PART 1: Properties of Seismic Waves
Hypothetical seismogram, indicating relevant features.
PART 1: Properties of Seismic Waves
Hypothetical seismogram, indicating relevant features.
SEISMIC WAVES Most earthquakes are caused by sudden movements on faults or by volcanic eruptions. The place where the disturbance starts below the surface is called the focus (or hypocenter). Most foci are restricted to depths less than about 100 km, except along subduction zones where foci can be as deep as 700 km. The kinetic energy (energy of motion) of the disturbance radiates in all directions away from the focus in the form of waves. Thus the leading edge of the wave, or wavefront, expands upward, laterally and downward. The wavefront first reaches the surface at the epicenter, which is directly above the focus, or very nearly so. There are different types of seismic waves:SURFACE WAVES: These are restricted to the lithosphere, and are responsible for most of the actual ground-shaking. Most of the kinetic energy of an earthquake is released in the form of surface waves. In fact practically all of the damage done by an earthquake is caused by surface waves. There are two main types of surface waves, L-waves and R-waves:
L-waves: "L" comes form the name of the mathematician, Love, who discovered these types of waves. However, it's helpful to think of "L" as standing for "lateral," which relates to the nature of the resulting gorund-shaking. The ground moves laterally, or shimmies side-to-side with no vertical component. Of the two types of surface waves, the L-waves are the most destructive. They can literally move the ground beneath a building faster than the building itself can respond, effectively shearing the base off of the rest of the building.BODY WAVES: These can pass into the deeper layers of the earth, beneath the lithosphere. There are two types of body waves, P-waves and S-waves. These waves are subject to reflection (echo) and refraction at boundaries between layers having different physical properties. The internal compositional layering of the earth (crust, mantle, core) and layering according to physical state (lithosphere=strong/rigid, asthenosphere=weak, mesosphere=strong, outer core=liquid, inner core=solid) are known from studying the reflection (echo) and refraction behavior of P-waves and S-waves. Neither of the body waves is reponsible for very much of the ground-shaking during an earthquake:R-waves: "R" stands for Rayleigh, the English physicist who first predicted this kind of seismic movement. Here, it's helpful to think of "R" as standing for "rolling," which relates to the nature of the ground-motion, like the motion of an anchored boat as it bobs up and down on sea waves. R-waves behave like sea-waves. As an R-wave passes beneath a building, for instance, the building first moves up and is then set back down.
P-waves: "P" stands for primary. These waves are the fastest of the wave types. After passing through the deep interior of the earth and re-emerging at the surface, P-waves are the first to arrive. P-waves are acoustic waves. Yes, they are a kind of sound wave. The energy propagates as a modulation or pulse of pressure.S-waves: "S" stand for secondary. These are the second fastest kind of seismic wave. The energy is carried by oscillations perpendicular to the path of propogation. This kind of motion can only be support by materials that are rigid. For this reason, S-waves cannot propogate through liquids. After passing through the deep interior of the earth, S-waves re-emerge at the surface only along paths that do not intersept the core. S-waves that intercept the core are stopped, the energy is absorbed completely, and none of the energy reaches the opposite side of the earth. Thus, a "shadow zone" is formed at the surface of the earth, defined by the area where S-waves are not detected.
VELOCITY The velocity of the P-wave, V(P), and the velocity of the S-wave, V(S), accelerate with depth down to the mantle-core boundary. There, as just described, the S-wave stops, and while the P-wave continues into the core, it does so at a much reduced velocity. At every depth in the lithosphere, asthenosphere and mesopshere, the velocity of the P-wave is nearly twice the velocity of the S-wave:V(P) ~ 1.8 V(S)
Near the surface V(P) ~ 7-8 km/s and V(S) ~ 4-5 km/s. Near the mantle-core boundary V(P) ~ 13 km/s and V(S) ~ 6.5 km/s. At any place on the surface where both the P-wave and the S-wave can be detected, there is a delay in time between the arrival of the P-wave and the arrival of the S-wave. From the perspective of an observer watching a seismometer, the first indication that an earthquake has happened somewhere is the arrival of the P-wave. A short period of time passes before the arrival of the S-wave. The delay in time is referred to as the S-P interval in the diagram at the top of this page. The diagram shows how to measure the S-P interval. The velocity of surface waves, V(R,L), is slightly slower than S-waves. Surface waves typically arrive immediately after the arrival of the S-wave, for practical purposes sometimes at the same time.
Amplitude is the maximum amount of deflection of the seismogram. This reflects the maximum amount of ground-shaking, as indicated in the hypothetical seismogram at the top of the page. Notice that most of the "ground-shaking" is associated with the surface waves. The observed maximum amplitude depends on how far the seismic station is from the epicenter, such that the observed maximum amplitude diminishes with distance from the epicenter. The hypothetical seismogram at the top of the page shows how to measure the amplitude. AMPLITUDE
Examine seismograms A and B below, and answer the questions starting below the seismograms. Enter your answers on your answer sheet. PROBLEMS
1.1 What is the arrival time for the P-wave?
1.2 What is the S-P interval?
1.3 What is the amplitude?
1.4 Which seismometer station was closer to the epicenter, A or B? Explain briefly.
1.5 Which seismogram records the most ground-shaking at the seismic station?
1.6 Give two reasons why these seismograms cannot be records of the same earthquake.
O.K., we're done here.
