We use seismic, tilt, lidar, thermal, and gravity data from 32 consecutive eruption cycles of Lone Star geyser in Yellowstone National Park to identify key subsurface processes throughout the geyser's eruption cycle. Previously, we described measurements and analyses associated with the geyser's erupting jet dynamics. Here we show that seismicity is dominated by hydrothermal tremor (~5-40 Hz) attributed to the nucleation and/or collapse of vapor bubbles. Water discharge during eruption preplay triggers high-amplitude tremor pulses from a back azimuth aligned with the geyser cone, but during the rest of the eruption cycle it is shifted to the east-northeast. Moreover,~4 min period ground surface displacements recur every 26 ± 8 min and are uncorrelated with the eruption cycle. Based on these observations, we conclude that (1) the dynamical behavior of the geyser is controlled by the thermo-mechanical coupling between the geyser conduit and a laterally offset reservoir periodically filled with a highly compressible two-phase mixture, (2) liquid and steam slugs periodically ascend into the shallow crust near the geyser system inducing detectable deformation, (3) eruptions occur when the pressure decrease associated with overflow from geyser conduit during preplay triggers an unstable feedback between vapor generation (cavitation) and mass discharge, and (4) flow choking at a constriction in the conduit arrests the runaway process and increases the saturated vapor pressure in the reservoir by a factor of~10 during eruptions.

[1] Taking advantage of large datasets of both gravity and elastic wave arrival time observations available for the Parkfield, California region, we generated an image consistent with both types of data. Among a variety of strategies, the best result was obtained from a simultaneous inversion with a stability requirement that encouraged the perturbed model to remain close to a starting model consisting of a best fit to the arrival time data. The preferred model looks essentially the same as the best-fit arrival time model in areas where ray coverage is dense, with differences being greatest at shallow depths and near the edges of the model where ray paths are few. Earthquake locations change by no more than about 100 m, the general effect being migration of the seismic zone to the northeast, closer to the surface trace of the San Andreas Fault.

We present newly compiled magnetic, gravity, and geologic datasets from the Parkfield region around the San Andreas Fault Observatory at Depth (SAFOD) pilot hole in order to help define the structure and geophysical setting of the San Andreas Fault (SAF). A 2‐D cross section of the SAF zone at SAFOD, based on new, tightly spaced magnetic and gravity observations and surface geology, shows that as drilling proceeds NE toward the SAF, it is likely that at least 2 fault bounded magnetic slivers, possibly consisting of magnetic granitic rock, serpentinite, or unusually magnetic sandstone, will be encountered. The upper 2 km of the model is constrained by an order of magnitude increase in magnetic susceptibility at 1400 m depth observed in pilot hole measurements. NE of the SAF, a flat lying, tabular body of serpentinite at 2 km depth separates two masses of Franciscan rock and truncates against the SAF.

Though shallow flow of hydrothermal fluids in Long Valley Caldera, California, has been well studied, neither the hydrothermal source reservoir nor heat source has been well characterized. Here a grid of magnetotelluric data were collected around the Long Valley volcanic system and modeled in 3‐D. The preferred electrical resistivity model suggests that the source reservoir is a narrow east‐west elongated body 4 km below the west moat. The heat source could be a zone of 2–5% partial melt 8 km below Deer Mountain. Additionally, a collection of hypersaline fluids, not connected to the shallow hydrothermal system, is found 3 km below the medial graben, which could originate from a zone of 5–10% partial melt 8 km below the south moat. Below Mammoth Mountain is a 3 km thick isolated body containing fluids and gases originating from an 8 km deep zone of 5–10% basaltic partial melt.

The Tualatin basin, west of Portland (Oregon, USA), coincides with a 110 mGal gravity low along the Puget-Willamette lowland. New gravity measurements (n = 3000) reveal a three-dimensional (3-D) subsurface geometry suggesting early development as a fault-bounded pull-apart basin. A strong northwest-trending gravity gradient coincides with the Gales Creek fault, which forms the southwestern boundary of the Tualatin basin. Faults along the northeastern margin in the Portland Hills and the northeasttrending Sherwood fault along the southeastern basin margin are also associated with gravity gradients, but of smaller magnitude. The gravity low refl ects the large density contrast between basin fi ll and the mafi c crust of the Siletz terrane composing basement. Inversions of gravity data indicate that the Tualatin basin is ~6 km deep, therefore 6 times deeper than the 1 km maximum depth of the Miocene Columba River Basalt Group (CRBG) in the basin, implying that the basin contains several kilometers of low-density pre-CRBG sediments and so formed primarily before the 15 Ma emplacement of the CRBG. The shape of the basin and the location of parallel, linear basin-bounding faults along the southwest and northeast margins suggest that the Tualatin basin originated as a pull-apart rhombochasm. Pre-CRBG extension in the Tualatin basin is consistent with an episode of late Eocene extension documented elsewhere in the Coast Ranges. The present fold and thrust geometry of the Tualatin basin, the result of Neogene compression, is superimposed on the ancestral pullapart basin. The present 3-D basin geometry may imply stronger ground shaking along basin edges, particularly along the concealed northeast edge of the Tualatin basin beneath the greater Portland area.on April 4, 2014 geosphere.gsapubs.org Downloaded from

