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Variation in Earthquake Motions
Earthquakes, then, have three different component motions: a backwards-forwards motion (‘P’ waves); a shear motion (‘S’ waves and Love waves); and a wave-like motion (Rayleigh waves).
But there are other factors which influence what type of displacement – and shaking – occurs at any point on the surface at any time.
- Distance from the point where the earthquake takes place is key. Because ‘P’ waves travel fastest, they will reach a given point first (and are the first to show on a seismogram). The further away the earthquake is from any point, the greater the time difference between the arrival of the ‘P’ waves and the others. It may, therefore, be possible to distinguish one type of movement from another at a distance. Close to an earthquake, the waves will arrive pretty much at the same time.
- Depth of an earthquake influences amount of damage and shaking. Deeper earthquakes occur further from the surface, and so the energy released is more likely to be dissipated before it reaches the surface. Here, the difference between body and surface waves is significant. In deeper earthquakes, the surface waves dissipate more rapidly and are weaker, so in deep earthquakes the ‘rolling’ motion’ is reduced.
- Local topography matters, too. It’s commonly known that some types of rock can cause amplification of earthquake waves – in other words, the same earthquake waves can cause greater shaking in a location where the ground is soft sediments than in a neighboring area where the ground is of solid rock – a case clearly demonstrated by the damage caused by the Mexico earthquake of 1985 where damage in Mexico City, built on a former lake bed, was much greater than in surrounding areas.
Unique Factors Make Earthquakes Feel Different
Taking the depth, distance from epicenter, and local topography along with other factors – such as the actual magnitude of an earthquake – it’s easy to see how location of the individual on the surface of the earth and of the earthquake within it, along with local factors, can sometimes make one earthquake feel very different from another.
Michigan Technological University. What Are Seismic Waves? Accessed 10 September 2012
National Institute of Standards and Technology. Earthquake Mexico 1985. Accessed 10 September 2012
Penn State University Department of Geosciences. Earthquake Effects. Accessed 10 September 2012
Smith, Peter J. and Rymer, Hazel. The Physical and Chemical Properties of the Earth. (2001). The Open University.
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