Review Questions and Answers; Earthquakes

An earthquake is groundshaking caused by a sudden cracking and rupturing of highly strained rock and quick, lateral and/or vertical movements of the blocks on either side of the rupture surface. Failure occurs when the cumulative strains finally exceed the internal, cohesive bonds that hold the rock together. Failure (rupture and block movements) is catastrophic, usually taking only a few tens of seconds to a minute, and a high percentage of the strain energy is converted to ground vibrations. Following the earthquake, strain begins gradually building again toward a future quake.

2. How are faults, foci, and epicenters related?

A fault is the plane or zone of fracture separating two blocks that are abruptly displaced during an earthquake. The focus is the point at depth, usually in a fault zone, where the displacement and sudden release of elastic energy initiate. It marks the initial rupture site associated with the earthquake. The epicenter is the point on the surface directly above the focus. These relationships are nicely shown in Figure 16.3.

3. Who was first to explain the actual mechanism by which earthquakes are generated?

H. F. Reid, a professor at Johns Hopkins University, based his elastic rebound idea on studies of the 1906 San Francisco earthquake.

4. Explain what is meant by elastic rebound.

Reid concluded that the quake was due to excess, elastic strain energy being suddenly (catastrophically) released as the highly overstrained rocks snapped back (rebounded) to a state of much lower strain. Cool lithospheric rocks have elastic limits large enough to support earthquake-causing, elastic strains; because they are much warmer, asthenospheric rocks begin deforming by flowage (plastic deformation) at much lower stress magnitudes; thus stored, elastic strain energies are too small in magnitude to produce a strong earthquake.

5. Faults that are experiencing no active creep may be considered "safe". Rebut or defend this statement.

Creep movements should reduce the likelihood of an earthquake by reducing the level of strain accumulated in the fault-zone rocks. A fault without creep may reflect two, fundamentally different conditions. First, the stresses may be too small to move the blocks and there is no accumulation of elastic strain; in this case, the fault is clearly inactive. In the other case, the fault is active but locked; the blocks cannot respond to tectonic stresses by moving, so elastic strain accumulates. A locked fault has a high probability for hosting a damaging, future quake, and such sites, known as seismic gaps, are of special interest in earthquake forecasting.

6. Describe the principle of a seismograph.

A seismograph produces scaled and timed records of ground motion, ground velocity, and ground acceleration that result from earthquake (seismic) waves moving through the instrument site. The sensing part of the instrument is usually placed underground in a rigid, concrete vault. The rigid structure vibrates in concert with the passing seismic waves; ground motions (displacement, velocity, and acceleration) are sensed by optical or electromagnetic sensors set in motion by the seismic vibrations. These variations are converted to electrical signals, amplified, and recorded as a function of time, giving an accurate record of the seismic wave patterns.

7. List the major differences between P and S waves.

P waves (primary or pressure waves) travel though matter as particle vibrations along lines parallel to the ray path or to the path traced by a point traveling with the wave front. P waves move through solids, liquids, and gases, the "vibrations" being in the form of pressure variations. S waves (secondary or shear waves) move by particle vibrations at right angles to the ray path; they are transmitted only by solids, not by liquids or gases. P waves always travel faster than S waves in a given material.

8. P waves move through solids, liquids, and gases, whereas S waves move only through solids. Explain.

Pressure variations (P waves) are transmitted through solids and fluids; the transmitting material need not have any shear strength. S waves are shear waves and can propagate only in materials with a finite, shear strength; thus S waves are transmitted only by solids, not by liquids and gases.

9. Which type of seismic wave causes the greatest destruction to buildings?

Surface waves have much higher amplitudes than body waves (P and S waves) and account for nearly all, dangerous, ground displacements and accelerations associated with nearby or more distant earthquakes. The horizontally-vibrating surface waves in general present more danger to buildings and other structures than surface waves with vertical motion. Close to the epicenter of a shallow-focus quake, the various waves more-or-less arrive at the same time, giving a cumulative jolt to the first, strong, ground motions.

