A. Earthquakes – What? Where? Why?
What is an earthquake?
An earthquake occurs when rocks break and slip along a fault in the earth. Energy is released during an earthquake in several forms, including as movement along the fault, as heat, and as seismic waves that radiate out from the “source” and causes the ground to shake, sometimes hundreds of km’s away.
What causes earthquakes?
Earthquakes occur from the deformation of outer, brittle portions of “tectonic plates”, the earth’s outermost layer of crust and upper mantle. Due to the heating and cooling of the rock below these plates, the resulting convection causes the adjacently overlying plates to move, and, under great stresses, deform. The rates of plate movements range from about 2 to 12 centimeters per year. Sometimes, tremendous energy can build up within a single, or between neighboring plates. If the accumulated stress exceeds the strength of the rocks making up these brittle zones, the rocks can break suddenly, releasing the stored energy as an earthquake.
Where do earthquakes occur?
Earthquakes occur all over the world; however, most occur on active faults that define the major tectonic plates of the earth. 90% of the world’s earthquakes occur along these plate boundaries (that represent about 10% of the surface of the earth). The “Ring of Fire” circling the Pacific Ocean, and including Canada’s west coast, is one of the most active areas in the world.
Can earthquakes be predicted?
With the present state of scientific knowledge, it is not possible to predict earthquakes and certainly not possible to specify in advance their exact date, time and location. However, a great deal of research is being conducted to develop reliable prediction methods.
Does a small earthquake mean that a larger earthquake is coming?
No, except for very rare exceptions. Every year, hundreds of earthquakes occur in Canada. Only a very tiny minority of these precede a larger earthquake. Although a large earthquake may be preceded by a foreshock, the occurrence of a small earthquake is not in itself a typical sign. About 1500 small earthquakes occur every year in Canada, whereas major earthquakes have occurred only a few times in this century.
A small earthquake, however, provides an ideal opportunity to offer reminders about safety measures to take before, during and after an earthquake.
Can humans trigger earthquakes?
Yes! Minor earthquakes have been triggered by human activities such as the filling of reservoirs, and the injection of fluids into wells for oil recovery or waste disposal. Such cases have been documented in many areas, including the United States, Canada, Japan, and India.
B. Earthquake Statistics – How Big? How Often?
How big can earthquakes be?
The largest earthquake ever recorded was magnitude 9.5! This earthquake struck just off the coast of southern Chile on May 22, 1960. It ruptured a segment of fault more than 1300 km long and caused a tsunami that caused destruction around the Pacific Ocean area, including Hawaii and Japan. This earthquake occurred along a subduction fault, where ocean floor is being pushed beneath a continent. This is the same plate tectonic setting as is found off the coast of southern British Columbia, Washington, Oregon, and northern California.
How often do earthquakes occur?
Earthquakes occur every day around the world. Each day there are about 1000 very small (magnitude 1-2) earthquakes on Earth (that is about one every 87 seconds!!!). Each year, on average, the Earth experiences 800 earthquakes capable of causing damage (magnitude 5-5.9), and 18 earthquakes of magnitude 7 or larger.
C. Megathrust Earthquakes
What is a megathrust earthquake?
A megathrust earthquake is a very large earthquake that occurs in a subduction zone, a region where one of the earth’s tectonic plates is thrust under another. The Cascadia subduction zone is located off the west coast of North America. From mid Vancouver Island to northern California the Juan de Fuca Plate is subducting beneath the North American Plate. The two plates are continually moving towards one another, yet become “stuck” where they are in contact. Eventually the build-up of strain exceeds the friction between the two plates and a huge megathrust earthquake occurs.
How often do megathrust earthquakes occur?
The recurrence time varies from subduction zone to subduction zone. In the Cascadia subduction zone 13 megathrust events have been identified in the last 6000 years, an average one every 500 to 600 years. However, they have not happened regularly. Some have been as close together as 200 years and some have been as far apart as 800 years. The last one was 300 years ago.
How big can they be?
Megathrust earthquake are the world’s largest earthquakes. The last Cascadia earthquake is estimated at magnitude 9. A megathrust earthquake in Chile in 1960 was magnitude 9.5, and one in Alaska in 1964 was magnitude 9.2.
Where do megathrust earthquakes occur?
The Cascadia fault, on which megathrust earthquakes occur, is located mostly offshore, west of Vancouver Island, Washington, and Oregon, although it does extend some distance beneath the Olympic Peninsula of Washington State. The large distance between the Cascadia fault and the urban centers limits the level of shaking that the urban areas are exposed to.
How do we know that megathrust earthquakes have occurred?
The sudden submergence of the outer coast when a megathrust earthquake occurs kills vegetation which can be dated. Megathrust earthquakes also cause underwater landslides off the continental shelf into the deep ocean. The landslide deposits can be recognized in core samples taken from the ocean floor.
