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Tom Parsons

Publications and source records attributed to Tom Parsons.

21 records · Page 2Linked to original sources
USGS1999

Wide-angle seismic recordings from the 1998 Seismic Hazards Investigation of Puget Sound (SHIPS), western Washington and British Columbia

This report describes the acquisition and processing of deep-crustal wide-angle seismic reflection and refraction data obtained in the vicinity of Puget Lowland, the Strait of Juan de Fuca, and Georgia Strait, western Washington and southwestern British Columbia, in March 1998 during the Seismic Hazards Investigation of Puget Sound (SHIPS). As part of a larger initiative to better understand lateral variations in crustal structure along the Cascadia margin, SHIPS participants acquired 1000 km of deep-crustal multichannel seismic-reflection profiles and 1300 km of wideangle airgun shot lines in this region using the R/V Thompson and R/V Tully. The Tully was used to record airgun shots fired by the Thompson in two different geometries: (1) expanding spread profiles (ESPs) and (2) constant offset profiles (COPs). Prior to this reflection survey, we deployed 257 Reftek and 15 ocean-bottom seismic recorders to record the airgun signals at far offsets. All data were recorded digitally on large-capacity hard disks. Although most of these stations only recorded the vertical component of motion, 95 of these seismographs recorded signals from an oriented 3-component seismometer. By recording signals generated by the Thompson's marine air gun array, operated in two differing geometries having a total volume of 110 and 79 liters (6730 and 4838 cu. in.), respectively, the arrays of wide-angle recorders were designed to (1) image the crustal structure, particularly in the vicinity of crustal faults and Cenozoic sedimentary basins, (2) determine the geometry of the Moho, and (3) image the subducting Gorda and Juan de Fuca plates. Nearly 33,300 air gun shots were recorded along several seismic lines. In this report, we illustrate the expanding spread profiles acquired using the Thompson and Tully, describe the land and ocean-bottom recording of the air gun signals, discuss the processing of the land recorder data into common receiver gathers, and illustrate the processed wide-angle seismic data collected using the Refteks and ocean-bottom seismometers. We also describe the format and content of the archival tapes containing the SEGY-formated, common-receiver gathers for the Reftek data. Data quality is variable but SHIPS appears to have successfully obtained useful data from almost all the stations deployed to record the airgun shots. Several interesting arrivals were observed: including refractions from the sedimentary basin fill in several basins, refractions from basement rocks forming the upper crust, Pg, refractions from the upper mantle, Pn, as well as reflections from within the crust and from the top of the upper mantle, PmP. We separately archived more than 30 local earthquakes recorded by the Reftek array during our deployment.

Washington↗
USGS1998

More than one way to stretch: A tectonic model for extension along the plume track of the Yellowstone hotspot and adjacent Basin and Range Province

The eastern Snake River Plain of southern Idaho poses a paradoxical problem because it is nearly aseismic and unfaulted although it appears to be actively extending in a SW-NE direction continuously with the adjacent block-faulted Basin and Range Province. The plain represents the 100-km-wide track of the Yellowstone hotspot during the last ∼16–17 m.y., and its crust has been heavily intruded by mafic magma, some of which has erupted to the surface as extensive basalt flows. Outside the plain's distinct topographic boundaries is a transition zone 30–100 km wide that has variable expression of normal faulting and magmatic activity as compared with the surrounding Basin and Range Province. Many models for the evolution of the Snake River Plain have as an integral component the suggestion that the crust of the plain became strong enough through basaltic intrusion to resist extensional deformation. However, both the boundaries of the plain and its transition zone lack any evidence of zones of strike slip or other accommodation that would allow the plain to remain intact while the Basin and Range Province extended around it; instead, the plain is coupled to its surroundings and extending with them. We estimate strain rates for the northern Basin and Range Province from various lines of evidence and show that these strains would far exceed the elastic limit of any rocks coupled to the Basin and Range; thus, if the plain is extending along with its surroundings, as the geologic evidence indicates, it must be doing so by a nearly aseismic process. Evidence of the process is provided by volcanic rift zones, indicators of subsurface dikes, which trend across the plain perpendicular to its axis. We suggest that variable magmatic strain accommodation, by emplacement and inflation of dikes perpendicular to the least principal stress in the elastic crust, allows the crust of the plain to extend nearly aseismically. Dike injection releases accumulated elastic strain but generates only the small earthquakes associated with dike propagation. The rate of dike emplacement required to accommodate the estimated longitudinal strain rate of the plain is roughly a composite width of 10 m every 1000 years for the geologically youngest and most active part of the plain. The locus of most rapid intrusion and strain has migrated toward Yellowstone and is now in the northeastern 100–150 km of the plain. Reduced magmatic input in the transition zone of the plain causes the transitional expression of seismicity and faulting there.

Tectonics↗