Tpd Diatomite and diatomaceous mudstone-The diatom flora in these strata (fossil locality 4) suggest a shallow, eutrophic, neutral to slightly alkalic, stagnant lacustrine setting (S. Staratt, written commun. 2006) Tplg Lignite and lignitic mudstone Tpwd Silicified (petrified) wood Tplm Limestone-Brackish or fresh water depositional setting, locally fossiliferous, present as lenses in the upper parts of basaltic andesite flows and breccias Tprb Breccia-Composed dominantly of angular, tectonically slickened clasts of rhyodacitic rocks with variable textures, interpreted to represent a fault scarp-derived breccia. Breccia includes clasts up to 3m in a comminuted matrix derived from rhyodacitic intrusives, flows and tuffs. Breccia locally includes: Tprg Gravel lenses-Intercalated in breccia, up to ~5m thick, unsorted to poorly sorted, unorganized to weakly segregated and cross-bedded. Gravel is composed of rounded to subangular pebbles to cobbles derived from Franciscan and related Mesozoic sources and from older Tertiary volcanics deposited in alluvial fans, debris flows and talus settings LATE TERTIARY VOLCANIC ROCKS Sonoma Volcanics (Pliocene and Miocene)-Rhyolitic to dacitic ash-flow and air fall tuff, andesitic water-lain tuff, and rhyolitic to basaltic flows and flow breccia. Regionally, the volcanic section becomes increasingly silicic up-section, and youngs from southwest to northeast, across the Rodgers Creek-Healdsburg and Maacama faults (McLaughlin and others, 2005; Fox and others, 1985). The Sonoma Volcanics consist of the following units. Tsd Dacitic flows-Mapped near the northeast corner of the Santa Rosa quadrangle and in the southeast half of the quadrangle along the Rodgers Creek Fault Zone. The dacite lacks macroscopic quartz or K-feldspar, commonly contains plagioclase phenocrysts, and rare hornblende Tsr Rhyolitic and rhyodacitic flows and intrusive rocks-Porphyritic to aphanitic, with phenocrysts of quartz and plagioclase. Includes the rhyodacitic rocks of Zamaroni Quarry (7.26±0.04 Ma), the rhyodacitic rocks of Cook Peak (7.94±0.02 Ma), and perlitic to banded rhyolitic to rhyodacitic flow rocks and obsidian in Annadel State Park (4.5±0.01 Ma) Tst Rhyolitic to dacitic and minor andesitic pumiceous tuff-Mostly ash-flows and minor air fall. This unit includes named and unnamed tephra layers of different ages in the Sonoma Volcanics (see Tephra data Tables 2.2 and 2.3, figure 2.2, and discussion in pamphlet) Tstw Crystal-rich rhyolitic to rhyodacitic welded tuff-Welded zones locally at tops of tuff layers that are overlain by flows of andesite or basalt Tstb Andesitic to rhyodacitic breccia (Pliocene)-Mapped locally in the northeast part of Santa Rosa 7.5' quadrangle, between the 4.83 Ma Lawlor Tuff and overlying basaltic andesite. Breccia consists of angular boulders and blocks of basalt, andesite, and vitric, porphyritic rhyodacite, in a lithic rhyo-dacitic pumiceous tuff matrix. The breccia may be associated with syn-volcanic faulting and (or) proximal pyroclastic venting Tsb Andesite, basaltic ...

A three‐dimensional (3‐D) electrical resistivity model of Mono Basin in eastern California, unveils a complex subsurface filled with zones of partial melt, fluid‐filled fracture networks, cold plutons, and regional faults. In 2013, 62 broadband magnetotelluric stations were collected in an array around southeastern Mono Basin from which a 3‐D electrical resistivity model was created with a resolvable depth of 35 km. Multiple robust electrical resistivity features were found that correlate with existing geophysical observations. The most robust features are two 300 ± 50 km3 near‐vertical conductive bodies (3–10 Ω m) that underlie the southeast and northeastern margin of Mono Craters below 10 km depth. These features are interpreted as magmatic crystal‐melt mush zones of 15 ± 5% interstitial melt surrounded by hydrothermal fluids and are likely sources for Holocene eruptions. Two conductive east dipping structures appear to connect each magma source region to the surface. A conductive arc‐like structure (< 0.9 Ω m) links the northernmost mush column at 10 km depth to just below vents near Panum Crater, where the high conductivity suggests the presence of hydrothermal fluids. The connection from the southernmost mush column at 10 km depth to below South Coulée is less obvious with higher resistivity (200 Ω m) suggestive of a cooled connection. A third, less constrained conductive feature (4–10 Ω m) 15 km deep, extending to 35 km is located west of Mono Craters near the eastern front of the Sierra Nevada escarpment and is coincident with a zone of sporadic, long‐period earthquakes that are characteristic of a fluid‐filled (magmatic or metamorphic) fracture network. A resistive feature (103–105 Ω m) located under Aeolian Buttes contains a deep root down to 25 km. The eastern edge of this resistor appears to structurally control the arcuate shape of Mono Craters. These observations have been combined to form a new conceptual model of the magmatic system beneath Mono Craters to a depth of 30 km.

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 2km-wide protruding northeast corner of the southern basin where damage from both the 1969 and 1906 earthquakes was concentrated. Ground-motion simulations of the 1969 and 1906 earthquakes, two events with opposing azimuths, using the gravitydefined basin surface, show enhanced ground motions along the northeastern edge of this corner, suggesting that basin-edge effects contributed to the concentration of shaking damage in this area in the past and may also contribute to strong shaking during future earthquakes.