4. Earthquakes

Elijah came to a cave, where he spent the night. But the LORD said to him, "What are you doing here, Elijah? Elijah replied, "I have zealously served the LORD God Almighty. But the people of Israel have broken their covenant with you, torn down your altars, and killed every one of your prophets. I alone am left, and now they are trying to kill me, too."

"Go out and stand before me on the mountain," the LORD told him. And as Elijah stood there, the LORD passed by, and a mighty windstorm hit the mountain. It was such a terrible blast that the rocks were torn loose, but the LORD was not in the wind.

After the wind there was an earthquake, but the LORD was not in the earthquake. And after the earthquake there was a fire, but the LORD was not in the fire. And after the fire there was the sound of a gentle whisper. When Elijah heard it, he wrapped his face in his cloak and went out and stood at the entrance of the cave. And a voice said, "What are you doing here, Elijah?"

An earthquake is simply a motion of the ground surface (ranging from a faint tremor to a wild motion capable of causing substantial rock displacement and destruction of buildings).

An earthquake is normally associated with a fault – cracks in the earth’s surface. As the earth’s crust moves (see plate tectonics in Chapter 3), the rocks on the surface move in relation to one another. Alas rocks do not always slide past each other smoothly or easily. They tend to stick … then move suddenly and jerkily when enough pressure builds up. Thus, earthquakes occur!

Many earthquakes occur every day, but are so slight they are only detectable by instruments (seismographs). See the links to the Canadian and U.S. government sites for updates on today's quakes. There is also global information available. It is estimated that there are 500,000 detectable earthquakes in the world each year. 100,000 of those can be felt, and 100 of them cause damage.

The December 26, 2004 earthquake off Sumatra, 9.0 on the Richter Scale, was one of the four strongest in this century; the strongest since the Alaska quake of 1964.

In North America, the West Coast of Canada and the United States are particularly vulnerable.

A. The Basics …

Earthquakes rarely are centered at the earth’s surface. They are usually centered deep under ground. The exact point, usually under the surface, where the earthquake originates is called the focus. The epicentre is the point on the earth's surface directly above the focus)

Most major earthquakes are associated with plate boundaries – compare Figure 3.6 (6^th ed., p.48) “Major lithospheric plate boundaries” with Figure 4.5 (6^th ed., p. 72) “World seismicity.” As you recall the various plate-to-plate collisions in plate tectonics (Chapter 3), this isn’t surprising! There are many other smaller faults located even within plates, however, so earthquakes can and do occur in many surprising places!

When an earthquake occurs, energy travels away from the focus as seismic waves. These are of three types (see Figure 4.6, 6^th ed., p. 72):

Magnitude is measured by the Richter Scale, developed in 1935 by Charles F. Richter, using a seismograph. It is a logarithmic scale (each whole number represents a 10X increase in wave amplitude). Thus the wave amplitude in a 6.0 earthquake is 10X that of a 5.0 quake.

In energy terms, each whole number represents a 31.5X increase in energy released.

Another scale often used is the Modified Mercalli Intensity Scale, it measures not only magnitude, but also the human/engineering effects of an earthquake. Intensity ratings are expressed as Roman numerals between I at the low end and XII at the high end.

II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings.

III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring.

IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock.

V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers.

VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage.

VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings.

VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change.

IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage.

X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly.

XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent.

XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.

Rating the Intensity of an earthquake's effects does not require any instrumental measurements. Thus seismologists can use newspaper accounts, diaries, and other historical records to make intensity ratings of past earthquakes, for which there are no instrumental recordings. Such research helps promote our understanding of the earthquake history of a region, and estimate future hazards.

B. Earthquakes as Natural Hazards

In this course we want to focus on earthquakes as they relate to us as human beings. Earthquakes really pose little direct danger to a person. People can't be shaken to death by an earthquake. Movies that show scenes with the ground suddenly opening up and people falling into fiery pits are just not true!

1. effect of ground shaking and movement. Buildings, highways, utilities, etc. can be damaged by the shaking itself or by the ground beneath them moving sideways or settling to a different level than it was before the earthquake (subsidence). Strong surface waves can make the ground heave and lurch, causing buildings to lean or tip over from all the movement. The ground shaking may also cause landslides, mudslides, and avalanches on steeper hills or mountains, all of which can damage buildings and hurt people (see Figures 4.11-4.15, 6^th ed., pp.76-79). The Northridge earthquake in 1994 in the San Fernando Valley triggered thousands of landslides in the Santa Susanna Mountains north of the epicenter.

