Earthquakes are shaking ground caused most often by sudden movements along cracks in the earth called faults. Some major faults acting for millions of years have offset rock layers by hundreds of Kilometers. Sedimentary rock layers originally are continuous, horizontal, and in superpositional order (oldest on bottom, youngest on top); however, fault movements cut rocks into discontinuous masses, and in places, fault deformation has tilted rock layers and even overturned the superpositional sequence. Geologists measure the 3-D orientation of rock layers via dip (angle and direction of inclination) and strike (compass bearing of rock cutting a horizontal plane).
Dip-slip fault types have dominantly vertical movements. Normal faults are due to extensional (pull-apart) forces. Reverse faults are due to compressional (push-together) forces. Strike-slip fault types have dominantly horizontal offsets. Straddling the fault, if the right-hand side moves toward you, it is a left-lateral fault. Bends (steps) in strike-slip faults cause the land to either uplift or down-drop. Another type of fault connects offset spreading-center segments-transform faults.
Earthquakes (seisms) disperse their energy in seismic waves that radiate away from the hypocenter or point of fault rupture. The point on the surface above the fault rupture is the epicenter. Some seismic waves pass through the body of the Earth-the P waves (Primary waves with a push-pull motion) and S waves (secondary waves with a shearing motion). Other seismic waves travel along the surface (Love and Rayleigh waves).
Earthquake energy is assessed by its magnitude. Different estimates of magnitude are derived from different methods based on local shaking (Richter scale), body waves (Mb),
Surface waves (Ms), or seismic moment (Mw). Earth has more than a million earthquakes each year, but more than 90 percent of the energy is released by the 12 to 18 largest events.
Tsunami are the biggest waves of all. Earthquakes, volcanic eruptions, landslides, and asteroid impacts disturb the deep ocean-water mass, sending off energetic waves. Earthquake- generated tsunami commonly travel almost 500 mph and may be spaced as much as 60 minutes apart. Tsunami slow in shallow water but may still be moving at freeway speeds. Local topography, as in harbors and inlets, may focus tsunami energy, creating waves over 100ft high that kill tens of thousands of people.
Seismic waves have different periods (time between cycles) and frequencies (number of cycles per unit of time): period= 1/frequency. P waves commonly have from one to 20 cycles per second; surface waves commonly have one cycle every 1to 20 seconds. Where the frequencies of seismic waves match the vibration frequencies of foundations and buildings, destruction may be great.
Earthquake effects on structures and people are assessed via the Mercalli Intensity Scale. Its variables are earthquake magnitude, distance from the hypocenter/epicenter, type of rock or sediment foundation, building style, and duration of shaking. Mercalli intensities are of more than just scientific interest because earthquakes don’t kill, buildings do.
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