Earthquake – A sudden release of stored energy. This energy has built up over long periods of time as a result of tectonic forces within the earth. Most earthquakes take place along faults in the upper 25 miles of the earth's surface when one side rapidly moves relative to the other side of the fault.

Seismic waves – shock waves generated by the release of built up energy Focus – Point seismic waves to radiate from. It is these seismic waves that can produce ground motion which people call an earthquake. Epicenter – Point on the earth surface directly above the focus.

Chang Heng’s Seismoscope

Seismometer - instruments that measure and record motions of the ground, including those of seismic waves generated by earthquakes

Types of waves generated: Body waves - W aves travel through the interior of the Earth ( P and S) Surface waves – Waves that travel just under the Earth's surface (Rayleigh and Love waves)

Body waves – P wave P waves - Longitudinal/ compressional waves which means that the ground is alternately compressed and dilated in the direction of propagation. In solids, these waves generally travel almost twice as fast as S waves, but are less destructive than the S waves and surface waves that follow them, due to their smaller amplitudes. Granite speed is ~5000 m/s.

Body waves – P wave P waves - Travels down into the earth rather than along the surface. P waves - primary,pressure, push-and-pull.

S waves -Transverse/shear waves, the ground is displaced perpendicularly to the direction of propogration. S waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses. Their speed is about 60% of that of P waves in a given material and are several times larger in amplitude than P waves for earthquake sources. Body waves – S wave

Surface waves– Rayleigh Rayleigh waves –aka ground roll; surface waves that travel as ripples with motions that are similar to those of waves on the surface of water. They are slower than body waves, roughly 90% of the velocity of S waves for typical homogeneous elastic media.

Surface waves– Love Love waves – waves that travel on the Earth's surface, the strength (or amplitude) of these waves decreases exponentially with the depth of an earthquake.

The difference in the arrival times of the P and S waves can be used to determine the distance to the event. You will need at least 3 seismographs.

Measuring Earthquakes

Richter’s Scale The Richter magnitude was originally calculated with this equation: M L = log 10 A − log 10 A 0 (delta), BUT, basically, the Richter magnitude measures an earthquake on a base-10 logarithmic scale. So a 9.0 earthquake is 10x larger than an 8.0 earthquake. Also known as the local magnitude scale.

Moment magnitude Seismologists now use the moment magnitude scale. Helps compare the energy released in earthquakes. MMS = rigidity of the Earth multiplied by the average amount of slip on the fault and the size of the area that slipped

Mercalli Intensity Scale Quantifies the effects of an earthquake on the Earth's surface, humans, objects of nature, and man-made structures on a scale of I through XII.

Differences in scales The MMS does not saturate at the upper end like the Richter scale; there is no upper limit to the possible measurable magnitudes. However, the Richter does not cluster low-energy earthquakes together. The Mercalli only measures the amount of destruction. Not quantitative and matters where you are locally.

Tsunamis How they are generated: Caused at subduction zones (convergent) boundaries, characterized by small wave height, long wave length (almost undetectable), problem is when it begins to shoal Monitored by seismologists using sensors on the equipment constantly monitor the pressure of the overlying water column. They use: P=ρgh Yet they are NOT: … greater strength, longer duration, not from the same source

Sumatra 2004 Earthquake Occurred in the Sunda Trench– where the Indo-Austrailian plate is subducting the Sunda plate MMS Longest duration of faulting ever observed, between minutes. Created vibrations that caused earthquakes in Alaska.