Earthquakes.ppt for Class observation 2023

measuring earthquake

1. Measuring Measuring
EarthquakesEarthquakes
Logarithmic scales and human Logarithmic scales and human
perceptionperception

2. How intense is an How intense is an
Earthquake?Earthquake?
The severity of an earthquake can be expressed in The severity of an earthquake can be expressed in
terms of both terms of both intensityintensity and and magnitudemagnitude..
IntensityIntensity
based on observed effects of ground shaking on people, based on observed effects of ground shaking on people,
buildings, and natural features. buildings, and natural features.
It varies from place to place within the disturbed region It varies from place to place within the disturbed region
depending on the location of the observer with respect to the depending on the location of the observer with respect to the
earthquake epicenter. earthquake epicenter.
MagnitudeMagnitude
related to the amount of seismic energy released at the related to the amount of seismic energy released at the
hypocenter of the earthquake. It is based on the amplitude of hypocenter of the earthquake. It is based on the amplitude of
the earthquake waves recorded on instruments which have a the earthquake waves recorded on instruments which have a
common calibration. The magnitude of an earthquake is thus common calibration. The magnitude of an earthquake is thus
represented by a single, instrumentally determined value. represented by a single, instrumentally determined value.

3. Intensity Scales Intensity Scales
(qualitative)(qualitative)
Poarid (Italian)– 1627; first attempt to classify Poarid (Italian)– 1627; first attempt to classify
earthquakes by intensity in 1627; four levels of intensity earthquakes by intensity in 1627; four levels of intensity
use to describe effects of earthquakes experienced at use to describe effects of earthquakes experienced at
different towns. different towns.
More attempts made in 18More attempts made in 18
thth
and 19 and 19
thth
centuries. centuries.
Early 1900’s Mercalli’s scale; 12 levels of intensity.Early 1900’s Mercalli’s scale; 12 levels of intensity.
Modified version (1931) published by Wood & Modified version (1931) published by Wood &
Neumann. Known as the Modified Mercalli Intensity Neumann. Known as the Modified Mercalli Intensity
Scale of 1931, it has become the standard used by the Scale of 1931, it has become the standard used by the
United States engineering seismology community (incl. United States engineering seismology community (incl.
National Geophysical Data Center and the U.S. National Geophysical Data Center and the U.S.
Geological Survey.)Geological Survey.)

4. Modified Mercalli ScaleModified Mercalli Scale
The The ModifiedModified Mercalli intensity scaleMercalli intensity scale is is
used for measuring the intensity of an used for measuring the intensity of an
earthquakeearthquake. .
Quantifies the effects of an earthquake on Quantifies the effects of an earthquake on
the Earth's surface, humans, objects of the Earth's surface, humans, objects of
nature, and man-made structures.nature, and man-made structures.
12 levels, with 1 denoting a weak 12 levels, with 1 denoting a weak
earthquake and 12 one that causes almost earthquake and 12 one that causes almost
complete destruction. (See handout)complete destruction. (See handout)

5. USGS Earthquake investigations.USGS Earthquake investigations.
USGS is the Federal agency responsible for collecting earthquake USGS is the Federal agency responsible for collecting earthquake
intensity data. intensity data.
Used "Earthquake Report“ questionnaire and field investigators to Used "Earthquake Report“ questionnaire and field investigators to
analyze earthquake damage in serious earthquakes. analyze earthquake damage in serious earthquakes.
Questionnaires sent to postmasters, National Weather Service Questionnaires sent to postmasters, National Weather Service
offices, military installations, etc. requesting a report of all offices, military installations, etc. requesting a report of all
effects of the earthquake in their area. Newspaper accounts, effects of the earthquake in their area. Newspaper accounts,
published scientific reports, and reports provided by published scientific reports, and reports provided by
seismology collaborators were also used.seismology collaborators were also used.
Expert observers may investigate and photograph the Expert observers may investigate and photograph the
damage.damage.
In the end, a seismologist analyzes each report and assigns In the end, a seismologist analyzes each report and assigns
intensities on the basis of the effects at each town;intensities on the basis of the effects at each town;
Intensity (or isoseismal) maps are then constructed for Intensity (or isoseismal) maps are then constructed for
earthquakes felt over large areas. earthquakes felt over large areas.

6. The Richter ScaleThe Richter Scale
Magnitude of most earthquakes is measured on the Magnitude of most earthquakes is measured on the
Richter scaleRichter scale, invented by Charles F. Richter in 1934. , invented by Charles F. Richter in 1934.
The Richter magnitude is calculated from the amplitude The Richter magnitude is calculated from the amplitude
of the largest seismic wave recorded for the of the largest seismic wave recorded for the
earthquake, no matter what type of wave was the earthquake, no matter what type of wave was the
strongest. strongest.