NS24A-01
Fault Connections and Probability of Multi-Fault Rupture in Southern California
Geology and past earthquakes show that faults are discontinuous at the surface, and are often represented by complex networks of traces and splays. It is difficult to quantify fault length from mapped faults due to these intricacies in fault networks, which means it is also difficult to estimate maximum magnitude from these faults. A correct estimate of maximum magnitude is important in order to determine the magnitude distribution of a given fault line. If the fault length is underestimated then the magnitude distribution of the fault will be misrepresented. In previous work, we have looked at past earthquakes and studied the rupture patterns of these events. First we compare fault length and rupture length for post 1975 earthquakes in Southern California. We found that rupture length and fault length were often unequal and multiple faults often ruptured in a single event. Furthermore, we created a set of guidelines for estimating length from mapped faults. Second, we studied the probability of multi- fault rupture based on the distances between mapped faults. The probabilities are derived from observations of ruptures that did or did not jump available step-overs. We found that an exponential model best fit the data, and that multi-fault rupture became unlikely at distances larger than 10 km. In this study I will examine the areas between mapped faults to look for possible fault-connections and then assess the probability of multi-fault rupture. I will use micro-seismicity to determine if there are features at depth connecting faults already mapped at the surface. In regions of interest I will also use remote sensing to check for any deformation or offset at the surface. Next, we will use the findings from our previous studies to examine the probability of multi-fault rupture. Finally, these probabilities will be used to determine the frequency of large events.
NS24A-02 INVITED
Basin Structure Beneath the Santa Rosa Plain, Northern California: Implications for Damage Caused by the 1969 Santa Rosa and 1906 San Francisco Earthquakes
Regional gravity data in the northern San Francisco Bay region reflect a complex basin configuration beneath the Santa Rosa plain that likely contributed to the significant damage to the city of Santa Rosa caused by the 1969 M 5.6, 5.7 Santa Rosa earthquakes and the 1906 M 7.9 San Francisco earthquake. Inversion of these data indicates that the Santa Rosa plain is underlain by two sedimentary basins about 2 km deep separated by the Trenton Ridge, a shallow west-northwest-striking bedrock ridge west of Santa Rosa. The city of Santa Rosa is situated above the 2 km-wide protruding northeast corner of the southern basin where damage from both the 1969 and 1906 earthquakes was concentrated. Using the gravity-defined basin geometry, ground motion simulations of the 1969 and 1906 earthquakes, two events with opposing azimuths, show enhanced ground motions along the northeastern edge of this corner suggesting that basin-edge effects contributed to the concentration of shaking damage in these events. This indicates that basin edge effects will likely contribute to strong shaking in this area during future earthquakes. Characterization of low-velocity basins and their detailed three-dimensional subsurface geometry is an essential component for characterizing the seismic hazard in the northern San Francisco Bay region and elsewhere.
NS24A-03
Seismic Imaging and Gravity Studies of the Eagle Rock and Raymond Faults in Arroyo Seco, Los Angeles County, California
In August 2007, we acquired high-resolution reflection and refraction data, using both P waves and S waves, across the Eagle Rock and Raymond faults in the Arroyo Seco, Pasadena and South Pasadena, respectively. The studies were conducted to determine the precise location of the faults, their dips, and their possible structural connections. The seismic profiles crossed geologically mapped projections of the faults near the center of the respective profiles. The northern profile, across the Eagle Rock fault, was 1200 m long and was recorded by 240 geophones at 5-m intervals. Seismic sources for the northern line were generated by Betsy-Seisgun ‘shots' at all recording locations and S-wave sources at 70 recording locations distributed symmetrically about the Eagle Rock fault. The southern line, across the Raymond fault, was 450 m long and recorded by 90 geophones at 5-m intervals. Seismic sources for the southern line were generated by combined accelerated weight drops and/or sledgehammer impacts at all recording locations, seisgun shots at a few locations, and S-wave sources at 70 locations. Preliminary shot gathers of P-wave data for both northern and southern profiles demonstrate abundant, robust reflectors. We used a tomographic velocity image from the P-wave data in order to image the structure of faults in both profiles. The resulting velocity models will be used to help migrate the acquired seismic reflection data. We will also create tomographic velocity images from the S-wave data in order to calculate Poisson's ratio and Vp/Vs ratio along the profiles. In addition, gravity data were acquired along both northern and southern profiles at 25-m intervals. Preliminary processing of the gravity data indicates lateral variations in rock density that we will combine with seismic results in the final interpretation.