10. Using Figure 16.13, determine the distance between an earthquake and a seismic station if the first S wave arrives 3 minutes after the first P wave.

The travel time versus distance curves for P and S waves are shown in the figure. To answer the question, you must find the point on the distance axis that corresponds to a difference in arrival times between the P and S waves of 3 minutes. On tracing paper make a line whose length is equal to a time interval of 3 minutes (0 to 3 minutes, 3 to 6 minutes, etc.) on the vertical or time axis. Then move the line onto the graph and find the distance on the horizontal axis where the time interval between the first P and S arrivals is 3 minutes. The correct distance is about 1900 kilometers.

11. Most strong earthquakes occur in a zone on the globe known as the ________ .

This is the circum-Pacific belt. It includes many earthquakes associated with active subduction zones around the Pacific rim (Fig. 16.15) where different parts of the Pacific oceanic crust are sinking into the mantle below island arcs or continental margins.

12. Deep-focus earthquakes occur several hundred kilometers below what prominent feature of the deep-ocean floor?

Deep-focus quakes occur below deep ocean trenches and, like the trenches, are directly associated with the subducting slab of oceanic lithosphere. With increasing depth, the intermediate- and deep-focus epicenters migrate farther toward the interior of the upper plate in correspondence to the dip of the subducting slab. The earthquakes are thought to occur in the sunken oceanic plate. Long after sinking into the mantle, interior parts of the slab stay cool enough to maintain some rigidity and component minerals can be "destabilized" and subject to very fast phase changes. Mantle rocks surrounding the slab are too "soft" to accumulate elastic strain energy, and mineral phases are more or less in equilibrium with respect to temperatures and pressures at those depths. Thus deep-focus earthquakes are associated only with subduction zones.

Foci at depths between 75 and 300 km are probably related to very rapid dehydration of minerals such as serpentine, made metastable by increased pressures as the slab sinks. Water is released to local volumes of the slab faster than it can migrate away, resulting in temporary, local, intergranular accumulations of liquid water, high pore pressures, and lowered mechanical strength. Sudden slippage in the "wet" dehydrated rocks and nearby rocks accommodates the volume reduction caused by dehydration and escape of the water toward the surface.

Interest in deeper-focus quakes (300 to 700 km) was greatly stimulated by the June, 1994, Bolivian earthquake, an unusually powerful, magnitude 8.2 event at a depth of 640 km. "Fault mechanisms" associated with quakes at these depths are still controversial and unresolved. Interest centers on catastrophic decomposition of metastable olivine to microcrystalline spinel in thin, tabular zones and temporary superplasticity and slip along the newly-formed, still microcrystalline, spinel zones (anticracking). The superplasticity disappears with coarsening of the spinel grains. Decomposition of olivine and spinel to perovskite-type phases, expected below 670 km, is endothermic, and does not support the anticracking mechanism. This may explain why earthquakes cease below 700 km, even though metastable olivine and spinel might exist temporarily to below this depth.

More recent analyses of the Bolivian quake data have focused on the slow rupture velocities and small percentage of total released strain energy (< 5 %) converted to elastic waves. This observation suggests that melting is important along the slip zone and that planar melt zones might allow rupture propagation and block slip to occur much like happens with brittle failure at shallow depths. Newer broadband seismic stations should help to resolve the source mechanisms of deep-focus quakes.

13. Distinguish between the Mercalli scale and the Richter scale.

The Richter earthquake-magnitude scale is based on standardized measurements of ground vibration amplitudes and the total energy released during the earthquake. It is a logarithmic, numerical scale; theoretical considerations of rock strength suggest that 9 is about the highest magnitude possible. The Richter scale is quantitative because magnitudes are based on measured wave amplitudes and a physically valid relationship between amplitude and wave energy. In recent years a newer magnitude scale based on the integrated seismic moment has been utilized. For low to moderate magnitude quakes originating along short fault segments, the two magnitude values are similar. For large earthquakes associated with extensive rupture zones, those with Richter magnitude 8+, the moment method gives higher values for the total energy released. These larger magnitudes are considered realistic since the Richter scale determination basically assumes that the seismic energy emanates from a single point rather than from the rupture surface.

