Search USGS⌕ Search

Geology topics

J.J. Zucca

Publications and source records attributed to J.J. Zucca.

5 recordsLinked to original sources
USGS2012

Active high-resolution seismic tomography of compressional wave velocity and attenuation structure at Medicine Lake Volcano, northern California Cascade Range

We determine compressional wave velocity and attenuation structures for the upper crust beneath Medicine Lake volcano in northeast California using a high-resolution active source seismic tomography method. Medicine Lake volcano is a basalt through rhyolite shield volcano of the Cascade Range, lying east of the range axis. The Pg wave from eight explosive sources which has traveled upward through the target volume to a dense array of 140 seismographs provides 1- to 2-km resolution in the upper 5 to 7 km of the crust beneath the volcano. The experiment tests the hypothesis that Cascade Range volcanoes of this type are underlain only by small silicic magma chambers. We image a low-velocity low- Q region not larger than a few tens of cubic kilometers in volume beneath the summit caldera, supporting the hypothesis. A shallower high-velocity high-density feature, previously known to be present, is imaged for the first time in full plan view; it is east-west elongate, paralleling a topographic lineament between Medicine Lake volcano and Mount Shasta. This lineament is interpreted to be the result of an old crustal weakness now affecting the emplacement of magma, both on direct ascent from the lower crust and mantle and in migration from the shallow silicic chamber to summit vents. Differences between this high-velocity feature and the equivalent feature at Newbeny volcano, a volcano in central Oregon resembling Medicine Lake volcano, may partly explain the scarcity of surface hydrothermal features at Medicine Lake volcano. A major low-velocity low- Q feature beneath the southeast flank of the volcano, in an area with no Holocene vents, is interpreted as tephra, flows, and sediments from the volcano deeply ponded on the downthrown side of the Gillem fault, a normal fault mapped at the surface north of the volcano. A high- Q normal-velocity feature beneath the north rim of the summit caldera may be a small, possibly hot, subsolidus intrusion. A high-velocity low- Q region beneath the eastern caldera may be an area of boiling water between the magma chamber and the ponded east flank material. These structural data are useful both for understanding Cascade Range volcanism and for geothermal development in progress on the volcano.

Journal of Geophysical Research Solid Earth↗
USGS2003

New signatures of underground nuclear tests revealed by satellite radar interferometry

New observations of surface displacement caused by past underground nuclear tests at the Nevada Test Site (NTS) are presented using interferometric synthetic aperture radar (InSAR). The InSAR data reveal both coseismic and postseismic subsidence signals that extend one kilometer or more across regardless of whether or not a surface crater was formed from each test. While surface craters and other coseismic surface effects (ground cracks, etc.) may be detectable using high resolution optical or other remote sensing techniques, these broader, more subtle subsidence signals (one to several centimeters distributed over an area 1-2 kilometers across) are not detectable using other methods [Barker et al., 1998]. A time series of interferograms reveal that the postseismic signals develop and persist for months to years after the tests and that different rates and styles of deformation occur depending on the geologic and hydrologic setting and conditions of the local test area.

Geophysical Research Letters↗
USGS1987

A geologic interpretation of seismic-refraction results in northeastern California

In 1981, the U.S. Geological Survey conducted a seismic-refraction experiment in northeastern California designed to study the Klamath Mountains, Cascade Range, Modoc Plateau, and Basin and Range provinces. Key profiles include 135-km-long, north-south lines in the Klamath Mountains and Modoc Plateau provinces and a 260-km-long, east-west line crossing all of the provinces. The seismic-velocity models for the Klamath and Modoc lines are comparatively homogeneous laterally but are quite different from each other. The Klamath model is finely layered from the surface to at least 14-km depth, consisting of a series of high-velocity layers (6.1–6.7 km/s), ranging in thickness from 1 to 4 km, with alternating positive and negative velocity gradients. A layer with an unreversed velocity of 7.0 km/s extends from 14 km to an unknown depth. The Modoc model, in contrast, is relatively thickly layered and has lower velocities than does the Klamath model at all depths down to 25 km. An upper layer, 4.5 km thick, of low-velocity material (2.1–4.4 km/s) overlies a basement with a considerably higher velocity (6.2 km/s). Velocity increases slowly with depth, with a small velocity step (to 6.4 km/s) at 11 km and a 7.0-km/s layer beginning at 25-km depth. Moho is probably 38–45 km deep under the Modoc Plateau, but its depth is unknown under the Klamath Mountains. A combined velocity-density model for the east-west line consists of a western part similar in configuration to the Klamath velocity model, an eastern part similar to the Modoc velocity model, and laterally changing velocity-density structure in between, in the Cascade Range. Beneath its upper layer, the velocity model for the Modoc Plateau is similar to that determined by other researchers for the adjacent Sierra Nevada. The velocity model is unlike those for rift areas, to which the Modoc Plateau has been compared by some authors. We theorize that beneath a veneer of volcanic and sedimentary rocks (the upper layer), the Modoc Plateau is underlain by a basement of granitic and metamorphic rocks that, like rocks in the Sierra Nevada, are the roots of one or more magmatic arcs. The fine layering in the Klamath seismic-velocity model is consistent with the geologic structure of the Klamath Mountains, characterized by imbricate thrusting of oceanic rock layers of various compositions and ages. Independent modeling of aeromagnetic data indicates that the base of the Trinity ultramafic sheet, the second major rock layer down in the structural sequence, corresponds to a velocity step to 6.7 km/s at 7-km depth in our model. The 6.7-km/s layer beneath the Trinity ultramafic sheet apparently corresponds to rocks of the central metamorphic belt, which are mafic schists. Rock units structurally deeper than rocks of the central metamorphic belt can be correlated with velocity layers below the 6.7-km/s layer, but with less certainty. In the model for the east-west line, the region of laterally changing velocity structure beneath the Cascade Range includes a 10-km step down to the east in the top of the 7.0-km/s layer. This region of lateral velocity change we interpret to be a fault, fold, or intrusive contact (or some combination of the three) between the stack of oceanic rock layers that underlie the Klamath Mountains and the buried roots of magmatic arcs inferred to underlie the Modoc Plateau. Magmas forming the modern Cascade Range arc apparently rise through this region.

GSA Bulletin↗