Ideally, don’t build near a fault! Realistically, build your power lines with some slack. And try to engineer your buildings in earthquake resistant ways.

Building earthquake resistant buildings is very tricky! For one thing, not all earthquakes move the same way (some shake up and down, some side to side), so a building designed to resist one type of movement may not survive the other! For a good introduction to building safer buildings, see the USGS factsheet. There are a host of good resources on line for making personal homes safer.

Unfortunately engineering specifics that are best for one spot are not always best for another! Variables such as

· design building in such a way that they do not have one natural frequency (when you shake a flagpole, you can get it really swinging if you get the right rhythm – you don’t want your building doing that! Using a variety of materials and designs prevents it from “really swinging” in sync with an earthquake!)

· using more “ductile” – flexible – materials helps (non-reinforced concrete, brick, and plaster are very brittle – reinforced concrete, steel, wood, and plastics are much more flexible.

· Keep the center of mass (center of gravity) in the center of the building, not off to one side (heavy weights off to the side – like a pendulum – start really swinging! … low, centered weights don’t swing so easily). The Transamerica Pyramid in San Francisco utilizes this low, central center of gravity principle.

· Braces – both against up and down and side-to-side movement – are always good!

Earthquake engineering is often a site specific science (a great career choice, too!)… check out these great sources …

Many ancient biblical cities -- including the Asia Minor (now Turkey) cities of Ephesus, Laodicea, and Hierapolis; and the Greek cities of Corinth and Athens -- were built in earthquake prone areas (the fault lines beneath Turkey are very similar to the San Andreas Fault, California). Consequently many of the cities suffered serious damage in antiquity ... stone buildings don't do earthquakes well!

2. Buildings can sink into the ground if soil liquefaction occurs. Liquefaction is the mixing of sand or soil and groundwater (water underground) during the shaking of a moderate or strong earthquake. When the water and soil are mixed, the ground becomes very soft and acts similar to quicksand. If liquefaction occurs under a building, it may start to lean, tip over, or sink several feet (see Figure 4.16, 6^th ed., p. 79). The ground firms up again after the earthquake has past and the water has settled back down to its usual place deeper in the ground. Liquefaction is a hazard in areas that have groundwater near the surface and sandy soil. This is a huge concern in the San Francisco area.

3. Another hazard is flooding. The ground can actually sink below water level (see Figure 4.19, 6^th ed., p. 81). An earthquake can potentially rupture (break) dams or levees along a river. The water from the river or the reservoir would then flood the area, damaging buildings and maybe sweeping away or drowning people. (Dams and reservoirs can actually cause earthquakes: dams are located in valleys (often formed by faults); the weight of the water in the reservoir can cause ground movement; the water can lubricate the fault to move.) For more on dams, see the Seismology Research Center.

4. Tsunamis and seiches can also cause a great deal of damage. A tsunami is what most people call a tidal wave, but it has nothing to do with the tides on the ocean. It is properly a seismic sea wave – a huge wave caused by an earthquake under the ocean.

When a subsea earthquake occurs, the ground movement sets up a wave in the ocean. The wave travels outwards in all directions … out to sea and back to shore. Waves travel at tremendous speeds, especially in deep water (see Figure 4.18, 6^th ed., p. 80). As tsunamis near land, their height increases in the shallow water, and they run up onto the shore.

Contrary to many artistic images of tsunamis, most tsunamis do not result in giant breaking waves (like normal surf waves at the beach that curl over as they approach shore). Rather, they come in much like very strong and very fast tides (i.e., a rapid, local rise in sea level). They can result in rising sea levels of many meters when they hit the shore and can do enormous damage to the coastline. Much of the damage inflicted by tsunamis is caused by strong currents and floating debris. The small number of tsunamis that do break often form vertical walls of turbulent water called bores. Tsunamis will often travel much farther inland than normal waves.

e.g. Sumatra, December 26, 2004 (information, animation). The vast majority of deaths in this tragic earthquake occurred as a result of the tsunami that swept across the Indian Ocean.

e.g. Alaska 1964 - when the waves hit Hawaii they were moving at 640 km/h with crests 80 km apart. At Hilo (NE Hawaii), the wave surges were 14-30 m high, deposited 4 1/2 m of silt in the harbour, fish were thrown into palm trees. 173 people were killed.

e.g. the only confirmed fatalities in a Canadian quake happened in a 7.2 quake in 1929 off Newfoundland. 27 people on the Burin Peninsula died in a tsunami caused by a large underwater landslide.