The Mercalli earthquake-intensity scale is based on visual observations; it gives an estimate of the groundshaking intensity in terms of human perceptions, eye witness accounts, and damage to buildings or other property. As such, it is a subjective scale; the ratings are strongly dependent on local site characteristics such as foundation stability and building design. The scale (from I to XII) utilizes Roman numerals.

14. For each increase of one on the Richter scale, wave amplitude increases __________ (10) times.

15. An earthquake measuring 7 on the Richter scale releases about ________ (30) times more energy than an earthquake with a magnitude of 6.

The magnitude is computed from the wave amplitude (standardized to what it would have been if measured 100 kilometers from the epicenter) and an accepted, mathematical relationship between the wave amplitude and the total energy released. Wave amplitudes of a magnitude 7 quake are about 10 times those associated with one of magnitude 6; about 30 times more energy is released. The Richter scale is logarithmic.

16. List four factors that affect the amount of destruction caused by seismic vibrations.

Many factors can be noted, particularly the amplitude of the ground displacement or acceleration, the length of time that shaking occurs, and the character of the groundshaking. In general, vertical ground motion is not so dangerous as lateral or horizontal shaking, and short period (high-frequency) vibrations are less dangerous than longer period vibrations. Stability of the foundation material, building design, and construction quality are also important factors.

17. What factor contributed most to the extensive damage that occurred in the central portion of Mexico City during the 1985 earthquake?

Weak foundation material and poor building design both contributed to the unusually severe damage at a site over 200 miles from the epicenter. The city is built on old lake beds and marshlands known to have existed when Cortez and the Spaniards routed Montezuma and his Aztec warriors early in the sixteenth century. Soft, unconsolidated, water-saturated sediments are extremely unstable with respect to vibrations; and groundshaking is greatly intensified over that in underlying solid rock. In the Mexico City case, intensified shaking caused slides, slumps, and liquefaction, leading to numerous foundation failures. In addition, heavy concrete floors in the upper stories of high-rise buildings were shaken loose from the walls, allowing them to drop down onto the next lower floor, causing that floor to drop. This process continued to ground level, leaving the heavy floor slabs stacked one atop the other. Left with no lateral support, the walls also collapsed.

18. The 1988 Armenian earthquake had a Richter magnitude of 6.9, far less than the magnitude of the great quakes in Alaska in 1964 and San Francisco in 1906. Nevertheless, the loss of life was greater in the Armenian event. Why?

Two factors account for the greater damage and loss of life from the Armenian quake. First, the damaged cities in Armenia were densely populated and very near the epicenter; and secondly, most people lived in large, multifamily, concrete slab, apartment buildings that collapsed or were heavily damaged. A 5.8 magnitude aftershock finished off many of the damaged or weakened buildings.

The Anchorage, Alaska, area was much less densely populated. The city had few high-rise structures; most people lived in wood frame, single family dwellings that hold together during groundshaking and do not collapse on their occupants. However, a few, taller, concrete slab buildings in downtown Anchorage did collapse; and the residential area of Turnagain Heights was heavily damaged by landslides and foundation failures (Figs. 16.19 & 16.26).

In 1906, the population of San Francisco was still fairly small. Building damage was concentrated in the downtown areas built on filled-in marshes and mudflats along San Francisco Bay; this ground is very unstable and numerous buildings collapsed. Elsewhere, buildings were damaged; but being massive, well-constructed, and only two or three stories high, they did not collapse. Damage from the 1989 Loma Prieta quake was concentrated in the same, unstable area (the Marina district). In the 1906 quake, fire did much more damage than the earthquake shaking; gas from broken pipes fueled the flames and broken water mains prevented effective fire-fighting. Much of the business district burned (Fig. 16.2) but few lives were lost.

