There are two main ways to measure earthquakes. The first of these is an estimate of the energy released, and the value is referred to as magnitude. This is the number that is typically used by the press when a big earthquake happens. It is often referred to as “Richter magnitude,” but that is a misnomer, and it should be just “magnitude.” There are many ways to measure magnitude—including Charles Richter’s method developed in 1935—but they are all ways to estimate the same number, which is proportional to the amount of energy released.

The other way of assessing the impact of an earthquake is to assess what people felt and how much damage was done. This is known as intensity. Intensity values are assigned to locations, rather than to the earthquake itself, and therefore intensity can vary widely, depending on the proximity to the earthquake and the types of materials underneath and the local conditions.

Earthquake Magnitude

Before we look more closely at magnitude we need to review what we know about body waves, and look at surface waves. Body waves are of two types, P waves, or primary or compression waves (like the compression of the coils of a spring), and S waves, or secondary or shear waves (like the flick of a rope). An example of P and S seismic wave records is shown in Figure 12.1.1. The critical parameters for the measurement of magnitude are labelled, including the time interval between the arrival of the P- and S-waves—which is used to determine the distance from the earthquake to the seismic station, and the amplitude of the S-waves—which is used to estimate the magnitude of the earthquake.

When body waves (P or S) reach Earth’s surface, some of their energy is transformed into surface waves, of which there are two main types, as illustrated in Figure 12.2.2. Rayleigh waves are characterized by vertical motion of the ground surface, like waves on water, while Love waves are characterized by horizontal motion. Both Rayleigh and Love waves are about 10% slower than S-waves (so they arrive later at a seismic station). Surface waves typically have greater amplitudes than body waves, and they do more damage.

Other important terms for describing earthquakes are hypocentre (or focus) and epicentre. The hypocentre is the actual location of an individual earthquake shock at depth in the ground, and the epicentre is the point on the land surface vertically above the hypocentre (Figure 12.2.3).

A number of methods for estimating magnitude are listed in Table 12.1. Local magnitude (ML) was widely used until late in the 20th century, but moment magnitude (MW) is now more commonly used because it gives more accurate estimates (especially with larger earthquakes) and can be applied to earthquakes at any distance from a seismometer. Surface-wave magnitudes can also be applied to measure distant large earthquakes.

Because of the increasing size of cities in earthquake-prone areas (e.g., China, Japan, California) and the increasing sophistication of infrastructure, it is becoming important to have very rapid warnings and magnitude estimates of earthquakes that have already happened. This can be achieved by using P-wave data to determine magnitude because P-waves arrive first at seismic stations, in many cases several seconds ahead of the more damaging S-waves and surface waves. Operators of electrical grids, pipelines, trains, and other infrastructure can use the information to automatically shut down systems so that damage and casualties can be limited.

The magnitude scale is logarithmic; in fact, the amount of energy released by an earthquake of M4 is 32 times higher than that released by one of M3, and this ratio applies to all intervals in the scale. If we assign an arbitrary energy level of 1 unit to a M1 earthquake the energy for quakes up to M8 will be as shown on the following table:

In any given year, when there is a large earthquake on Earth (M8 or M9), the amount of energy released by that one event will likely exceed the energy released by all smaller earthquake events combined.

Earthquake Intensity

The intensity of earthquake shaking at any location is determined not only by the magnitude of the earthquake and its distance, but also by the type of underlying rock or unconsolidated materials. If buildings are present, the size and type of buildings (and their inherent natural vibrations) are also important.

Intensity scales were first used in the late 19th century, and then adapted in the early 20th century by Giuseppe Mercalli and modified later by others to form what we know call the modified Mercalli intensity scale (Table 12.4). Intensity estimates are important because they allow us to characterize parts of any region into areas that are especially prone to strong shaking versus those that are not. The key factor in this regard is the nature of the underlying geological materials, and the weaker those are, the more likely it is that there will be strong shaking. Areas underlain by strong solid bedrock tend to experience much less shaking than those underlain by unconsolidated river or lake sediments.

An example of this effect is the 1985 M8 earthquake that struck the Michoacán region of western Mexico, southwest of Mexico City. There was relatively little damage in the area around the epicentre, but there was tremendous damage and about 5,000 deaths in heavily populated Mexico City some 350 kilometres from the epicentre. The key reason for this is that Mexico City was built largely on the unconsolidated and water-saturated sediment of former Lake Texcoco. These sediments resonate at a frequency of about two seconds, which was similar to the frequency of the body waves that reached the city. For the same reason that a powerful opera singer can break a wine glass by singing the right note, the amplitude of the seismic waves was amplified by the lake sediments. Survivors of the disaster recounted that the ground in some areas moved up and down by about 20 centimetres every two seconds for over two minutes. Damage was greatest to buildings between 5 and 15 storeys tall, because they also resonated at around two seconds, which amplified the shaking.

An intensity map for the 1946 M7.3 Vancouver Island earthquake is shown in Figure 12.2.4. The intensity was greatest in the central island region where, in some communities, chimneys were damaged on more than 75% of buildings, some roads were made impassable, and a major rock slide occurred. The earthquake was felt as far north as Prince Rupert, as far south as Portland Oregon, and as far east as the Rockies.

Image Descriptions

Figure 12.2.1 image description: P-waves and S-waves from a small (M4) earthquake near Vancouver Island in 1997. The P-wave arrived in 0.7 seconds with an amplitude ranging from negative 0.7 millimetres per second to 1.1 millimetres per second and lasting until the arrival of the S-wave. The S-wave arrived at 8.7 seconds, with a minimum amplitude of negative 2.8 millimetres per second and a maximum amplitude of 2.7 millimetres per second. The S-wave’s net amplitude gradually decreased over the next 5 seconds. [Return to Figure 11.3.1]

Figure 12.2.4 image description: The graduated intensity of the 1945 M7.3 Vancouver Island earthquake based on the modified Mercalli intensity scale. The area surrounding the epicentre of the earthquake which included central Vancouver Island ranged between a very strong (7) and severe (8) intensity. The next ring included the northern and southern parts of Vancouver Island, as well as a part of the main land coast including Vancouver and much of the Sunshine coast a strong (6) intensity. The next ring, which reached experienced a moderate (5) intensity, included Seattle and much of the BC interior. The outermost ring ranged between not felt (1) and light (4) intensity. It was felt as far north as Prince Rupert and the southern tip of Haida Gwaii, south eastern BC, and as far south as north western Oregon. [Return to Figure 12.2.4]

Media Attributions

• Figure 12.2.1: © Steven Earle. CC BY.
• Figure 12.2.2 (left): “Rayleigh Wave.” Adapted by Steven Earle. Public domain.
• Figure 12.2.2 (right): “Love Wave” © Nicoguaro. Adapted by Steven Earle. CC BY.
• Figure 12.2.3: © Steven Earle. CC BY.
• Figure 12.2.4: “Vancouver Island, British Columbia June 23, 1946 – Magnitude 7.3,” © National Resources Canada. Used under the terms allowing for non-commercial reproduction. This reproduction is a copy of an official work that is published by the Government of Canada. This reproduction has not been produced in affiliation with, or with the endorsement of the Government of Canada.

Text Attributions

• Table 12.3: The modified Mercalli intensity scale © Wikipedia. Adapted by Steven Earle. CC BY-SA.