NS23A-01
Near-Surface Seismic Reflection and GPR Imaging of the Active Emigrant Peak Fault, Fish Lake Valley, NV
Multifaceted near-surface geophysical studies of active faulting in the Eastern California Shear Zone are being conducted at the University of Kansas. During the summer of 2006 shallow seismic reflection and GPR data sets were acquired across the active Emigrant Peak fault on the east side of Fish Lake Valley, Nevada. This fault is a normal fault that aids in the transfer of regional right-lateral deformation associated with the Death Valley/Fish Lake Valley fault zone. Locally a 20 m high scarp marks the trace of the main fault across a large, active alluvial fan. The GPR experiment produced a pseudo-3D image approximately 500m by 115m in size with a bin size of 1m by 5m. Depth penetration was dependent on antenna frequency, but reached approximately 25m in the dry alluvial fan sediments. Two 2-D seismic lines were acquired with a depth penetration of approximately 200m using a 30.06 caliber rifle source. The main line was over 400m in length and the cross line over 150m in length. CMP bins were 0.25m in size. Both data types were processed to migrated images and imported into an industry-standard reflection interpretation package. Analysis of the GPR volume allowed the interpretation of numerous normal faults parallel to the main Emigrant fault both near the main scarp and as ‘off-fault' deformation. Some are down-to-the-basin ‘growth faults' and some are antithetic in nature. Faults were only mapped if they were continuous across many x-lines. The migrated seismic images contain numerous reflections, grouped in packages of short reflectors of different amplitudes and dip orientations. The GPR fault planes were transferred onto the seismic data and correlated with obvious breaks in dip and amplitude between the reflection packages. After basic interpretation of the faults the stratigraphic changes across the fault planes were analyzed on the seismic data to estimate offsets at different depths for each fault. Currently, we are working to estimate a quantitative cumulative depth/throw history for each fault using both data sets. The seismic data allows us to quantify cumulative fault throw vs. depth at scales of meters to tens of meters. The GPR is more limited. The GPR data is, however, absolutely necessary to confidently identify the faults. The two data types are thus complimentary, especially in areas of complex ‘off-fault' deformation and active sedimentation.
NS23A-02
Neotectonic Evolution of Normal Faults Within the Taupo Rift (New Zealand) Revealed Using High-Resolution 3D GPR Data
The complex shallow subsurface structures of active fault zones are generally difficult to characterize using traditional paleoseismic techniques like surface mapping and trenching. High-resolution 3D ground penetrating radar (GPR) is an excellent tool for imaging these structures, because GPR data can reveal both the complicated patterns of deformation across the fault zone and along-strike variations in fault morphology. We have acquired several 3D GPR data sets over a ~ 250 x 150 m region that crosses five steeply dipping normal faults within the active Taupo Rift of New Zealand. The kinematically linked faults have ruptured a late Quaternary stratigraphic sequence of fluvial sediments and volcanic tephras. A paleoseismic trench excavated at the same location has allowed us to correlate well-dated tephra units with at least three prominent reflections picked from the migrated and depth-corrected GPR volumes. By mapping offsets of these horizons across the faults, we have determined their tectonic evolution over the past ~ 20 ka. From the GPR data we have measured variations in displacement along the length of each fault and with time. We have also observed distributed strain between faults, including fault-related folding and progressive tilting of hanging-wall strata.
