Ch5 Geology

Unreinforced masonry

the most dangerous types of homes are those constructed of unreinforced masonry because they offer very little resistance to lateral shearing motion.

Elastic rebound theory

Explains how earthquakes originates from a rock body deforms and accumulates such that it reaches its elastic limit at which point the rock suddenly fails and releases its stored energy

Intraplate Earthquake

Geologists refer to earthquakes that occur far from a plate boundary or active mountain belt as intraplate earthquakes.

Seismic Gaps

Geologists use the term seismic gap to refer to those sections of an active fault where the strain has not been released for an extended period of time. Seismic gaps can be useful in predicting what areas are most likely to experience a large earthquake.

Mercelli Intensity Scale

He created what is known as the Mercalli intensity scale, whereby earthquakes are ranked based on a set of observations most people could report objectively, particularly the type of damage sustained by buildings. A modified version of Mercalli's rankings and standardized observations is listed in Table 5.1. Note how the intensity scale ranks earthquakes from I to XII, with XII representing total destruction.

Elastic rebound theory

In addition to faults, we also need to consider the elastic property of the rock itself. Recall that when rocks are more ductile (less brittle), they tend to accumulate less strain, and instead undergo plastic deformation (Chapter 4). This is important because an earthquake requires a rock body rigid enough to accumulate strain. It turns out that earthquakes do not occur deeper than 435 miles (700 km) below the Earth's surface because the rocks become so ductile that they deform only by plastic flow, and hence do not rupture.

liquafaction

In areas where saturated conditions exist close to the surface (i.e., a high water table—Chapter 11), ground shaking can cause a phenomenon called liquefaction, where sand-rich layers of sediment behave as fluid. As illustrated in Figure 5.27A, compacted sand grains are normally in contact with one another, hence are able page 144to support the weight of overlying sediment and human structures.

What does Primary wave do?

In primary waves , or P-waves, the particles vibrate in the same direction the wave is traveling, causing rocks to alternately compress and decompress with each passing wave.

liquefaction problem

Liquefaction is a serious problem because while subsurface sand layers are in the liquid state, heavy objects sitting on the surface are left unsupported, allowing them to sink or topple over (Figure 5.27B). In hilly terrain, liquefaction can cause slopes to become unstable, triggering different types of landslides geologists refer to as mass wasting (Chapter 7). The increased water pressure within the saturated sediment can also cause geysers of liquefied sand to erupt onto the surface, creating what are called sand blows

Focal Depth and Wave Attenuation

Moreover, the energy of the corresponding seismic waves steadily decreases as they travel away from the focus, a process referred to as wave attenuation.

________ that travel through solid earth materials as _____ as opposed to less scientific term _____________ waves.

Regardless of their origin, we will refer to vibrational waves that travel through solid earth materials as seismic waves, as opposed to the less scientific term of earthquake waves.

Predictions

Short-term predictions, therefore, are not yet a reality, and some seismologists believe they never will be. What we have been successful at is predicting earthquakes on a long-term basis using statistical probabilities. These long-term predictions are similar to weather forecasts in that they give the probability of an earthquake occurring within a given time period. In this section we will take a brief look at the current status of both short- and long-term earthquake predictions.

there are ________ types of surface waves why is there motion complicated?

Similarly there are two basic types of surface waves, but here the motion is more complicated because of the way the waves interact with the ground surface. As indicated in Figure 5.6, the particles in Rayleigh waves move back and forth in the direction of the wave, plus they move up and down, creating a rolling motion similar to ocean waves. In Love waves the ground moves back and forth and side to side at the same time. In essence then, both types of surface waves force the ground to move in two different directions at the same time, something human structures are not normally designed to handle. Consequently, surface waves cause far more structural damage than do body waves.

Emergency managers have learned to: (important)

have learned how to coordinate medical personnel, police, and firefighters in the aftermath of a major earthquake. They also know the value of education programs designed to teach citizens basic steps for surviving an earthquake and minimizing damage to their property.

Engineers today know how to: (Important)

how to design and construct buildings capable of withstanding the violent ground shaking associated with powerful earthquakes.

Epicenter

is the point on the surface that lies directly above the focus, which means it is also the closest point to where the strain energy was released (Figure 5.6).

