Earthquakes

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Earthquakes are caused by the movement of one tectonic plate against another and can also be induced through human activity including the disposal of liquid waste from oil and gas wells.  The west coast of the US has historically had a much higher risk of earthquakes due to the fault or boundary between the Pacific and North American plates that runs up and down the west coast.

Overview#

The earth's surface consists of seven major tectonic plates that move between ¼-4” per year. In addition to the major plates, there are dozens of smaller plates known as minor (1-20 million km2) and microplates (< 1 million km2).  Boundaries between plates are known as faults, and when plates move, the jagged boundaries often stay stuck together causing the energy to build up as the plates continue to move.  Earthquakes result from the sudden movement or slipping of one tectonic plate against another at a fault when the energy from the movement of the plates eventually overwhelms the friction holding the plate boundaries in place.

Major and Minor Tectonic Plates

There are three basic categories of faults or boundaries between plates--convergent, divergent, and transform. Convergent boundaries are where two plates push together, divergent boundaries are where two plates spread apart, and transform boundaries are where two plates slide past each other in a horizontal motion.

When a thinner, denser, and more flexible oceanic plate converges or collides with a continental plate, the oceanic plate sinks beneath the more rigid continental plate and can form deep ocean trenches like the one off the coast of South America.  This is known as subduction. During this process, the rocks pulled down under the continent begin to melt. Sometimes the molten rock rises to the surface through the continent, forming a line of volcanoes.  The horseshoe-shaped region where the Pacific plate sinks beneath continental plates to the west, north, and south is known as the Ring of Fire for the 800-1,000 volcanoes that have formed.  About 90% of all earthquakes and 80% of all major earthquakes occur along the Ring of Fire as well.  Major earthquakes along the Ring of Fire include the 1960 Alaska earthquake and the earthquake that produced the devastating Indian Ocean Tsunami in 2004.  Another form of convergent boundary occurs when two continental plates collide.  Because neither plate is stronger than the other, both plates crumple and are pushed up.  This process forms mountain ranges including the Himalayas, Andes, and Alps.  

Divergent boundaries, also known as spreading centers, allow molten rock from the earth’s mantle to escape and form new crust.  The Great Rift Valley, the Red Sea, and the Gulf of Aden were all formed through this process.  Earthquakes associated with divergent boundaries are relatively small.

When plates move horizontally past each other at a transform boundary, the grinding action can tear blocks of crust from the boundaries of each plate, deforming or “transforming” the boundary.  The San Andreas Fault in California is an example of a transform boundary.  This type of fault also causes earthquakes including the infamous 2010 Haiti and 1906 San Francisco earthquakes.  

Earthquake severity is measured and expressed in more than one way.  In the US, we use both an intensity scale and a magnitude scale; the same or similar scales are used in other parts of the world.  Earthquake intensity is a qualitative measure, and in the US we use the Modified Mercalli Intensity scale which is an arbitrary ranking from I-X based on observed effects ranging from imperceptible shaking to catastrophic destruction.

Modified Mercalli Intensity Scale

Earthquake magnitude was originally measured using the Richter scale, but in 1979 a version that more accurately measures larger earthquakes called Moment Magnitude (Mw) became the most widely used measure of earthquake size.  Like the Richter scale, Moment Magnitude is a logarithmic scale, which means that for every whole number you move up on the scale, the amount of energy or motion recorded increases 10 times.  For example, an earthquake with an Mw of 3 is ten times more powerful than an earthquake with an Mw of 2.  The largest earthquake ever recorded occurred in Chile in 1960 with an Mw of 9.5 and the 1906 earthquake in San Francisco CA was an Mw of 7.8.

The probabilistic map below for the US was produced by the USGS and shows the expected number of damaging earthquakes caused by tectonic plate movement over a 10,000-year period.  Areas outlined in the central and eastern US have experienced a dramatic increase in what are known as induced earthquakes--those caused by human activity.  Between 1973-2008 there was an average of 25 earthquakes per year in the central and eastern US with a magnitude of 3 or higher.  Since 2009 the average has increased to 58 per year, and since 2013 the average has been at least 100 per year.  The dramatic increase in earthquakes in this region is primarily a result of the increased number of wastewater injection wells that are used to dispose of liquid waste from oil and gas wells, including hydraulic fracturing.

Your Home and Earthquakes#

Health and Safety#

The Centers for Disease Control (CDC) recommends several steps to prepare for earthquakes including knowing the signs, learning the safe spots, and planning and preparing.  The signs of an earthquake include roaring or rumbling sounds that gradually get louder as well as possible ground movement that includes rolling, shaking, or violent jolts.  Because most injuries and deaths during earthquakes are caused by falling building material or heavy objects, safe spots are areas away from walls and under structures like sturdy tables.  To prevent injury or possible death, the CDC and Ready.gov recommend dropping to your hands and knees, covering your head and neck, and holding onto your shelter.  In addition, creating a family emergency communications plan with contact information for family members and other important contacts, planning and practicing evacuation routes, and assembling an emergency supply kit that includes an ax, shovel, broom, rope, sturdy shoes, gloves, fire extinguisher, and a signaling device like a whistle in addition to normal emergency supply kit items like water, food, and medication are recommended.

Property Values#

The effect of earthquakes on property values is highly variable by region and changes over time.  A 2016 study by the Federal Reserve Bank of Cleveland using sales data from 2006-2014 found that housing prices in Oklahoma fell 3-4% after experiencing an earthquake with an intensity of 4-5 on the Modified Mercalli Intensity scale.  When the intensity was above 6, the prices fell down to 9.8%.  A meta-analysis by VU University in Amsterdam of 20 studies from different parts of the world including California and Oklahoma between 1983 and 2016 found that the risk of smaller earthquakes (magnitude of < 3) lowered housing prices by 1.3% and stronger earthquakes (magnitude > 7) lowered prices by 2.9%.

What You Can Do#

The CDC recommends securing hazards in your home, inspecting and securing your home’s structures, and knowing how and where to shut off utilities.  Anything that can move or fall should be secured including appliances, furniture, windows, and large hanging decorations.  Poisons, solvents, and toxic materials should be stored in well-ventilated areas, out of reach of children, and away from water sources.  The structure of your home can be strengthened at all points of connection between beams, posts, joists, and plates.  Chimneys and roofs should be checked for loose bricks or tiles, and the roof adjacent to chimneys should be reinforced to prevent falling chimney bricks from penetrating the roof.

Evaluate your earthquake risk and homeowners insurance policy.  Standard homeowners insurance does not cover damage from earthquakes.  Earthquake insurance is available nationwide, and California law requires insurance companies to offer add-on earthquake coverage.

AreaHub’s Knowledge Center is updated regularly and provides information drawing upon scientific studies and sources.

Sources#

• US Geological Survey: “The Science of Earthquakes.”
• California Academy of Sciences: “Plate Boundaries: Convergent, Divergent, and Transform.”
• USGS: “The Modified Mercalli Intensity Scale.”
• Plate Boundaries and Natural Hazards, Geophysical Monograph: “Introduction to Plate Boundaries and Natural Hazards.”
• USGS: “Where do Earthquakes occur.”
• USGS: “Introduction to the National Seismic Hazards Maps."
• USGS: “Induced Earthquakes.”
• Federal Reserve Bank of Cleveland: “Earthquakes and Housing Prices: Evidence from Oklahoma.”
• VU University, Amsterdam: “Earthquakes and house prices: a meta-analysis.”
• CDC: “Preparing for an Earthquake.”