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.

Earthquake Mitigation

Mitigating the effects of earthquakes involves careful zoning and planning, but the most

visible mitigation is adapting to the hazard via seismic engineering.

San Francisco Bay Area residents and visitors observe gigantic construction projects

along their roads and bridges each day. Many of these projects are “seismic retrofits.”

A retrofit is a change in design and construction so that there are improvements; seismic

retrofit means changes are made to a structure to reduce or eliminate loss of life and

property during an earthquake. We retrofit buildings and roads that were built using

older techniques with designs that are less safe. Generally, it is cheaper and less

disruptive to retrofit before hand than try to repair a structure damaged by an

earthquake. There are many ways a structure can be retrofitted, but two main ideas are

most common. Sometimes, the best approach is to make a building stronger. Walls and

foundations are designed to support the weight of the rest of the building pushing down

on them. Earthquake shaking, however, pushes buildings side-to-side—a direction that

they are not always designed to withstand. Shear walls and cross bracing (Pictures 1

and 2) provide strength and stiffness to resist future earthquakes. Shear walls can

strengthen individual houses the same way they do for large buildings.

Another way to protect a building is to isolate it from the ground—a lot like adding

shock absorbers to its foundation. The ground can move back and forth during shaking,

but the building stays still (Pictures 3 and 4). Because each building has unique

architecture and a unique setting, there is a different retrofit solution that's right in each

case. Earthquake engineers are people who come up with creative new ways to make

these buildings safer than ever before.

Examples of base isolators – “shock absorbers” for a structure. The one below is being installed under a bridge. (USGS)

Find base isolation videos (YouTube)

Find seismic retrofit videos (YouTube)

The Salt Lake City – County Building was one of the first structures in the United States retrofit with base isolators. The ornate sandstone structure was completed in 1894. (USGS)

From the National Information Service for Earthquake Engineering University of California, Berkeley:

Earthquake Engineering can be defined as the branch of engineering devoted to mitigating earthquake hazards.

The General Goals in Seismic-Resistant Design and Construction for structures other than response-critical facilities (hospitals, police, fire,

and communications, which are held to a higher standard) are as follows:

a. To prevent non-structural damage in frequent minor ground shaking

b. To prevent structural damage and minimize non-structural damage in occasional moderate ground shaking

c. To avoid collapse or serious damage in rare major ground shaking

Structural components are those parts that hold up the building. Generally, these include foundations, walls, floors, and roofs. Non-structural

components are everything else – heating and cooling systems, plumbing, doors, windows, awnings, false ceilings, partitions, etc.

Elements of Seismic Structural Design

1. Building, including superstructure and non-structural components, should be light and avoid unnecessary masses.

2. Building and its superstructure should be simple, symmetric, and regular in plan and elevation to prevent significant torsional forces,

avoiding large height-width ratio and large plan area.

3. Building and its superstructure should have a uniform and continuous distribution of mass, stiffness, strength and ductility, avoiding

formation of soft stories (one level that is weaker than the others).

4. Superstructure should have relatively short spans and avoid use of long cantilevers (overhangs).

5. The non-structural components should either be well separated so that they will not interact with the rest of the structure, or they should

be integrated with the structure.

6. Superstructure should be designed so that it can deform in a predictable, orderly way.

7. Superstructure should be provided with balanced stiffness and strength between its members, connections and supports.

8. The stiffness and strength of the entire building should be compatible with the stiffness and strength of the soil foundation.

9. The whole substructure (foundation) and superstructure (frame) should be tied together so that they can work as a unit.

10. During construction, quality of structural materials and workmanship must be controlled.

Find seismic shake table videos (YouTube)

U.S. Earthquake Risk

As the map below shows, earthquakes in the United States are not just a west coast problem. Damaging earthquakes have happened all across the

country, including unexpected places like Missouri and South Carolina. But the risk for states like Florida, Texas, and Minnesota is quite low.

This particular map shows the risk from shaking, measured as horizontal acceleration as a percentage of gravity (g). This kind of calculation

depends heavily on the historical record, so that some unexpected quakes could happen, though they are much less likely.

Because it is a major plate boundary,

the west coast of the United States

has high and widespread earthquake

hazards, and earthquakes are

common. The San Andreas fault

through California is the best-known

hazard, but the Cascadia subduction

zone off the coast of Oregon and

Washington has the potential to

generate great earthquakes and

tsunamis.

red = highest

blue = lowest

Earthquakes Cannot Be Predicted An article by seismologists Robert J. Geller, David D. Jackson, Yan Y. Kagan, Francesco Mulargia Science 14 March 1997:

Vol. 275 no. 5306 p. 1616 DOI: 10.1126/science.275.5306.1616

R. J. Geller is at the Department of Earth and Planetary Physics, Faculty of Science, Tokyo University, Yayoi 2-11-16, Bunkyo-ku, Tokyo 113, Japan. E-

mail:bob@global.geoph.s.u-tokyo.ac.jp. D. D. Jackson and Y. Y. Kagan are at the Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095-

1567, USA. E-mail:djackson@ucla.edu and ykagan@ucla.edu. F. Mulargia is at the Dipartimento di Fisica, Settore di Geofisica, Universita di Bologna, Viale Berti Pichat 8,

40127 Bologna, Italy. E-mail:mulargia@ibogfs.df.unibo.it

Earthquake prediction is usually defined as the specification of the time, location, and magnitude of a future earthquake within stated limits.

