Lecture 8 earthquakes 2 NF Flashcards
What are the different types of seismic waves?
P(primary) compressional waves
S (secondary) shear waves
P and S wave velocity controlled by composition and temperature.
Shear (S) waves cannot travel through a liquid (only solid) so do not pass through the earths core and are slower than P waves.
Shear waves cannot travel through a liquid - shadow zone caused by outer core (beyond 104°)
Refraction of P waves by slower outer liquid core also causes a P wave shadow (104-140°)
Waves may change type (e.g., P to S) as reflected at major boundaries.
A variety of wave paths are possible for seismic waves through the earth.
Seismometers (seismographs):
- Detect the passage of seismic waves.
- Use a system of masses and springs. The sensor
must be firmly anchored to bedrock. The sensor site must be carefully chosen (in a concrete vault, down a borehole or at the bottom of a deep mine).
Seismograms (seismic signature):
- Originally recorded on a smoked drum or paper roll. Record rotates continuously, pen gradually moves across the recording drum as well as leaving wiggles, and a solenoid is used to deflect the pen for 1 second each minute. Time is therefore accurately recorded.
- Now electronic sensors and digital recording
systems connected to worldwide data networks.
Slide 9 confused :(
Most observatories record three components of ground motion: vertical,
north-south (horizontal) and east-west (horizontal).
Modern seismometers use the ‘mass-on-a-spring’ principle to cause
movement of a coil relative to a magnet.
- Induces a voltage across the coil, which can be measured and
recorded or negative feedback loop necessary to hold magnet in place.
- Voltage proportional to the velocity of the coil with respect to the
magnet, so the amplitude of the signal recorded in the seismogram is
proportional to the velocity of ground motion.
What is the WWSSN
1960s installation of the World-Wide Standard Seismic Network (WWSSN):
- Identical sets of seismometers around the world, all linked to a common time standard.
- Allowed global plate boundary maps to be created.
- Used arrival times of different seismic waves at different stations to accurately determine location.
Locating earthquakes
Seismic wavefronts propagate out from
the hypocentre or focus, the waves
take longer to reach more distant
seismographs.
P-waves travel faster than S-waves, so
time difference between the two also
increases with increasing distance.
Hence the ‘P to S’ time difference can
be used to determine epicentral
distance from a recording station.
By determining epicentral distance to
several different seismic observatories
(stations), it’s possible to determine the
location of the epicentre. A minimum of
3 recording stations is necessary.
Triangulation.
What can be used to calculate focal depth?
time difference between P and S wave arrivals.
Earthquake magnitude scales
based on two assumptions:
(i) for a given source/receiver geometry, the “larger” of two earthquakes will generate larger
amplitude arrivals; and
(ii) amplitudes of arrivals behave in a “predictable” or homogeneous/linear manner.
What is the Richter scale Magnitude
scale based on the amplitudes of seismic waves. It is a logarithmic scale: an increase of 1 in magnitude results from an increase of a
factor of 10 in the amplitude.
when is the Richter scale wrong
- developed in California so may not be relevant for other locations.
or for those recorded with modern instruments.
The constants of other magnitude scales (next) are fixed to try and ensure that different scales match. It is found that this magnitude scale is invalid for larger magnitude earthquakes (M>8). Scientifically the Moment Magnitude is preferred as more precise across the magnitude range.
how can the local earthquake magnitude be estimated?
ML is usually determined using empirical graphical methods, as shown on the right. The Richter scale is calibrated for certain types of instruments. So, by determining the S-P time interval and the amplitude of the largest wave, these points can be joined to estimate the local earthquake magnitude.
What is surface wave magnitude
Beyond 600 km epicentral distance, records of shallow earthquakes are dominated by surface waves (although amplitudes are strongly influenced by source depth).
For shallow earthquakes, the preferred measurement of magnitude is the Surface wave magnitude.
deep earthquakes don’t generate as many surface waves.
Body wave magnitude
Used for deep focus (>50 km) earthquakes
Mb and Ms equal for magnitude 6.5; Mb>Ms for
smaller magnitudes; Mb<Ms for larger magnitudes.
Give An alternative measure of the elastic energy released by an earthquake
Seismic moment. This gives a measure of the size of an earthquake which depends on the earthquake mechanism, rather than the amplitudes of seismic waves it generates.
It is also something that we can add up (e.g., if slip happens on different areas of a fault at different times.
What does unbound mean when referencing the moment scale.
Since it depends on slip and area only. It depends on total permanent deformation.
What is the preferred magnitude scale for seismologists
- Moment magnitude
more accurate at higher magnitudes and better represents the deformation occurring.
What is earthquake energy
The cumulative frequency-magnitude plot
indicates that the number of events falls by a factor of ~10 for each unit increase in magnitude.
Since seismic moment increases by a
factor of 30 for a unit magnitude increase,
we can deduce that almost all of the moment release is expected to occur in the largest few earthquakes.
Essentially a magnitude increase of 1 is
equivalent to 30 x increase in energy.
What is the measure of earthquake intensity?
In 1902, an Italian seismologist, G. Mercalli, developed an intensity scale based on felt effects of earthquakes.
* Earthquake intensity is a local estimate of the intensity of ground motion and of the resulting damaging effects. It is based on detailed written descriptions of the damage associated with each intensity level.
* It is a result of the size and location of the earthquake + local effects (soil conditions, ground composition, liquefaction, bbuilding type and quality, etc)
* It can be used to produce intensity maps of the area affected by an earthquake.
* Maps are based on isoseismals — contours of equal intensity of shaking.
* The more modern European Macroseismic Scale (EMS) defines 12 intensities
Positives and negatives of earthquake intensity
EQ intensity is useful for historical
events:
- Intensity can be used to estimate
earthquake magnitude and location
For modern mitigation:
- Intensity gives indication of local
ground conditions.
Problems:
- Relies on accuracy of reporting
- Relies on presence of people to
record effects
Earthquakes summary
Different types of seismic wave are produced by
earthquakes with particular wave form, velocity
and propagation patterns
2) Methods of recording and locating earthquakes
including errors
3) Estimating scale of an earthquake and energy
released
4) Different earthquake magnitude scales,
applicability and errors
5) Earthquake intensity and its applicatio
