HIGH-RESOLUTION GPR AND SEISMIC REFLECTION SURVEYING OF SHALLOW FAULT DEFORMATION

Because historical and instrumental records of seismicity in most parts of the world are much shorter than the average recurrence intervals of large earthquakes, the seismic hazard posed by many active faults is not well understood. In fact, some of the most devastating earthquakes of recent times (e.g., Kashmir, 2005; Sichuan, 2008; Haiti, 2010; Christchurch, 2011) have occurred on faults that were either previously unknown, or whose threat had not been recognized. In places where seismic hazard is poorly defined, conventional paleoseismic techniques based on surface mapping, trenches, and boreholes can provide important details on the structure, kinematics, and timing of past fault movements. These methods are most effective in places where 1) the faults are well-expressed in the landscape, 2) the shallow deformation zone is relatively narrow, and 3) fault-zone structures do not change appreciably along strike. More complex shallow fault deformation is often difficult to characterize using these methods. Here, we show how high-resolution ground-penetrating radar (GPR) and seismic reflection surveys can help overcome these shortcomings and provide valuable complementary information to paleoseismic investigations. We present geophysical data acquired over active faults from two different tectonic settings in New Zealand: 3-D GPR data over a zone of normal faults within the Maleme Fault Zone and 2- and 3-D GPR and seismic reflection data across a northern section of the transpressive Alpine Fault Zone. Our data sets show new details of fault deformation at these locations, including temporal variations in fault displacement patterns, along-strike variations in fault-zone morphology, and evidence for diffuse off-fault deformation. Furthermore, by integrating the geophysical data with other geological observations, we obtain new and refined fault slip-rate estimates that can be used for regional seismic hazard assessments.