How fast earthquake

Major Faults in New Zealand. Monitoring Earthquakes. Seismic Activity. Ground Deformation. Other earthquake questions.

Can earthquakes be predicted? Earthquake weather? How long does an earthquake last? In some instances reflections from the boundary between the mantle and crust may induce strong shaking that causes damage about km from an earthquake we call that boundary the "Moho" in honor of Mohorovicic, the scientist who discovered it.

A seismic reflection occurs when a wave impinges on a change in rock type which usually is accompanied by a change in seismic wave speed. Part of the energy carried by the incident wave is transmitted through the material that's the refracted wave described above and part is reflected back into the medium that contained the incident wave. When a wave encounters a change in material properties seismic velocities and or density its energy is split into reflected and refracted waves.

The amplitude of the reflection depends strongly on the angle that the incidence wave makes with the boundary and the contrast in material properties across the boundary. For some angles all the energy can be returned into the medium containing the incident wave.

The actual interaction between a seismic wave and a contrast in rock properties is more complicated because an incident P wave generates transmitted and reflected P- and S-waves and so five waves are involved. Likewise, when an S-wave interacts with a boundary in rock properties, it too generates reflected and refracted P- and S-waves. I mentioned above that surface waves are dispersive - which means that different periods travel at different velocities.

The effects of dispersion become more noticeable with increasing distance because the longer travel distance spreads the energy out it disperses the energy. Usually, the long periods arrive first since they are sensitive to the speeds deeper in Earth, and the deeper regions are generally faster.

A dispersed Rayleigh wave generated by an earthquake in Alabama near the Gulf coast, and recorded in Missouri. The mathematics behind wave propagation is elegant and relatively simple, considering the fact that similar mathematical tools are useful for studying light, sound, and seismic waves. We can solve these equations or an appropriate approximation to them to compute the paths that seismic waves follow in Earth. The diagram below is an example of the paths P-waves generated by an earthquake near Earth's surface would follow.

The paths of P-wave energy for a shallow earthquake located at the top of the diagram. The main chemical shells of Earth are shown by different colors and regions with relatively abrupt velocity changes are shown by dashed lines. The curves show the paths of waves, and the lines crossing the rays show mark the wavefront at one minute intervals. Note the curvature of the rays in the mantle, the complexities in the upper mantle, and the dramatic impact of the core on the wavefronts.

We have already discussed the main elements in Earth's interior, the core, the mantle, and the crust. By studying the propagation characteristics travel times, reflection amplitudes, dispersion characteristics, etc. Great progress was made quickly because for the most part Earth's interior is relatively simple, divided into a sphere the inner core surrounded by roughly uniform shells of iron and rock. Models that assume the Earth is perfectly symmetric can be used to predict travel times of P-waves that are accurate to a few seconds for a trip all the way across the planet.

The diagram below is a plot of the P- and S-wave velocities and the density as a function of depth into Earth. The top of the Earth is located at 0 km depth, the center of the planet is at km. Velocity and density variations within Earth based on seismic observations. The main regions of Earth and important boundaries are labeled. Several important characteristics of Earth's structure are illustrated in the chart. First note that in several large regions such as in the lower mantle, the outer core, and inner core, the velocity smoothly increases with depth.

The increase is a result of the effects of pressure on the seismic wave speed. Although temperature also increases with depth, the pressure increase resulting from the weight of the rocks above has a greater impact and the speed increases smoothly in these regions of uniform composition.

The shallow part of the mantle is different; it contains several important well-established and relatively abrupt velocity changes. In fact, we often divide the mantle into two regions, upper and lower, based on the level of velocity heterogeneity.

In this depth range the minerals that make up the mantle silicate rocks are transformed by the increasing pressure. The atoms in these rocks rearrange themselves into compact structures that are stable at the high pressures and the result of the rearrangement is an increase in density and elastic moduli, producing an overall increase in wave speed.

The two largest contrasts in material properties in the Earth system are located near the surface and the core-mantle boundary. Both are compositional boundaries and the core-mantle boundary is the larger contrast.

Other sharp contrasts are observable, the inner-core outer-core boundary is relatively sharp, and velocities increase from the liquid to the solid. More recent efforts have focused on estimating the lateral variations in wave speed within the shells that make up the reference model. These approaches are often based on seismic tomography, which is a way of mapping out the variations in structure using observations from large numbers of seismograms.

The basic idea is to use observed delayed or early arrival times delayed with respect to the reference model to locate regions of relatively fast and relatively slow seismic wave speed. An example of an S wave is wiggling or shaking a rope which is tied down at one or both ends. Both P and S waves travel outward from an earthquake focus inside the earth.

The waves are often seen as separate arrivals recorded on seismographs at large distances from the earthquake. The direct P wave arrives first because its path is through the higher speed, dense rocks deeper in the earth. Many open cracks in the earth, set off from the main fault, were observed and thought to be the work of a supershear shock wave. Das says that these cracks could be used as a "diagnostic tool" to look for further evidence of supershear earthquakes.

Her analysis is detailed in the Aug. To get a supershear quake you need a very long, straight section of a strike-slip fault one in which the two sides of the fault slide past one another, instead of under or over each other to rupture because, as Das puts it, "earthquakes are like cars.