How do we know that we will have another one in the future?
The deformation of the crust in a predictable pattern can be detected by very careful geodetic measurements using Global Positioning Satellites, precise leveling, micro-gravity measurements and changing distance measurements using laser technology.
If the shaking of a magnitude 7 is 10 times greater than a magnitude 6 and 100 times greater than a magnitude 5, is the shaking from a magnitude 9 100 times greater than a magnitude 7
No. Earthquake shaking, in the frequencies that damage buildings, increases to a maximum between a magnitude 7 and 8 earthquake, then the shaking simply involves a bigger area. However, the duration of shaking for a megathrust earthquake is much longer. It can be several minutes. This long duration can result in damage to some types of buildings that might not be damaged at the same strength of shaking produced by a smaller earthquake.
If a magnitude 6.9 earthquake can devastate Kobe, Japan, what would a magnitude 9 megathrust earthquake do to Vancouver?
The Kobe earthquake was right beneath the city and the megathrust earthquake will be about 150 kilometers from Vancouver. The damage pattern would be very different. We can get a good example of the kinds of damage Vancouver can expect to experience if we look at what happened to Anchorage, Alaska, during the 1964 magnitude 9.2 megathrust earthquakes. Anchorage is about the same distance from the Alaska subduction fault. Small buildings generally had little or no damage, unless they were affected by land sliding. Almost all the damage involved large buildings or large structures such as bridges.
Will Vancouver Island sink when a megathrust earthquake occurs?
No. Vancouver Island is part of the North American plate. The fact that there is water between Vancouver Island and the mainland is function of the current position of sea level. However, the west coast of Vancouver Island will drop as much as a meter or two when the next megathrust earthquake occurs.
Are megathrust earthquakes our biggest earthquake hazard?
No. Inland earthquakes, which are not as big but can be much closer to our urban areas and occur much more frequently, are our biggest earthquake hazard.
Why do megathrust earthquakes cause tsunamis?
The thrusting motion of megathrust earthquake causes large vertical movement on the sea floor and this displaces a large volume of water which travels away from the undersea motion as a tsunami.
Is all of coastal BC vulnerable to tsunamis from a megathrust earthquake?
No. Just the coast exposed to the open Pacific is vulnerable to damaging tsunamis waves. The areas vulnerable to tsunamis are indicated in the red-tabbed pages of the telephone books published for the coastal communities of British Columbia.
If we have lots of little earthquakes will they relieve the stress building up for a megathrust earthquake?
No. It takes many, many small earthquakes to release the amount of energy equivalent to a large earthquake. The amount of energy released increases about 40 times every time there is an increase of one unit on the magnitude scale. Thus, if we consider a small earthquake at the felt level, about magnitude 2, there would have to be 40x40x40x40x40x40x40 of these earthquakes to release the amount of energy as one magnitude 9 event. That is about one million small earthquakes a day, every day, for 500 years. That level of earthquake activity is not observed.
D. Measuring Earthquakes
Are there other magnitude scales?
Yes. The Richter scale was developed by Charles Richter in the 1930’s for use in southern California using one type of seismograph. As seismographs were deployed around the world, and as different types of seismometers were developed, it became apparent that the Richter scale was strictly valid only over a certain distance range and for limited range of shaking frequency.
What is the difference between earthquake magnitude and intensity?
An earthquake has a single magnitude value (e.g., magnitude=6.3) that is assigned based on the amount of energy released by that earthquake. The magnitude is determined using a seismograph, recording the amplitude of the ground shaking, and correcting this measurement for the distance from the earthquake source. Intensity refers to how the earthquake is felt at various locations, and a range of values (between I and XII) will be assigned. Intensity values are generally largest close to the epicenter of the earthquake, and will generally decrease with increasing distance from the epicenter (note however, that local soil conditions may increase the level of shaking).
E. Earthquake Effects
How do earthquakes cause damage?
Most earthquake damage is caused by ground shaking. The magnitude or size of an earthquake, distance to the earthquake focus or source, type of faulting, depth, and type of material are important factors in determining the amount of ground shaking that might be produced at a particular site. Where there is an extensive history of earthquake activity, these parameters can often be estimated.
The magnitude of an earthquake, for instance, influences ground shaking in several ways. Large earthquakes usually produce ground motions with large amplitudes and long durations. Large earthquakes also produce strong shaking over much larger areas than do smaller earthquakes. In addition, the amplitude of ground motion decreases with increasing distance from the focus of an earthquake. The frequency content of the shaking also changes with distance. Close to the epicenter, both high (rapid) and low (slow)-frequency motions are present. Farther away, low-frequency motions are dominant, a natural consequence of wave attenuation in rock. The frequency of ground motion is an important factor in determining the severity of damage to structures and which structures are affected.
How can we minimize the damage caused by earthquakes?