NS23A-03
High Resolution 3D Seismic Reflection Imaging Across the Northern Alpine Fault, New Zealand
The Alpine Fault is a major transform structure that delineates the boundary between the Australian and Pacific plates through the South Island of New Zealand. Geological evidence suggests that ~470 km of dextral strike-slip movement has occurred along its length. Although it has not been affected by major ruptures during the 150 years of recorded history, large earthquakes (magnitude >7.5) have the potential to cause significant damage in inhabited regions. At our study site, the northern section of the fault juxtaposes Haast schist against west-coast sedimentary basement rocks. Recent lateral and smaller dip-slip components of movement are indicated by offset Quaternary river terraces at the site, and ground-penetrating radar data image a steeply dipping fault zone in the shallow subsurface (<15 m depth). The dip and fault structure at greater depths is uncertain. We have conducted a high-resolution 3D seismic reflection survey to image fault zone structure beyond the sedimentary cover into basement rock to ~200 m depth. An area of roughly 184 x 500 m was surveyed using a pseudo-3D shooting configuration that yielded ~50-fold data at a 4 x 2 m spacing. In addition, we recorded a 370 m-long ultra-high resolution seismic reflection line that provided ~60-fold data at 0.25 m intervals. Application of relatively standard seismic reflection processing techniques has yielded high-quality stacked sections that reveal sedimentary layering in shallow river gravels and a strong reflection from the sediment-basement contact. The dipping basement reflection is offset by ~40 m across the main trace of the Alpine fault. We speculate that basement faulting has offset an erosional surface that formed during the most recent major period of glaciation. Such an interpretation results in a Quaternary dip-slip rate in accord with other estimates along the fault in this region. A second basement discontinuity suggests the existence of a subsidiary fault strand approximately 300 m from the main fault.. We intend to employ more sophisticated processing techniques in an attempt to image more steeply dipping structures within the fault zone.
NS23A-04 INVITED
HIGH-RESOLUTION SEISMIC REFLECTION STUDIES OF ACTIVE FAULTS: A CASE STUDY FROM WASHINGTON STATE
In the past five years, new high-resolution seismic surveys have filled in gaps in our understanding of active structures beneath the Puget Lowland region of Washington State. The extensive forests have made recognition of active faults difficult, but new Light Distance and Ranging (LIDAR) detailed topographic data have made a major breakthrough in mapping active faults. Extensive regional and high-resolution marine seismic surveys have been fundamental to understanding the tectonic framework of the area. These marine profiles, however, lack coverage beneath water bodies that large ships cannot navigate and beneath city streets underlain by late Pleistocene glacial deposits that are missing from the waterways. Recent land surveys and profiles in restricted waterways can therefore bridge the gap between paleoseismic and marine geophysical studies, and test elements of models proposed by regional-scale geophysical studies. We have also been venturing into more congested areas to seismically image faults in key urban locations. Results from recent surveys have: 1) documented new faults that had long been suspected in the Olympia area; 2) clarified the relationship between the LIDAR scarps and observed structures across the Tacoma fault zone; 3) provided a window into structures beneath the north and eastern portions of the western Tacoma fault zone; 4) documented deformation along the Seattle fault near a paleoseismic trench; 5) mapped the eastern part the Seattle fault zone beyond its previously mapped limits; and 6) documented multiple fault strands in the Seattle fault zone in the cities of Bellevue and Seattle. The results better constrain interpretations of paleoseismic data collected on these faults, and provide targets for future paleoseismic studies.
NS23A-05
Integrated Geophysical and Geological Fault Assessment at a Hazardous-Waste Landfill: Fluorspar Area Fault Complex, Central United States
Federal and Commonwealth of Kentucky regulations require proposed hazardous waste facilities undergo a surface-fault rupture hazard assessment prior to issuing construction permits. Permanent ground deformation may expose below-ground structures such as landfills and settling ponds, as well as above-ground structures such as tanks and incinerators to rupture and/or topple failure, and thus potential uncontrolled contaminant release. Regulations prohibit placing new hazardous waste facilities within 61 m (200 ft) of a Holocene-active fault. However, identifying and characterizing active faults in areas lacking geomorphic expression is a challenging task, as exemplified in and near the New Madrid seismic zone and Fluorspar Area fault complex (FAFC). In the mid-continent, surface manifestations of active faults are generally impeded by thick sequence of relatively weak, water-saturated Mississippi embayment sediment overlying bedrock. The soft sediment overburden and long recurrence interval between large earthquakes conceal neotectonic structures in bedrock and commonly fail to produce significant or noticeable geomorphic features. A proposed hazardous-waste landfill in western Kentucky is located within the upper Mississippi embayment and above the late Proterozoic-early Cambrian FAFC, an area also coincident with diffuse microseismicity. Integrated geophysical and geological methodologies were essential for a surface-fault rupture assessment. Nearly 1 km of SH-wave seismic reflection data were collected and interpreted for evidence of late Quaternary deformation. Five significant high-angle anomalies were interpreted to extend within approximately 7 m of the ground surface, near the upper limit of the seismic sampling. Eighty-six, densely spaced, continuous cores, each 9.1 m deep, intersected these features. Stratigraphic and chronological analyses were performed on the cores to assess the presence or absence of structure, and to determine the near-surface extent and age of the features. The upper 10 m of sediment ranged between nearly16 ka and greater than 125 ka. Interpretations of geologic cross sections indicate that the most abrupt elevation changes were constrained to post-date a 53.6 to 75.5 ka loess deposit; however, no perceptible displacement was found at the base of a younger loess dated between 16.6 and 23.5 ka. Collectively, these analyses indicate an absence of Holocene deformation beneath the proposed landfill site.
