ALBERT

Description: We use data from two seismic networks and satellite interferometric synthetic aperture radar (InSAR) imagery to characterize the 22 December 2003 M (sub w) 6.5 San Simeon earthquake sequence. Absolute locations for the mainshock and nearly 10,000 aftershocks were determined using a new three-dimensional (3D) seismic velocity model; relative locations were obtained using double difference. The mainshock location found using the 3D velocity model is 35.704 degrees N, 121.096 degrees W at a depth of 9.7+ or -0.7 km. The aftershocks concentrate at the northwest and southeast parts of the aftershock zone, between the mapped traces of the Oceanic and Nacimiento fault zones. The northwest end of the mainshock rupture, as defined by the aftershocks, projects from the mainshock hypocenter to the surface a few kilometers west of the mapped trace of the Oceanic fault, near the Santa Lucia Range front and the 〉5 mm postseismic InSAR imagery contour. The Oceanic fault in this area, as mapped by Hall (1991), is therefore probably a second-order synthetic thrust or reverse fault that splays upward from the main seismogenic fault at depth. The southeast end of the rupture projects closer to the mapped Oceanic fault trace, suggesting much of the slip was along this fault, or at a minimum is accommodating much of the postseismic deformation. InSAR imagery shows approximately 72 mm of postseismic uplift in the vicinity of maximum coseismic slip in the central section of the rupture, and approximately 48 and approximately 45 mm at the northwest and southeast end of the aftershock zone, respectively. From these observations, we model a approximately 30-km-long northwest-trending northeast-dipping mainshock rupture surface--called the mainthrust--which is likely the Oceanic fault at depth, a approximately 10-km-long southwest-dipping backthrust parallel to the mainthrust near the hypocenter, several smaller southwest-dipping structures in the southeast, and perhaps additional northeast-dipping or subvertical structures southeast of the mainshock plane. Discontinuous backthrust features opposite the mainthrust in the southeast part of the aftershock zone may offset the relic Nacimiento fault zone at depth. The InSAR data image surface deformation associated with both aseismic slip and aftershock production on the mainthrust and the backthrusts at the northwest and southeast ends of the aftershock zone. The well-defined mainthrust at the latitude of the epicenter and antithetic backthrust illuminated by the aftershock zone indicate uplift of the Santa Lucia Range as a popup block; aftershocks in the southeast part of the zone also indicate a popup block, but it is less well defined. The absence of backthrust features in the central part of the zone suggests range-front uplift by fault-propagation folding, or backthrusts in the central part were not activated during the mainshock.

Description: Near urban areas and extending approximately 60 km along the eastern margin of the Livermore Valley, the Greenville fault is the easternmost right-lateral strike-slip fault of the San Andreas system in the greater San Francisco Bay area. Notwithstanding the 1980 Livermore earthquake sequence (mainshock M (sub L) 5.9) on the Greenville fault, there is no record of recency or of Holocene rates of activity on the Greenville fault, yet this fault exhibits clear geomorphic evidence of late Quaternary faulting. In trenches parallel and normal to the fault through alluvial fan deposits at the Laughlin Road site only pedogenic carbonate was available for Formula dating. Therefore, we applied several photon-stimulated luminescence (PSL) sediment-dating procedures to the silt and sand fractions of six samples. The polymineral-fine-silt multi-aliquot age estimates are generally inaccurate, but the single-grain quartz (SGQ) and multigrain quartz single-aliquot regenerative-dose (SAR) ages from sand grains are in stratigraphic sequence. In trench 3A these SAR ages range from 125+ or -11 yrs (before 2007) within the topmost unit L to 13.45+ or -0.79 ka in the base of the lowermost channel-fill unit G. The SAR PSL results demonstrate the importance of the use of SGQ dating for such sediments and provide the first numerical ages used to constrain the slip rate on the Greenville fault. Trench exposures reveal that unit G is an alluvial sequence infilling a paleochannel offset in a right-lateral sense along the northern Greenville fault. Age estimates from upper and middle unit G bracket deposition of subunit Gb are between 11.12+ or -0.55 ka and 10.64+ or -0.85 ka and those from middle and lower unit G bracket deposition of subunit Go are between 11.12+ or -0.55 ka and 13.45+ or -0.79 ka. These SAR PSL age estimates and measurements of the lateral offset constrain a preliminary slip-rate estimate to about 2 mm/yr or higher for the northern Greenville fault zone.

Description: Recent upward revision of the 1872 Owens Valley earthquake from M (sub w) 7.4-7.5 to 7.7-7.9 implies either additional unrecognized rupture length or anomalously strong ground motions associated with this event. We investigate the first possibility through paleoseismic trenching south of the mapped surface rupture in the Haiwee area, where historical accounts suggest significant surface deformation following the earthquake. Trenching focused on a prominent north-striking scarp, herein termed the Sage Flat fault, expressed in Pleistocene alluvial fans east of Haiwee Reservoir. Surficial mapping and ground-based Light Detection and Ranging (lidar) surveying suggest that this fault accommodates east-down normal motion, and possibly a comparable amount of dextral slip. Trenching and luminescence dating brackets the timing of the most recent surface-rupturing earthquake between approximately 25.7 and 30.1 ka, and provides evidence for an earlier event predating this time. In combination with scarp profiling, these dates also suggest a maximum rate of normal, dip-slip fault motion up to approximately 0.1 mm/yr over this period. Although we discovered no evidence for recent surface rupture on the Sage Flat fault, a series of subvertical fractures and fissures cut across young trench stratigraphy, consistent with secondary deformation associated with seismic shaking. As such, we suggest that possible ground disturbance in the Haiwee area during the 1872 event primarily reflected ground shaking or liquefaction-related deformation rather than triggered slip. In addition, we infer a structural and kinematic connection between the Owens Valley fault and oblique-dextral faults north of Lower Cactus Flat in the northwestern Coso Range, rather than a west-step into northern or western Rose Valley. Consideration of these structures in the total extent of the Owens Valley fault suggests a length of 140 km, of which at least 113 km ruptured during the 1872 event. Online Material: Procedural details and expanded results from the OSL sample analyses, as well as high-resolution paleoseismic trench logs.