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

Abstract

[1] 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.

Shallow fault segmentation of the Alpine fault zone, New Zealand revealed from 2- and 3-D GPR surveying

“…On this figure, the southwestward continuation of fault strands s1 and s3 and the region of transpressive folding into the area of dense beech forest are inferred. The subsidiary fault strand s2 has also been imaged on an ultra-highresolution reflection seismic profile that was acquired along a line approximately coincident with GPR profile C-C′ (Kaiser et al, 2009;Figs. 2a and 8b).…”

“…1a and 2b) imaged a steeply dipping fault plane (~80°) in the near surface (<15 m) and revealed evidence for off-fault folding and tilting within a broad~50-m-wide zone of deformation (McClymont et al, 2008a). From interpretations of a recently acquired ultra-high-resolution reflection seismic profile over the same area, Kaiser et al (2009) have demonstrated that the fault dips at 75-80°to the southeast and offsets vertically the dipping surface of underlying Triassic and Paleozoic basement units by~35 m.…”

Neotectonics of the Sevier Desert basin, Utah as seen through the lens of multi-scale geophysical investigations

“…The geophysical data used in this study span a range from relatively deep-penetration P-wave seismic reflection to horizontally-polarized shear-wave (SH) seismic reflection to 200-MHz ground penetrating radar with resolution on the order of 10 cm. Thus, geophysical data have been acquired over a range of resolutions to better connect surface geology to deeper features (Bruno et al, 2010a,b;Kaiser et al, 2009;McBride et al, 2010b). In order to characterize the range of structural styles in the Sevier Desert basin, we chose three study sites, the Clear Lake fault scarp, Tabernacle Hill, and Devils Kitchen (Fig.…”

Flower structures and Riedel shears at a step over zone along the Alpine Fault (New Zealand) inferred from 2‐D and 3‐D GPR images

“…The pattern of faulting at Inchbonnie (i.e., positive flower structures and small step over zones) is unlike the arrangement of alternating strike‐slip and reverse faults observed along the Alpine Fault southwest of our study site [ Norris and Cooper , 1995; Cox and Sutherland , 2007]. By comparison, left‐stepping overlapping fault strands are seen in GPR and high‐resolution seismic reflection data acquired across the Alpine Fault at Calf Paddock to the northeast [ Kaiser et al , 2009; McClymont et al , 2010; Kaiser et al, unpublished manuscript, 2011]. However, fault strands within and outside of the step over zones at Inchbonnie are more numerous and complex than those to the northeast.…”

“…However, fault strands within and outside of the step over zones at Inchbonnie are more numerous and complex than those to the northeast. A noticeable difference between the Inchbonnie and Calf Paddock sites is the thickness of sediments ruptured by the Alpine Fault; ∼200 m at the former (based on preliminary processing of high‐resolution seismic reflection data) versus ∼60 m at the latter [ Kaiser et al , 2009, 2011]. It is possible that the sedimentary cover at Calf Paddock is not thick enough to allow the formation of distinct flower structures and multiple fault strands within the step over zones.…”