456 ENGINEERING GEOLOGY/Aspects of Earthquakes Aspects of Earthquakes A W Hatheway, Rolla, MO and Big Arm, MT, USA ß 2005, Elsevier Ltd All Rights Reserved Introduction The main product arising from the work of engineering geologists is their site characterizations for engineered works In terms of the impact of earthquakes and their mitigation, design engineers, architects, and planners need to both accommodate people and provide functionality With these goals in mind, engineering geologists develop three-dimensional descriptions of the ground to the depth that will be affected by the static loads resulting from the engineered works and by the expected levels of earthquake-induced ground motion The damaging effects of earthquakes can be mitigated, but the earthquakes themselves cannot yet be controlled or stopped Engineering geology can be applied in a variety of ways to assist in mitigating the effects of earthquakes; for instance by recommending the avoidance of sites that are likely to experience ground displacement as a result of fault movement; grading the site to a configuration that promotes stability of the ground mass; improving the engineering characteristics of the ground so as to resist damaging deformation; controlling groundwater that is likely to experience excess pore pressure leading to ground failure under the involved structural loads; and creating green space to isolate active faults and possible seismically induced slope failures Engineering geologists also record details of and analyse the damage done by earthquakes (Figure 1) In this context, the role of the geologist is expanded from considering the known structural-loading conditions imposed by the designers to considering the unknown, but anticipated, ground motion during a hypothetical earthquake felt to be appropriate to the role and function of the project Worst-case-earthquake design predictions are appropriate for critical facilities or for projects housing dependent populations Critical facilities are those projects for which the consequences of failure are intolerable Worst-case-earthquake design is based on the concept of the maximum credible earthquake (MCE), which is the greatest magnitude and duration of strong motion that can reasonably be expected to occur Ancillary to this concept is risk analysis to define magnitudes and durations of strong motion for which there are cost benefits and for which damage can be tolerated Figure Engineering geological mapping records evidence of earthquake motion such as this railroad deformation indicating north to south shortening of the ground along the rail right of way Most evidence of earthquake damage is fragile and likely to be destroyed by human activity within hours or days (Photographed by Richard J Proctor, then Chief Engineering Geologist, Metropol itan Water District of Southern California, at San Fernando Valley, California, February 1971.)