Earthquake-engineered buildings therefore are________ (important)

less likely to collapse and crush their occupants.

Moment Magnitude Scale

moment magnitude scale, which is based on similar types of seismogram measurements as Richter's, but is more accurate over a wide range of magnitudes and geologic conditions.

seismic waves originate at the point of ______ called _____

seismic waves originate at the point of rupture called the focus—also called the hypocenter.

Natural Vibration Frequency

A building swaying back and forth is technically considered to be vibrating—similar to how a guitar string vibrates. Moreover, the building will vibrate at a fixed frequency called its natural vibration frequency; frequency is the number of times the motion is repeated in a set amount of time.

Tsunami

A tsunami is a series of ocean waves that form when energy is suddenly transferred to the water by an earthquake, volcanic eruption, landslide, or asteroid impact. The majority of tsunamis, however, form during subduction zone earthquakes when tectonic plates abruptly move and displace large volumes of seawater.

Earthquakes & Buildings

During an earthquake, buildings are subjected to lateral shear stress due to the horizontal ground motion and their own inertia. This lateral shear force causes structures built on slabs

Mass wasting

Earthquakes also serve as one of the basic triggering mechanisms for mass wasting, which is where earth materials move downslope due to gravity

Brittle material reach their elastic limit when______ (important)

Moreover, when brittle materials reach their elastic limit, they undergo permanent deformation by fracturing, whereas ductile materials deform by flowing plastically

What does a secondary wave do?

On the other hand, particles in secondary waves , or S-waves, vibrate perpendicular to the wave path, which creates a shearing (side-to-side) motion.

Pancaking

Once a floor becomes detached, it naturally falls onto the one below, which can cause additional floors to fail in a cascading manner that engineers call pancaking. The result is either a total or partial collapse of the structure in which few people survive.

Seismic Hazard

One statistical approach involves the creation of seismic hazard maps, which incorporate information on past seismic activity, magnitudes, and displacement rates on faults. For example, the seismic hazard map of the United States in Figure 5.34 shows the potential strength of page 150horizontal ground motion compared to the strength of gravity—commonly called g-force.

Retrofitting

Retrofitting projects are most common for those buildings whose continued operation would be critical in the aftermath of a major earthquake. Examples include emergency command and control centers, hospitals, and police and fire stations. Retrofitting is also becoming common in areas where people have only recently realized that they lie in an area with a history of major earthquakes. This typically occurs in places that were settled after the last major earthquake, thus no one page 153ever saw much need to incorporate seismic engineering. Examples include the Pacific Northwest and New Madrid seismic zones (see the hazard map in Figure 5.34), where a powerful earthquake has not occurred in living memory.

Although primary and secondary waves travel along the same path the compressional nature of ______________ waves allows them to travel _______________

Although primary and secondary waves travel along the same path, the compressional nature of the P-waves allows them to travel faster; hence they are the first to arrive at any given point. Later in this chapter you will see how the early arrival of P-waves serves as the basis for earthquake early-warning systems.

Important

Although the intensity scale is a qualitative measure since it is based on subjective human observations, it still provides a useful means of comparing the strength of different earthquakes.

Base Isolation

Another important element is base isolation, where the amount of shear force that can act on a structure is minimized. Base isolation involves placing rubber bearings (dampers) between the structural skeleton and the foundation supports. Because the skeleton is no longer connected to the ground in a rigid manner, more of the lateral shearing force will be directed on the rubber bearings and less to the building.

earthquakes

At around 5:00 p.m. in the afternoon of January 12, 2010, Earth's crust suddenly shifted beneath Port-au-Prince, the capital city of Haiti. The resulting earthquake unleashed waves of vibrational energy, causing buildings throughout this impoverished city to quickly collapse (Figure 5.1). The Haitian government estimated that 316,000 people were killed and over a million left homeless (independent groups estimate a much lower death toll, ranging from 47,000 to 158,000).

Magnitude scales

Because there was such a large range in the amplitude data (i.e., some waves were very small and others quite large), they used a logarithmic scale and called it the magnitude scale. Their original magnitude scale, shown in Figure 5.11, is now known as the Richter magnitude scale.

