Location and geometry of the Wellington Fault (New Zealand) defined by detailed three-dimensional georadar data

Earthquakes with surface-wave magnitudes of 7.3-7.9 are estimated to be associated with the rupture of the Wellington Fault at relatively regular intervals of 500-770 years. The last such earthquake probably happened between AD 1510 and 1660. Along its southern segment, the Wellington Fault passes through Wellington, New Zealand's capital, and the densely populated Hutt Valley. It is considered to be a highly hazardous structure. To map the shallow geometry of the Wellington Fault, we have collected 3-D ground-penetrating radar (georadar) data at two sites along the fault in the Hutt Valley. At one site, the first ever georadar fault plane reflections from an active strike-slip fault are observed. They coincide with conspicuous diffractions generated by abrupt truncations of structures against the fault plane. These georadar data provide the most vivid shallow images of any active fault surveyed to date. At the second site, apparent offsets of lineaments in the sedimentary sections on either side of the fault are consistent with similar to20 m of dextral displacement estimated from the offset of a nearby terrace riser. The dips and minimum depth extents of the primary zones of fault displacement at the two sites are 55-75degreesSE and similar to20 m and 72-84degreesSE and similar to12 m, respectively. Although the georadar-defined zones of faulting are a few meters wide, prominent reflection fabrics suggest that shearing, fracturing, and crushing extend for several tens of meters on either side of the fault.