Abstract - Over millions of years, active faults grow relatively uniformly and may reach kilometre-scale cumulative displacements due to many large magnitude earthquakes. Displacement rates on many individual faults nevertheless become increasingly variable on shorter time scales which may reflect a combination of variable earthquake behaviour (i.e. recurrence interval and single event slip) and sampling artefacts. We examine the relations between fault displacement rates on short (<25 kyr) and long (>1 Myr) timescales in the Taranaki Rift, New Zealand, using trench and coastal outcrop together with onshore and offshore deep (~<5 km) seismic-reflection lines. Comparison between paleoearthquake and million-year (seismic-reflection) displacements on faults in the Taranaki Rift shows that the number of faults in the system, their lengths, displacement rates and earthquake recurrence interval differ significantly for different observational time windows. More specifically, short-term fault data (< 25 kyr) generally overestimate the long-term average displacement rates of individual faults and underestimate the number of active faults as well as their total length and their associated earthquake recurrence interval. Active faults that displace the ground surface by <~1 m are not routinely resolved, resulting in underestimates of fault lengths by up to 85% and incomplete temporal and spatial sampling of paleoearthquakes on faults with short-term displacement rates of ~<0.1 mm/yr. Variations of earthquake recurrence interval and slip of up to an order of magnitude may also contribute to undersampling of those faults which have experienced few earthquakes during the Holocene. Seismic reflection data have been used to improve estimates of maximum fault rupture lengths and to identify potential low displacement rate (<0.1 mm/yr) active faults that were not previously recognised. The integrated dataset provides improved estimates of seismic hazard in the rift. Our analysis may also have application in offshore regions where bathymetry and seismic reflection data are available.
Abstract of talk given to:
Submarine Paleoseismology: The Offshore Search of Large Holocene Earthquakes, European Sciences Foundation, Obergurgl, September 2010.