This document was modified and supplemented by BYU-Idaho faculty. All illustrations are from USGS unless otherwise noted.

Learning Objectives Your goals in studying this chapter are to:

• Understand exactly what an earthquake is and

the related terminology.

• Understand the types of faults and seismic waves

• Understand how earthquakes are measured.

• Understand the kinds of damage earthquakes

can cause.

• Understand earthquake mitigation measures,

including basic principles of seismic engineering.

• Understand earthquake risk in the United States.

• Understand the limitations of earthquake

prediction.

• Understand earthquake preparedness.

How Earthquakes Cause Damage Earthquakes cause damage in several ways that are discussed in the following sections: • Shaking • Fire • Liquefaction • Tsunami • Seiche • Landslides A different set of these occurs in each earthquake depending on magnitude, depth, location, types of building construction, and local geology. We learn best about the causes of damage through case studies, which each illustrate the different conditions that can lead to damage in different ways.

Mexico City sustained heavy damage from an earthquake in 1985. (University of California)

Shaking The factors that affect the intensity of shaking are: 1) Magnitude. Larger magnitude earthquakes release more energy into the ground, which causes seismic waves with higher energy and higher amplitudes (= harder shaking). 2) Distance. Shaking decays with distance. The same goes for depth – deeper earthquakes are essentially farther away, and do less damage. 3) Local soils. Soft soils amplify shaking. The most severe shaking occurs in “fill,” where bays or marshy areas have been filled in for construction purposes. Solid bedrock shakes considerably less (sometimes more than 15X less!) than soft soils. At greater distances, intensity is also affected by the overall composition of the crust (see the map below). Most of the western U.S., for example, is highly faulted and made of heterogeneous materials. Seismic waves lose energy every time they cross a boundary into a different material, so in the West seismic waves lose energy before they can travel very far. In the Midwest and East, however, the crust is more uniform, and seismic waves travel longer distances before losing energy. Earthquakes like those in New Madrid, Missouri, in 1811-1812 resulted in more widespread Mercalli intensities across the region compared to the 1992 Landers quake of the same magnitude.

soft soils animation

This map shows soft soil areas (including bay fill in gray) in the San Francisco Bay Area. These areas are most vulnerable to hard shaking and liquefaction in an earthquake. The south end of the San Francisco bay is known as “Silicon valley,” and several important technology companies are located on soft soils, including Google, Apple, Microsoft, Adobe, Facebook., and many others A large earthquake here could have widespread consequences for the internet.

Fire Fire did most of the damage in the 1906 San Francisco earthquake. Some fires were arson, but most started from oil lamps, wood stoves, and chimney fires. (USGS)

The fires in San Francisco in 1906 spread unchecked because liquefaction had severed the underground water pipes. Fire has also been a major factor in the Lisbon, Portugal quake in 1755 and the Kobe, Japan earthquake in 1995. Mitigating fires today involves homeowners more than most disaster mitigation measures because homes are so numerous, and experience shows that many fires start in homes. Mitigating earthquake-caused fires involves proper and careful installation and examination of gas-powered appliances and their supporting piping. Gas or propane water heaters and forced-air heaters must be strapped to the structure of the house (not just to sheetrock) to prevent them from falling over and breaking the gas lines. Broken gas lines are the most common cause of house fires after earthquakes. Other gas appliances must be connected with adequate flexible piping, which is also a building code requirement. Fireplaces, space heaters, wood stoves, and other potential sources of fire must be used in accordance with the manufacturer’s guidelines, regularly inspected for problems, and operated with care and common sense. It is also a good idea to have fire extinguishers in your home. After an earthquake, check around your building carefully. If you smell gas, you should immediately turn off the gas valve that leads into your building. Wait for clearance from the gas company, and they will inspect appliances and turn the gas back on for you.

San Francisco, 1906 (USGS)

Tsunami (source: NOAA, modified) Figure 1. Click to see an animation of a tsunami generated by an earthquake. A tsunami (plural is also “tsunami” or “tsunamis”) is a set of ocean waves caused by any large, abrupt disturbance of the sea, typically an upward or downward displacement of the seafloor, or a landslide. If the disturbance is close to the coastline, local tsunamis can demolish coastal communities within minutes. A very large disturbance can cause local devastation AND export tsunami destruction thousands of miles away. The word tsunami is a Japanese word, represented by two characters: tsu, meaning, "harbor", and nami meaning, "wave". Tsunami rank high on the scale of natural disasters. Since 1850 alone, tsunami have been responsible for the loss of over 420,000 lives and billions of dollars of damage to coastal structures and habitats. Most of these casualties were caused by local tsunamis that occur about once per year somewhere in the world. For example, the December 26, 2004, tsunami killed about 130,000 people close to the earthquake and about 58,000 people on distant shores. Predicting when and where the next tsunami will strike is currently impossible before the tsunami starts. Once the tsunami is generated, forecasting tsunami arrival and impact is possible through modeling and measurement technologies.

