Application of high resolution DEM data to detect rock damage from geomorphic signals along the central San Jacinto Fault
Introduction
Understanding the structure of active fault zones is important for many branches of earth sciences, including earthquake and fault mechanics and crustal hydrology. Because faults grow and evolve as a result of crustal stresses generated by multiple earthquakes, there are many connections between the processes that govern earthquake ruptures and the structural properties of fault zones. In a typical fault structure, the principal slip zone is surrounded by gouge and embedded within a tabular or wedge shaped damage zone which can extend to several kilometers depth and several hundred meters width (Ben-Zion and Sammis, 2003, and references therein). However, most of the movement across the fault is accommodated within the narrow localized principal slip zone (Rockwell and Ben-Zion, 2007). The broad damage zones are observed in gravity and geodetic surveys around active faults (e.g., Stierman, 1984, Hamiel and Fialko, 2007), and are also seen in numerical modeling of evolving fault zone structures (Finzi et al., 2009), but they appear to accommodate only minor adjustments in the long term motion of faults (e.g., Chester and Chester, 1998, Rockwell and Ben-Zion, 2007). Clarifying the in-situ properties of fault zone structures can provide important information on the mechanisms that generate each structural component, as well as the stress fields that are operative during and between earthquakes.
In this paper we examine the effect of seismic-induced rock damage on the development of drainages near an active fault zone. In the past, studies that looked at the interaction between active faults and geomorphology (e.g. Whipple, 2004, Densmore et al., 2007) used the tectonic factor only as the source of relief, which caused increased erosion as a result of the increased elevation difference. It was recently suggested by Molnar et al. (2007) that tectonics play an important role in causing rapid erosion of hillslopes by fragmenting the upper crust down to the scale of boulders or smaller. They suggested that fracture density is an important factor affecting rock erodibility. It was shown that both micro- and macrofracture density increase in proximity to faults (Chester and Chester, 1998, Wilson et al., 2003), and that along active faults there is a low-velocity zone that is seen in fault zone trapped waves and is associated with intense damage (Li et al., 1999). Another recent observation regarding fault zones was the existence of highly damaged or pulverized bodies of rock along active strike-slip faults, at distances of up to hundreds of meters away from the fault (Dor et al., 2006a) and with increasing intensity of pulverization closer to the fault (Rockwell et al., 2009). The pulverized rocks undergo intense fracturing in the microscale which reduces their grain size significantly (Rockwell et al., 2009). This reduction in grain size can have an effect of decreasing the rock permeability by increasing moisture retention in the near surface, thereby decreasing infiltration capacity, which in turn increases runoff and promotes the initiation of channels. Along with the general decrease in the strength of the “rock”, the end result may be to cause higher drainage density where damage is more intense, i.e. close to the fault.
Theoretical results indicate that on a fault that separates different elastic solids, ruptures tend to propagate in a wrinkle-like pulse predominately in the direction of slip on the compliant side of the fault (e.g. Weertman, 1980, Ampuero and Ben-Zion, 2008). Such ruptures produce dynamic dilation at the tip that propagates in the direction of motion on the more compliant side of the fault, and dynamic compression at the tip propagating in the other direction. Due to these opposite rupture tip changes in normal stresses, ruptures tend to propagate in the direction of motion of the block with slower seismic velocities at depth, which is referred to as the preferred direction. On bimaterial faults that produce a preferred propagation direction for earthquake ruptures, most of the rock damage is expected to accumulate on the stiffer side, which persistently experiences a tensile stress field during earthquake ruptures (Ben-Zion and Shi, 2005), as it is easier to damage rocks under tension than under compression (Fig. 1). If there is no preferred rupture direction, such as in a homogenous solid, superposition of the damage generated by many earthquakes is expected to be approximately symmetric across the fault. The existence of a preferred propagation direction for ruptures on large faults can have fundamental consequences for many aspects of earthquake physics and estimates of seismic shaking hazard in major metropolitan areas near large faults (e.g. Ben-Zion, 2001).
