Abstract - The relation between maximum displacement and length provides important information about the scaling properties of faults and earthquakes. Displacement-length plots typically reveal slopes of 1-1.5 for ancient fault systems and 1 for historical earthquakes. Active normal faults in the Taupo rift, however, have slopes of 0.5-0.7 for paleoearthquakes and faulted volcanic surfaces ranging up to ca. 60 ka in age. These low slopes are attributed to two principal factors: sampling bias and fault interaction/intersection. The bulk of our data are from a digital elevation model of displaced geomorphic surfaces with a vertical resolution of ~2 m. Displacements less than ~2 m are not routinely imaged in the model resulting in an under-sampling of small faults and a reduction in lengths of at least 100-200 m. Both of these sampling artefacts introduce a systematic under-sampling of low displacement-length faults particularly for those faults with lengths of <1 km, and therefore lead to a decrease in the slope of the displacement-length relation. Fault interaction and displacement transfer in the rift locally enhances the growth of short faults (e.g., <2 km) that are often immediately adjacent to, and at the terminations of, the larger faults. Although many of these short high-displacement faults may not be physically linked (i.e. hard-linked) with other structures, their displacements are transferred to adjacent faults by deformation of the intervening rock volume, a kinematic scenario referred to as soft-linkage. Whatever the nature of linkage between faults (i.e. hard- or soft-), complementary displacement transfer permits all faults to be components of a single kinematically coherent system, with faults therefore displaying interdependent, and complex, earthquake histories. The accentuated displacement transfer associated with fault interactions and intersections, nevertheless means that related faults are marked by higher than average displacement changes and by high displacement-length ratios, irrespective of time scale. Since the rift comprises many (>100) short interacting/intersecting faults they are together responsible for a decrease in the slope of the displacement-length relation. As extension continues to accrue across the rift, we suggest that such short faults are either temporally or permanently abandoned, or are incorporated into larger faults, scenarios which result in an increase in the slope of both the finite fault and earthquake displacement-length relations with fault system maturity.
Abstract of talk given to:
International Union of Geodesy and Geophysics XXIV, Perugia, July 2007.