Seismic Waves. Wave motion is perhaps most familiar to us from our observations of waves on water. When a stone is thrown into a pool of water, the surface of the water is disturbed where the stone strikes, and ripples move outwards from the place of its’ disturbance.
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Wave motion is perhaps most familiar to us from our observations of waves on water
When a stone is thrown into a pool of water, the surface of the water is disturbed where the stone strikes, and ripples move outwards from the place of its’ disturbance.
This wave train is produced by movements of the particles of water in the vicinity of the ripples. The water however, does not actually flow in the direction in which the ripples travel (think of a cork on the surface). Earthquake waves are quite analogous to those caused by that of a stone thrown into a pool.
Waves traveling outwards from 1964 Great Alaskan Earthquake
Only S-waves exhibit the tree. Hold the line tight and shake it rapidly from side to side. You can see what looks like waves running down the clothesline towards the tree distorting the shape of the clothesline. Similarly, when S waves pass through rock they distort its shape. S waves cannot pass through liquid or air, and you would not feel them aboard a ship at sea.phenomenon called polarization. As S-waves travel through the Earth, they encounter structural discontinuities that refract or reflect them and polarize their vibrations.
When an S wave is polarized so that the particles of rock move only in a horizontal plane, it is denoted by the symbol SH. When the particles of rock all move in the vertical plane containing the direction of propagation, the S wave is called an SV wave
The Speed of P and S Waves tree. Hold the line tight and shake it rapidly from side to side. You can see what looks like waves running down the clothesline towards the tree distorting the shape of the clothesline. Similarly, when S waves pass through rock they distort its shape. S waves cannot pass through liquid or air, and you would not feel them aboard a ship at sea.
The actual velocities at which P and S-waves travel depend on the densities and inherent elastic properties of the rocks through which they travel.
Wave speed depends on the measures of only two elastic properties, called elastic moduli: the incompressibility (k) and the rigidity (m) of the rock.
When a uniform pressure is applied to the surface of a cube of rock, its’ volume is reduced and a measure of its’ change in volume per unit volume is its; incompressibility. This type of deformation occurs when P-waves propagate through the Earth’s interior.
In general, as k h,P-wave velocity h
A second type of deformation occurs when equal but opposite tangential pressures are applied to opposite faces of a cube of rock. The cube will deform by shearing out of its’ shape without any change in volume.
This same strain occurs when a cylindrical core of rock is twisted by equal and opposite pressures applied at opposite ends. The greater the resistance, the greater the rigidity.
Because S-waves propagate by shearing the rock, the rigidity gives a measure of their speed.
In general, as mh, S-wave velocity h
k = modulus of incompressibility (or bulk modulus)
Granite – k = 27 x 1010 dyne/cm2
water – k = 2.0 x 1010 dyne/cm2
m = modulus of rigidity
Granite – m = 1.6 x 1010 dyne/cm2
water – m = 0.0 dyne/cm2
P-wave velocity – a = (k + 4/3 m)/r
S-wave velocity – b = m/r
Where r = density of the rock through which the wave is traveling
Granite 5.5 km/s 3.0 km/s
Water 1.5 km/s 0.0 km/s
Because of the great pressures inside the Earth, the rock density increases everywhere with depth.
As a consequence, it would appear that the position of the density term in the denominator of each formula would cause both the P and S-wave velocities to decrease with depth. However, both incompressibility and rigidity increase more rapidly than rock density with depth.
A typical recording of an earthquake records a typical “train” of seismic waves. First the P waves, followed in succession by the S and surface waves.
An earthquake can be compared to a symphony orchestra, with many instruments producing sound waves that vibrate at both high and low frequencies. A seismologist can separate out the complex waveforms into simple sinusoidal components using high-speed computers.
Focal Depth several different seismograph stations, we are able to locate the
Charles Richter, inventor of the magnitude scale.
Charles Richter has explained that: several different seismograph stations, we are able to locate the
"Magnitude can be compared to the power output in kilowatts of broadcasting station. Local intensity on the Mercalli scale is then comparable to the signal strength on a receiver at a given locality; in effect the quality of the signal. Intensity like signal strength will generally fall off with distance from the source, although it also depends on the local conditions and the pathway from the source to the point."
Great; M > =8Major; 7 < =M < 7.9Strong; 6 < = M < 6.9Moderate: 5 < =M < 5.9Light: 4 < =M < 4.9Minor: 3 < =M < 3.9Micro: M < 3
Magnitude Approximate Equivalent
4.0 1010 tons
5.0 31800 tons
6.0 1,010,000 tons
7.0 31,800,000 tons
8.0 1,010,000,000 tons
9.0 31,800,000,000 tons
Ground rupture, Hector Mine Earthquake
Damage caused by Whittier Earthquake
Compare the fault area of the magnitude 7.3 (top) with that of the magnitude 5.6 (smallest one near the bottom).
