Processed strong-motion records from the Imperial Wildlife Liquefaction Array, Imperial County, California, recorded during the Superstition Hills earthquakes, November 24, 1987

Insight into liquefaction by system identification

“…This transducer is being singled out for several reasons: analysis of sand boil ejecta after the 1981 Westmorland earthquake indicated that unit B1 lique®ed (Holzer et al, 1989); P5 most clearly indicated that the excess pore pressure exceeded the total overburden pressure; it was located at the top of the layer expected by the experts at the USGS to liquefy during the Superstition Hills shaking and therefore (by some theories) would be at the point of initial liquefaction (Scott, 1986;Florin & Ivanov, 1961); and it is believed that P5 is the transducer in the array most likely to have performed correctly (Hushmand, Scott & Crouse, 1991, 1992. (Brady et al, 1989) with the pore pressure ratio r u (ratio of excess pore pressure u to initial effective overburden stress " ó v ) history for piezometer P5, Superstition Hills earthquake. At time t 0 (subscript time from ®rst arrival in seconds) the initial shear wave (S-wave) energy from the ®rst subevent arrived, and a very slight increase in pore pressure was recorded by P5.…”

“…The soil pro®le of the Wildlife site above layer B1 is relatively uniform, and the particle motion due to S-waves is geologically restrained to the horizontal. In addition, the porepressure history (Brady et al, 1989) indicated that location P5, at the top of layer B1, was the point of initial liquefaction. Once the pore pressure in layer B1 begins to rise, the sand in this layer is very soft in comparison to the rest of the system.…”

“…In general, the materials in unit B grade from coarsest at the bottom (7 m) to ®nest at the top. Brady et al (1989) note that a wood fragment recovered from a depth of 6 m was estimated to be about 230 years old by carbon-14 analysis. Unit B is believed to be a point-bar deposit, on the concave side of a meander curve (Bennett et al, 1984).…”

“…Complete pore pressure records were made during both temblors at ®ve different depths (Brady et al, 1989). Records show that an increase in pore water pressure was measured during both the Elmore Ranch and Superstition Hills earthquakes.…”

“…The shaking was strong enough to induce liquefaction at the Wildlife site, which suffered more than 2 m of lateral spread towards the Alamo river, and sand boils occurred over at least 33 ha (Holzer, Youd & Hanks, 1989). Accelerograms from buried and surface transducers, as well as pore pressure±time histories, were captured for both the Elmore Ranch and Superstition Hills earthquakes (Brady, Mork, Seekins & Switzer, 1989). This is the only full input±output data set known to exist for a site experiencing liquefaction, and the only set of records complete enough to allow an unambiguous SI analysis.…”