NS23A-06 INVITED
Multi-Methods High-Resolution Seismic Imaging of Active Fault Zones
Traditional methods of seismic imaging in active fault zones can be challenging because of large, abrupt velocity variations and disrupted strata within the fault zones. Such challenges are often more pronounced in the near surface, where accurate images are needed for trenching, ground-water evaluation, hazard evaluation, and other uses. We use a combination of seismic P- and/or S-wave refraction tomography and reflection imaging to characterize fault zones at scales ranging from a few meters to several kilometers. Most fault zones are characterized by reductions in P- and S-wave velocities ranging from about 10 to 35 percent, but in places with thin vadose zones or shallow water tables, these velocity anomalies may not be observed with P waves alone. To identify fault zones in such thin vadose-zone areas and in massive (non reflective) rocks, we use P- and S-wave data to develop images of Vp/Vs and Poisson's ratio, which are particularly sensitive to damaged rock. In addition to imaging the velocity anomalies, the detailed tomographic velocity models are used to accurately stack and migrate the reflection images. The combined velocity and reflection images can, therefore, define multiple characteristics of fault zones. In locations where borehole, trench, and other physical data are present, the combined seismic imaging method has been shown to be highly accurate. Comparison of the combined imaging method with traditional reflection imaging at various sites along the San Andreas other faults emphasizes the structural details added with the combined imaging method.
NS23A-07 INVITED
Shallow Dip of Two Great Basin Normal Faults Demonstrated by Shallow Seismic Reflection With Refraction Tomography
Whether normal earthquake faulting can ever occur on planes dipping as shallowly as 30° is a subject of debate. We have investigated suspect low-angle normal faults in the Great Basin with a combination of two near- surface seismic imaging techniques. Shallow seismic reflection imaging combined with first-arrival travel-time tomography has proved effective in detailing normal-fault geometries from the surface to about 1 km depth on two major Great Basin normal faults. Additional constraints provided by gravity surveys leave no doubt that the basin- forming normal faulting occurred and continues to occur on fault planes dipping less than 40°. The 1954 Dixie Valley earthquake ruptured the southern segment of the Dixie Valley fault in central Nevada. Abundant geologic and paleoseismic evidence points to a 30° dip for the rupture. A smooth, straight fault-plane reflection extends at that dip from within 2 m of the surface to 0.5 km depth in a series of seismic reflection images. Below 0.5 km the fault plane is concealed below a strong capping basalt reflection, though we observe basin stratigraphy terminating against the fault in a rollover geometry. Computing a velocity tomography section coincident with the reflection section, from first arrivals on the reflection records with SeisOpt\textregistered @2DTM (\copyright Optim, 2007), shows that the basin-bottom velocity contrast is coincident with the 30°-dipping reflection and stratigraphic terminations. Although the northern Dixie Valley fault certainly dips steeply at the geothermal plant 45 km north, the southern Dixie Valley 1954 rupture must be shallow-dipping. At the West Ruby Mts. range-front fault in eastern Nevada, a smaller-scale program similarly combining shallow reflection with refraction tomography constrained the geometry of that fault to 100 m depth. Superimposing the reflection image on the tomographic section allows easy identification of basin-bottom, fault, and basin- stratigraphic reflections. Classic tilted gravity-slide blocks 20-30 m wide riding on a detachment are clear in the combined image. This geometry suggests progressive abandonment of slide blocks updip, with the most recent, prehistoric scarp the most downdip of a series of scarps.