A 3D measurement of the offset in paleoseismological studies

Abstract

The slip rate of a seismogenic fault is a crucial parameter for establishing the contribution of the fault tothe seismic hazard. It is calculated from measurements of the offset of linear landforms, such channels,produced by the fault combined with their age. The three-dimensional measurement of offset in buriedpaleochannels is subject to uncertainties that need to be quantitatively assessed and propagated into theslip rate. Here, we present a set of adapted scripts to calculate the net, lateral and vertical tectonic offsetcomponents caused by faults, together with their associated uncertainties. This technique is applied hereto a buried channel identified in the stratigraphic record during a paleoseismological study at the ElSaltador site (Alhama de Murcia fault, Iberian Peninsula). After defining and measuring the coordinates ofthe key points of a buried channel in the walls of eight trenches excavated parallel to the fault, we(a) adjusted a 3D straight line to these points and then extrapolated the tendency of this line onto asimplified fault plane; (b) repeated these two steps for the segment of the channel in the other side of thefault; and (c) measured the distance between the two resulting intersection points with the fault plane.In doing so, we avoided the near fault modification of the channel trace and obtained a three-dimen-sional measurement of offset and its uncertainty. This methodology is a substantial modification ofprevious procedures that require excavating progressively towards the fault, leading to possible under-estimation of offset due to diffuse deformation near the fault. Combining the offset with numericaldating of the buried channel via U-series on soil carbonate, we calculated a maximum estimate of the netslip rate and its vertical and lateral components for the Alhama de Murcia fault.