Prediction would have to be reliable (few false alarms and few failures) and accurate (small ranges of uncertainty in space, time, and magnitude)

to justify the cost of response. Previous Perspectives in Science may have given a favorable impression of prediction research, and the news

media and some optimistic scientists encourage the belief that earthquakes can be predicted. Recent research suggests to us that this belief is

incorrect.

An earthquake results from sudden slip on a geological fault. Such fracture and failure problems are notoriously intractable. The heterogeneous

state of the Earth and the inaccessibility of the fault zone to direct measurement impose further difficulties. Except during a brief period in the

1970s , the leading seismological authorities of each era have generally concluded that earthquake prediction is not feasible. Richter, developer of

the eponymous magnitude scale, commented as follows in 1977: "Journalists and the general public rush to any suggestion of earthquake

prediction like hogs toward a full trough... [Prediction] provides a happy hunting ground for amateurs, cranks, and outright publicity-seeking

fakers”. This comment still holds true.

For large earthquakes to be predictable, they would have to be unusual events resulting from specific physical states. However, the consensus of a

recent meeting was that the Earth is in a state of self-organized criticality where any small earthquake has some probability of cascading into a

large event. This view is supported by the observation that the distribution of earthquake size is invariant with respect to scale for all but the

largest earthquakes. Such scale invariance is ubiquitous in self-organized critical systems. Whether any particular small earthquake grows into a

large earthquake depends on a myriad of fine details of physical conditions throughout a large volume, not just in the immediate vicinity of the

fault. This highly sensitive nonlinear dependence of earthquake rupture on unknown initial conditions severely limits predictability. The

prediction of individual large earthquakes would require the unlikely capability of knowing all of these details with great accuracy. Furthermore,

no quantitative theory for analyzing these data to issue predictions exists at present. Thus, the consensus of the meeting was that individual

earthquakes are probably inherently unpredictable.

Empirical earthquake prediction would require the existence of observable and identifiable precursors that would allow alarms to be issued with

high reliability and accuracy. There are strong reasons to doubt that such precursors exist. Thousands of observations of allegedly anomalous

phenomena (seismological, geodetic, hydrological, geochemical, electromagnetic, animal behavior, and so forth) have been claimed as earthquake

precursors, but in general, the phenomena were claimed as precursors only after the earthquakes occurred. The pattern of alleged precursors tends

to vary greatly from one earthquake to the next, and the alleged anomalies are frequently observed at only one point, rather than throughout the

epicentral region. There are no objective definitions of "anomalies," no quantitative physical mechanism links the alleged precursors to

earthquakes, statistical evidence for a correlation is lacking, and natural or artificial causes unrelated to earthquakes have not been compellingly

excluded. In other fields threshold signals have often been erroneously claimed as important physical effects; most if not all "precursors" are

probably misinterpreted as well. Unfortunately, each new claim brings a new set of proposed conditions, so that hypothesis testing, which is what

separates speculation from science, is nearly impossible.

Chinese seismologists claimed that the 4 February 1975 Haicheng (magnitude = 7.3) earthquake was successfully predicted and that "very few

people were killed". However, an official publication in 1988 states there were 1328 deaths and 16,980 injured. This disparity casts doubt on

claims for the Haicheng prediction. China's Cultural Revolution was still taking place in 1975. An American delegation's report captures the

remarkable atmosphere: "Earthquake prediction was not a minor experiment.... Indeed, belief in earthquake prediction was made an element of

ideological orthodoxy that distinguished the true party liners from right wing deviationists." The possibility that political pressures caused

inaccuracies in claims for the Haicheng prediction cannot be excluded. An intense swarm of microearthquakes, many of which were large enough

to be felt by local residents, began over 24 hours before the main shock. These microearthquakes might well have induced some spontaneous

evacuation. At least 240,000 people died in the 1976 Tangshan, China, earthquake, which was not predicted.

Varotsos and co-workers claim to be able to predict earthquakes in Greece on the basis of geoelectrical observations, but our analyses show their

claims to be without merit. Some of the geoelectrical signals are artifacts of industrial origin, and there is no compelling evidence linking any of

the geoelectrical signals to earthquakes. Controversy lingers primarily because Varotsos's claims have not been stated as unambiguous and

objectively testable hypotheses.

Is prediction inherently impossible or just fiendishly difficult? In practice, it doesn't matter. Scientifically, the question can be addressed using a

Bayesian approach. Each failed attempt at prediction lowers the a priori probability for the next attempt. The current probability of successful

prediction is extremely low, as the obvious ideas have been tried and rejected for over 100 years. Systematically observing subtle phenomena,

formulating hypotheses, and testing them thoroughly against future earthquakes would require immense effort over many decades, with no

guarantee of success. It thus seems unwise to invest heavily in monitoring possible precursors.

Seismology can, however, contribute to earthquake hazard mitigation. Statistical estimates of the seismicity expected in a general region on a time

scale of 30 to 100 years [as opposed to "long-term predictions" of specific earthquakes on particular faults within a few years] and statistical

estimates of the expected strong ground motion are important data for designing earthquake-resistant structures. Rapid determination of source

parameters (such as location and magnitude) can facilitate relief efforts after large earthquakes. Warnings of tsunamis produced by earthquakes

also contribute significantly to public safety. These are areas where earthquake research can greatly benefit the public.

The city of Tangshan in northeastern China was destroyed by a shallow M7.1 earthquake in July, 1976. Death toll estimates are as high as 655,000. (whoi.edu)

Earthquake Preparedness

The documents (PDF format) linked here provide excellent guidelines for

preparing you, your family, your home, and your neighborhood for an

earthquake. While some of the information was developed for California, the

principles apply anywhere.

click to read the pamphlet