Field-based Quantification of Empirical Relationships Between Pore-pressure Change and Seismic Shaking Using Data from the Chi-Chi (Mw = 7.6) Earthquake, Taiwan

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Alluvial fans over the world are often shaken by large earthquakes that have led to some of the worst historical landslides even on very gentle slopes (e.g., the Turnagain Heights landslide, Anchorage, Alaska, in the 1964 Alaskan earthquake). However, systematic field evidence for coseismic change in pore-pressure in alluvial fans was not available until after the 1999 Chi-Chi, Taiwan, earthquake. The 1999 Chi-Chi (Mw=7.6) earthquake in central Taiwan was recorded by a dense network of modern seismological, geodetic and hydrologic instruments. Widespread coseismic pore-pressure changes, captured for the first time by 188 hydrologic wells in a network of 70 hydrologic stations, occurred across a large alluvial fan (the Choshui River fan) west of the epicenter of the earthquake. Ground-motion on this fan, both during the main-shock and during several large (M 6) aftershocks of the Chi-Chi earthquake was recorded by 80 broadband strong-motion accelerometers at high sampling rate, and the data was promptly published. A complex, yet systematic, pattern in the coseismic change in pore-pressure emerged from the digitized water-level records that led to a hypothesis that the coseismic pore-pressure change was due to the nonlinear mechanical response of sediments to seismic shaking. The hypothesis, however, was based on an empirical relationship from a small number of laboratory experiments performed on small samples under applied stresses with magnitude and frequency greatly different from that during earthquake shaking. The availability of both the ground-motion data and the hydrological data from the Chi-Chi earthquake has created a rare opportunity for a quantitative field test of the laboratory-based relationship between the pore-pressure change in sediments and seismic shaking. This project combines both the hydrological data and the seismological data from the Chi-Chi earthquake in a search of the empirical relationship between the two data sets that would be the first field-based relationship between pore-pressure change and seismic shaking, and would greatly facilitate our understanding and assessment of liquefaction and landslide potential in alluvial fans during earthquakes, and would thus be of great value to the seismic engineering community.