of the Australian continent since 1968. Geological and geophysical data, along-fault surface slip distributions, and fault orientations relative to regional compressive stresses, indicate these events may have ruptured along shallow pre-existing Precambrian bedrock structures. Rupture termination is likely controlled by obliquely orientated structures. No unambiguous geological evidence for preceding surface-rupture on these source faults has been found. Palaeoseismic trenching and geomorphic-geochronologic studies provide evidence of absence of analogous surface ruptures over timescales from 10³ to 10⁶ yrs. The absence of evidence for recurrence has implications for applying terms such as ‘active’ and ‘slip-rate’ for these faults. The historic record of faulting in the centre and west of the continent differs from palaeoseismically active faults identified in the south-east, which show recurrence over 10³-10⁴ year timescales. The limited number of studies to date indicate non-periodic recurrence of large events on individual faults, and in deforming regions, raising another set of challenges for seismic hazard assessments. This non-homogenous fault behaviour across the Australian ‘stable continental region’ highlights complexities in using fault-based inputs to assess seismic hazard in low-strain interiors and raises questions for how strain accumulates and dissipates in these crustal settings.