19. In addition to the destruction created directly by seismic vibrations, list three other types of destruction associated with earthquakes.

Fires, such as those that followed the 1906 San Francisco quake, and secondary effects such as landslides, dam failures, and tsunamis can be more dangerous than the groundshaking. Damaged utilities, water and sewer systems, and waste disposal systems leave an earthquake ravaged area without reliable communications and electricity and vulnerable to serious, public health problems. In addition, hazardous materials, toxic chemicals, and radioactive substances may be released from broken pipelines, damaged storage facilities, and in truck and train accidents.

20. What is a tsunami? How is one generated?

Tsunami are very-long period, long wavelength surface waves in the oceans generated by sudden displacement of large volumes of seawater. Most tsunami are caused by sudden vertical motion of the seafloor during earthquakes. However, tsunami associated with the 1883 Krakatau volcanic eruption are known to have been triggered by high-volume, extremely fast moving pyroclastic flows entering the ocean. Sudden caldera collapse would also generate tsunami. The waves that devastated ancient Crete were evidently generated by pyroclastic flows and caldera collapse associated with the eruption of Santorini Volcano on the Aegean Island of Thera. Farther back in time, truly gigantic tsunami that struck Pacific Basin coastlines were evidently triggered by massive-scale landslides in the Hawaiian Islands.

At sea, tsunami are not noticeable; their long wavelengths (kilometers) and modest amplitudes (a few meters) are easily lost in the normal open-ocean swell. However, as they enter shallower waters along a coastline, the wave slows, amplitudes grow, and huge waves pile up at the beachline and surge inland. As if that were not enough trouble, the water then rushes back out to sea, retracing the initial areas of destruction. Needless to say, tsunami can be very dangerous and destructive.

Tsunami generation and runup are very sensitive to the amplitude, period, and duration of sea-bottom groundshaking, and to coastal bathimetry. Thus occasionally, localized, unusually destructive tsunami are associated with seemingly modest-magnitude earthquakes.

21. Cite some reasons why an earthquake with a moderate magnitude might cause more extensive damage than a quake with a high magnitude.

In addition to magnitude, earthquake casualties and damage depend on many other natural and cultural factors. Aftershocks can slow or stop rescue efforts and cause additional damage and casualties. Amplified ground shaking and liquefaction intensify damage to structures built on quick clay and soft, unconsolidated, and/or water-saturated foundation materials. Landslides, rockslides, and rolling boulders can account for far more deaths and injuries than are attributable directly to the groundshaking. In the 1998 Afghanistan earthquakes, hillside villages were bombarded by rolling boulders shaken down from above or lost entirely to landslides and avalanches. Time of year, the climatic season, remoteness, and time of day are important. Many survivors of the first 1998 Afghanistan earthquake later succumbed to the freezing nighttime conditions and a lack of food and shelter. A shock during the day when most people are out-of-doors results in far fewer injuries and fatalities than one at night when families are sleeping in their homes. The appalling death tolls associated with earthquakes in the magnitude 5 to 6 range in countries such as Iran, Afghanistan, India, and Morocco are mainly due to collapse of weakly bonded mud and rock homes. The walls fail, the homes collapse, and the unlucky inhabitants are entombed under heavy rocks and rubble.

As in the 1906 San Francisco earthquake, fire is a constant threat; gas lines and water mains break, streets, highways, and bridges may be impassable, and there may be no practicable way to fight the fires. Electrical power failures, damage to sewage plants, unsafe and/or unavailable drinking water, and other results of infrastructure damage all contribute to poor sanitation and increase the threat of sickness and epidemics. In highly sophisticated societies like the U. S., accidents involving toxic chemicals, explosives, oil and gas storage and refining facilities, and nuclear power plants, etc., increase the chances of dangerous secondary effects following a quake. Finally a tsunami may be made more dangerous by some unusual combination of seafloor displacement, seafloor topography, and coastal configuration.

