Earthquakes are a natural phenomenon which carry with them a great human cost: according to National Geographic, there are approximately 10,000 fatalities a year worldwide that are directly attributable to earthquakes; furthermore, these quakes can also spur on other natural disasters like mudslides and tsunamis (National Geographic, 1). Because of this, seismology, or the study of earthquakes, is important in human terms. This paper will seek to discuss the seismological concepts of “faults”, “foci” and “epicenters” and also discuss the safety issues surrounding faults which have no active creep.
FAULTS, FOCI, AND EPICENTERS
In the study of earth science, the terms “faults”, “foci”, and “epicenters” are inter-related terms in regards to seismology, or the study of earthquakes. The United States Geological Survey Glossary defines a fault as a fracture in the Earth’s crust, the slippage and movements of which cause earthquakes; a focus – or hypnocenter – is the point beneath the earth where the earthquake originates, and the epicenter is the point on the earth’s surface located immediately above the focus (United States Geological Survey, 1).
THE SAFETY OF FAULT LINES WITH NO ACTIVE CREEP
A creep is geology is slow, and more or less continuous movement along a fault (Southern California Earthquake Center, 4), but the question of whether or not a fault is safe if it has no active creep is somewhat more complicated as an issue. On one hand, if a fault is free of creep because the stresses placed on the blocks of the earth’s crust around it are not enough to move it, the fault is considered inactive. On the other hand, a fault may be considered active but locked if the lack of activity comes from the fact that the blocks are unable to respond to tectonic stress: the elastic strain can accumulate, making it more likely to be a site of future seismic activity (Geology Classroom, 1). Another term for a locked fault is a “seismic gap”; the USGS defines this as a section of a fault which has produced no earthquake activity in the past, but which in the future has the capacity to produce earthquakes (United States Geological Survey, 1). In short, just because there is no active creep along the fault line, that does not mean that is safe or that is will not produce seismic activity at some point in the future.
Understanding the difference in seismological terms such as “faults”, “foci” and “epicenter” are important in a serious study of seismology. A knowledge of the relative safety (or lack of it) of fault lines, even those without an active creep, is also good knowledge to have, as it has wider implications for patterns of human settlement, the best construction of buildings along a fault, and education programs and warning systems to minimize loss of life during these natural disasters.
Geology Classroom. “Review Questions and Answers”. Retrieved from : www.geoclassroom.com/101online
National Geographic Magazine. “Earthquake Profile”. Retrieved from http://environment.nationalgeographic.com/environment/natural-disasters/earthquake-profile
Southern California Earthquake Center. “Wallace Creek Interpretive Trail: A Geologic Guide to the San Andreas Fault along Wallace Creek” Retrieved from www.scec.org/wallacecreek
United States Geological Survey. “Earthquake Glossary”. Retrieved from http://earthquake.usgs.gov/learn/glossary