Environmental Engineering Reference
In-Depth Information
fractures. Fault identification is an important element in studies that aim to evaluate the
probability of earthquake occurrence and magnitude.
Modern earthquake engineering places a lot of emphasis on evaluations of fault criteria,
particularly to arrive at estimated values of magnitude and ground motion. There is much
to learn, however, about faults. Some faults considered as dead or inactive have been the
location of quakes, such as the White Wolf Fault in California and the Ramapo Fault in
New Jersey. Some faults are buried and undetected such as the fault beneath the
Northridge event.
The North Antolian Fault in Turkey has been the subject of a number of studies because
of its unusual history (Okumura et al., 1993). Similar to the San Andreas in length, its strike-
slip form, and its long-term movement rates, the fault has been the location of eight events
of
M
6.8 to 7.8 between 1939 and 1999 along a 900 km length
(Figure 11.25).
Of particular
interest is the fact that rather than being located in a few areas, the earthquakes have been
migrating westward in sequence from Erzincan (1939,
M
7.4). The
Izmit event is reported to have resulted in at least 13,000 deaths and estimates of up to 35,000
missing (Associated Press release). Radiocarbon dating in deep trenches dug at several loca-
tions by a joint Japan-Turkey research team recognized eight events dating from about 30
B
.
C
.
with a recurrence interval estimated between 200 and 300 years (Okumura et al., 1993).
Correlations have been made from earthquake data in some geographic regions (princi-
pally in the United States) to develop a number of relationships:
7.8) to Izmit (1999,
M
Length of fault rupture vs. earthquake magnitude
●
Distance from the causative fault vs. the acceleration due to gravity
●
Fault displacement vs. magnitude
●
Fault Study Elements
During engineering studies for seismic design, the following aspects related to faulting are
considered:
Positive identification that a fault (or faults) is present
●
Fault activity: establish the “capable fault” by judging if it is potentially active or
inactive
●
Displacement amount and form (dip-slip, strike-slip, etc.) that might be expected
●
Earthquake magnitude that might be generated by rupture (related generally to
length)
●
Estimated site acceleration after attenuation from the capable fault
●
Fault Activity (The Capable Fault)
Significance
In recognition that shallow-focus events are associated with faulting, but that many
ancient faults are not under stress and therefore are “dead” or inactive and not likely to be
the source of a shock, it becomes necessary, in order to predict a possible earthquake, to
identify a fault as active, potentially active, or inactive (dead). Seismic design criteria are
often based on the identification of active or potentially active faults (capable faults) and
their characteristics.
The Capable Fault
U.S. Nuclear Regulatory Commission (NRC, 2003) defines to the
Capable Tectonic Source
as
a tectonic structure that can generate both vibratory ground motion and tectonic surface
