Groundwater-strain coupling before the 1999 Mw 7.6 Taiwan Chi-Chi earthquake

•Precursory signals were found at 78% wells before the Mw 7.6 Chi-Chi earthquake.•The signals are consistently comprised by three phases in noise-free records.•Uplift (decrease) of groundwater levels when strain rates are negative (positive).•A well coupling between groundwater levels and surface strain was discovered.

SummaryThe coupling of pre-earthquake anomalous phenomena between long-term groundwater levels recorded at 42 monitoring stations and time-varying surface strain derived from 16 GPS stations was found in the Choshuichi Alluvial Fan before the 1999 Mw 7.6 Chi-Chi earthquake in Taiwan. The noise-free groundwater-level anomalies consistently comprised by a sequence of decrease, rise and flat phases, which agree very well with changes in strain rates computed from the GPS stations. These coupling agreements show that in addition to compression, tension can be generated before a thrust earthquake occurrence as well. This case demonstrates that short-term surface deformation as signals against noise and accuracy of pre-earthquake anomalous phenomena can be simultaneously examined by using multiple-parameter crosscheck for significantly reducing the uncertainty of earthquake precursory evaluation.