Using case studies from the 2010-2011 Canterbury, New Zealand earthquake sequence, this study assesses the accuracies of paleoliquefaction back-analysis methods and explores the challenges, techniques, and uncertainties associated with their application. While liquefaction-based back-analyses have been widely used to estimate the magnitudes of paleoearthquakes, their uncertain efficacies continue to significantly affect the computed seismic hazard in regions where they are relied upon. Accordingly, their performance is evaluated herein using liquefaction data from modern earthquakes with known magnitudes. It is shown that when the earthquake source location and mechanism are known, back-analysis methods are capable of accurately deriving seismic parameters from liquefaction evidence. However, because the source location and mechanism are often unknown in paleoseismic studies, and because accurate interpretation is shown to be more difficult in such cases, new analysis techniques are proposed herein. An objective parameter is proposed to geospatially assess the likelihood of any provisional source location, enabling an analyst to more accurately estimate the magnitude of a liquefaction-inducing paleoearthquake. This study demonstrates the application of back-analysis methods, provides insight into their potential accuracies, and provides a framework for performing paleoliquefaction analyses worldwide.
Data from the 2010-2011 Canterbury earthquake sequence (CES) provides an unprecedented opportunity to assess and advance the current state of practice for evaluating liquefaction triggering. Towards this end, select case histories from the CES are used herein to assess the predictive capabilities of three alternative CPT-based simplified liquefaction evaluation procedures: Robertson and Wride (1998); Moss et al. (2006); and Idriss and Boulanger (2008). Additionally, the Liquefaction Potential Index (LPI) framework for predicting the severity of surficial liquefaction manifestations is also used to assess the predictive capabilities of the liquefaction evaluation procedures. Although it is not without limitations, use of the LPI framework for this purpose circumvents the need for selecting “critical” layers and their representative properties for study sites, which inherently involves subjectivity and thus has been a point of contention among researchers. It was found that while all the assessed liquefaction triggering evaluation procedures performed well for the parameter ranges of the sites analyzed, the procedure proposed by Idriss and Boulanger (2008) yielded predictions that are more consistent with field observations than the other procedures. However, use of the Idriss and Boulanger (2008) procedure in conjunction with a Christchurch-specific correlation to estimate fines content showed a decreased performance relative to using a generic fines content correlation. As a result, the fines correction for the Idriss and Boulanger (2008) procedure needs further study.
In practice, several competing liquefaction evaluation procedures (LEPs) are used to compute factors of safety against soil liquefaction, often for use within a liquefaction potential index (LPI) framework to assess liquefaction hazard. At present, the influence of the selected LEP on the accuracy of LPI hazard assessment is unknown, and the need for LEP-specific calibrations of the LPI hazard scale has never been thoroughly investigated. Therefore, the aim of this study is to assess the efficacy of three CPT-based LEPs from the literature, operating within the LPI framework, for predicting the severity of liquefaction manifestation. Utilising more than 7000 liquefaction case studies from the 2010–2011 Canterbury (NZ) earthquake sequence, this study found that: (a) the relationship between liquefaction manifestation severity and computed LPI values is LEP-specific; (b) using a calibrated, LEP-specific hazard scale, the performance of the LPI models is essentially equivalent; and (c) the existing LPI framework has inherent limitations, resulting in inconsistent severity predictions against field observations for certain soil profiles, regardless of which LEP is used. It is unlikely that revisions of the LEPs will completely resolve these erroneous assessments. Rather, a revised index which more adequately accounts for the mechanics of liquefaction manifestation is needed.
The focus of the study presented herein is an assessment of the relative efficacy of recent Cone Penetration Test (CPT) and small strain shear wave velocity (Vs) based variants of the simplified procedure. Towards this end Receiver Operating Characteristic (ROC) analyses were performed on the CPT- and Vs-based procedures using the field case history databases from which the respective procedures were developed. The ROC analyses show that Factors of Safety (FS) against liquefaction computed using the most recent Vs-based simplified procedure is better able to separate the “liquefaction” from the “no liquefaction” case histories in the Vs liquefaction database than the CPT-based procedure is able to separate the “liquefaction” from the “no liquefaction” case histories in the CPT liquefaction database. However, this finding somewhat contradicts the assessed predictive capabilities of the CPT- and Vs-based procedures as quantified using select, high quality liquefaction case histories from the 20102011 Canterbury, New Zealand, Earthquake Sequence (CES), wherein the CPT-based procedure was found to yield more accurate predictions. The dichotomy of these findings may result from the fact that different liquefaction field case history databases were used in the respective ROC analyses for Vs and CPT, while the same case histories were used to evaluate both the CPT- and Vs-based procedures.
