Chapter 8 Notes
Earthquake are common on this planet. Thousands of small ones occur every day. There are more than a million detectable earthquakes every year.
Earth Shaking is caused by a rapid release of stored stress energy that builds up due to plate motions and interactions, most of which are due to tectonic forces.
Most quakes are small, large quakes destroy buildings and kill people. There have been 3.5 million deaths in the last 2000 years, there are several hundred large quakes every year. Note: The earthquakes do not kill people, collapsing buildings and tsunamis do.
Most earthquake damage is due to ground shaking. The ground shaking can last 10’s of seconds and is what collapses buildings.
Quakes also spawn devastating tsunamis.
-Indian Ocean 230,000 dead and Eastern Coast of Japan 20,000 dead
Faults: planar breaks in the crust. Most faults are sloping. Vertical faults are rare. The type of fault depends on the relative motion of blocks. (Normal, thrust, reverse, strike slip)
Normal Fault: the hanging wall moves down relative to the footwall. It most often results from extension like in a continental rift or passive margin.
Thrust fault: a special kind of a reverse fault with a lower angle. 30 degrees instead of 60 degrees.
Reverse fault: The hanging wall moves up relative to the footwall. It usually results from compression. The slope of a reverse fault is steep. Occurs in mountain belts.
Strike slip fault: along a strike slip fault, one block slides laterally across the other block. Tend to be vertical.
The displacement is the amount of movement across a fault. Offset fences, roads, streams. Quakes greater than a magnitude 6 can have meters of displacement.
Faults are found in many places in the crust and include both active and inactive faults.
A fault trace is the place that a fault…
Faults form when tectonic forces add stress to rock. As stress is added to rock it bends slightly without breaking. Continental stress causes small cracks to develop and grow, eventually progressing to the point of failure. Stored elastic energy is released all at once, creating an earthquake.
Hypocenter vs. Epicenter
The hypocenter or focus of the quake is the location where the fault slip occurs underground on a fault surface.
The epicenter is the land surface directly abive the hypocenter. Maps portray the location of epicenters.
Seismic body waves
P waves
-Travel by compressing and expanding the material parallel to the wave travel direction. P waves are the fastest seismic waves and they travel through solid, liquids, and gases. Wave propagation is direction and vibration direction are parallel.
S waves
-Travel by moving material back and forth, perpendicular to the wave travel direction. S waves are slower than p waves. S waves only travel through solids, not liquids or gases. Propagation direction is perpendicular. Shear and secondary.
L waves
-Surface waves travel along the earths exterior. Surface waves are the slowest traveling waves and the most destructive. Like s waves these propagate perpendicularly.
R waves
- A combination of p and s waves that travel along the land surface. They cause the ground to ripple up and down like water and are high amplitude that cause intense shaking.
Locating Earthquakes
****P waves and s wave arrival time can be graphed. A travel time curve plots the increasing delay in arrivals. The time gap between the s and p wave arrival times yields distance to the epicenter.
When at least 3 stations record the earthquake, drawn circles can locate the quake epicenter.*****
Earthquake size is described in two ways.
Intensity: Proxied by the severity of damage observed in the field.
Magnitude: The amount of ground motion that is measured on a seismograph.
Intensity and magnitude are related in an approximate way, even though they are very different measurements.
The modified Mercalli intensity scale is a subjective determination that assigns Roman numerals to differing degrees of damage.
Damage intensity decreases with distance from the epicenter….
Calculating the richter magnitude
- Measure S-P time from seismogram
- Measure Peak Wave amplitude from seismogram
- Draw line and read off the quake magnitude
Quake locations and depths are linked to plate tectonic boundaries.
Shallow is less than 60km intermediate is 60-300km, deep is 300-660km depth. Shallow quakes occur at divergent plate boundaries. Intermediate and deep quakes occur at convergent boundaries. Shallow are most common, then intermediate, and deep are the most rare. Become deeper as you go West.
Divergent plate boundary: Mid Ocean Ridges
Two types of faulting typify mid-ocean ridges; normal faults at the spreading ridge axis and strike slip faults along the transforms.
MOR Earthquakes are shallow and relatively small.
Convergent plate boundaries are the cause for bigger earthquakes. Mega thrust quake would be greater than a magnitude 8. Megathrust quakes are related to tsunamis and can be quite deadly. COnvergent plate boundaries can have both shallow, intermediate, and deep earthquakes.