[1] 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 groundpenetrating 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 $20 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 55-75°SE and $20 m and 72-84°SE and $12 m, respectively. Although the georadardefined 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.

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[1] To characterize buried portions of the San Andreas Fault (SAF) near its northern extremity in California, three trenches were excavated perpendicular to it and a 3-D georadar data set was acquired. Similar sequences of unconsolidated fluvial sediments, Franciscan serpentinite and active fault zones were observed in the three trenches. Fault traces in the trenches were initially assumed to be part of a single SAF strand. However, the georadar data demonstrated the existence of two SAF strands, one of which was exposed in the central and southern trenches and the other in the northern trench. Offset of a linear-trending georadar feature (possibly a fluvial paleochannel) suggests that 4.5 -5.5 m of horizontal displacement occurred across the eastern SAF strand, either during the 1906 San Francisco earthquake or earlier. Georadar reflections from flat-lying and trough-shaped sedimentary boundaries are vertically offset by 0.2 -0.3 m along both fault strands.

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