Tsunami damage in Indonesia, 2004. (USGS)

Tsunamis are difficult to illustrate because they travel entirely under water, with only tiny disturbance of the surface. The ocean surface does not rise up with seafloor displacement – have you ever tried to lift water? Tsunamis are pulses of dense water, like the pulse you feel in the water when someone swims by you. Nearly all illustrations get this wrong, even the ones by the experts!

Tsunami are most commonly generated by earthquakes in marine and coastal regions. Major tsunami are produced by large (greater than 7 on the Richer scale), shallow focus (< 30km depth in the earth) earthquakes associated with the movement of oceanic and continental plates. They frequently occur in the Pacific, which is surrounded by convergent plate boundaries. When a plate boundary fault ruptures, it provides a vertical movement of the seafloor that allows a quick and efficient transfer of energy from the solid earth to the ocean (try the animation in Figure 1). When a powerful earthquake (magnitude 9.1) struck the coastal region of Indonesia in 2004, the movement of the seafloor produced a tsunami in excess of 30 meters (100 feet) along the adjacent coastline, killing more than 240,000 people. From this source, the tsunami radiated outward and within 2 hours had claimed 58,000 lives in Thailand, Sri Lanka, and India. Underwater landslides associated with smaller earthquakes are also capable of generating destructive tsunami. The tsunami that devastated the northwestern coast of Papua New Guinea on July 17, 1998, was generated by an earthquake that registered 7.0 on the Richter scale that apparently triggered a large underwater landslide. Three waves measuring more than 7 meter high struck a 10-kilometer stretch of coastline within ten minutes of the earthquake and landslide. Three coastal villages were swept completely clean by the waves, leaving nothing but sand and 2,200 people dead. Other large-scale disturbances of the sea that can generate tsunami are explosive volcanoes and asteroid impacts. The eruption of the volcano Krakatoa in the East Indies on Aug. 27, 1883 produced a 30-meter tsunami that killed over 36,000 people. In 1997, scientists discovered evidence of a 4km diameter asteroid that landed offshore of Chile approximately 2 million years ago that produced a huge tsunami that swept over portions of South America and Antarctica.

Tsunami damage in Indonesia, 2004 as seen from a satellite. The waves rushed up hillsides as far as 100 feet locally, destroying buildings, farms, and forests. (NASA)

Any vertical movement of the seafloor immediately sends a pressure wave (very like a p-wave) out in all directions. The resulting tsunami propagates as a set of waves with great wavelengths (~100 km). The waves are unseen as they travel across the deep ocean basins, and only become visible when they encounter shallow water (<100’s m deep). In shallow water, the pulses slow down and pile up higher. At the coast, an arriving tsunami is like a rapidly rising tide, and less commonly looks like big breakers. In some places, the first part of the tsunami wave train may be a low, causing the sea to retreat. The wave heights and directions determined by the adjacent coastline geometry. Because each fault displacement is unique, every tsunami has unique wavelengths, wave heights, and directionality (Figure 2 shows the propagation of the December 24, 2004 Sumatra tsunami.) From a tsunami warning perspective, this makes the problem of forecasting tsunamis in real time daunting. Figure 2. Click to see the propagation of the December 24, 2004 tsunami in the Indian Ocean. Warning Systems. Since 1946, the tsunami warning system has provided warnings of potential tsunami danger in the pacific basin by monitoring earthquake activity and the passage of tsunami waves at tide gauges. However, neither seismometers nor coastal tide gauges provide data that allow accurate prediction of the impact of a tsunami at a particular coastal location. Monitoring earthquakes gives a good estimate of the potential for tsunami generation, based on earthquake size and location, but gives no direct information about the tsunami itself. Tide gauges in harbors provide direct measurements of the tsunami, but the tsunami is significantly altered by local bathymetry and harbor shapes, which severely limits their use in forecasting tsunami impact at other locations. Partly because of these data limitations, 15 of 20 tsunami warnings issued since 1946 were considered false alarms because the tsunami that arrived was too weak to cause damage. Figure 3. Click to see a real-time deep ocean tsunami detection system responding to a tsunami generated by seismic activity.

Tsunami coming over the protection wall in northern Japan, March 11, 2011. The walls were not built tall enough to mitigate damage from this large of a wave. In addition, locally the ground dropped more than a meter, effectively lowering the tsunami walls. (L.A. Times)

