Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk Assessmentsoil, typically hundreds of feet, but up to 12 miles), and loss of bearing strength (soildeformations causing structures to settle or tip). Liquefaction can cause severe damage toproperty.• Lateral Spreads are a type of landslide, but are distinctive because they usually occur onvery gentle slopes or flat terrain and occur in a rapid fluid-like flow movement, caused byliquefaction. Ground failure is usually triggered by rapid ground motion, such as thatexperienced during an earthquake, but can also be artificially induced. When coherentmaterial, either bedrock or soil, rests on materials that liquefy, the upper units may undergofracturing and extension and may then subside, translate, rotate, disintegrate, or liquefy andflow. Lateral spreads are almost always discussed in conjunction with liquefaction.• Landslides occur as a result of horizontal seismic inertia forces induced in the slopes by theground shaking. The most common earthquake-induced landslides include shallow, disruptedlandslides such as rock falls, rockslides, and soil slides. Debris flows are created whensurface soil on steep slopes becomes totally saturated with water. Once the soil liquefies, itloses the ability to hold together and can flow downhill at very high speeds, taking vegetationand/or structures with it. Slide risks increase after an earthquake during the wet season.Landslides are further addressed in Section 5.3.12 (Slope Failure).• Tsunamis: As an Oceanic Plate is subducted beneath a Continental Plate, it sometimesbrings down the lip of the Continental Plate with it. Eventually, too much stress is put on thelip and it snaps back, sending shockwaves through the earth’s crust, causing a tremor underthe sea, known as an undersea earthquake. Factors that affect tsunami generation from anearthquake event include magnitude (generally, a 7.5 magnitude [M] and above), depth ofevent (a shallow marine event that displaces the seafloor), and type of earthquake (thrust asopposed to strike-slip). Tsunamis are further addressed in Section 5.3.16 (Tsunami).The severity of an earthquake can be expressed in terms of intensity and magnitude. Intensity isbased on the damage and observed effects on people and the natural and built environment. Itvaries from place to place depending on the location with respect to the earthquake epicenter,which is the point on the Earth’s surface that is directly above where the earthquake occurred.The severity of intensity generally increases with the amount of energy released and decreaseswith distance from the fault or epicenter of the earthquake. The scale most often used in the U.S.to measure intensity is the Modified Mercalli (MM) Intensity Scale. As shown in Table 5-5, theMM Intensity Scale consists of 12 increasing levels of intensity that range from imperceptible tocatastrophic destruction. Peak ground acceleration (PGA) is also used to measure earthquakeintensity by quantifying how hard the earth shakes in a given location. PGA can be measured ing, which is vertical acceleration due to gravity.Magnitude is the measure of the earthquake strength. It is related to the amount of seismicenergy released at the earthquake’s hypocenter, the actual location of the energy released insidethe earth. It is based on the amplitude of the earthquake waves recorded on instruments, knownas the Richter magnitude test scales, which have a common calibration.5-17