9.8: Earthquake Risk
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Factors that Determine Shaking
Earthquake magnitude is an absolute value that measures pure energy release. Intensity, however, i.e. how much the ground shakes, is determined by several factors.
Earthquake Magnitude—In general, the larger the magnitude, the stronger the shaking and the longer the shaking will last.
This table is taken from the USGS and shows scales of magnitude and Mercalli Intensity and descriptions of shaking and resulting damage.
|Magnitude||Modified Mercalli Intensity||Shaking/Damage Description|
|1.0 – 3.0||I||Only felt by a very few.|
|3.0 – 3.9||II – III||Noticeable indoors, especially on upper floors.|
|4.0 – 4.9||IV – V||Most to all feel it. Dishes, doors, cars shake and possibly break.|
|5.0 – 5.9||VI – VII||Everyone feels it. Some items knocked over or broken. Building damage possible.|
|6.0 – 6.9||VII – IX||Frightening amounts of shaking. Significant damage especially with poorly constructed buildings|
|≥ 7.0||≥ VIII||Significant destruction of buildings. Potential for objects to be thrown in the air from shaking.|
Location and Direction—Shaking is more severe closer to the epicenter. The severity of shaking is influenced by the location of the observer relative to the epicenter, the direction of rupture propagation, and the path of greatest rupture. The severity of shaking is influenced by the location of the observer relative to the epicenter, the direction of rupture propagation, and the path of greatest rupture.
Local Geologic Conditions—Seismic waves are affected by the nature of the ground materials through which they pass. Different materials respond differently to an earthquake. Think of shaking a block of Jello versus a meatloaf, one will jiggle much more when hit by waves of the same amplitude. The ground’s response to shaking depends on the degree of substrate consolidation. Solid sedimentary, igneous, or metamorphic bedrock shakes less than unconsolidated sediments.
This video shows how different substrates behave in response to different seismic waves and their potential for destruction.
Seismic waves move fastest through consolidated bedrock, slower through unconsolidated sediments, and slowest through unconsolidated sediments with a high water content. Seismic energy is transmitted by wave velocity and amplitude. When seismic waves slow down, energy is transferred to the amplitude, increasing the motion of surface waves, which in turn amplifies ground shaking.
Focus depth—Deeper earthquakes cause less surface shaking because much of their energy, transmitted as body waves, is lost before reaching the surface. Recall that surface waves are generated by P and S waves impacting the Earth’s surface.
Factors that Determine Destruction
Just as certain conditions will impact the intensity of ground-shaking, several factors affect how much destruction is caused.
Building Materials—The flexibility of a building material determines its resistance to earthquake damage. Unreinforced masonry (URM) is the material most devastated by ground shaking. Wood framing fastened with nails bends and flexes during seismic wave passage and is more likely to survive intact. Steel also has the ability to deform elastically before brittle failure. The Fix the Bricks campaign in Salt Lake City, Utah has good information on URMs and earthquake safety.
Intensity and Duration—Greater shaking and duration of shaking causes more destruction than lower and shorter shaking.
Resonance—Resonance occurs when seismic wave frequency matches a building’s natural shaking frequency and increases the shaking happened in the 1985 Mexico City Earthquake, where buildings of heights between 6 and 15 stories were especially vulnerable to earthquake damage. Skyscrapers designed with earthquake resilience have dampers and base isolation features to reduce resonance.
Resonance is influenced by the properties of the building materials. Changes in the structural integrity of a building can alter resonance . Conversely, changes in measured resonance can indicate a potentially altered structural integrity.
These two videos discuss why buildings fall during earthquakes and a modern procedure to reduce potential earthquake destruction for larger buildings.
A long hiatus in activity along a fault segment with a history of recurring earthquakes is known as a seismic gap. The lack of activity may indicate the fault segment is locked, which may produce a buildup of strain and a higher probability of an earthquake recurring. Geologists dig earthquake trenches across faults to estimate the frequency of past earthquake occurrences. Trenches are effective for faults with relatively long recurrence intervals, roughly 100s to 10,000s of years between significant earthquakes. Trenches are less useful in areas with more frequent earthquakes because they usually have more recorded data.
This video shows the distribution of significant earthquakes on Earth during the years 2010 through 2012. Like volcanoes, earthquakes tend to aggregate around active boundaries of tectonic plates. The exception is intraplate earthquakes, which are comparatively rare.
Subduction Zones—Subduction zones, found at convergent plate boundaries, are where the largest and deepest earthquakes, called megathrust earthquakes, occur. Examples of subduction-zone earthquake areas include the Sumatran Islands, Aleutian Islands, west coast of South America, and Cascadia Subduction Zone off the coast of Washington and Oregon. See Chapter 2 for more information about subduction zones.
