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5 Collateral Hazards Collateral hazards include fault ruptures, landslides, liquefac- tion, tsunamis, and seiches. As noted in an earlier section, collateral hazards can cause very significant losses. For example, most of the Tosses experienced in the Alaskan earthquake of 1964 resulted from collateral hazards, with landslides causing major property loss and tsunamis causing 119 of the 131 fatalities (National Research Coun- cil, 1972~. In principle, the overall method for predicting losses that might result from these hazards is much the same as that for losses caused by shaking of facilities. The first question m: How extensive and severe might collateral hazards be? For example, how large an area might be affected by landsliding, and how far and how rapidly might the earth move? These are questions for earth scientists, such as geologists. The next question m: What damage and losses would result? These primarily are questions for engineers. Where structures are impacted directly by severe collateral hazards, losses may be quite large. Sometimes the effect of a hazard may be indirect. For exa~nple, the Hebgen earthquake in Montana in 1959 triggered a landslide that blocked the Madison River, and a lake began to form behind this earthen mass. Considerable effort was expended to alleviate concern about potential downstream flooding when this potentially erodible "dam" eventually overtopped. 4s
46 FAULT RUPTURE A surface fault rupture involving several feet of movement will cause major damage (50 percent loss ratio or greater) to almost all houses or small structures sited directly on the fault, and generally even greater damage to larger structures. However, well-built buiTcI- ings located within only a few feet of a well-defined fault rupture may experience little or no damage. Losses to buildings directly attributable to surface rupture usually are a small fraction of total losses associated with an earthquake, and can be predicted with sat- isfactory accuracy once the path of the rupture is defined. In the eastern United States, it appears that potential faults are deeply buried and hence fault rupture probably does not extend to the sur- face. Hence this particular hazard generally can be ignored in this large portion of the country. [AND SLIDES AND LIQUEFACTION Landslides and liquefaction are consequences of ground shaking. A rational procedure for evaluating liquefaction and landslide-caused Tosses as a result of a scenario earthquake is described in ATC-13 and involves the following steps: 1. Identify soil, geologic, and topographic conditions potentially susceptible to such failures as a result of an earthquake. 2. Select relations expressing the likelihood of failure, or the fraction of susceptible area expected to fail, as a function of the intensity of ground shaking. 3. Select additional relations giving Toss ratios for buildings and other facilities located at failed areas. These steps parallel those for the direct effect of ground shaking upon buildings (Figure 1-2~. Identification of areas where these hazards may occur is a major inventory-type problem. The USGS has prepared, or helped prepare, detailed geological hazard maps for several metropolitan areas (e.g., Borcherdt, 1975; Youd et al., 1978; Ziony 1985), and is collaborat- ing in the development of such maps for additional areas. Generally these maps shower} areas within which there is a significant likelihood that landsliding or liquefaction might be triggered by an earthquake, and it is not necessarily expected that actual hazards will occur in all such areas or over all of a given area. Preparing a good map of collateral geological hazards is a time-consuming and expensive task.