Seiches (pronounced SAYSHes) are small tsunamis. They occur on lakes that are shaken by the earthquake and are usually only a few feet high, but they can still flood or knock down houses, and tip over trees. A seiche is also what happens in the swimming pools of Californians during and after an earthquake. It is "an internal wave oscillating in a body of water" or, in other words, it is the sloshing of the water in your swimming pool, or any body of water, caused by the ground shaking in an earthquake. It may continue for a few moments or hours, long after the generating force is gone.

There are great online resources about tsunamis, including general information, specific case studies, and research studies. Start with the USGS Tsunami Page.

5. Another earthquake hazard is fire. These fires can be started by broken gas lines and power lines, or tipped over wood or coal stoves. They can be a serious problem, especially if the water lines that feed the fire hydrants are broken, too. For example, after the Great San Francisco Earthquake in 1906, the city burned for three days. Most of the city was destroyed and 250,000 people were left homeless.

More recently, the 1994 Northridge (California) earthquake resulted in very little fire damage, while the (almost) identical 1995 Kobe (Japan) earthquake resulted in 5500 buildings being burned. The difference appears to have been a combination of

Most of the hazards to people come from man-made structures themselves and the shaking they receive from the earthquake. The real dangers to people are being crushed in a collapsing building, drowning in a flood caused by a broken dam or levee, getting buried under a landslide, or being burned in a fire.

The world's deadliest recorded earthquake occurred in 1557 in central China. It struck a region where most people lived in caves carved from soft rock. These dwellings collapsed during the earthquake, killing an estimated 830,000 people. In 1976 another deadly earthquake struck in Tangshan, China, where more than 250,000 people were killed.

C. Earthquake Prediction

Alas the complexity of the earth’s tectonic structure and activity has made prediction notoriously difficult! The San Andreas fault, for instance, is NOT a single, continuous fault, but rather is actually a fault zone made up of many segments. Movement may occur along any of the many fault segments along the zone at any time. The San Andreas fault system is more that 1300 km (800 miles) long, and in some spots is as much as 16 km (10 miles) deep. Southern California has about 10,000 earthquakes per year; most cannot be felt. Several hundred are over 3.0; 15-20 over 4.0 each year.

In 1969 in Tianjin, China, a zookeeper reported to the earthquake prediction office, that they had seen unusual behaviour from some of the animals. A few hours later a magnitude 7.4 earthquake struck the region. This is the only well-documented case in which animal behaviour led to a successful prediction. People have suggested that unusual behaviour by catfish, eels, other fish, frogs, snakes, turtles, sea birds, chickens, other birds, dogs, cats, deer, horses, cows, rats and mice have all precursed earthquakes … but no proof!

One well-known successful earthquake prediction was for the Haicheng, China earthquake of 1975, when an evacuation warning was issued the day before a M 7.3 earthquake. In the preceding months changes in land elevation and in ground water levels, widespread reports of peculiar animal behavior, and many foreshocks had led to a lower-level warning. An increase in foreshock activity triggered the evacuation warning. Unfortunately, most earthquakes do not have such obvious precursors. In spite of their success in 1975, there was no warning of the 1976 Tangshan earthquake, magnitude 7.6, which caused an estimated 250,000 fatalities.

Recent US (Loma Prieta/Northridge) and global quakes (Kobe/Turkey) have come with no obvious warnings.

Understanding the cycle of earthquakes in a particular region can help scientists predict the next earthquake. Consider the information in your text on the earthquakes in north Turkey (Figures 4.23 and 4.24, 6^th ed., p. 83-84). For the past 150 years, earthquakes of about magnitude 6 have occurred an average of every 22 years on the San Andreas fault near Parkfield, California. Using a set of assumptions about fault mechanics and the rate of stress accumulation, the USGS made a precise Parkfield prediction - of a 6.0 earthquake between 1988 and 1992. Though that prediction was not fulfilled, a 6.0 earthquake is still expected at Parkfield (don’t move there!!!).

