Applications of high resolution topography in tectonic geomorphology

Abstract

In recent years, sub-meter scale topography data have become increasingly available, mostly from laser scanning methods and satellite stereophotogrammetry. These data have increased the extent to which we can remotely document and analyze tectonic features, and allow us to capture higher resolution details. In particular, we can use DEMs to carefully map surface deformation from ground-rupturing earthquakes---both at the fault and in the near-field---producing detailed records of rupture patterns, slip magnitude, damage zone properties, and scarp preservation; these characteristics can then be considered with dynamic rupture processes and the earthquake cycle. In this thesis, we approach tectonic questions with high resolution topography data, observing the geomorphic signatures of recent earthquakes, and developing routines that extract rupture information from modern surfaces. The three body chapters consist of independent journal manuscripts connected by this common theme. In Chapter 2, we demonstrate a low-cost and logistically practical procedure for independently creating high resolution (sub-decimeter) topography data, rather than relying on industrial methods. This method builds on photogrammetry to resolve surface shape from overlapping photographs and a few georeferencing points, producing sufficient quality elevation data to make geometric measurements. Recovered elevations are comparable to those from traditional laser scanning methods to within reported errors. We demonstrate our methodology at two tectonic sites in California: (1) a slip rate site, where fluvial features are offset by the southern San Andreas fault Banning strand; and (2) a section of the 1992 $M_w$~7.2 Landers earthquake scarp, which is undergoing continuous degradation monitoring. This method has become commonplace in tectonics, and among other geologic applications. In Chapter 3, we revisit the densely vegetated 1959 M$_w$~7.2 Hebgen Lake earthquake surface rupture with newly acquired lidar topography data. We produce dense throw distributions along the major faults activated by the earthquake, in most places observing offsets that greatly exceed 1959 measurements. This suggests that---although the scarps do not consistently express a distinct, muliti event topographic signal---we have captured at least one paleo-earthquake, in agreement with trenching results. We compute roughness along the throw distribution for each fault, finding a smoother distribution for a fault on steep talus slopes that exploits weak bedding planes, which we interpret to reflect slip from only the most recent earthquake. We treat the scarp as the source's planar intersection with the topography, from which we recover shallow fault dip. We resolve highly segmented structures over wavelengths of 100s of meters, and are unable to fit continuous scarps to a single plane. Segment dip averages range $\sim$30-45$^\circ$, much shallower than dips from seismology and geodesy, suggesting anti-listric source geometry that exploits inherited Laramide structures near the surface. Our results have cautionary implications when interpreting paleo-earthquake magnitude and source geometry from morphologically simple scarps. In Chapter 4, we use a pair of lidar datasets spanning the 2010 $M_w$~7.2 El~Mayor--Cucapah earthquake to reveal shallow fault geometry near the northern rupture extent. The earthquake accommadated NW-SE right-lateral shear along the Pacific-North American plate boundary, and also had a normal component. Models mostly agree on moderate to steeply dipping source fault geometry except where a road cut reveals that locally, the Paso Superior fault dips at $<$20$^\circ$. We use a 3D displacement field from Iterative Closest Point (ICP) lidar differencing to determine whether near-field deformation in the road cut proximity corresponds to a shallowly dipping structure. We compute fault dip using heave and throw ratio derived from displacement profiles projected onto the primary rupture. We fit planes to four continuous surface ruptures near the road cut. We model elastic dislocation, inverting surface deformation for simplified, homogenous planar sources. We consistently find moderate to steep dips at distance from the road cut, but shallow dips near or $<$20$^\circ$ for a $\sim$2~km fault length centered on the fault exposure. Our results suggest that the shallowly dipping Paso Superior fault did activate during the 2010 event, and postulates that other low-angle normal faults observed in the geologic record may activate during earthquakes. Taken together, these results show how high resolution topography can be used to understand the structures activated by modern earthquakes. Single, post-event datasets can be used to interpret historic or prehistoric ruptures, with the precaution that scarps may appear morphologically simple, while dataset pairs that capture near-fault surface displacement can provide additional constraints on shallow structures.