Geology, GSG 1030
Earthquake Analysis
Earthquakes are events of sudden ground-shaking, which in many cases can be very devastating.  Earthquakes happen for a number of reasons (see below).  In this exercise, you will learn how to analyze the record of an earthquake.  The exercise refers to earthquakes in and around the Caribbean basin, an area that experiences many earthquakes every year.  The map below shows the locations of earthquakes (epicenters) in the Caribbean region.  Note how many noteworthy earthquakes (Magnitude > 3.5) occured in just the twenty-two years from 1967 to 1999.  Note also the relationship between the epicenters and plate boundaries.  For more details, visit the seismic hazard website for the Caribbean Disaster Mitigation Project (CDMP).
***Haiti Earthquake, 12 January 2010, LINKS: [1], [2], [3]***

CAUSES of EARTHQUAKES

 ACTIVE FAULTS (You may want to review the different types of faults in your text):
Movement on Normal Faults is accompanied by vertical movement where two lithosphere plates are being pulled apart (tension).  The movement is normally (pardon the pun) downward.  This is the characteristic type of fault at a divergent plate boundary. Divergent plate boundaries are marked by oceanic ridges.

Movement on Reverse Faults are accompanied by vertical movement where two lithospheric plates are being pushed together (compression).  The movement is usually upward.  This is the characteristic type of fault at a convergent plate boundary.  You will recall that at a convergent plate boundary one lithospheric plate is being shoved beneath the edge of another plate.  The process is called subduction. Convergent plate boundaries involving oceanic lithosphere are marked by a deep, oceanic trenches, which very nearly mark the actual position of the plate boundary on the sea floor.

Movement of Strike-Slip Faults involves no vertical movement.  The movement is horizontal.  This is the characteristic type of fault at a transform plate boundary.


VOLCANIC ERUPTIONS.  In ocean basins there are three situations of concern:

A Volcanic Island Arc is formed above the subduction zone adjacent to a convergent plate boundary, hence is parallel to the oceanic trench but offset from the trench by 300 km or more.  The magma is typically andesitic to rhyolitic.  Volcanic eruptions can be catastrophically explosive, producing earthquakes.

Hot Spot volcanism in the oceanic basins involves basaltic magma.  The locations of hot spots, which are formed above mantle plumes, are quite independent of plate boundaries.  An example is the island of Hawaii.  The newest eruptive center near Hawaii is the submarine volcano of Loihi, just off the south coast of Hawaii.  There are no such hots spots in the Caribbean basin.

Basaltic volcanism occurs along divergent plate boundaries, where new oceanic crust is formed at oceanic ridges.


METEOR IMPACTS:

Catastrophic meteor impacts in the past are now well documented.  One of the largest and best documented happened 65 million years ago and marked the close of the Cretaceous Era.  The site of the impact was the Yucatan Peninsula in Mexico.
THE EXERCISE
The exercise is organized into three parts.  In the first part you will learn about the properties of earthquakes and seismic waves.  In the second and third parts of the exercise, you will learn how to locate an epicenter and determine the earthquake's magnitude.

PART 1.  Properties of earthquakes and seismic waves. For printing a copy of the ANSWER SHEET.

PART 2.  Analysis of an Earthquake: Step by Step.  For printing a copy of the ANSWER SHEET.  NOTE: You will need a compass (for drawing circles).

PART 3.  Analysis of another Earthquake: You're on your own.  For printing a copy of the ANSWER SHEET.  NOTE: You will need a compass (for drawing circles).


Web site designed by R. N. Abbott, Jr., Department of Geology, Appalachian State University, Boone, NC 28608. [e-mail]

(2004)