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Stephen F. Obermeier

Publications and source records attributed to Stephen F. Obermeier.

13 recordsLinked to original sources
USGS2009

Chapter 7 using liquefaction‐induced and other soft‐sediment features for paleoseismic analysis

This chapter focuses on the methodology for determining whether observed sediment deformation had a seismic shaking or a nonseismic origin. The chapter emphasizes features developed from the process of liquefaction, which is the transformation of a granular material from a solid state into a liquefied state as a consequence of increased pore-water pressure. Geophysical methods including electrical resistivity and electromagnetic induction and ground-penetrating radar are refined sufficiently to be used with some success to locate buried liquefaction features. Paleoliquefaction investigations are useful to engineers and planners because of the high shaking threshold required to develop liquefaction features. The threshold is a horizontal acceleration on the order of 0.1 g for strong earthquakes, even in highly susceptible sediment. Features having a liquefaction origin can be developed at earthquake magnitudes as low as about 5 but a magnitude of about 5.5–6 is the lower limit at which liquefaction effects become relatively common. Seismic liquefaction effects described in the chapter are caused mainly by cyclic shaking of level or nearly level ground. Primary seismological factors contributing to liquefaction are the amplitude of the cyclic shear stresses and the number of applications of the shear stresses.

International Geophysics↗
USGS2005

Revised magnitude-bound relation for the Wabash Valley seismic zone of the central United States

Seismic hazard assessment in the central United States, and in particular the Wabash Valley seismic zone of Indiana-Illinois, frequently relies on empirical estimates of paleoearthquake magnitudes ( M ). In large part these estimates have been made using the magnitude-bound method. Existing region-specific magnitude-bound relations rely heavily on only a few historical earthquakes in the central United States and eastern Canada that induced reported liquefaction features. Recent seismological studies have suggested smaller magnitudes than previously presumed for some of these earthquakes, however, and other studies have reinterpreted site-to-source distances to liquefaction features associated with some of these earthquakes. In this paper, we re-examine historical earthquakes ( M > ∼5) that occurred in the central and eastern United States and eastern Canada; some of these earthquakes triggered liquefaction and others did not. Based on our findings, we reinterpret the region-specific magnitude-bound relation for the Wabash Valley. Using this revised magnitude-bound relation, we present magnitude estimates for four prehistoric earthquakes that occurred in the Wabash Valley seismic zone during Holocene time.

Illinois, Indiana↗
USGS2003

Geotechnical Analysis of Paleoseismic Shaking Using Liquefaction Features: Part I. Major Updating of Analysis Techniques

A new methodology is proposed for the geotechnical analysis of strength of paleoseismic shaking using liquefaction effects. The proposed method provides recommendations for selection of both individual and regionally located test sites, techniques for validation of field data for use in back-analysis, and use of a recently developed energy-based solution to back-calculate paleoearthquake magnitude and strength of shaking. The proposed method allows investigators to assess the influence of post-earthquake density change and aging. The proposed method also describes how the back-calculations from individual sites should be integrated into a regional assessment of paleoseismic parameters.

Open-File Report↗
USGS1989

The New Madrid earthquakes: An engineering-geologic interpretation of relict liquefaction features

Earthquake-induced sand blows and sand-filled fissures are present in a belt 40 to 60 km. wide that extends from near Charleston, Mo., southward to about 20 km. south of Marked Tree, Ark. This region of earthquake-induced sand blows and other liquefaction-related features is almost exclusively in the St. Francis Basin, an alluvial lowland that typically has a thin (2 to 8 m thick), clay-bearing topstratum underlain by about 30 to 60 m of unconsolidated sand (the substratum). Liquefaction of the substratum sands has made the sand blows. The sand blows and other liquefaction-related features on the ground surface in the St. Francis Basin are almost certainly results of the New Madrid earthquakes of 1811-12. In this report, geologic and engineering properties of the alluvium are used in combination with a map showing the bounds of the liquefaction-related features to locate approximately the epicentral zones for two of the major shocks: the earthquakes of December 16,1811, and February 7,1812. Properties used for the analysis included the Standard Penetration Resistance of the substratum sands, characteristics of the sand's grain size, thickness of the topstratum, and the thickness of the post-Tertiary alluvium. The method of analysis relies largely on the evaluation of the liquefaction potential of the sands. This is done by using the Standard Penetration Test blow counts and by devising a method that uses all possible combinations of liquefaction potential and a realistic relation between attenuation of earthquake accelerations and distance from the epicenter (or more correctly, energy-release center). Two interpreted 1811-12 energy-release centers generally agree well with zones of seismicity defined by modern, small earthquakes. Bounds on accelerations are placed at the limits of sand blows that were generated by the 1811-12 earthquakes in the St. Francis Basin. Conclusions show how the topstratum thickness, sand size of the substratum, and thickness of alluvium affected the distribution of sand blows in the St. Francis Basin.

