SILLS AS INDICATORS OF STRENGTH OF SEISMIC SHAKING

Nearly horizontal sand intrusions, or sills, are commonly observed at sites where steeply dipping sand dikes form due to seismic liquefaction. These sills commonly develop along the base of a fine-grained surficial cap that overlies the liquefied source sand. Sills also often occur within clay/silt stringers immediately beneath a thicker, more massive cap. Where liquefaction has been severe, sills can cut through source sands at depth, as well as within the cap. Photographic examples are shown at http://pubs.usgs.gov/openfile/of98-488/

Based on field paleoliquefaction searches by Obermeier in many parts of the U.S., thin sills are often found in relative abundance where even small dikes are rare or virtually absent. This is especially the case at the outermost limits of dikes, far from the earthquake epicenter. Where both dikes and sills are observed at an exposure, it is tacitly assumed by some that both types of structures develop by the same process, i.e., liquefaction causing fluidization, in turn causing the intrusions.

We suggest, however, that complete (or nearly complete) liquefaction is not required to form both dikes and sills. (Complete liquefaction is defined as the state where the porewater pressure carries the full weight of the overburden.) Dikes form as a result of a rapid buildup of porewater pressure during liquefaction, which both fluidizes the source sand and hydraulically fractures the overlying cap (Obermeier, 1996, Eng. Geol., 44, 1-76). This requirement for forming dikes is shown by the clear association of the engineering properties of liquefiable sand with the formation of sand dikes, as illustrated in the cyclic stress method relating SPT blowcount, seismic demand, and the presence of sand blows vented through dikes.

In contrast, forming sills requires fluidization, but not hydraulic fracturing of the cap. And fluidization does not necessarily require a rapid buildup of porewater pressure, thereby also eliminating the requirement of complete liquefaction. Even relatively slow buildup of porewater pressure during shaking can be adequate for fluidization, especially in layered sediments containing thin clay or silt stringers, which decrease the rate of porewater pressure dissipation.