In more affluent countries, advanced seismic networks, scientific studies of past seismic activity, up-to-date land use zoning regulations, and strict building codes drastically reduce earthquake damages and casualties. Experience gained from previous earthquakes is constantly being used to upgrade building codes, to recommend ways to strengthen older buildings and infrastructure, and to increase overall earthquake preparedness. These activities all pay big dividends in lowered damage costs and lower casualties. Collapsed buildings are mostly to blame for the appalling death tolls often associated with moderate magnitude earthquakes in less affluent countries. Relatively new, modern, top-heavy, concrete-slab apartment buildings in Russia and Mexico and traditional stone and mud homes in North Africa, Iran, and India are basically deathtraps during earthquakes.

22. Can earthquakes be predicted?

Generous research funding and advances in monitoring, computing, , and other fields of geophysics seemed to put earthquake predication "within reach" in the 1970s and 80s. Studies of strain monitoring and precursor phenomena suggested reliable prediction was a realistic goal. Anomalous strain rates, anomalous uplift, subsidence, tilting, variations in the ratio of S-wave to P-wave velocities, anomalous variations in electrical and magnetic characteristic of rocks, and odd or unusual animal behavior seemed adequate for prediction. We needed careful monitoring and observations "at the right time in the right place" (in the epicentral area of a future quake before it strikes) so that the different precursory phenomena could be identified and directly tied to the impending earthquake. There were successes and some notable failures (see Chapter 16 under the heading Can Earthquakes Be Predicted); some failures were of the "where did that one come from" kind and some well-predicted quakes failed to materialize. Failed predictions had strongly negative economic and social consequences, and diverted the public's attention from earthquake preparedness.

In recent years, the dream of exact prediction has faded, to be replaced with the notion that earthquakes are inevitable and that society's efforts should be directed toward minimizing damage and casualties. Thus zoning regulations, sensible land use decisions (use the trace of a major active fault as open parklands, not for high rise apartments!), careful mapping of areas with risky foundation materials, strict compliance with tough building codes, retrofitting older building to bring them "up to snuff", making highways and bridges strong enough to survive a quake intact, preparation and training for disaster response, etc., are all worthwhile investments of pubic funds and will pay dividends when an earthquake strikes or should some other, unforeseen calamity or disaster strike with or without warning.

Forecasting today is directed more toward identifying risks, increasing preparedness, and reducing casualties and damage. These responsibilities are best handled by teams of professionals with diverse training and experience. Earth scientist are members of these teams, contributing their scientific expertise and advice to help make prudent decisions in the long-term public interest.

23. What is the value of long-range earthquake forecasts?

Paleoseismicity studies, historical seismicity studies, geologic and geophysical studies, and high- and low-tech strain monitoring enable earthquake risks and anticipated maximum ground accelerations to be determined in a general way.

Two examples illustrate today's earthquake risk studies. Based on historical accounts, tree ring dates, forest ecology, and sound geologic studies, the destructive tsunami that struck Japan in January of 1700 is directly attributed to a powerful earthquake that struck the coastal region of Oregon and Washington. The regions major metropolitan areas, such as Portland and Seattle, would be heavily damaged by a repeat performance (Jacoby and others, GEOLOGY, v. 25, 999-1002). The Dead Sea transform zone has been relatively quiet for hundreds of years. However, careful geologic, archaeological, and historical work has documented a powerful earthquake, May 20, 1202, along the segment north of Lake Tiberias (Sea of Galilee) (Ellenblum and others, 1998, GEOLOGY, v. 26, p. 303-306).

By educating the public, such studies enhance prospects for increased disaster preparedness and reduced fatalities, injuries, and property damages when the next powerful earthquake inevitably does strike.