460 ENGINEERING GEOLOGY/Aspects of Earthquakes mounted a field effort to observe and map the damage of the magnitude 8.3 event as a long series of Professional Papers, the content of which has served as the ongoing model for the application of engineering geology to earthquake mitigation learning Subsequent American earthquakes have been given similar, yet lesser overall, attention, for instance the San Fernando (California) earthquake of February 1971 (magnitude 6.6) and the Loma Prieta event of 17 October 1989 (magnitude 7.1) located just south of San Francisco (California) on a San Andreas splinter fault made to confirm the absence of active faults within the footprint of the power block (i.e the location of the reactor and the critical cooling linkage) The concept of avoiding active fault traces was promoted so that the risk of damage to the reactor and its containment as a result of rupture-type earthquake ground motion could be minimized As a result, the technique of exploratory trenching advanced from the original method of observation from hand-dug pits (Table 4) Purpose As an integral part of site characterization in seismogenic regions, attention should be given to identifying geological conditions that may make the site susceptible to physical damage from strong motion (Table 5) Strictly speaking, the purpose of engineering geological mapping in this context is to record the physical nature of ground-rupture earthquakes and to elucidate the related types of motion-induced damage (Table 1) Once defined, these can be applied on a worldwide basis, wherever similar conditions of tectonics and near-field geology and topography occur Engineering geological mapping for earthquake mitigation (Table 2) determines how engineered works and human safety can be protected by judicious design considerations, most of which are governed by geological conditions related to the site characterization Geological Profile (or Ground Profile) This term geological profile has taken over in engineering geology from ‘geological section’ and considers only the depth of influence of engineering works (generally less than 15 m; Table 3) Exploration Trenches and Trench Logging During the peak of nuclear power-plant siting and construction in the 1970s considerable effort was Site Characterization Post-Event Surveys Certain elements of the earth media are particularly susceptible to being lifted, shifted, toppled, or cracked by earthquake strong motion The patterns of damage reveal much about the frequency characteristics of the incident ground motion and the relative duration of the strong motion Particularly affected are fine soils, boulders on slopes, blocks of rock defined by joints, overly steep stream and shore banks and cliffs, and hillside masses saturated with groundwater Geologists have but hours to locate, photograph, and map these features before they are destroyed, first by human visitors and soon after by rainfall and other natural erosive agents (Table 6) Cultural features within the built environment (including engineered works) offer additional potential Table Engineering geological mapping of earthquake effects Elements Purpose Important considerations Stratigraphy Identify and describe the geological formational units to be expected in design and construction Define character of groundwater, as it is affected by ground motion and diminishes the shear strength of earth media to resist dynamic deformation Delimit observable or likely subsurface bounds of each detectable hard rock unit Individual engineering geological units Groundwater regime Rock mass characterization Presence of weak rock Basis of definition, including why the rock is determined to be weak Potential problems related to sedimentological, structural, or geomorphological conditions Portions of surface or subsurface that appear to be related to mapped patterns of earthquake damage Perched water Vadose zone and fluctuations Peizometric surface Potentiometric surface Identify bodies of discontinuity bounded rock masses that may become unstable from shaking Recommendations as to how and why such weak rock may pose problems to design and/or construction, operation, and maintenance Use special map symbols to portray these features; the Geological Society of London Engineering Geomorphological map symbols are ideal for this purpose