ALBERT

Description: The recent earthquakes in Christchurch, New Zealand, show that active faults, capable of generating large - magnitude earthquakes, can be hidden beneath the Earth’s surface. Here we combine near - surface paleoseismic data with deep (〈5 km) onshore seismic - reflection lines to identify sub - resolution active faults and to explore the relations between fault growth over short (〈27kyr) and long (〉1Ma) timescales in the Taranaki Rift, New Zealand. Displacement rates vary temporally on individual faults by in excess of an order of magnitude over timescales of thousands to millions of years. These changes are attributed to fault interactions rather than to changes in regional strain rates. During the Holocene fault displacement rates were both faster (~50%) and slower (~50%) than their million - year averages. The short - term fault data are incomplete and biased towards the faults that have moved fastest during the Holocene. The integration of different timescale atasets provides a basis for identifying active faults not observed at the ground surface, estimating maximum fault - rupture lengths, inferring maximum short - term displacement rates and improving earthquake hazard assessment.

Description: The recent Mw 7.1 Darfield Earthquake produced rupture of the ground surface along a fault that was not known to exist prior to the earthquake. How many more active faults remain undiscovered, how best to identify these faults and what hazard they might pose are all important questions arising from the Canterbury earthquakes. In this talk we use aerial photograph, fault trenching, seismic reflection, LiDAR and historical seismicity information post 1845 to cast light on the first two of these questions for New Zealand. On the Taranaki Peninsula, where active normal faults generally have slow displacement rates 〈0.5 mm/yr, seismic reflection lines reveal that 〈50% of active faults produce mappable traces and that even where active traces have been mapped these constitute 〈50% of the sub-surface fault length. Similar statistics also apply on the Rangitaiki Plains in the Bay of Plenty, where a recently published LiDAR digital elevation model reveals many more active normal faults with longer traces than was previously identified from 1:16000 aerial photographs and field mapping. In common with the Taranaki example the rates of sedimentation over much of the plains are comparable to the fault displacement rates (0.2-2 mm/yr) and fault scarp burial may be common. To further test the incompleteness of the geological record historical large magnitude earthquakes since 1845 are considered for all of New Zealand. Of the onshore events with magnitudes 〉7 only about half show evidence of surface rupture along the primary fault surface and have the potential to be recorded in the future geological record. For the historical data, however, the main reason active faulting was not observed is that either ground surface rupture did not occur or was below the detection threshold. Together, lack of surface rupture and scarp concealment (or destruction) by surface processes are likely to mean that many active faults, particularly with slower displacement rates (〈0.5 mm/yr), have not yet been discovered. The Taranaki and Bay of Plenty studies suggest, however, that acquisition of data, including LiDAR and seismic reflection lines, can significantly reduce this knowledge gap.

Description: The catastrophic earthquakes that recently (September 4th, 2010 and February 22nd, 2011) hit Christchurch, New Zealand, show that active faults, capable of generating large-magnitude earthquakes, can be hidden beneath the Earth’s surface. In this study we combine near-surface paleoseismic data with deep (〈5 km) onshore seismic- reflection lines to explore the growth of normal faults over short (〈27 kyr) and long (〉1 Ma) timescales in the Taranaki Rift, New Zealand. Our analysis shows that the integration of different timescale datasets provides a basis for identifying active faults not observed at the ground surface, estimating maximum fault-rupture lengths, inferring maximum short-term displacement rates and improving earthquake hazard assessment. We find that fault displacement rates become increasingly irregular (both faster and slower) on shorter timescales, leading to incom- plete sampling of the active-fault population. Surface traces have been recognised for 〈50% of the active faults and along 50% of their lengths. The similarity of along-strike displacement profiles for short and long time inter-vals suggests that fault lengths and maximum single-event displacements have not changed over the last 3.6 Ma. Therefore, rate changes are likely to reflect temporal adjustments in earthquake recurrence intervals due to fault interactions and associated migration of earthquake activity within the rift.