Asymmetric patterns of rock damage across large strike-slip faults have recently been documented at several localities over several scales. In southern California, Dor et al. (2006b) observed significant asymmetric distribution of damage elements on one side of the principle slip surfaces of the San Jacinto, Punchbowl and San Andreas faults at a scale of a few meters. Lewis et al., 2005, Lewis et al., 2007 found clear asymmetry of low-velocity damaged fault zone layers from analysis of seismic fault zone trapped and head waves along sections of the San Jacinto and San Andreas faults. The fault zone layers imaged in these studies are about 100 m wide and extend through the top few km of the crust. Dor et al. (2008) observed damage asymmetry at scales ranging from sub-meter to over a km across parts of the North Anatolian Fault in Turkey, consistent with the (opposite) rupture directions of the 1943 and 1944 earthquakes on the fault, which are thought to represent long term preferred propagation directions on the corresponding sections of the North Anatolian Fault.
The asymmetry of rock damage across faults may be expressed by differences in the surface hydrology of the drainage systems on the opposite sides of the fault. The increase in damage to the rocks is expected to correlate with more erosion on the one hand, and higher drainage density on the other hand. If indeed the damage is significantly higher on one side compared to the other, then we expect to see the influence of the damage asymmetry in the drainages on both sides of the fault. In general, the erosion intensity and drainage density are affected by many other intrinsic and extrinsic parameters, which include climate, rock unit, soil type, slope, aspect, relief, land use, basin development stage, etc. However, in cases where those variables are similar across terrains, the different levels of rock damage across the fault may be the influential factor on erosion and the development of drainage patterns.
In this paper, we study two neighboring terrains, which lay on the two sides of a major fault — the San Jacinto Fault. Quantitative comparison of geomorphic parameters related to erosion and drainage patterns is used to study the underlying distribution of rock damage, and to examine the symmetry properties of damage across the fault. Because the chosen terrains are approximately similar in their generic parameters that may affect erosion, having similar climate, geology and geomorphic history, earthquake-induced damage may be invoked to explain any observed differences in the erosion intensity and drainage patterns that are indicated by the analyses done in this work.
Section snippets
Regional setting
The San Jacinto Fault (SJF) is one of the major branches of the San Andreas Fault (SAF) system in southern California, and extends from the Transverse Ranges southeastward into the Salton Trough (Fig. 2). It is presently the most seismically active fault in southern California, with a geologic slip rate of 12–14 mm/yr (Rockwell et al., 1990) and post-Cretaceous cumulative offset of ~ 24 km (Sharp, 1967). The SJF is a young fault, but its age is poorly constrained to 1–2.5 Ma (Matti and Morton, 1993
Data
We use two datasets with different resolutions to examine the geomorphic parameters of the study area. The first is a 30 m pixel DEM derived from SRTM (Shuttle Radar Topography Mission) data, obtained from the USGS seamless server (http://seamless.usgs.gov). This will be referred to as the SRTM dataset. The second is a 1 m pixel DEM derived from the point cloud data of the B4 LiDAR (Light Distance And Ranging, a.k.a. ALSM (Airborne Laser Swath Mapping)) project, obtained from the GEON portal (//www.geongrid.org
SRTM dataset
We select 19 drainages NE of the fault and 15 drainages SW of the fault for the comparison of parameters. The chosen drainages are all contained within the buffer area and the igneous–metamorphic rock units. Fig. 6 illustrates the drainage locations, the rock units and the Hi values. The comparison results are summarized in Table 2. Two parameters are compared across the fault for each rock unit — the hypsometric integral (Hi) and Drainage Density (Dd). Looking at the study area as a whole, the
Discussion
We have presented an analysis of the geomorphology near the San Jacinto Fault using several different methods to explore drainage properties. We show that drainage density increases in close proximity to the fault, and demonstrate that some geomorphic differences between the drainages on the two sides of the fault cannot be explained by the usual factors that control drainage morphology. This suggests that fault related damage is a likely factor that can produce those differences. Here we
Conclusions
The rock damage signal along the Clark strand of the San Jacinto Fault was inferred from observations derived from two elevation datasets and a suit of hydrological and GIS tools, and there appears to be a correlation between the amount of damage across and within the fault zone and the drainage properties in those areas. The results from the lower resolution SRTM data point to a ~ 1 km wide damage zone centered on the main strand of the San Jacinto Fault. The use of high resolution topography
Acknowledgements
We thank Ken Hudnut, the GEON portal website and the people behind it (www.geongrid.org) for access to the B4 LiDAR data. This paper benefited greatly from the comments of Ramon Arrowsmith and an anonymous reviewer.
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