Lecture on Earthquakes result from Seismic Waves www.AssignmentPoint.com

Earthquakes result from seismic waves generated from motion along a fault, such as the San Andreas shown in the middle of the image.

Earthquakes are the most destructive natural forces on Earth Earthquakes are the most destructive natural forces on Earth. The Alaskan earthquake of 1964 was one of the three most powerful seismic events in the past century. The image was taken in down town Anchorage.

Earthquakes occur along active faults where energy is stored and suddenly released when the stresses along the fault exceed the frictional forces resisting motion. The longer the time interval (seismic gap) between seismic events the greater the magnitude of the earthquake.

During an earthquake event seismic (energy) waves propagate concentrically from the initiation point of fault motion. Body waves (P- and S- waves) travel through the earth and are subject to refraction as they propagate through different compositonal zones of the earth. Remember the difference in properties of P- versus S- waves. Surface waves are created from residual energy as the body waves reach and interact at the surface of the earth. Surface waves are restricted to the surface of the earth and can cause significant damage to structures during an earthquake event.

Seismic wave velocities according to the density of the material in which they propagate. P-waves travel the fastest (~6-7 km/ second) within the lithosphere. S-waves travel ~3 km/second within the lithosphere. Surface waves travel with the slowest velocity. If you know the velocity of seismeic waves you can determine the focus or epicenter of the earthquake using the difference in arrival time between P- and S-waves at a given seismic station. Think about two trains leaving Olympia at 2:00 pm and travels 60 miles to Seattle. One train travels at 30 mph and the other at 60 mph. How long will each take to travel to Seattle? What about if they travel 120 miles? Note how the difference in arrival time increases the greater the distance the two trains travel.

If you measure the difference in arrival times of P- versus S-waves at three different seismic stations you can determine the focus or epicenter of an earthquake using the intersection point of the three arcs.

Earthquake magnitude is dependent upon the amplitude and frequency of seismic waves. Because the amplitude and frequency of seismic waves are inversely related, the difference in energy release between each unit of magnitude scale is 30 times.

Seismic hazards are found in many different regions of the U. S Seismic hazards are found in many different regions of the U.S. Note that some hazards occur in the middle of tectonic plates on old re-activated faults. The Puget Lowland is an area with high seismic risk.

Hanshin Expressway Kobe Japan

Tsunamis may be generated by vertical offset of the ocean floor along subduction zone earthquakes or by landslides into the water column.

There are three settings where earthquakes occur in Washington state.

Cultus Bay Scatchet Head Sandy Hook As the floor of the Puget Sound was vertically offset, a tsunami was generated that inundated seveal of the south-facing embayments on Whidbey Island. A sand sheet (light gray near the top of the section) is preserved in Cultus Bay sediment record. Radiocarbon dating indicates that deposition of the sand sheet is coincident with uplift of the Seattle Fault 1100 years ago.

Geologist are now aware that there can be major subduction zone earthquakes along our coastline that are capable of generating magnitude 8-9 seismic events. Offset along the Cascadia subduction zone can cause major coastal subsidence and tsunami events along with the expected ground shaking.

In between major seismic events the North American and Juan de Fuca plates are “stuck” together and storing energy. Accurate GPS data show that the coastline of the Pacific Northwest is rising because of the compressive forces as the energy is stored along the fault zone. During a subduction zone earthquake event the plates become “unglued” and motion may occur along large section of the subduction zone (stretching from British Columbia to California).

The above schematic cartoon (Brian Atwater, USGS) provides an overview of the sequence of events that led to the formation of the “Ghost Forest.”

Following the large earthquake event and coeval coastal subsidence, there were low lying victims, who died of salinity poisoning, and as well as survivors who witnessed the event.

By cross-dating the tree-ring patterns of the victim and witnesses, the timing of the earthquake can be determined to within one year.

Based on cross-dating of tree-ring width patterns it was determined that the most recent offset of the Cascadia subduction zone occurred in 1700 AD.

Yes, tsunami deposits and coastal subsidence events indicate that there was a major (magnitude 8-9) subduction zone earthquake along the Cascadia subduction zone on January

Sediment records exposed along the Washington coastline indicate that there were 7 major earthquake events along the Cascadia subduction zone over the past 3500 years. What is the recurrence interval of earthquakes along the Cascadia subduction zone? Is the timing of earthquake events predictable within years or decades?