Collision Zones—Collisions between converging continental plates create broad earthquake zones that may generate deep, large earthquakes from the remnants of past subduction events or other deep-crustal processes. The Himalayan Mountains (northern border of the Indian subcontinent) and Alps (southern Europe and Asia) are active regions of collision-zone earthquakes. See Chapter 2 for more information about collision zones.
Transform Boundaries—Strike-slip faults created at transform boundaries produce moderate-to-large earthquakes, usually having a maximum moment magnitude of about 8. Transform fault boundaries create moderate and large earthquakes, usually having a maximum magnitude of about 8. The San Andreas fault (California) is an example of a transform-boundary earthquake zone. Other examples are the Alpine Fault (New Zealand) and Anatolian Faults (Turkey). See Chapter 2 for more information about transform boundaries.
Divergent Boundaries—Continental rifts and mid-ocean ridges found at divergent boundaries generally produce moderate earthquakes. Examples of active earthquake zones include the East African Rift System (southwestern Asia through eastern Africa), Iceland, and Basin and Range province (Nevada, Utah, California, Arizona, and northwestern Mexico). See Chapter 2 for more information about divergent boundaries.
Intraplate Earthquakes—Intraplate earthquakes are not found near tectonic plate boundaries, but generally occur in areas of weakened crust or concentrated tectonic stress. The New Madrid seismic zone, which covers Missouri, Illinois, Tennessee, Arkansas, and Indiana, is thought to represent the failed Reelfoot rift . The failed rift zone weakened the crust, making it more responsive to tectonic plate movement and interaction. Geologists theorize the infrequently occurring earthquakes are produced by low strain rates
Secondary Hazards Caused by Earthquakes
Most earthquake damage is caused by ground shaking and fault-block displacement In addition, there are secondary hazards that endanger structures and people, in some cases after the shaking stops.
Liquefaction—Liquefaction occurs when water-saturated, unconsolidated sediments, usually silt or sand, become fluid-like from shaking. The shaking breaks the cohesion between grains of sediment, creating a slurry of particles suspended in water. Buildings settle or tilt in the liquified sediment, which looks very much like quicksand in the movies. Liquefaction also creates sand volcanoes, cone-shaped features created when liquefied sand is squirted through an overlying and usually finer-grained layer.
This video demonstrates how liquefaction takes place.
Tsunamis—Among the most devastating natural disasters are tsunamis, earthquake-induced ocean waves. When the seafloor is offset by fault movement or an underwater landslide, the ground displacement lifts a volume of ocean water and generates the tsunami wave. Ocean wave behavior, which includes tsunamis, is covered in Chapter 12. Tsunami waves are fast-moving with low amplitude in deep ocean water but grow significantly in amplitude in the shallower waters approaching the shore. When a tsunami is about to strike land, the drawback of the trough preceding the wave crest causes the water to recede dramatically from shore. Tragically, curious people wander out and follow the disappearing water, only to be overcome by an oncoming wall of water that can be upwards of a 30 m (100 ft) high. Early warning systems help mitigate the loss of life caused by tsunamis.
Landslides—Shaking can trigger landslides (see Chapter 10). In 1992 a moment magnitude 5.9 earthquake in St. George, Utah, caused a landslide that destroyed several structures in the Balanced Rock Hills subdivision in Springville, Utah .
Seiches—Seiches are waves generated in lakes by earthquakes. The shaking may cause water to slosh back-and-forth or sometimes change the lake depth. Seiches in Hebgen Lake during a 1959 earthquake caused major destruction to nearby structures and roads.
This video shows a seiche generated in a swimming pool by an earthquake in Nepal in 2015.
Land Elevation Changes—Elastic rebound and displacement along the fault plane can cause significant land elevation changes, such as subsidence or upheaval. The 1964 Alaska earthquake produced significant land elevation changes, with the differences in height between the hanging wall and footwall ranging from one to several meters (3–30 ft). The Wasatch Mountains in Utah represent an accumulation of fault scarps created a few meters at a time, over a few million years.
8. Starr AM (2015) Ambient resonance of rock arches. M.S. thesis, University of Utah
9. Hildenbrand TG, Hendricks JD (1995) Geophysical setting of the Reelfoot rift and relations between rift structures and the New Madrid seismic zone. U.S. Geological Survey, Washington; Denver, CO
10. Christenson GE (1995) The September 2, 1992 ML 5.8 St. George earthquake, Washington County, Utah. Utah Geological Survey, Salt Lake City, Utah