To date, no generally useful method of predicting earthquakes has yet been found. It may never be possible to predict the exact time when a damaging earthquake will occur, because when enough strain has built up, a fault may become unstable, and any small background earthquake may or may not continue rupturing and turn into a large earthquake. While it may eventually be possible to accurately diagnose the strain state of faults, the precise timing of large events may continue to elude us. .

D. Earthquake Control?

The only major “control” approach suggested has been lubricated “locked” faults so that built up pressure is relieved in many small movements rather than pressuring building up until a major movement – a large earthquake – occurs. Unfortunately, no-one can really predict what the consequence of fluid injection into a fault might be. Yes, it may in fact work: many small tremors may result and built up pressure may be relieved. Or it may not work! People may trigger the big one! This method has never been used! The potential dangers are too unpredictable … and potentially too catastrophic!

E. Earthquake Preparedness

We may not be able to predict quakes or to control them, but we do know where earthquake hazards are high. And we can be as prepared as possible. Future earthquake damage can be greatly reduced by:

- If you're inside your home, stay there. Get out of the kitchen... safer places are inside halls, in corners, in archways. Take cover under a heavy table, desk or any solid furniture that you can get under and hold onto. Protect your head and face. Doors may slam on your fingers if you're in a doorway. Avoid areas near windows.

- If you're in a yard outside your home, stay there and get clear of buildings and wires that could fall on you.

- Don't go outside where you may be hit by falling debris... sidewalks next to tall buildings are particularly dangerous.

- Avoid elevators... if you're in an elevator when an earthquake happens, hit all floor buttons and get out when you can. High rise residents will hear fire alarms go off and electricity may fail.

- If you're in a vehicle, pull over to the side (leave the road clear), away from bridges, overpasses and buildings. Stay in your vehicle.

- If you're in a crowded public place, take cover and watch that you don't get trampled. In shopping centres, take cover in the nearest store and keep away from windows, skylights and display shelves of heavy objects.

remain in a protected place until the shaking stops.

- Anticipate aftershocks... they may occur soon after the first quake.

- Try to remain calm and help others.

· Communally … one of the big differences between the total devastation of the Kobe earthquake and the Northridge earthquake was government preparedness. In California the government had an effective emergency plan -- to clear the highways and mobilize emergency services -- and had developed more earthquake resistant water systems.

Just for interest, the average rate of motion across the San Andreas Fault Zone during the past 3 million years is 56 mm/yr (2 in/yr). This is about the same rate at which your fingernails grow. Assuming this rate continues, scientists project that Los Angeles and San Francisco will be adjacent to one another in approximately 15 million years. So don't buy a house in San Francisco expecting a free ride to L.A. ... yet!

F. So Where’s the Danger?

In the United States we tend to associate quakes with California! True, many major quakes have occurred there. However look at Figure 4.29 (6^th ed., p. 90) and 4.30 (6^th ed., p. 91). Other major danger areas are:

Hawaii and Alaska don’t show on these maps, but each experienced many quakes each year. Hawaii’s tend to be associated with volcanic activity. Alaska, located on several faults, experiences severe quakes quite regularly (including a 7.9 on the Denali Fault in November 2002).

Canada is less tectonic-ly “active” than the US … but we still have over 300 quakes per year (nothing California’s 10,000!). Figure 4.23 (6^th ed., p. 92) shows the two most active regions in Canada:

Less well known are earthquake prone regions off Newfoundland and in the Yukon and high Arctic. See the Earthquake Map of Canada for all the details of where earthquakes have occurred.

On a completely different note ... given the devastating tragedies associated with earthquakes, how do we know there is a God? Do you ever get questions about that? I do! Here are some thoughts about science and faith from Dr. John Polkinghorne, former Cambridge Professor of Mathematical Physics and now President of Queen's College, Cambridge. He is also an ordained Anglican minister and a Fellow of the Royal Society ...