47 If these maps are not already available, it is necessary to construct an approximate hazards map using the expertise of experienced local geotechnical engineers and geologists. ATC-13 summarizes the gen- eral principles to be followed, and more specific guidance is available in a recent compilation of papers by USGS (Ziony, 1985) and a report on liquefaction by the National Research Council (1986~. Working Paper F contains additional discussion of these matters. Liquefaction The word liquefaction has been used to cover several differ- ent types of phenomena associated with the increase in pore water pressures in cohesionIess soils during earthquake shaking, with a resulting decrease of strength and/or stiffness. One common mani- festation is lateral spreading of nearly level ground toward adjacent dips or other low points. Such lateral spreading can disrupt road- ways, canals, pipelines, and so on, and will also damage buildings in the area affected by the spreading. Liquefaction can also cause more dramatic flow slides from steeper slopes, which occurred at the lower Van Norman Dam during the 1971 San Fernando earthquake (Seed et al., 1975), at Irene tailings dams in Chile in 1965 (Dobry and Alvarez, 1967), and at the waterfront in Seward, Alaska in 1964 (Seed, 1973~. Obviously such slides are extremely damaging both to any structures on the slide area or In the path of the slide. Still another manifestation of liquefaction is the appearance of sand boils (small volcanoes emitting sand and water) on the surface of level ground. Where such sand boils occur, there can be excessive settlements of facilities. There may also be large differential horizon- tal movements between only slightly distant points on the surface, resulting in damage to highway, pipelines, and so on. I.iquefaction has been observed repeatedly during earthquakes in California, but it has also been observed elsewhere. An enormous area was affected by massive liquefactions during the 1811-1812 New Madrid, Missouri earthquakes, and the phenomenon was very evident during the 1886 Charleston, South Carolina earthquake. Liquefac- tion Is a potential problem in any area where a Toss estimation is being made. The liquefaction hazard maps are based on general indicators of liquefaction susceptibility: geological age, the manner in which cohe- sioniess soil is deposited, and the depth to the water table. Detailed procedures involving drilling and sampling have been developed for
48 evaluating the liquefaction susceptibility of specific sites; such efforts are beyond the scope of a loss estimation study, although it may be possible to make good use of pre-existing data to supplement geological information. Approaches developed for California to iden- tify liquefaction susceptible areas (Youd and Perkins, 1978) can be used, with proper judgment and knowledge of local conditions, to guide preparation of approximate maps for areas in other parts of the country. ATC-13 contains relations giving estimated fractions of lique- faction susceptible areas experiencing different damage states (i.e., degrees of liquefaction) as a function of MMI. These were prepared using expert opinion, and are generally applicable throughout the United States. A very recent paper by Youd and Perkins (1988) presents a method for mapping liquefaction severity index (LSI), which is related to the extent and severity of liquefaction phenomena within liquefaction susceptible soils. The specific relations suggested in this paper are derived from experiences in California, and cannot be extrapolated directly to other parts of the country. The same ATC-13 report also gives damage ratios for the losses to buildings affected by liquefaction. These clearly are engineering judgments, but they do provide some guidance. Combining this damage Formation with maps of liquefaction susceptible areas and estimations of extent and severity of liquefaction provides property loss estimates from these collateral hazards. It is essential to keep in mind that different manifestations of liquefaction (e.g., lateral spreading and sand boils) have quite different implications concerning damage. This method is untried and must be used with caution and judg- ment and tailored to local situations, but it does permit systematic evaluation of potentially important losses. [~48]i~e8 Here landslides is used to cover all permanent earth movements other than those involving saturated cohesionIess soils. In a very comprehensive review of earthquake-induced landslides to date, Keefer (1984) reviewed 40 earthquakes worldwide and ranked the abundance of different types of landslides. He listed disrupted soil slides as very abundant; soil slumps, soil lateral spreads, soil block slides, and soil avalanches as abundant; and rapid soil flows as moderately common. Of these types of landslides involving soils,
49 only soil lateral spreads and rapid soil flows most likely involve liquefaction or failure of sensitive clays. The most notable example of failure of sensitive clays in this country occurred in Anchorage, Alaska in 1964 (Seed, 1973~. Technical methods for assessing specific sites are available, but generally are beyond the means of a broad area loss estimate study. Since a combination of very sensitive clay and very strong shaking must be present if such failures are to occur, local geotechnical and geological experts should be able to decide whether or not this particular problem is present. Soil falls, disrupted soil slides, and soil avalanches can occur as failures of dry, cohesive or cohesioniess soils (Keefer, 1984) and were assessed as particularly common to abundant in the 1811- 1812 New Madrid (Missouri) and 1906 San Francisco earthquakes. Likewise, landslides In cohesive soils are not restricted to sensitive clays, as shown by Keefer's historic review. Soil slumps, for example, have been at least moderately common in many earthquakes where sensitive clays do not exist. Landslides in rock (e.g., rockfaBs and debris flows) are expected only in steep terrain, in rock formations experiencing a loss of strength during ground sharing. It is very difficult to determine, even with the most careful testing and evaluation, whether or not a slide is likely, in relation to ground-shaking intensity, at a given rock site. Thus it is necessary to rely very heavily on past experience and judgment. AT~13 contains results from expert opinion con- cerning the probability of such events. The historical review of 40 worldwide earthquakes by Keefer (1984) formed the basis for develop ing methods to identify seismically induced landslide susceptibility. These methods have subsequently been applied to several California metropolitan areas, including San Mateo County (Wieczorek et al., 1988) and the Los Angeles Region (Wilson and Keefer, 1985~. Of the 40 earthquakes in the review, only 10 were from California. At least a half dozen were from other regions in the United States, including Alaska, Hawaii, Missouri, Montana, South Carolina, and Washing- ton, with the rest of the historical earthquakes from other parts of the world having a variety of geologic and climatic conditions. ATC-13 also provides estimates for the likelihood of damage to facilities, given that landsliding occurs - again from expert opinion. The warning in the last paragraph under liquefaction applies even more to landslides in rock. Locally steep rock slopes, particularly in closely jointed and weakly cemented rocks, are highly susceptible to seismically induced rockfalIs and rockslides as noted by Keefer (19843.
