Crustal seismic reflection profile across the alpine fault and coastal plain at Whataroa, South Island

Abstract

Crustal seismic reflection data recorded across the Alpine Fault and coastal plain at Whataroa in 1998 are used to derive shallow (4 seconds two-way time or s twt) and deep (14 s twt) seismic reflection images and a simple refraction model. A single 25 km long, 636 channel receiver array recorded energy from 50 kg shots fired at 1 km intervals with intervening shots of 2.5 kg at about 300 m intervals along the profile. Record quality varied from good to poor. Across the Alpine Fault, basement lies at about 300Á500 m (0.2Á0.5 s twt), and a weak basement reflection may be attributed the Alpine Fault. Basement deepens sharply at about 6 km to the northwest of the fault, and a 3 km section of probably Miocene and younger sediments is imaged to the northwest. The southern boundary of the coastal basin probably corresponds to the South Westland Fault. The deeper section images a distinct band of strong reflectivity at a depth of about 9Á10 s twt, the base of which is inferred to be Moho. Depth conversion gives a horizontal Moho across the whole profile at a depth of about 27 km.

Advanced seismic imaging techniques characterize the Alpine Fault at Whataroa (New Zealand)

“…For the larger‐scale Whataroa98 P wave model (Figure b) there have been previous analyses using ray tracing of first break refraction arrivals by Davey []. Comparing the final P wave velocity model derived for the Whataroa98 data set in this study with the results obtained by Davey [], Figure shows their compatibility.…”

“…The final velocity model obtained by interpolation and extrapolation of the Whataroa98 data set result from Figure b is overlain by contour lines derived by Davey []. In principal, the shown P wave velocity structures coincide and reveal a deeper sedimentary basin in the northwest and a shallow basement in the southeast.…”

“…According to previous studies [e.g., Garrick and Hatherton , ; Davey , ], a 100–500 m thick sediment layer is expected above the basement in the Whataroa valley. Consequently, the sharp increase to higher velocities at an approximate depth of 400 m could be interpreted as the change from sediment to basement.…”

“…As discussed earlier, this HVL is evident for all used starting models including a model that is structurally similar to that of Davey. Additionally, Davey [] states that the base of the deepest sediment layer as well as the basement velocity is only poorly confined. Thus, we assume that the travel time tomography presented here is sensitive enough to detect this intrabasement feature and shows more detail than the previous model by Davey [].…”

Seismically invisible fault zones: Laboratory insights into imaging faults in anisotropic rocks

“…They conclude that seismic anisotropy is a pervasive feature throughout much of the South Island crust, where many faults exist. This strong anisotropy may explain, for example, why guided wave studies of the Alpine Fault have indicated a 10-40% velocity reduction, whereas refraction studies have implied a much smaller velocity reduction of 4-16% and the fault has been imaged as a weak reflector [Eccles et al, 2015;Davey, 2010;Eberhart-Phillips and Bannister, 2002].…”