Earthquakes - Geotechnical Engineering Investigation - page 898

11.1.3 Objectives and Scope

Objectives
The objectives of this chapter are to summarize and interrelate all of the aspects of earth-
quakes including their causes, characteristics, and surface effects, to provide a basis for
recognizing the in hazard potential, for investigating quakes comprehensively, and for
minimizing their consequences.
There is no field in geotechnical engineering in which the state of the art is changing
more rapidly, and it is expected that some of the concepts and methodology presented
may quickly become obsolete.

Scope
The earthquake phenomenon is described in terms of its geographic distribution, its
location as determined by focus and epicenter, its force as measured by intensity and
magnitude, attenuation of the force with distance, and its causes and predictability.
In addition, ground and structural response to its forces, including effects on the geo-
logical environment such as faults and crustal warping, liquefaction and subsidence,
slope failures, tsunamis, and seiches are also considered. Structural response is treated
only briefly in this text as a background to the understanding of those elements of earth-
quake forces that require determination for the analysis and design of structures during
investigation.

11.2 Earthquake Elements

11.2.1 The Source

Tectonic Earthquakes
General
Tectonic earthquakes are those associated with the natural overstress existing in the crust
as described in Appendix A. This overstress is evidenced by crustal warping, faults, and
residual stresses in rock masses as well as earthquakes. It is generally accepted that large
earthquakes are caused by a rupture in or near the Earth's crust that is usually associated
with a fault or series of faults, but primarily along one dominant fault termed the causative
fault (see Section 6.5).
Most earthquakes result from motion occurring along adjacent plates comprising the
Earth's crust or lithosphere, such as the Pacific Plate “subducting” ( Figure 11.2) beneath
the North American Plate. The plates are driven by the convection motion of the material
within the Earth's mantle, which in turn is driven by heat generated within the Earth's
core. Motion along adjacent plates is constricted by friction, which causes strain energy to
accumulate (Scawthorn, 2003).
Elastic rebound theory is described by Richter (1958): “The energy source for tectonic
earthquakes is potential energy stored in the crustal rocks during a long growth of strain.
When the accompanying elastic stress accumulates beyond the competency of the rocks,
there is fracture; the distorted blocks then snap back toward equilibrium, and this pro-
duces an earthquake.” Earthquakes at very shallow focus may be explained by the elastic
rebound theory, but the theory does not explain deep-focus events.