In this same way, low frequency earthquake waves can be recorded thousands of miles away from the earthquake source. A commonly used earthquake scale is the surface wave magnitude, or MS, which measures the largest deflection (amplitude) of the needle on the seismograph for a surface wave that has a frequency of about 20 seconds.
San Francisco Earthquake, 1906
Alaska Earthquake, 1964
Acceleration, Velocity, Displacement (Image courtesy of Charles Ammon, Penn State)
When you step on the accelerator in the car or put on the brakes, the car goes faster or slower. When it is changing from one speed to another, it is accelerating (faster) or decelerating (slower). This change from one speed, or velocity, to another is called acceleration. During an earthquake when the ground is shaking, it also experiences acceleration. The peak acceleration is the largest acceleration recorded by a particular station during an earthquake.
g is the acceleration of gravity 9.8 (m/s2) or the strength of the gravitational field (N/kg) (which it turns out is equivalent). G is the proportionality constant 6.67x10-11 (N-m2/kg2) in Newton's law of gravity. On the other hand, the force of gravity, or F = mg, at the surface of the earth, or F = GMm/r^2 at a distance r from the center of the earth (where r is greater than the radius of the earth). When there is an earthquake, the forces caused by the shaking can be measured as a percentage of gravity, or percent g.
Giuseppe Mercalli, developer of the Mercalli Intensity Scale.
Masonry A. Good workmanship, mortar, and design; reinforced especially laterally, and bound together by using steel, concrete, etc.; designed to resist lateral forces.
Masonry B. Good workmanship and mortar; reinforced. but not designed in detail to resist lateral forces.
Masonry C. Ordinary workmanship and mortar, no extreme weaknesses like failing to tie at corners, but neither reinforced nor designed against horizontal forces.
Masonry D. Weak materials, such as adobe; poor mortar; low standards of workmanship; weak horizontally.
Earthquake intensities are based on a post-earthquake survey of a large area: damage is noted, and people are questioned about what they felt. An intensity map is a series of concentric lines, irregular rather than circular, in which the highest intensities are generally, but not always, closest to the epicenter of the earthquake.
The Mercalli intensity scale is also useful in “predicting” what might happen given a scenario earthquake. Since ground conditions and building types are known for a given location, given an earthquake of a certain magnitude, it is possible to determine the Mercalli intensities at that location.
The above scenario is for a theoretical magnitude 7.4 earthquake on the southern San Andreas fault. Computer calculated Mercalli intensities are color coded for easier recognition.
As mentioned, it is possible to relate earthquake intensity to the maximum amount of ground acceleration (peak ground acceleration, or PGA) that is measured with a special seismograph called a strong-motion accelerograph. This is shown on the previous MMI scale. Acceleration is measured as a percentage of the Earth’s gravity. A vertical acceleration of 1 g would be just enough to lift you (or anything else) off the ground. Obviously this would have a major impact on damage done by an earthquake at a given site.
Intensity map for the 1994 Northridge earthquake. Note that damage is strongly related to both PGA and PGV at the bottom of the map.
In the early days of seismology, it was enough to locate an earthquake accurately and to determine its magnitude. But seismic waves contain much more information, including the type of faulting. The seismogram shows that the first motion of an earthquake P wave is either a push toward the seismograph or a pull away from it. With the network of seismographs in California, its possible to determine the push or pull relationship at many stations, leading to information about whether the earthquake was on a reverse fault, a normal fault, or a strike-slip fault.
1st motion of P wave up is a compression (push) towards the seismograph
1st motion of P wave down is an extension (pull) away from the seismograph
The first motions of P waves show which way the fault moved during an earthquake.
C = Compression
E = Extension
For a left lateral strike-slip fault, the fault plane solution will look like this. The circle, or “beach ball”, is actually a 2-D representation of a 3-D fault zone, with the red areas indicating areas in compression, the white being areas in extension.
Areas in extension
Areas in compression
Note that a secondary, or auxiliary plane is also present in these cases.
Typical Fault Plane Solutions solution will look like this. The circle, or “beach ball”, is actually a 2-D representation of a 3-D fault zone, with the red areas indicating areas in compression, the white being areas in extension.
Records a compression everywhere
Records an extension everywhere
Aftershock zones can be defined in two different ways...
An aftershock is actually just a normal earthquake in every physical detail. Out of context, there is no way to tell the difference between any arbitrary earthquake and an "aftershock". The only real difference between the two is that an aftershock follows closely in the wake of a larger earthquake, and in roughly the same location as its predecessor. That larger, initial earthquake is usually referred to as the "mainshock".
More specifically, there are two guidelines for labelling an earthquake as an aftershock. First, it must occur within an "aftershock zone." This is sometimes defined as within one fault-rupture length of the mainshock rupture surface, or alternatively, within an area defined by seismologists based upon early aftershock activity. Second, it must occur within that designated area -- the "aftershock zone" -- before the seismicity rate in that area returns to its "background", meaning pre-mainshock, level. If an earthquake meets these two criteria, seismologists consider it an "aftershock."