50 These slopes need not be particularly high; a steep slope 10-20 m high could generate a damaging rockfalI. Many metropolitan areas are naturally hilly and in combination with highway cuts, hillside excavations for building foundations create sufficiently hazardous conditions for rockfall and rockslides during earthquakes. Within the United States, Los Angeles, Memphis, Portland, Salt Lake City, San Diego, San Francisco, and Seattle are a few examples of large metropolitan areas with recognized potential for seismically induced landslides involving rock. TSUNAMIS Tsunami inundation areas for the West Coast of the United States, including Alaska and Hawaii, are available in the form of FEMA flood maps (FIRM or Flood Insurance Rate Maps). For the East Coast, the potential hurricane storm surge exceeds possible tsunami heights, and tsunami hazard zones are thus of less impor- tance and are not plotted on FIRMs. Other maps by USGS or the U.S. Army Corps of Engineers are also available and show the extent of expected tsunami run-up for the West Coast. A mean recurrence interval for the tsunami of 100 or 500 years, or both, is commonly used as a basis for developing these maps. Even though tsunami hazard maps are available, there are two basic difficulties to overcome in estimating losses. First, as with ground failures, one must assess the degree of damage to various structures that might be caused by a range of water heights and velocities. All structures located within the shaded tsunami run-up zones on a map are not expected to be subjected to the same effects. Second, the tsunami map obtained will generally be predicated on a variety of causative earthquakes that may be located thousands of miles away, and thus tsunami losses may be unrelated to the losses associated with a local earthquake scenario. Generally, the major tsunami risk in California, Oregon, Washington, and Hawaii is posed by distant earthquakes, such as large Alaskan or Chilean events. In southern Alaska, a local earthquake could cause a tsunami, and the earthquake and tsunami losses might thus be combined in a particular scenario. SElCHES Here seiches refers to waves induced in lakes, reservoirs, and so on as a result of ground shaking (or perhaps because of permanent
51 tectonic movement of the underlying earth). If these waves are of sufficient amplitude, facilities along the waterfront may be damaged and there might be overtopping and thus damage to any earth dam · · retaping a reservoir. A specialized study Is needed to determine whether or not there might be a problem and if so the extent of it. Fortunately, seiches are usually not a problem.
~2 Electron equlp~ent tat ~ not anchored securely to the ~und~tlon can Awe and ~D over, as b~ppened to this unit durlug the 1971 San Fernando eartb- quake (~ 6.6~. Able earthquake did extensive damage to electric power ~clUtles, Beech brlOges, and water and gas dbtrlbutlou systems. Boas comply ~ G. Collapse of an Flood control tower ~ Anchorage' Alaska durlog the 1964 e~rthqu~e (~ 8.3~.6]. One emu w" kH6d In the collapse and one was l~ured. All Or traffic control and ~v~o~ serapes were disrupted in the Anchorage area due to d~ to telephone' lute~hone, and te~typewrher - 1~, ~ =~ ~ ~ ~~
