Engineering Geology and Geohazards Lecture 12: Measuring Earthquakes Flashcards
Define Earthquake magnitude
Magnitude is a measure of the energy released at
the focus of an earthquake.
- The best known magnitude scale is the Richter
magnitude. - Richter showed that the larger the energy of the
earthquake, the larger the amplitude of ground motion at a given distance.E
Describe the Richter scale
It is a logarithmic scale so each level is one order
of magnitude larger that the previous. So an
increase of one on the magnitude scale means a
10-fold increase in amplitude and ~31-fold
increase in energy released.
Recorded as the amount the needle of the then standard seismometer is deflected, adjusted for distance.
- Richter used observations of earthquakes in
southern California to determine a reference
event; the magnitude of an earthquake is
calculated by comparing the maximum amplitude
of the signal with this reference event at a specific
distance.
What is the equation for the RIchter scale
ML = log10A(mm) + (Distance correction factor)
* Where A is the peak amplitude of needle deflection
measured directly from the paper record of a WoodAnderson seismometer.
Describe the limitations of the Richter scale
calibrated his scale on earthquakes in southern
California, so potential problem in applying
elsewhere.
- Originally based on records from a specific
seismograph: Wood-Anderson seismograph.
Other instruments have since been calibrated for
comparability though. - Local Magnitude (ML) estimate of earthquake size
is unreliable for measurements taken at a distance
>600 km from the epicentre. - Upper limit for reliable calculation is ~ML6.8
- Only events with ML>4.6 are recorded globally.
What are some alternative magnitude scales
- Body wave magnitude, Mb, is determined by
measuring the amplitude of P-waves from
teleseismic earthquakes. - Surface wave magnitude, Ms, is determined by
measuring the amplitude of surface waves. (not suitable for deep earthquakes because they
generate disproportionately weaker surface waves) - Moment magnitude (Seismic moment) Mw
(w=work accomplished). Designed to best
represent the total amount of seismic energy
released in an earthquake.
HOWEVER - Richter scale still used for M>3.5 earthquakes
Define seismic energy
Potential energy is stored in the seismogenic
lithosphere in the form of elastic strain. During an
earthquake, this stored energy is transformed and
results in
1. faulting and deformation,
2. heat,
3. radiated seismic energy (body waves and surface
waves).
Define the seismic moment
The seismic moment M0 is a measure of the total
amount of energy that is transformed during an
earthquake. Only a small fraction of the seismic
moment M0 is converted into the radiated seismic
energy that seismometers register.
Describe the moment magnitude scale
introduced in 1979 to address shortcomings in the Richter scale
Advantage of MW is that it is clearly related to physical properties of the source, since the seismic moment (M0) is based on the area of fault rupture, the average amount of movement, and the force required to overcome the friction holding the rocks together.
Moment Magnitude scale does not saturate at the upper end; there is no upper limit to the possible measurable magnitudes. However, this has the side-effect of low-energy earthquakes clustering together
How many earthquakes happen per year
About 3 million earthquakes per year, of which about 0.5 million are large enough to be felt.
- Exponential decrease in earthquake size with increase in number.
*No evidence that the numbers of earthquakes are increasing, or decreasing.
Why are Earthquake amplitude scales different from earthquake intensity scales
The intensity of the movement at the surface (and therefore the
effects of an earthquake), will depend not only on the energy
released at the focus, but also on the focal depth and the type of
rocks and topography in the area.
List some earthquake intensity scales
-Modified Mercalli intensity scale (MMS)
-the European Macroseismic scale (EMS)
-the Japanese intensity scale (JMA).
Most have 12 degrees of intensity.
