Moving plates exert stress (force/area) on the rocks that make up the
lithosphere. Several factors control whether on not this lithosphere
breaks, producing an earthquake.
- Strength of rock.
The strength of any solid material is the stress it can withstand
without deforming or breaking. A rock is only as strong as its weakest
- Water within rock
Pores and cracks in many rocks are filled with fluids like groundwater,
which aids slippage. Fluid weakens a rock, so a rock with lots of
fluid (especially fluid under pressure) breaks at lower stresses than
a rock with less fluid. In the 1960s: Rocky Mountain Arsenal, Rangely
Oil Field (Colorado)
- Temperature. A
cold rock shatters or breaks; a warm rock bends or flows (Big Hunk
candy bars). Temperature increases downwards in the earth. The transition
from breaking to flowing (earthquake cut-off depth) is about 15-20
km in western California.
When rocks break to produce an earthquake, they move along a fault
Geologists recognize three main types of faults: reverse (or thrust),
normal, and strike-slip.
Your book (p. 45-47) attributes each fault type to a particular stress,
as shown below:
geologists attribute each fault type to specific orientations of tensional
stress and compressional stress (show in figure at left; click to zoom
Each type of faulting is characteristic of, but not limited to, one
type of plate boundary. All three types are present in California, despite
the dominance of right-lateral strike-slip faults.
use scissors and a hand-out sheet (right, click to zoom in) to
demonstrate why California (and specifically, the Bay Area) has normal
and reverse/thrust faults.
Geologists can identify evidence that faulting, and thus earthquakes,
have occurred in the past. Most common evidence: Abrupt termination
of rock layers that originally were continuous.
What is the definition of active, anyway? In California: an active
fault is one that has ruptured one or more times in last 11,000
years, or two or more times in the last 700,000 years. [CDMG maps
of faults in the Bay Area - ordering
Some methods geologists use to identify faults (some don't always indicate
- Trenching and 14C ("carbon-14") dating are
used to date layers & movements.
- Rocks along fault may be ground up (brecciated), polished,
- Ground-up rocks are easier to erode, so linear gullies or valleys
- Fault scarps (escarpments), which will be eroded away if
fault is no longer active.
- Offset surface features such as streams, roads, etc.
Whatever the means, a rupture starts at the focus or hypocenter
(usually below the surface), then progresses outward within the fault
The epicenter is the point on the surface vertically above the
If we wanted to describe to someone else the exact location of an epicenter,
what coordinates would we need to specify? How about for the focus?
CDMG Map Ordering Information:
San Francisco-San Josť quadrangle geologic and fault maps (order
RGM005) $17 from
Department of Conservation, Division of Mines and Geology
P.O. Box 2980, Sacramento, CA 95812-2980
check or credit card; see http://www.consrv.ca.gov/dmg/pubs/index.htm