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The mobile south flank of Kīlauea Volcano hosts two normal fault systems, the Koa'e fault system (KFS) and the Hilina fault system (HFS). In historical time, at least three M>6.5 earthquakes have occurred on the basal detachment of the Kīlauea Volcano's south flank, with the most recent being the 4 May 2018 M6.9 earthquake. Here we analyze kinematic Global Positioning System data collected from 2001 to 2017 and interferometric synthetic aperture radar data before, during, and after the 2018 M6.9 earthquake to determine the crustal motion across the HFS and KFS faults. Our results indicate that the HFS faults did not significantly slip during the interseismic period from 2007 to 2011. Despite its substantial magnitude, interferometric synthetic aperture radar (InSAR) data show that the 2018 M6.9 earthquake triggered subcentimeter level slip along sections of the previously mapped HFS branches. Up to 20 cm of offset occurred on what appears to be a newly formed (or previously unknown) fault near the eastern end of the HFS. During the 3 months following the M6.9 earthquake, up to ~30 cm of slip occurred along the KFS, which helps accommodate rapid large‐scale subsidence of Kīlauea's summit region as large volumes of summit reservoir magma fed the lower East Rift Zone eruption. The HFS appears to activate only in concert with large earthquakes on the basal detachment. The KFS, on the other hand, moves both seismically during small local earthquakes and aseismically in response to nearby earthquakes and caldera subsidence.

Faults several kilometers long near Pear Lake in Sequoia National Park, California nucleated on preexisting dikes weakened by closely-spaced echelon joints. These joints essentially parallel much more widely spaced regional joints. Fault nucleation by dike fracturing can explain how a nearly planar fault can grow to lengths of several kilometers or more. We suggest that opening mode fractures, including dikes, might serve more commonly as nuclei for faults than is generally appreciated. The longest fault in the study area, the Pear Lake fault, has a trace length of ∼7 km and a maximum measured lateral separation of 86–98 m. A sharp decrease in slip occurs near the center of the trace of the Pear Lake fault where another fault branches from it. Mechanical analyses demonstrate that a sharp decrease in slip is expected where a fault branches. All the faults near Pear Lake show non-elliptical slip distributions. Based on our mechanical analyses, we infer that the observed slip distributions reflect a mechanical interaction among the faults. The geometry and distribution of structures that precede faulting (i.e. dikes and joints) control to a large extent the structure of a fault network and hence the ultimate slip distribution on faults in the network.

Field observations of strike slip faults in jointed granitic rocks of the central Sierra Nevada, California, combined with a mechanical analysis of fault interaction, provide insight into how fault terminations vary with scale. We document here a strike-slip fault system 2–3 km long. Clustered about the west end of the fault system are several dozen faults that parallel the three main fault zones in the system. We interpret this cluster of small faults as a barrier that inhibited growth of fault zones in the fault system. A two-dimensional mechanical analysis shows that a cluster of small faults flanking the tip of a large fault zone will tend to diffuse the stress concentration near the fault zone tip—an analogous effect in engineering is known as crack-tip shielding. Near-tip stress concentrations promote fault growth, and processes that decrease these stress concentrations inhibit fault growth. As faults lengthen and grow, they interact with features at greater distances and over a broader area, so the potential for tip shielding effects will increase as fault length increases. This effect can account for why the mechanisms and character of fault terminations would tend to vary as a function of scale.

International audience; Abstract Available from http://www.agu.org

[1] A suite of new techniques in thermochronometry allow analysis of the thermal history of a sample over a broad range of temperature sensitivities. New analysis tools must be developed that fully and formally integrate these techniques, allowing a single geologic interpretation of the rate and timing of exhumation and burial events consistent with all data. We integrate a thermal model of burial and exhumation, (U-Th)/He age modeling, and fission track age and length modeling. We then use a genetic algorithm to efficiently explore possible time-exhumation histories of a vertical sample profile (such as a borehole), simultaneously solving for exhumation and burial rates as well as changes in background heat flow. We formally combine all data in a rigorous statistical fashion. By parameterizing the model in terms of exhumation rather than time-temperature paths (as traditionally done in fission track modeling), we can ensure that exhumation histories result in a sedimentary basin whose thickness is consistent with the observed basin, a physically based constraint that eliminates otherwise acceptable thermal histories. We apply the technique to heat flow and thermochronometry data from the 2.1-km-deep San Andreas Fault Observatory at Depth pilot hole near the San Andreas fault, California. We find that the site experienced

Received 26 October 2004; revised 11 February 2005; accepted 10 March 2005; published 16 June 2005. [i] Observations of surface deformation allow us to determine the kinematics of faults in the San Francisco Bay Area. We present the Bay Area velocity unification (BAVU, bay view), a compilation of over 200 horizontal surface velocities computed from campaign-style and continuous Global Positioning System (GPS) observations from 1993 to 2003. We interpret this interseismic velocity field using a three-dimensional block model to determine the relative contributions of block motion, elastic strain accumulation, and shallow aseismic creep. The total relative motion between the Pacific plate and the rigid Sierra Nevada/Great Valley (SNGV) microplate is 37.9 ± 0.6 mm yr -1 directed toward N30.4°W ± 0.8° at San Francisco (±2σ). Fault slip rates from our preferred model are typically within the error bounds of geologic estimates but provide a better fit to geodetic data (notable right-lateral slip rates in mm yr -1 : San Gregorio fault, 2.4 ± 1.0; West Napa fault, 4.0 ± 3.0; zone of faulting along the eastern margin of the Coast Range, 5.4 ± 1.0; and Mount Diablo thrust, 3.9 ± 1.0 of reverse slip and 4.0 ± 0.2 of right-lateral strike slip). Slip on the northern Calaveras is partitioned between both the West Napa and Concord/Green Valley fault systems. The total convergence across the Bay Area is negligible. Poles of rotation for Bay Area blocks progress systematically from the North America-Pacific to North America-SNGV poles. The resulting present-day relative motion cannot explain the strike of most Bay Area faults, but fault strike does loosely correlate with inferred plate motions at the time each fault initiated.

Large earthquakes generate frictional heat, and the magnitude of heating is related to the slip magnitude, the applied effective normal stress, and the frictional strength of the fault. We looked for evidence of this heating in apatite fission-track age and track-length distributions of samples from adjacent to and within the San Gabriel fault zone in southern California. The fault is thought to be an abandoned major trace of the San Andreas fault system active from 13 to 4 Ma and has since been exhumed from depths of 2-5 km. At our sample locality, as much as 40 km of total slip is thought to have accumulated along a localized ultracataclasite layer just 1-8 cm thick. We see no evidence of a localized thermal anomaly in either fission-track ages or track lengths-even in samples within just 2 cm of the ultracataclasite. Because of the absence of any measurable impact on fission tracks, we have been able to use forward modeling of heat generation, heat transport, and fission-track annealing to constrain the frictional properties of the fault. We find that either there has never been an earthquake with >4 m of slip at this locality, or the average apparent coefficient of friction must have been