NS24A-04
Geophysical Characterization of Yucca Fault in Yucca Flat, Nevada Test Site, Nevada
Yucca Fault is a north-striking, east-dipping normal fault located in the eastern half of Yucca Flat, an elongate basin on the Nevada Test Site in the Basin and Range province. The fault offsets alluvial material in the shallow subsurface and has a prominent surface scarp; abundant drill-hole data record offsets of 200 to 400 m on underlying Miocene volcanic rocks and Paleozoic carbonate bedrock. Understanding the geometry and physical properties of the Yucca Fault and other fault zones in Yucca Flat is an important component in assessing the role of faults in the groundwater flow and transport at the Nevada Test Site. From 2005 to 2007, we conducted near- surface geophysical studies across the surface trace of the Yucca Fault to determine the feasibility of the chosen methods in helping to evaluate the extent of footwall damage and to investigate possible hanging wall splay faults. In particular, we selected an area of the fault that exhibited marked sinuosity and multiple splays that result from the interaction of en echelon linked segments. We acquired direct current (dc) resistivity, compressional (P) and shear wave seismic, ground magnetic, and audio magnetotelluric (AMT) data. The dc resistivity data, including both two-dimensional and three-dimensional data sets, show a 200 ohm-m higher resistivity on the hanging wall than on the footwall. These data sets provide the best high-resolution detail of the subsurface geometry of the fault zone for the top 30 to 35 m. These data correlate well with detailed surface fracture mapping, where subtle changes in the shallow resistivity structure are interpreted to be a result of structural disruption of caliche cementation within the alluvium. The P-wave seismic data show a lower velocity zone that corresponds with the surface expression of the fault. The shear wave data, however, appear to suffer from converted (P-wave) arrivals produced by the shallow caliche layer and therefore do not provide useful information. The total-field magnetic data exhibit a relatively sharp 180 nT transition across the fault, which we interpret to be due to the fault offset of alluvial fill and underlying Miocene welded tuff. Two AMT profiles were collected coincident with two of the dc resistivity profiles, and they exhibit lateral variations in resistivity that correlate with the mapped surface expression of Yucca Fault. At shallow depths, the AMT data show the same resistivity trend across the fault as do the dc resistivity data, but the AMT data show the opposite transition at greater depths. The results of these geophysical methods encompass varying scales of resolution and depths of investigation and aid in the characterization of fault effects in the alluvium and underlying volcanic rocks.
NS24A-05
Near-surface geophysical surveying of the Green Valley fault, Fairfield CA
We conducted seismic and ground-penetrating (GPR) surveys of the strike-slip Green Valley fault zone with the goals of imaging buried stream channels offset by the fault and determining the long-term slip rate of the fault. Recording geometry and line locations were selected based on results from previous geophysical surveys and lithologic logs from a 5 m deep pit excavated the previous year. We recorded a grid of nine fault-parallel lines, 90 m long and spaced 5 m apart, using both seismic refraction and GPR. Stratigraphy was resolved in the upper 2-3 m of the GPR profiles and may be confidently correlated between lines. Ground-truth information was provided by the 5 m deep pit and two new 3 m deep trenches. In addition GPR lines were recorded at the trench locations prior to trenching. We conducted particle-size analyses of sedimentary units exposed in the pit. Sediments in the upper 5 m have a bimodal particle size distribution with distinct sandy and silty fractions at 3φ and 7-8φ. In general, the silty fraction is dominant, but two sandy layers a few tens of cm thick were observed in both the pit and trenches and were evidently imaged in the GPR profiles. GPR penetration at the site of the new survey is limited by a highly-conductive clay layer at a depth of 2-3 m below the surface. Seismic refraction surveys provide velocity information to depths of 6-8 m, where a sharp increase in velocity occurs. While previous seismic refraction surveys have helped to constrain the location of the Green Valley fault, the new data with closer line spacing may prove more useful for defining paleodrainage geometry. Auger boreholes and cone-penetration testing (CPT) provide stratigraphic information to 15 m depth that will help constrain velocity models derived from the seismic refraction surveys.
