Earthquakes

Characteristics


Earthquakes are geologic events that involve movement or shaking of the Earth's crust. Earthquakes are usually caused by the release of stresses accumulated because of the rupture of rocks along opposing fault planes in the Earth's outer crust. These fault planes generally follow the outlines of the continents.
Earthquake
Earthquakes can result from crustal strain, volcanoes, landslides or the collapse of caverns. 

Earthquakes can affect hundreds of thousands of square miles causing damages to property, loss of life and disruption of social and economic infrastructure in the affected area.

Cause


Most earthquakes occur as a result of the release of accumulated stresses. These stresses build up when two opposing tectonic plates of the Earth's crust become lodged on one another. The areas where these blocks meet are known as fault lines.

The areas of greatest instabilities occur at the perimeters of the lines where two plates are moving in opposite directions and at opposite speeds. When the plates become locked together energy begins to accumulate. Eventually, this energy will exceed the rock's strength and a rupture will occur. The rocks on both sides of the fracture will snap releasing the stored energy which generates seismic waves.

Duration & Timing


Earthquakes may last only a few seconds or may continue for up to several minutes. They can occur at any time of the day or night and at any time of the year. 

Damage


Although thousands of earthquakes occur in the United States each year, most are too small to affect us. Earthquakes of larger magnitude, however, which release more energy during fault ruptures, can be hazardous, exposing us to the risk of harm or loss.

The primary risk to people is what the ground motions can do to the natural and man-made environments.

The level of damage depends upon the amplitude and duration of the shaking, which are directly related to the earthquake size, distance from the fault, site and regional geology. 

Other damaging earthquake effects include landslides, the down-slope movement of soil and rock (mountain regions and along hillsides), and liquefaction, in which ground soil loses the ability to resist shear and flows much like quick sand. In the case of liquefaction, anything relying on the substrata for support can shift, tilt, rupture or collapse.

Magnitude and Intensity Scales

Magnitude 


Magnitude measures the energy released at the source of the earthquake. Magnitude is determined from measurements on seismographs.

Each unit increase in magnitude typically corresponds to a ten-fold increase in wave amplitude, or a 244-fold increase in energy intensity. 

Intensity 


Intensity measures the strength of shaking produced by an earthquake at a certain location. Intensity is determined from effects on people, human structures and the natural environment.
Magnitude Modified Mercalli Intensity Abbreviated Modified Mercalli Intensity Scale
1.0-3.0 I I: Not felt except by a very few under especially favorable conditions.
3.0-3.9 II-III II: Felt only by a few persons at rest, especially on upper floors of buildings

III:Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated
4.0-4.9 IV-V IV: Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

V: Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
5.0-5.9 VI-VII VI: Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

VII: Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
6.0-6.9 VIII-IX VIII: Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.

IX: Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
7.0 + VII + X: Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent

XI: Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly

XII: Damage total. Lines of sight and level are distorted. Objects thrown into the air

Earthquake Safety

  1. Before
  2. During
  3. After

Before an Earthquake



  • Look around places where you spend time.  Identify safe places such as under a sturdy piece of furniture or against an interior wall in your home, office or school so that when the shaking starts, you Drop to the ground, Cover your head and neck with your arms, and if a safer place is nearby, crawl to it and Hold On. 
  • Practice how to “Drop, Cover, and Hold On!
  • To react quickly you must practice often. You may only have seconds to protect yourself in an earthquake.
  • Before an earthquake occurs, secure items that could fall and cause injuries (e.g., bookshelves, mirrors, light fixtures).
  • Store critical supplies (e.g., water, medication) and documents.
  • Plan how you will communicate with family members, including multiple methods by making a family emergency communication plan.
  • When choosing your home or business, check if the building is earthquake resistant per local building codes. 

Additional Resources



Want to learn more about earthquakes? Visit these links for a more in depth look:

  1. USGS Earthquakes Program
  2. USGS: Modified Mercalli Intensity Scale
  3. USGS: Magnitude vs. Intensity
  4. Ready.gov: Prepare for Earthquakes
  5. Dropcoverholdon.org (en español)