Transcript Earthquakes
Earthquakes Earthquakes Friction between moving plates works against convection currents, producing stress When the plates can no longer resist the stress there is an earthquake – a massive release of energy that shakes the crust Divergent Plate Boundary Animation Earthquakes It is very difficult to predict the timing, exact location, and strength of earthquakes By understanding earthquakes we can design warning systems and building structures to be better prepared when they do occur All three types of boundaries are found in BC At the convergent boundary where the Juan de Fuca Plate is wedged against the North American Plate, energy is built up over centuries and can cause earthquakes when this energy is released Describing Earthquakes The FOCUS is the location INSIDE the Earth where an earthquake starts Energy release begins at the focus The EPICENTER is the point on Earth’s surface directly above the focus Earthquakes occur at various depths, depending on the type of plate interaction • Earthquakes at the surface tend to cause more damage • Over 90% of earthquakes have foci that are less than 100 km deep Seismic Waves Energy released by an earthquake produces vibrations called SEISMIC WAVES Body waves travel underground Surface waves travel on the Earth’s surface SEISMOLOGY is the study of these waves. These waves reveal the source and strength of an earthquake. Primary Waves (P waves) Type of body wave Fastest seismic waves First to be sensed Ground stretches and squeezes in direction of wave travel Travels through solids, liquids and gases Animation Secondary Wave (S-waves) Type of body wave Slower seismic wave Second to arrive Ground motion is perpendicular to direction of wave travel Travels through solids, but not liquids Usually causes more structural damage than P-waves because Swaves are larger Animation Surface Waves (L – waves) Travels along the Earth’s surface (surface wave) Slowest seismic wave Last to arrive (last to be felt) Like ripples in a pond Body waves change in the Earth’s layers They speed up or slow down or are refracted (bent), depending on what layer they are travelling through This helps scientists determine Earth’s composition Measuring Earthquakes Seismometers (aka seismographs) are devices that measure the amount of ground motion caused by an earthquake A seismometer produces a record of ground motion called a seismogram These provide info such as the time of the earthquake, how long it lasted, and the amount of ground shaking Magnitude = a number that rates the strength (energy) of an earthquake With each 1-step increase on the magnitude scale, the size of the seismic waves is 10x larger