Earthquakes: Occurrence, Measurement, and Plate Boundaries

Where Earthquakes Occur

• Most earthquakes occur in the oceans and along the edges of continents where tectonic plates meet
• Few earthquakes occur in the middle of a continent

Earthquakes and Plate Boundaries

• Earthquakes result from the buildup and release of stress along plate boundaries
• Earthquakes release energy, causing vibrations in the ground from movement along breaks (faults) in Earth's crust
• Fault: A break in Earth's crust where movement occurs
• Forces moving tectonic plates push/pull rocks along a fault, blocks of rock on either side may move past each other when forces are large enough
• Movement can be vertical or horizontal
• Earthquake size depends on the force applied to the fault
• The greater the force, the greater the chance of a large, destructive earthquake occurring
• Earthquakes can occur anywhere between Earth's surface and depths of more than 600 km

How Earthquakes are Measured

• Earthquakes range from barely noticeable vibrations to devastating waves of energy
• Magnitude describes the amount of energy released

Earthquake Magnitude

• Richter magnitude scale: a numerical rating system measuring the energy/magnitude of largest seismic waves produced by an earthquake
• Scale is based on the height/amplitude of earthquake waves, measured using a seismograph
• Richter scale uses amount of ground motion at a given distance to determine magnitude
• Each increase of one unit on the Richter scale represents 10 times the ground motion
• Example: A magnitude 8 earthquake produces 10 times greater shaking than a magnitude 7 earthquake, and 100 times greater shaking than a magnitude 6 earthquake

Amounts of Energy

• For each increase of one unit on the scale, an earthquake releases 32 times more seismic energy
• Example: a magnitude 6 earthquake releases approximately 1,024 times more energy than a magnitude 4 earthquake

Moment Magnitude Scale

• A rating scale that measures energy released by an earthquake
• The scale considers the size of the fault that breaks, the motion along the fault, and rock strength
• Both scales measure earthquake magnitude and produce similar readings
• Moment magnitude scale utilizes newer technologies for more accurate measurement

Modified Mercalli Scale

• Way to measure and describe an earthquake by examining damage from shaking, which is directly related to intensity/strength
• Measures earthquake intensity based on descriptions of its effects on people and structures
• Scale ranges from I (not felt) to XII (destroys everything)
• Higher numbers indicate greater effects

Earthquake Damage Factors

• Amount of damage from an earthquake depends on its strength, the nature of surface materials, structure design, and distance to the epicenter
• Epicenter: point on Earth's surface directly above where energy is first released
• Areas closest to the epicenter often suffer the most damage

What Factors Affect the Amount of Damaged Caused by an Earthquake?

• Earthquake hazards: factors determining how much damage an earthquake causes
• Identification helps prevent damage and loss of life

Structural Failure

• Resonance: seismic waves can generate resonance in structures "tuned" to the same frequency, resulting in structural failure
• Structures can fail when supporting walls are weak, or building height causes violent sway

Structural Damage

• Pancaking happens when ground floor supporting walls fail
• This causes upper floors to fall and collapse
• Building Height structural failure result from the height of a building
• Tall buildings sway with a natural period that depends on their heights
• Seismic waves with the same period as a tall building can cause the building to sway violently and collapse

Land and Soil Failure

• An area’s geology can contribute to structural failure
• An earthquake produces more damage on loose sediment than solid bedrock
• Liquefaction: shaking from an earthquake causes wet soil to act like a liquid
• Liquid-like ground is not strong enough to support heavy buildings
• Buildings can sink into the ground, causing the building to collapse
• Liquefaction is responsible for most building damage after Earthquakes occurs

Tsunami

• Tsunami: a large ocean wave generated by vertical motion of the seafloor during an earthquake
• Far from shore, a tsunami has a short wave height, often less than 30 cm high, and wavelengths can be hundreds of kilometers long
• Nearing short, a tsunami slows down and grows higher; can rise as high as 30 meters
• A tsunami can cause major damage

Earthquake Risk

• Earthquake risk is based on how often an area experiences earthquakes
• The highest risk of earthquakes occurs near tectonic plate boundaries of the western states
• Transform plate boundary in California and the convergent plate boundaries in Oregon, Washington, and Alaska have the highest earthquake risks

Risk Without Plate Boundaries

• Some parts of the central and eastern United States have high earthquake risk because of past activity, based on geologic history
• Areas that have experienced earthquakes in the past will likely experience earthquakes again

Predicting Earthquakes

• Earthquakes cannot be predicted reliably, but scientists can determine where they are most likely to occur in the United States
• This helps cities plan for future earthquake events
• Areas at high risk for earthquakes help drive the need for earthquake-safe technologies

Reducing the Impacts of Earthquakes

• Study the area - past earthquake activity, the geology around a fault, the population density, and the building design
• Engineers use these risk assessments to design buildings that can withstand the shaking during an earthquake
• City and state government officials use risk assessments to help plan and prepare for future earthquakes
• It is important for scientists to learn as much as possible about earthquakes to try and reduce their impact on society

Explore the distribution and measurement of earthquakes. This lesson elucidates how most earthquakes occur along tectonic plate boundaries and in the oceans. Learn about the relationship between earthquakes, plate movement, forces along faults, and the methods used to measure their intensity.