Seismic Data

By using seismic data to locate focal points and epicenters of earthquakes, geologists have a powerful tool for studying Earth's interior. In fact, it was through the analysis of seismic data that scientists were able to define the boundaries of tectonic plates (Chapter 4). From a hazard perspective, seismic studies are important as they help scientists understand how and where rock strain is being relieved along fault zones, which is useful in making long-term earthquake predictions.

What are the two types of body waves?

Consequently, there are two types of body waves shown inFigure 5.6. Primary waves and Secondary waves, P-waves and S-waves.

Early Warning Signs

Depending on the distance back to the epicenter, the early warning may range from a few seconds to a minute or more (beyond 1 minute the epicenter is usually far enough away that damage is minimal). A warning on the order of seconds may not seem significant, but it can be enough for people to seek shelter or pull off the highway and avoid accidents. It may also be enough time for doctors to suspend doing delicate surgery, or for people doing hazardous work to keep from being injured. An early warning can also allow utility and transportation networks to shut down in a controlled manner. For example, only seconds are needed for preprogrammed systems to close valves on gas lines, thereby reducing the risk of uncontrolled fires. Likewise, trains can be programmed to automatically stop. Electric utilities can also shut down critical control systems on electrical grids and at power plants. This not only prevents damage to the systems themselves, but allows electrical service to be restarted much more quickly.

Foreshocks Main shock Earthquake precursors

Small earthquakes called foreshocks will sometimes precede the major release of energy, known as the main shock, which can be followed by a series of less powerful aftershocks. In addition to the foreshocks, phenomena called earthquake precursors sometimes occur just prior to the main shock. Precursors include such things as changes in land elevation, water levels and dissolved gases in wells, and unusual patterns of low-frequency radio waves.

transform boundaries

The San Andreas is actually a large transform fault that separates the Pacific and North American plates, but is often referred to as a fault zone due to the network of interlocking faults found on either side. This means that as tectonic forces cause strain to accumulate along page 135the boundary, some of the strain is distributed among the different fault

Seismic engineering

The basic approach in this branch of engineering, known as seismic engineering, is to: (a) provide greater structural strength with respect to the lateral shearing forces generated during an earthquake; and (b) minimize the amount of shear force that can develop on the structure in the first place.

Elastic rebound theory

The elastic rebound theory not only explains how earthquakes occur, but can account for why some areas experience repeated earthquakes. Recall from our discussion on plate tectonics (Chapter 4) that Earth's tectonic forces operate over long periods of geologic time. This means that when a rock body ruptures, the strain may be released, but the tectonic forces are still present. After each rupturing event, the strain slowly rebuilds until the elastic limit of the rock body is again reached, thereby producing repeated earthquakes as shown in Figure 5.5. Because faults and fractures represent weak zones within a rock body, rupturing generally takes place where the strain overcomes the frictional resistance along a fault or fracture. Since the rocks in tectonically active areas usually contain numerous faults, the sudden release of strain along one fault can alter the distribution of strain on the others. This redistribution of strain commonly produces a series of smaller earthquakes calledaftershocks, which may continue to occur for days or weeks after the primary earthquake, often called the main shock

Planning and education

The first step is to determine the level or severity of risk in a given area. In the United States this task has largely been performed by the USGS through the construction of various hazard maps, particularly the seismic hazard map shown in Figure 5.34. Based on the hazard assessment, state and local government agencies will typically develop building codes that require appropriate levels of seismic engineering in buildings and other structures. The goal here is to reduce structural damage and the loss of life. Also important are local zoning ordinances that keep homes and key facilities, such as hospitals, schools, dams, and fuel storage areas, from being built across known faults In addition to taking steps to minimize structural damage, it is equally important that individual citizens, businesses, and local emergency services know page 156exactly what to do both during and after an earthquake.

Resonance

The matching of frequency then leads to the phenomenon called resonance, whereby the amplitudes of the individual waves combine as shown in Figure 5.23A. Here the increased amplitude. It turns out that buildings around 10-20 stories high are the most susceptible to resonance. On the other hand, tall skyscrapers are unlikely to experience resonance as their vibration frequency is beyond the frequency range of most seismic waves.