Arkansas, Missouri, Tennessee↗
USGS1985

Geologic evidence for recurrent moderate to large earthquakes near Charleston, South Carolina

Multiple generations of earthquake-induced sand blows in Quaternary sediments and soils near Charleston, South Carolina, are evidence of recurrent moderate to large earthquakes in that area. The large 1886 earthquake, the only historic earthquake known to have produced sand blows at Charleston, probably caused the youngest observed blows. Older (late Quaternary) sand blows in the Charleston area indicate at least two prehistoric earthquakes with shaking severities comparable to the 1886 event.

South Carolina↗
USGS1978

Relationship of landslides to fractures in Potomac Group deposits, Fairfax County, Virginia

Landsliding is a common problem in eastern Fairfax County, an area underlain by Potomac Group (Lower Cretaceous) Coastal Plain deposits of silt and clay interbedded and interfingered with sand and gravel. The slides commonly are present in clay and silt that, on the basis of laboratory tests, appear to be much too strong to have failed. However, the very plastic silt and clay deposits are commonly cut by long continuous to short discontinuous high-angle and subhorizontal joints, shears, and faults. These fractures can contribute to sliding. Failure along faults and shears takes place because the relative movement has greatly weakened and softened the clay and silt. The mechanism for failure along the joints is less obvious but may be related to infilling along joints, slight movement due to erosional unloading, swelling of clays, and softening along joints. Field investigations suggest that the three best developed high-angle joint sets in eastern Fairfax County have trends in the northeast quadrant. Lineaments plotted on a lineament map prepared from Landsat imagery also trend northeast parallel to the regional joint orientation. This regional orientation suggests that some of the fractures are tectonic in origin. Therefore, they should be anticipated in a broad zone many miles wide in the vicinity of the Fall Line. In addition, fractures of unknown origin contributing to landsliding, have been observed in massive Potomac Group clay and silt deposits. Irrespective of origin, these fractures require careful investigation and engineering to prevent slope failure at critical locations during and after major excavation.

Virginia↗
USGS1977

Mineralogical, chemical, and physical properties of the regolith overlying crystalline rocks, Fairfax County, Virginia: a preliminary report

Undisturbed cores of saprolite developed on crystalline rocks of the Piedmont Province in Fairfax County, Virginia have been obtained using a combination of Shelby tubes, Denison sampler, and modified diamond core-drilling. The principal purpose of the core study is to correlate variations in chemistry, mineralogy and texture with engineering properties throughout the weathering profile. Coring sites were chosen to obtain a maximum depth of weathering on diverse lithologies. The rocks investigated include pelitic schist, metagraywacke, granite, diabase and serpentinite. Four to twelve samples per core were selected, depending on thickness of 1) the weathering profile (from about 1 m in serpentinite to more than 30 m in pelitic schist) and on 2) megascopic changes in saprolite character for analysis of petrography, texture, clay mineralogy andd major element chemistry. Shear strength and compressibility were determined on corresponding segments of core. Standard penetration tests were performed adjacent to coring sites to evaluate engineering properties in situ. Geochemical changes of saprolite developed from each rock type follow predictable trends from fresh rock to soil profile, with relative Increases in Si, Ti, Al, Fe3+ and H20; variable K; and relative loss of Fe 2+, Mg, Ca, and Na. These variations are more pronounced in the weathering profiles over mafic and ultramafic rocks than metagraywacke. Clay minerals in granite, schist and metagraywacke saprolite are kaolinite, dioctahedral vermiculite, interlayered micavermiculite, and minor illite. Gibbsite is locally developed in near-surface samples of schist. Standard penetration test data for the upper 7 m of saprolite over schist and metagraywacke suggest alternations between stronger and weaker horizons than probably reflect variations in lithology including the presence of quartz lenses. Results for granite saprolite are most consistent but indicate lower strength. Shear strength increases fairly regularly downward in the weathering profile. The engineering behavior of diabase saprolite is controlled by a dense, plastic, near-surface clay layer (montmorillonite and kaolinite)overlying rock which is weathered to a granular state (grus), while engineering properties of serpentinite are determined by a very thin weathering profile.

Open-File Report↗