We can best minimize the damage caused by future earthquakes by understanding what causes earthquakes, where they will occur, how often they occur, how large they can be, how earthquake waves travel through the earth, what influences the level of ground shaking at a site, and by designing structures to withstand the shaking that would be expected during an earthquake. In Canada, the Geological Survey of Canada operates a network of seismograph stations to continuously monitor earthquake activity, and provides seismic hazard maps for the national Building Code of Canada so that buildings and critical structures can be designed to withstand earthquakes.
F. Earthquake Engineering and Seismic Hazard
Can buildings be designed to withstand earthquakes?
Yes! Engineers can, and are, designing earthquake-resistant structures.
What is the safest type of structure?
The safest type of structure is a modern, well-designed, and well-constructed building. Generally, wood-frame houses perform very well during an earthquake. However, even these structures are prone to damage from soil failure, chimneys may be damaged or collapse, windows may break, interior walls may crack, and those houses not securely bolted to their foundation may fail at or near ground level.
Unreinforced masonary structures (those not seismically upgraded) are generally more vulnerable to earthquake damage. For some photos of damage caused to unreinforced masonary structures during the M=7.3 Vancouver Island earthquake of 1946.
G. Earthquakes – What To Do… Before, During, and After
What should I do during an earthquake?
Falling objects pose the greatest danger during a major earthquake. In Canada, no house has ever collapsed during an earthquake. However, many types of objects may fall and cause damage or injuries. Of prime concern, therefore, is protection from falling objects such as framed pictures, light fixtures, plaster from ceilings or the upper part of walls, or chimneys which may fall outside or through the roof into the house.
When an earthquake occurs,
1. turn away from windows and other glass. Windows may break and glass shards can fly great distances;
2. take cover under a sturdy desk, table, or door frame to prevent injury from falling debris;
– if you are outside, try to keep to open areas well clear of buildings and power lines;
– if you are driving, pull over and stop your vehicle, preferably in an open area.
After an earthquake, follow emergency radio broadcasts carefully, and restrict your telephone calls to genuine emergencies.
What should I do after a strong earthquake?
1. Stay calm.
2. Help the injured, if any.
3. Speak calmly with family members, especially children about what has just happened, in order to
4. Stay tuned to the radio and follow instructions.
5. Use the telephone only in an emergency.
6. Do not enter damaged buildings
7. To prevent fire, check the chimneys or have them checked before using the furnace or fireplace.
Check all gas lines.
Will more shocks be felt after a strong earthquake?
For several hours, or even days, after a strongly felt earthquake, it is quite possible that people may feel more shocks. However:
In most cases, these shocks (called aftershocks) will be smaller; therefore, the shaking will be weaker.
Aftershocks do not mean that a stronger earthquake is coming.
Aftershocks are normal; they show that the earth’s crust is readjusting after the main earthquake.
The number of felt aftershocks is quite variable and thus cannot be predicted. There might be several per day, or only several per week. However, the number of aftershocks, and their magnitude, will normally decrease with time.
It is impossible to predict either the number or the magnitude of aftershocks that might occur. These vary greatly from one region to another, according to many factors which are not yet well-understood.
H. Earthquakes, Volcanoes, and Tsunamis
Is there a link between earthquakes and volcanoes?
Yes. There are both indirect links (the movements of tectonic plates cause both earthquakes and volcanoes) and direct links (earthquakes signify the movement of lava towards the surface and sometimes indicate the imminent eruption of a volcano.
When will an earthquake cause a tsunami?
Earthquakes can trigger a tsunami when there is significant vertical movement of the sea-floor over a large area. Earthquakes at plate boundaries beneath the ocean, especially subduction earthquakes, such as the M=9 earthquake of January 26, 1700 Located to the west of Vancouver Island are particularly effective in generating tsunamis.
The rapid vertical movement of the seafloor (either up or down) displaces a large mass of water from its equilibrium position, and generates a large wave (tsunami). Tsunami’s can have wavelengths of 100’s of km’s and travel very fast on the open ocean (e.g., 700 km/hr). As they approach the shore they slow down, begin to lose energy, and the wave height increases, in some cases to 10-20 m or more. Tsunamis can travel across the ocean, and hence a large earthquake anywhere in the Pacific may affect Canada’s west coast.
Not all underwater earthquakes cause tsunamis. Most earthquakes beneath the ocean are too small to generate a tsunami. Even some very large earthquakes, such as the M=8 earthquake located just to the west of the Queen Charlotte Islands in 1949 do not generate tsunamis. In the case of this earthquake the movement of the seafloor was a horizontal slip, and not the vertical motion required to generate a tsunami.
Tsunami’s can also be generated by submarine landslides (that may be triggered by earthquakes), or underwater volcanic eruptions. However, these waves tend to lose energy quickly and their effects, although perhaps significant, are much localized.