Stephen F. Obermeier

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.

Evidence of Strong Earthquake Shaking in the Lower Wabash Valley from Prehistoric Liquefaction Features

Earthquake-induced liquefaction features in Holocene sediments provide evidence of strong prehistoric shaking, magnitude mb 6.2 to 6.7, in the Wabash Valley bordering Indiana and Illinois. The source of the one or more earthquakes responsible was almost certainly in or near the Wabash Valley. The largest event is interpreted to have occurred between 7500 and 1500 years ago on the basis of archeological, pedological, and stratigraphic relations.

Sand boils induced by the 1993 Mississippi River flood: Could they one day be misinterpreted as earthquake-induced liquefaction?

In areas that are seismically active but lacking clear surficial faulting, many paleoearthquake studies depend on the interpretation of ancient liquefaction features (sand blows) as indicators of prehistoric seismicity. Sand blows, however, can be mimicked by nonseismic sand boils formed by water seeping beneath levees during floods. We examined sand boils induced by the Mississippi River flood of 1993 in order to compare their characteristics with sand blows of the New Madrid earthquakes of 1811–1812. We found a number of criteria that allow a distinction between the two types of deposits. (1) Earthquake-induced liquefaction deposits are broadly distributed about an epicentral area, whereas flood-induced sand boils are limited to a narrow band along a rivers levee. (2) The conduits of most earthquake-induced sand blows are planar dikes, whereas the conduits of flood-induced sand boils are most commonly tubular. (3) Depression of the preearthquake ground surface is usual for sand blows, not for sand boils. (4) Flood-induced sand boils tend to be better sorted and much finer than sand-blow deposits. (5) Source beds for earthquake-induced deposits occur at a wide range of depths, whereas the source bed for sand boils is always near surface. (6) Materials removed from the walls surrounding the vent of a sand blow are seen inside sand blows, but are rarely seen inside sand boils. In general, flood-induced sand boils examined are interpreted to represent a less-energetic genesis than earthquake-induced liquefaction.

Chapter 7 Using liquefaction-induced features for paleoseismic analysis

Publisher Summary This chapter describes the methodology for determining whether sediment deformation has a seismic shaking origin or a nonseismic origin. The chapter focuses on the process of liquefaction, which is the transformation of a granular material from a solid state into a liquefied state because of increased pore-water pressure. The chapter describes various manifestations of liquefaction-induced deformation in fluvial and near shore marine deposits and the application of criteria for establishing an earthquake origin. The systematic study of paleoliquefaction is a young discipline. Accordingly, some of the physical parameters that control effects of liquefaction in the field are not completely understood. Seismic liquefaction effects described in the chapter are caused mainly by the 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. These factors are related to field conditions of shaking amplitude and duration of strong shaking. Both peak acceleration and duration generally correlate with the earthquake magnitude. Analytical engineering methods for evaluating variable and irregular cyclic stress applications typical of real earthquakes are well developed and yield results acceptable for engineering analysis providing that shaking amplitude-time records can be reasonably bracketed. The chapter discusses the way to set limits for the strength of prehistoric shaking.

Chapter 7 Using Liquefaction‐Induced and Other Soft‐Sediment Features for Paleoseismic Analysis

Publisher Summary 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.

Liquefaction Evidence for Repeated Holocene Earthquakes in the Coastal Region of South Carolinaa

Features thought to have originated from earthquake-induced liquefaction have been discovered throughout much of the coastal region in South Carolina and in extreme southeastern North Carolina. Nearly all these liquefaction features are sandblows presently manifested as filled craters. Prehistoric craters near Charleston formed in long-separated episodes at least three times within the past 7200 years. Ages of dated craters far from Charleston, beyond the farthest 1886 earthquake sandblows, differ from ages of craters near Charleston. Insufficient data have been collected to determine whether ages of all craters far from Charleston differ from ages of craters near Charleston. Both the size and relative abundance of pre-1886 craters are greater in the vicinity of Charleston (particularly in the 1886 meizoseismal zone) than elsewhere, even though the susceptibility to earthquake-induced liquefaction is approximately the same at many places throughout this coastal region. These data indicate that, in this coastal region, the strongest earthquake shaking during Holocene time has taken place repeatedly near Charleston.