ENGINEERING GEOLOGY/Aspects of Earthquakes 459 Seismotectonic Zonation Zonation is a technique whereby known and predicted earthquake damage can be assigned to areas for planning, zoning, and regulatory purposes A number of concepts apply to zoning Each determination should be made on site-specific needs Active Faults Where ground rupture or the associated effects of surficial damage have been noted it is appropriate to assign a status of ‘active’ faulting State-of-the-art determination is represented by the Alquist–Priolo Active Fault Zones, legislatively mandated in 1969 by the California Legislature These zones are named, delineated, and maintained by the California Geological Survey and constitute 400 m wide strips centred along the best-identified trace or centre line of the subject fault Zones are plotted on : 24 000 US Geological Survey topographical quadrangle bases and are available for reference on the world wide web Credible Faults The definition of credible faults was an innovation of the nuclear power plant safety programme of the US Nuclear Regulatory Commission (USNRC); credible faults are known faults or areas where there is strong geomorphological or structural geological evidence of fault displacement at least once during the Holocene and more than once in the past 500 000 years The half-million-year bracket is an artefact of the fusilinid age determination resolution affinity of California marine seashore terraces, and was applied to the Diablo Canyon Nuclear Power Station (NPS) in the early 1960s The concept, however, is not applicable elsewhere, though it is frequently specified Credible faults are considered capable of experiencing ground rupture and propagation of ground motion along their entire known length Bonilla was the first to associate ground-rupture length with the magnitude of the causative event an active fault has not been identified, a most likely location is selected for each seismogenic zone; the expected ground motion is attenuated from the hypocentre to the site This becomes the MCE for each of the candidate source faults or zones Finally, the style of earthquake to be represented for each of the candidates is evaluated, in terms of vector displacement, duration of strong motion, and peak ground acceleration Attenuation This assessment looks at the ground motion from the hypocentre to the project Waves of ground motion move through the earth media, are concentrated in the bedrock, and there suffer the physical effects of trying to displace the surfaces of innumerable rock discontinuities encountered during the passage to the site This infinite number of surface-to-surface grinding processes consumes much of the energy of the earthquake Attenuation curves have been developed empirically for a variety of geological terranes and are published in the literature, particularly in the reports of the former US Army Waterways Experiment Station (now Engineer Research and Development Command, Vicksburg, MS, USA) Each geological terrane has its own directional effect on attenuation The greatest effect is generally exerted perpendicular to strongly foliated older metamorphic terranes, such as the north-east-foliated Lower Palaeozoic sequence of New England, USA Comparison of Candidate Design Earthquakes A selection of potentially impacting design earthquakes are defined in terms of the parameters listed above Each candidate is moved through a defined scenario and attenuated from each stipulated source to the site The worst cases are then declared the MCEs, and their characteristics are used as input in seismic-withstand design Deterministic Design Input Engineering Geological Mapping Design-level input by engineering geologists forms the basis for carrying actual earth science evidence into ‘seismic withstand’ design by civil and other engineers This geological input consists of the following process of analysis and evaluation First, the area around the project is seismically zoned, and credible faults are identified or a most likely epicentre for a candidate earthquake is assigned for each seismogenic zone identified Then, where active faults are defined, the design-input ground motion is considered to occur at the location closest (measured perpendicular to the fault trace) to the site Where A primary method for addressing and meeting seismic threats uses engineering geological mapping, which records the evidence of past earthquake damage and uses this alongside geological conditions Earthquake damage was generally not mapped in detail until the Alaskan ‘Good Friday’ earthquake of 1964 The former (regrettably now defunct) Branch of Engineering Geology responded to this event under the constructive leadership of its Director, the late Edwin B Eckel, also later the Executive Director of the Geological Society of America Eckel’s lobbying in Washington DC was immediate, and the Survey