Forecasting impacts. Recently developed real-time, deep ocean tsunami detectors (Figure 3) will provide the data necessary to make tsunami forecasts. The November 17, 2003, Rat Is. tsunami in Alaska provided the most comprehensive test for the forecast methodology. The Mw 7.8 earthquake on the shelf near Rat Islands, Alaska, generated a tsunami that was detected by three tsunameters located along the Aleutian Trench-the first tsunami detection by the newly developed real-time tsunameter system. These real-time data combined with the model database were then used to produce the real-time model tsunami forecast. For the first time, tsunami model predictions were obtained during the tsunami propagation, before the waves had reached many coastlines. The initial offshore forecast was obtained immediately after preliminary earthquake parameters (location and magnitude Ms = 7.5) became available from the West Coast/Alaska TWC (about 15-20 minutes after the earthquake). The model estimates provided expected tsunami time series at tsunameter locations. When the closest tsunameter recorded the first tsunami wave, about 80 minutes after the tsunami, the model predictions were compared with the deep-ocean data and the updated forecast was adjusted immediately.. These offshore model scenarios were then used as input for the forecast for Hilo Bay. The tsunami recorded nearly half a meter (peak-to-trough) signal at the Hilo gage. Model forecast predictions for this tide gage are compared with observed data in Figure 4. The comparison demonstrates that amplitudes, arrival time and periods of several first waves of the tsunami wave train were correctly forecasted. More tests are required to ensure that the inundation forecast will work for every likely tsunami. When implemented, such forecasts will be obtained even faster and would provide enough lead time for potential evacuation or warning cancellation for Hawaii and the U.S. West Coast. Figure 4. Coastal forecast at Hilo, HI for the 2003 Rat island quake, showing comparison of the forecasted (red line) and measured (blue line) gage data. Mitigation. The recent development of real-time deep ocean tsunami detectors and tsunami inundation models has given coastal communities the tools they need to reduce the impact of future tsunamis. If these tools are used in conjunction with a continuing educational program at the community level, at least 25% of the tsunami related deaths might be averted. The best mitigation tools are education, zoning (to keep people and structures high above sea level wherever possible), evacuation plans, tsunami walls, and warning systems.

A seiche is a wave in a lake caused by displacement of the lake bottom. In 1959, Hebgen Lake near West Yellowstone, Montana, dropped nearly 6 meters on the side near the Hebgen Lake normal fault in a M7.3 earthquake. The resulting seiche damaged roads and lakeshore cabins, permanently changed the shoreline, and went over the Hebgen dam several times (see photos).

(USGS)

Liquefaction (source: Utah Geological Survey) What is liquefaction? Liquefaction may occur when water-saturated sandy soils are subjected to earthquake ground shaking. When soil liquefies, it loses strength and behaves as a viscous liquid (like quicksand) rather than as a solid. This can cause buildings to sink into the ground or tilt, empty buried tanks to rise to the ground surface, slope failures, nearly level ground to shift laterally tens of feet (lateral spreading), surface subsidence, ground cracking, and sand blows. Why is liquefaction a concern? Liquefaction has caused significant property damage in many earthquakes around the world, and is a major hazard associated with earthquakes in Utah. The 1934 Hansel Valley and 1962 Cache Valley earthquakes caused liquefaction, and large prehistoric lateral spreads exist at many locations along the Wasatch Front. The valleys of the Wasatch Front are especially vulnerable to liquefaction because of susceptible soils, shallow ground water, and relatively high probability of moderate to large earthquakes. Where is liquefaction likely to occur? Two conditions must exist for liquefaction to occur: (1) the soil must be susceptible to liquefaction (loose, water-saturated, sandy soil, typically between 0 and 30 feet below the ground surface) and (2) ground shaking must be strong enough to cause susceptible soils to liquefy. Northern, central, and southwestern Utah are the state's most seismically active areas. Identifying soils susceptible to liquefaction in these areas involves knowledge of the local geology and subsurface soil and water conditions. The most susceptible soils are generally along rivers, streams, and lake shorelines, as well as in some ancient river and lake deposits. How is liquefaction potential determined? The liquefaction potential categories shown on this map depend on the probability of having an earthquake within a 100-year period that will be strong enough to cause liquefaction in those zones. Highliquefaction potential means that there is a 50% probability of having an earthquake within a 100-year period that will be strong enough to cause liquefaction. Moderatemeans that the probability is between 10% and 50%, low between 5 and 10%, andvery low less than 5%. What can be done? To determine the liquefaction potential and likelihood of property damage at a site, a site-specific geotechnical investigation by a qualified professional is needed. If a hazard exists, various hazard-reduction techniques are available, such as soil improvement or special foundation design. The cost of site investigations and/or mitigation measures should be balanced with an acceptable risk.

Liquefaction damage from the 1989 Loma Prieta earthquake near San Francisco, California. (USGS)

The diagram at left shows how liquefaction can lead to formation of a “sand blow” or “mud volcano.” The photo below shows apartment buildings that sank when liquefaction occurred under them in the Niigata, Japan earthquake in 1964. Liquefaction mitigation involves putting building foundations on deep pilings, and is quite expensive. (USGS)

Landslides We will study the broader category of mass wasting and specific causes of landslides in a later chapter. Landslides are very common in earthquakes, triggered when shaking causes loose or unstable ground to move downhill. The Hebgen Lake earthquake in 1959 (not far north of BYU-Idaho) caused a massive avalanche that killed dozens of sleeping campers and blocked the Madison River, forming Quake Lake (see image below, left).

Japan, 2007 (USGS)

China, 2008 (USGS) “Madison Slide”, 1959 (USGS)