NS24A-06
High Resolution Seismic Reflection Survey Across a Segmentation Transfer Zone in the Ostler Fault Zone, South Island, New Zealand
The majority of the ~45 mm/yr of oblique convergence between the Pacific and Australian plates in the South Island of New Zealand is accommodated on the Alpine Fault. Further convergence is distributed onto structures east of the Alpine Fault. The Ostler Fault Zone is one of these structures, accommodating 1 - 2 mm/year of east- west compression. The fault consists of a series of predominantly west-dipping, highly segmented, surface rupturing thrust faults that transect Quaternary glacial outwash terraces in the Mackenzie Basin. This study focuses on the Benmore section of the fault zone, where folding and a series of small faults accommodate displacement in a transfer zone between two non-overlapping fault segments. Although fault traces have been extensively mapped at the surface and ground-penetrating radar data image steeply dipping faults at depths less than 5 metres, little is known of the structure at greater depths. We have conducted a high resolution seismic reflection survey to determine the structure of the fault, and associated deformation in the hanging wall, and to track lateral variations in these structures across the transfer zone. Twelve 1.2-km-long seismic lines were recorded perpendicular to the fault strike, covering approximately 1.6 km of fault length. Two additional tie lines were recorded parallel to strike. We used a 240 channel acquisition system with 3 and 6 m receiver and shot spacings, respectively. In addition, we recorded a 340-m-long ultra-high-resolution line with 0.5 m receiver and 1m shot spacing. Application of standard seismic reflection processing techniques to these high quality data sets reveals dipping sedimentary layering in both the hangingwall and the footwall down to 650 ms two way travel time, where a strong horizontal reflection occurs. Additional, weaker, horizontal reflections are recorded down to 1300 ms ttwt. Structures identified so far indicate that compression in the hanging wall of the fault is accommodated by both folding and subsidiary faulting, varying laterally across the transfer zone. More sophisticated processing techniques are being applied in an attempt to image steeper dipping structures.
NS24A-07
High-Resolution Seismic Imaging of Active Strike-Slip Faults in Coastal and Offshore Southern California
Identifying and locating active faults in coastal and offshore areas require high-resolution geophysical methods due to the subaqueous environment. Multi-channel seismic reflection profiling using high-frequency (100-10,000 Hz) energy sources and state-of-the-art digital streamers provide excellent high-resolution images of subsurface structure in layered coastal and offshore sediments. Other geophysical methods, such as electrical resistivity, and geotechnical methods, such as cone penetrometer methods, supplement the seismic profiles to map fault traces onto adjacent subaerial sites. In some cases, transition zone seismic methods can be used where subaqueous energy sources are recorded both by hydrophones in the water and geophones on the adjacent subaerial sites, or vice versa with land-based seismic sources. For seismic hazard evaluations, the geophysical studies must be combined with geologic ground truth typically obtained from boreholes. Coastal geology, where land meets sea and changing sea levels shift sediment depositional environments from subaerial to subaqueous produce complex imaging challenges. Careful data acquisition and processing must be used to match acoustic source/receiver combinations with anticipated subsurface geologic structure and to achieve imaging requirements. Shallow gas and other buried geologic anomalies sometimes disrupt the subsurface image, which complicates the geologic interpretation. We present several examples of high- resolution seismic profiles used to identify and locate active (Holocene) fault traces in coastal and offshore areas of southern California necessary for earthquake fault rupture evaluations (Alquist-Priolo Earthquake Fault Zones).