Ultrahigh‐resolution seismic reflection imaging of the Alpine Fault, New Zealand

Abstract

High‐resolution seismic reflection surveys across active fault zones are capable of supplying key structural information required for assessments of seismic hazard and risk. We have recorded a 360 m long ultrahigh‐resolution seismic reflection profile across the Alpine Fault in New Zealand. The Alpine Fault, a continental transform that juxtaposes major tectonic plates, is capable of generating large (M > 7.8) damaging earthquakes. Our seismic profile across a northern section of the fault targets fault zone structures in Holocene to late Pleistocene sediments and underlying Triassic and Paleozoic basement units from 3.5 to 150 m depth. Since ultrashallow seismic data are strongly influenced by near‐surface heterogeneity and source‐generated noise, an innovative processing sequence and nonstandard processing parameters are required to produce detailed information on the complex alluvial, glaciofluvial and glaciolacustrine sediments and shallow to steep dipping fault‐related features. We present high‐quality images of structures and deformation within the fault zone that extend and complement interpretations based on shallow paleoseismic and ground‐penetrating radar studies. Our images demonstrate that the Alpine Fault dips 75°–80° to the southeast through the Quaternary sediments, and there is evidence that it continues to dip steeply between the shallow basement units. We interpret characteristic curved basement surfaces on either side of the Alpine Fault and deformation in the footwall as consequences of normal drag generated by the reverse‐slip components of displacement on the fault. The fault dip and apparent ∼35 m vertical offset of the late Pleistocene erosional basement surface across the Alpine Fault yield a provisional dip‐slip rate of 2.0 ± 0.6 mm/yr. The more significant dextral‐slip rate cannot be determined from our seismic profile.