"Science is very impressive. We all enjoy, every day of our lives, the new things made possible by the advance of science. Science also enlightens our minds and enlarges our imaginations. We know that we are the inhabitants of an unremarkable planet, circling an unremarkable star, in a universe that contains at least 10,000 million million million stars. Science tells us what makes the stars shine, why water is wet, how genetic information is conveyed from one generation to the next. It's a story of astonishing achievement, and perhaps the most impressive thing about it is that we can all agree on the answers. The dust really does settle. Not only does science answer questions, it does so to universal satisfaction.

"It would be foolish to deny that there's a striking contrast with religion. We'll not all agree on the answer to the most fundamental religious question of them all: is there a God? Although the different faiths clearly refer to a common human experience of the spiritual, they seem to say such different things about it. Is the individual human self of unique value and significance [so say Judaism, Christianity and Islam] or is it, in fact, an illusion [so says Buddhism] or is it recycled through reincarnation [so says Hinduism]? Is suffering something to be excepted or avoided? And so on.

"The conclusion seems clear. Science is based on facts and leads to real knowledge. When religion is just based on opinion. It may help you or me to live our lives -- religion may be true for me your true for you -- but it's not just plain true, pure and simple. So it may seem, but I believe that such a conclusion would be a fundamental mistake of the most disastrous kind. If I thought it were true, I would not be a religious person. How could something really help one in one's life if it were just a personal illusion? The only the true can be a real basis for living -- and facing death.

"Two mistakes lead to the false conclusion that science and religion in faulty encounter of fact with mere opinion. One is a mistake about science. The other is a mistake about religion. Let's take science first.

"Many people's impression of how science progresses is that a prediction is made, and experiment is performed, and a great new discovery has been made. In actual fact, it is all a good deal more subtle and more interesting than that. In the first case, the facts that concern scientists are already interpreted facts. Most of the time you can't see directly what's happening. You have to infer it from the things you can see, and that inference requires the use of theoretical interpretation.

He goes on to give an example from elementary particle physics the study of the smallest bits of matter. In a bubble chamber particles passing through trigger a chain of little bubbles which make their pass visible as scientists study the patterns they have to interpret what actually occurred. "Without the interpretation, the patterns were just a mess. Now the problem is this. In order to make the interpretation, you have to know some science already. You can't just stare at the world; you have to view it from a chosen point of view. Choosing the point of view involves an act of intellectual daring in betting that things might be this way. This means that in science, experiment and theory, fact and interpretation, are always mixed up with each other... Someone once said that scientists wear spectacles behind the eyes -- it's not just what they see that the way they see it that counts. In other words, science uses a mixture of fact and opinion. Of course there are reasons for the opinions, and opinions can be revised when they don't seem to work very well, but you can do without them.

"Everyone knows that religion involves faith. Many people seem to think that faith involves shutting one size, greeting one's teeth, and believing six impossible things before breakfast, because the Bible or the Pope or some other unquestionable authority tells us so. Not at all! Faith may involve a leap, but it's a leap into the light, not the dark. The aim of the religious quest like that of the scientific quest is to seek motivated belief about what is the case. We have already said that religion can only be of real value if it's actually true. It's not a technique for whistling in the dark to keep our spirits up.

"There is a whole string of evidential questions that can be addressed. How reliable is the New Testament? What can we actually know about Jesus? Are their reasons for believing the claims that he was raised from the dead? What are we to make of that strange phenomenon, the Christian Church, that has given rise both to St. Francis and the Inquisition? For faith to be possible, rational responses to these questions are required.

"I believe that science and religion are intellectual cousins under the skin. Both are searching for motivated belief. Neither can claim absolutely certain knowledge, for each must base its conclusions on an interplay between interpretation and experience. they are both part of the great human endeavor to understand." (John Polkinghorne, Quarks, Chaos and Christianity [New York: Crossroad Publishing Co., 1994], pages 1 -- 12.)

In lecture five we will continue this discussion from Dr. Polkinghorne. Do you agree? Why? Why not? Feel free to discuss this quote on the course discussion site, www.nicenet.org ...

This page is the intellectual property of the author, Bruce Martin, and is copyrighted © 2005 by Bruce Martin. This page may be copied or printed only for educational purposes by students registered in EAS 205 (Taylor University College). Any other use constitutes a criminal offence.

Scripture quotations marked (NLT) are taken from the Holy Bible, New Living Translation, copyright © 1996. Used by permission of Tyndale House Publishers, Inc., Wheaton, Illinois 60189. All rights reserved