Search

The 2017 Pohang earthquake [the second largest local magnitude (ML) of 5.4 since 1978] caused significant damage: numerous sand boils and a few building settlements were observed in rice paddies and residential areas, respectively, representing unprecedented case histories of earthquake-triggered liquefaction and cyclic softening. This study evaluated liquefaction triggering and cyclic softening potentials using three in situ tests [standard penetration test (SPT), cone penetration test (CPT), and downhole (DH) test for shear wave velocity (VS)] and laboratory tests (grain size and soil indices) for the observed sand boils and building settlements. We selected six sites, four of which had sand boils (Sites 1, 2, 3, and 4), and two of which had experienced building settlements that may have resulted from cyclic softening (Sites 5 and 6). The SPT, CPT, and VS adequately assessed liquefaction triggering [i.e., factor of safety (FS)<1 or ∼1 ] at Sites 1 through 4 (except for VS at Sites 1 and 2), where sand boils were prevalent. The cyclic softening potential was fairly evaluated from the SPT and CPT (FS<1 or ∼1 at several depths) at Sites 5 and 6, consistent with the building settlement, whereas VS led to FS>2 at all depths. The site-specific cyclic stress ratio through the maximum shear stress ratio computed from site response analysis appropriately evaluated the liquefaction triggering and cyclic softening at the considered sites. The results of the soil index test are consistent with the liquefaction and cyclic softening susceptibility criteria for fine-grained soils. We publicly provide the field and laboratory measurements in this study to enrich case history data on liquefaction and cyclic softening induced by intermediate-size earthquakes (e.g., a moment magnitude, M<6), which might significantly contribute to geotechnical earthquake engineering and engineering geoscience communities. [ABSTRACT FROM AUTHOR]

Levees provide vital functions for water delivery and flood protection. However, they present unique challenges for seismic design because their great length makes engineering evaluation of stability at closely spaced regular intervals impractical. Accordingly, relatively broad, empirically-driven risk assessment tools have the potential to serve as effective screening tools. We are undertaking a large data collection and synthesis effort to support the development of such tools, with the initial focus being on levee performance from the 2007 Mw6.6 Niigata Chuetsu-oki earthquake in Japan. Naturally, ground shaking is a key variable in this process, so the reliable estimation of ground shaking hazards fromseismic networks is an essential element of the case history analysis. We postulate thatdirect application of Krigingtechniques can produce biased ground motion estimates due to variable site conditions. Accordingly, we apply Kriging to residuals of ground motion prediction equations (GMPEs), which remove the average site effect. The resulting maps of residuals can be readily applied with the GMPE to produce ground motion maps that properly reflect spatial variations of geologic conditions.The proposed procedure produces ground motions near levees that are lower in some areas than those produced bydirect Kriging.

This paper describes the relationship between bedrock depth (D) and site fundamental frequency (f0) in the Nakdonggang delta region in the southeastern part of the Korean peninsula. We collected borehole logs to confirm the thickness of the sediments and estimated the f0 at over 200 locations across the delta using the horizontal-to-vertical spectral ratio (HVSR) method. We developed an f0 map of the study area by spatially interpolating the f0 values using the Ordinary Kriging method. The bedrock depth in the main delta showed a power-law dependence on the f0. The derived f0–D model predicted much shallower bedrock depths compared with similar studies from other parts of the world. This was attributed to the fact that the Nakdonggang delta region is composed of relatively low Vs Holocene sediments. With an f0 map, the derived model could enable a quick estimation of the bedrock depth, which could help to determine the site class in the Nakdonggang delta region according to the Korean Seismic Design Standard (KDS 17 10 00). [ABSTRACT FROM AUTHOR]