Vast majority of earthquakes are ______ or ______ in nature which is why they are found along______________

The vast majority of earthquakes though are tectonic or magmatic in nature, which explains why most earthquakes are found along plate boundaries

Magmatic Earthquakes

There are also many magmatic earthquakes that are caused when magma forces its way up through the crust (Chapter 6).

Ground Amplification

This abrupt decrease in velocity causes wave amplitude to increase, creating a phenomenon known as ground amplification. Ground amplification is a serious problem because it greatly increases the level of shaking, which, in turn, increases the risk of structural failure. This brings up another problem, namely, how seismic waves behave in sedimentary basins, which are depressions in the crust filled with sediment and sedimentary rocks.

Model Of occurrence Foreshocks Precursors

This model then can explain the occurrence of some foreshocks and the following earthquake precursors: Increase in foreshocks—microcracks forming prior to complete rupture, or main shock. Slight swelling or tilting of the ground surface—microcracks increasing the rock volume. Decreased electrical resistance—water entering new void spaces is more conductive than surrounding minerals. Fluctuating water levels in wells—water entering new cracks causes water levels to lower; levels rise when voids close again. Increased concentration of radon gas in groundwater—new cracks allow the gas, a radioactive decay product of uranium, to escape from rocks and enter wells. Generation of radio signals—changes in rock strain or movement of saline groundwater.

subduction earthquakes

This subduction zone not only produces the volcanic arc (Chapter 4) known as the Cascade Mountain Range, it also generates subduction zone earthquakes, which form when an oceanic plate is overridden by another plate. Subduction zones are important because they are capable of generating extremely powerful and devastating earthquakes. For example, the ten largest earthquakes ever recorded were all subduction zone earthquakes, and of these, seven were magnitude 9 or higher. The possibility of such an earthquake along the Cascadia subduction zone is rather frightening since a magnitude 9 will unleash 32 times more energy than a magnitude 8. Keep in mind that throughout history, earthquakes in the magnitude 8 range have proven to be sufficiently powerful enough to level entire cities.

The sudden release of strain, lasting anywhere from several seconds to a few minutes is transformed into ___________________________that _____________________ (important) In addition the release of energy begins at the __________________

This sudden release of strain, lasting anywhere from several seconds to a few minutes, is transformed into vibrational wave energy that radiates outward, causing the ground to shake in what is called an earthquake. In addition, the release of energy generally begins at a point called the focus, also called the hypocenter, which is typically located where the rock is the weakest, such as along a preexisting fracture plane. While the strain is being released, rocks on either side of the fracture will begin moving in opposite directions, creating what geologists refer to as displacement. Note that when rocks are displaced along a fracture, the fracture technically becomes a fault

Tectonic earthquakes

Today scientists understand that most earthquakes are caused by the buildup of strain associated with tectonic forces, and hence are often referred to as tectonic earthquakes.

One of the poorest nations________________ does not have resources such as for natural disasters. (important)

Unfortunately, Haiti is one of the poorest nations in the world and simply does not have the resources or political stability needed to institute building codes and effective emergency management plans.

Wealthy nations have resources such as: (important)

Wealthy nations, like the United States, have the resources and organizational structure needed to prepare for a major earthquake and thereby reduce the risk.

body waves

When a rock body is actually rupturing and being displaced along a fault, it generates seismic waves

Surface waves

When body waves reach the surface, the energy is not transferred to the atmosphere, but instead begins to move along the land surface

Earthquakes and humans

While earthquakes are an important part of the Earth system, the reality is they have always posed a number of hazards to humans. Prior to the development of agriculture and cities, the primary hazards people faced from earthquakes were landslides and tsunamis. While these hazards are significant, they are relatively minor compared to problems that developed once people started living and working in buildings, particularly as buildings became progressively taller and heavier over time. Another key factor is that exponential population growth has resulted in many more people living in earthquake hazard zones compared to the past.

ground fissures

earthquakes also create large open cracks called ground fissures over a wide area of the landscape (faults themselves do not open up like fissures). Ground fissures typically develop close to the surface in loose sediment where there is little resistance to the rolling and stretching motion associated with surface waves. Unlike ground displacement that occurs directly along the fault trace, open fissures have the potential to affect a greater number of structures because they develop over a much broader area.