Tectonic Forces

4.4 Earthquake Risk

Ground Shaking, Location, and Direction

In general, the larger the magnitude, the stronger the shaking, and the longer the shaking will last. However, other factors influence the level of shaking. Closer earthquakes will inherently cause more shaking than those farther away. The location of the epicenter and direction of rupture will influence how much shaking is felt. The direction that the rupture propagates along the fault influences the shaking. The path of most significant rupture can intensify shaking in effect known as directivity.

The nature of the ground materials affects the properties of the seismic waves. Varied materials respond differently to an earthquake. Think of shaking jello versus shaking a meatloaf; one will jiggle much more to the same amount of shaking. The response to shaking depends on their degree of consolidation; lithified sedimentary rocks, and crystalline rocks shake less than unconsolidated sediments and landfill. This is because seismic waves move faster through consolidated bedrock, move slower through unconsolidated sediment, and move slowest through unconsolidated materials with high water content. Since the energy is carried by both velocity and amplitude, when a seismic wave slows down, its amplitude increases, which in turn increases seismic shaking. Energy is transferred to the vertical motion of the surface waves.

Depth of Focus

The focus is the place within the Earth where the earthquake starts, and the depth of earthquakes influences the amount of shaking. Deeper earthquakes cause less shaking at the surface because they lose much of their energy before reaching the surface. Recall that most of the destruction is caused by surface waves, caused by the body waves reaching the surface.

What Determines Destruction? Building Materials

Building material choices can influence the amount of damage caused by earthquake shaking. The flexibility of building materials relates to their resistance to damage by earthquake waves. Unreinforced Masonry (URM) is the most devastated by ground shaking. Wood framing held together with nails that can bend and flex with wave passage are more likely to survive earthquakes. Steel also can deform elastically before brittle failure. The Salt Lake City campaign “Fix the Bricks” has useful information on URMs and earthquake safety.

Shaking Intensity and Duration

More significant shaking and duration of shaking will cause more destruction than less shaking and shorter shaking. Resonance is when the frequency of seismic energy matches a building’s natural frequency of shaking, determined by properties of the building, and intensifies the shaking’s amplitude. This famously 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. Changes in the structural integrity of a structure could alter its resonance. Conversely, changes in measured resonance can indicate potential changes in structural integrity. (9 Crustal Deformation and Earthquakes – An Introduction to Geology, n.d.)

Earthquake Recurrence

Geologists dig earthquake trenches across some faults to measure ground deformation and estimate the frequency of occurrence of past earthquakes. Trenches are useful for faults with prolonged recurrence intervals (100s to 10,000s of years), which is the period be-tween significant earthquakes. In areas with more frequent earthquakes and more measured earthquake data, trenches are less necessary. A long hiatus in earthquake activity could indicate the buildup of stress on a specific segment of a fault with strain held in place by friction, which would indicate a higher probability of an earthquake along that segment. This hiatus of seismic activity along a length of a fault (i.e., a fault that is locked and not having any earthquakes) is known as a seismic gap.

Secondary Hazards Caused by Earthquakes Liquefaction

Liquefaction is when saturated unconsolidated sediments (usually silt or sand) is liquefied from shaking. Shaking causes loss of cohesion between grains of sediment, reducing the effective stress resistance of the sediment. The sediment flows very much like the quicksand presented in movies. Liquefaction creates sand volcanoes, which is when liquefied sand is squirted through an overlying (usually finer-grained) layer, creating cone-shaped sand features. It may also cause buildings to settle or tilt.

Earthquake-induced tsunamis have caused many of the more recent devastating natural disasters. Tsunamis form when earthquakes offset the seafloor in the ocean subsurface. This offset can be caused by fault movement or underwater landslides and lifts a volume of ocean water generating the tsunami wave. Tsunami waves travel fast with low amplitude in deep ocean water but are significantly amplified as the water shallows as they approach the shore. When a tsunami is about to strike land, the water in front of the wave along the shore will recede significantly, tragically causing curious people to wander out. This receding water is the drawback of the trough in front of the tsunami wave, which then crashes onshore as a wall of water upwards of a hundred feet high. Warning systems have been established to help mitigate the loss of life caused by tsunamis.

Landslides

Shaking can trigger landslides (see landslide section for more information). One example is the 1992 magnitude 5.9 earthquake in St. George, Utah. This earthquake caused the Spring-dale landslide, having a scarp that offset and destroyed several structures in the Balanced Rock Hills subdivision.

Seiches

Seiches are waves on lakes generated by earthquakes, which cause sloshing of water back and forth and, sometimes, even changes in elevation of the lake. A seich in Hebgen Lake during the 1959 earthquake caused significant destruction to structures and roads around the lake.

Land Elevation Changes

Significant subsidence and upheaval of the land can occur about the slippage that causes earthquakes. Land elevation changes are the result of the relaxation of stress and subsequent movement along the fault plane. The 1964 Alaska earthquake is an excellent example of this. Where the fault cuts the surface, the elevation of one side causes a fault scarp that maybe a few feet to 20 or 30 feet in height. The Wasatch Mountains represent an accumulation of fault scarps of a couple of dozen feet at a time over a few million years.

Human-Induced Earthquakes

Can humans create earthquakes? Not intentionally, but the answer is yes, and here is why. If a water reservoir is built on top of an active fault line, the water may lubricate the fault and weaken the stress built up within it. This may either create a series of small earthquakes or potentially create a massive earthquake. Also, the sheer weight of the reservoir r’s water can weaken the bedrock causing it to fracture. Then the obvious concern is if the dam fails. Earthquakes can also be generated if humans inject other fluids into a fault, such as sew-age or chemical waste. Finally, nuclear explosions can trigger earthquakes. One way to determine if a nation has tested a nuclear bomb is by monitoring the earthquakes and energy released by the explosion.

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Physical Geography and Natural Disasters by R. Adam Dastrup, MA, GISP is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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