EARTHQUAKE VIBRATIONS AND

BUILDING DAMAGES

Earthquake vibrations can cause significant damage to buildings and structures.  They can also create minor vibrations that cause building owners to carefully dissect their property for any type of damage that may or may not have been there before.  The qualified professional engineers at RFE have the ability to determine if these damages are caused by the actual earthquake vibrations, or if there are other preexisting conditions.

Surface level ground vibrations generated from earthquakes can damage buildings as the ground vibrations travel under a building's foundation.  Earthquakes are measured in both magnitude and intensity.

Magnitude is a quantitative measurement of the ground movement at the epicenter of the earthquake, as recorded by a seismograph.  The seismograph amplifies and measures ground motion. 

The United States Geological Survey (USGS) has placed seismographs at various locations throughout each county of each state. The time, location, and magnitude of an earthquake are determined from the data recorded by the numerous seismograph recording stations.  The magnitude of an earthquake is determined from the logarithm of the amplitude of the ground waves recorded by the seismograph, calculated from an energy formula.  Adjustments are included for the variation in distance between the various seismograph stations and the epicenter of the earthquake.

On the Richter scale, magnitude is expressed in dimensionless whole numbers and decimal fractions.  For example, a 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3.  Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in the measured amplitude of the ground movement.  As an estimate of energy, each whole number step in the Richter magnitude scale corresponds to the release of about 31 times more energy than the prior number. (1)(2)

Intensity is the qualitative measurement of building damages, visually observed at the particular location or structure being investigated for vibration damages.  Visual observation and recording of  building materials cracking is the basis for the qualitative assessment of vibration level intensity.

The Modified Mercalli Intensity Scale, developed in 1931, is used to qualitatively determine the intensity of an earthquake on individual buildings.  The scale consists of a graduated series of certain key responses, such as people awakening, movement of furniture, and damage structures.

The Modified Mercalli Intensity value assigned to a specific site after an earthquake is a more meaningful measure of severity than the magnitude (Richter number) because the intensity refers to the effects actually experienced at each building.  After the occurrence of widely felt earthquakes, the National Earthquake Information Center (NEIC), a division of the USGS, mails questionnaires to postmasters in the disturbed area, requesting the information so that the intensity values can be assigned.  The results of this postal canvas and information furnished by other sources are used to assign an intensity with the felt area.  The maximum observed intensity generally occurs near the epicenter.

MECHANICS OF GROUND VIBRATIONS

Three primary factors determine the intensity of an earthquake:

Ground vibration causes damage by differential displacement.  As vibration waves pass under a structure, they will move the foundation of the structure up and down, from side to side, and back and forth.  If this movement of the structure could be in its entirety, there would be no damage.  It is the differential movement that actually causes the damage. Generally, while the lower portion of the structure is in motion due to vibration, the top of the structure is in its original position at rest.  As a result, diagonal stresses are rapidly imposed on the walls and ceilings, first in one direction, and then in another. (3)

The first evidence of vibration damage appears as the lengthening and widening of pre-existing plaster or drywall cracking, and is identified as Threshold vibration intensity level.

If the diagonal stresses and motion are intense enough, the walls open up in two sets of diagonal cracks (sometimes called scissor cracks), at about 45 degrees to the horizontal, and about 90 degrees to each other.  Similar cracks may also result in ceilings due to torsional vibration.  Because plaster or drywall is usually the weakest material in the building, it is the first material to form new cracks.  The formation of the new diagonal cracking is identified as Minor vibration intensity level.

Major level vibration intensity can cause cracking of concrete and masonry. (3)

Because of the graduated intensity level scale, without the presence of Threshold level drywall and/or plaster cracking, and clear evidence of Minor level scissor cracking of plaster and/or drywall, Major level damages to masonry, concrete, and foundations will not occur. Without clear visual evidence of Threshold and Minor level drywall and plaster damages, it should be assumed that masonry, concrete and foundation damages were pre-existing, before the earthquake.

HUMAN RESPONSE TO GROUND VIBRATIONS

Ground vibrations are highly perceptible to humans.  During even minor earthquakes,

windows and dishes might rattle, knickknacks and pictures might move or fall if not securely fastened, and hanging lamps might sway.  These effects can last from one to three seconds or more, depending upon the distance from the epicenter, geologic influences and other factors.  Despite these sometimes startling effects, there is no direct correlation between how an earthquake shake "feels" or sounds and its potential for causing structural damage to a home or building.  In fact, cultural stresses (e.g., doors slamming, kids jumping, people pounding nails) and natural stresses (e.g., sunlight, wind, rain, temperature and humidity fluctuations and changes in soil moisture) can place greater stresses on a home or building than earthquake vibrations."  (4)

RFE has been involved in hundreds of earthquake/vibration inspections with decades of experience in these types of claims.  The qualified professional engineers at RFE have the ability to determine if these damages are caused by the actual earthquake vibrations, or if there are other preexisting conditions. Through thorough site inspections, testing, and analysis, the staff at RFE are able to determine the How and Why of the loss for our Clients.

REFERENCES:

(1)         "Earthquakes and Volcanoes," Volume 21, Number 1, 1989; Spence, Sipkin, Choy

(2)         "Severity of an Earthquake," 1989-288-913, U.S. Printing Office; U.S. Geological Survey General Interest Publication.

(3)         "Structure Response and Damage Produced by Ground Vibration From Surface Mine Blasting," BM Report Investigation 8507, 1980; Siskind, Stagg, Kopp, Dowding.

(4)         "Blasting in Ohio's Quarries and Surface Coal Mines," Ohio Department of Natural Resources, Division of Mineral Resources Management, June 2004.