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Figure 1‐4 This 45-foot deep sinkhole formed in Monticello, FL, after Tropical Storm Debby produced excessive rains and flooding. David Fine/FEMA.

      1.2.5 Seismic and Volcanic Hazards

      Seismic hazards are hazard agents produced by the movement of tectonic plates that float on magma. Earthquakes occur along fault lines where landmasses move apart, collide or slide against each other laterally. When this movement occurs, waves travel in and on top of the earth. These waves emanate from the geographic origin of the earthquake, known as a focal point. The location on the earth’s crust directly above the focal point is called the epicenter. The intensity of an earthquake is described by use of the Richter scale, a measurement of the registered shaking amplitudes. In contrast, the Mercalli scale is used to denote the physical observation of damages that result from the displacement of the earth’s crust (e.g., broken windows, cracked walls, falling pictures, etc.).

      Earthquake risk is probability the highest in locations surrounding the ring of fire (i.e., countries positioned around the Pacific Rim). For instance, Japan has earthquakes on a constant basis and Chile has had the most powerful earthquake recorded in history, registering an amazing 9.5 on the Richter scale. Nevertheless, earthquakes occur in many locations around the world, and they can be extremely damaging and deadly. Tens of thousands and even hundreds of thousands have perished in earthquakes in Mexico City, Russia, India, and Iran. Haiti suffered a 7.0‐magnitude quake in 2010, which killed between 200,000 and 300,000 people. In 2013, the Sichuan province in China was the epicenter of a major earthquake. The movement of tectonic plates coupled with poor construction took the lives of 200 people. Building codes have historically been weak in such countries, resulting in structural collapses and the crushing of their inhabitants.

      In the United States, there are major fault lines in California, Utah, Illinois, South Carolina and in New England. While some of these fault lines are not active, earthquake faults along the Pacific Coast slip frequently and have destroyed gas and water lines, roads and bridges, and homes and other structures. The 1989 Loma Prieta and 1994 Northridge earthquakes killed scores of individuals. In the Midwest, the New Madrid fault stretches from Arkansas to Missouri and Tennessee. Earthquakes in this area have changed the course of the Mississippi River in the past. Additional destructive slips in this area are projected to occur in the future.

      Earthquakes may trigger other natural hazards. For instance, while tsunamis may be associated with underwater landslides and asteroids that impact the oceans, they result most often from earthquake hazards. If fault lines slip under the ocean, the accompanying seismic waves displace water, which races vertically and horizontally away from the focal point. When these “harbor” or “tidal” waves reach land, they become amplified on the surface. The resulting waves or series of waves may travel hundreds of feet to a few miles inland. They move rapidly (as fast as 500 mph) and may reach one or two stories in height. The waves consequently level many of the buildings and much of infrastructure that lies in their path.

Tsunamis result in a number of drownings and may sweep their victims out to sea as they recede. Hawaii and the northwestern coast of the United States are prone to tsunamis. One tsunami struck Hilo, Hawaii in 1946 and another affected Alaska in 1964. Several deaths resulted in each event. However, the most powerful tsunami in history occurred on January 4, 2005. The Sumatra earthquake registered over 7.0 on the Richter scale; it sent powerful tsunami waves to over 12 countries surrounding the Indian Ocean. Over 300,000 people died from this tragic event. Another major tsunami occurred after an earthquake struck Japan in 2011. The tsunami that was generated had waves reaching over a 100 feet in some locations. The Tohoku earthquake and tsunami damaged the Fukushima nuclear power plant and caused the release of radioactive material into the air. Almost 16,000 people were killed as a result of these combined hazards. This event illustrates how complex some disasters can be.

      Volcanic activity is another type of natural hazard and it is closely related to earthquakes and the movement of magma within the earth’s crust. Magma may bubble up through fissures in the earth surface, creating a cone with a reservoir of lava. These mountainous craters may vent superheated gasses and emit lava flows down the side of the cone. In the United States, volcanic activity is present mainly in the Northwest and in Hawaii.

Volcanic eruptions can be particularly deadly, as was the case with Mt. St. Helens in 1980 (Figure 1‐5). A bulge developed over time on the north face of Mt. St. Helens and eventually the growing pressure gave way in a violent explosion. Tons and tons of soil, lava, and mud were sent down the side of the mountain and into the valley and rivers below. Fifty‐seven people were killed in the incident, being vaporized immediately, buried under volcanic debris or drowned in lahars (violent mudflows). Volcanic ash also rained down on communities around the volcano and even in nearby states. This made some vehicles inoperable and caused a cleanup nightmare. In addition, the logging industry in this area was severely disrupted for a period of time due to the Mt. St. Helens eruption.

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