Types and Causes of Induced Seismicity - Induced Seismicity Potential in Energy Technologies - page 17

criterion. Lacking these estimates, predicting how close or how far the fault system is from
instability remains difficult, even if the orientation of the fault is known. This implies that
the magnitude of the increase in pore pressure that will cause a known fault to slip cannot
generally be calculated. Nonetheless, understanding how different factors contribute to
slip initiation is valuable because it provides insight about whether fluid injection or with-
drawal may be a stabilizing or a destabilizing factor for a fault (in other words, whether
fluid injection or withdrawal causes the difference between the driving shear stress and the
shear strength to increase or decrease). Any perturbation in the stress or pore pressure that
is associated with an increase of the shear stress magnitude and/or a decrease of the normal
stress and/or an increase of the pore pressure could be destabilizing; such a perturbation
brings the system closer to critical conditions for failure. A large body of evidence suggests
that the state of stress and pore pressure are often not far from the critical conditions where
a small destabilizing perturbation of the stress and/or of the pore pressure could cause a
critically oriented fault to slip (Zoback and Zoback, 1980, 1989).

Magnitude of a Seismic Event

The moment magnitude scale, designated M , is directly related to the amount of crustal
energy released during a seismic event (Hanks and Kanamori, 1979). This energy can be
thought of as the total force released during the earthquake times the average fault displace-
ment over the fault rupture area (see also the section Earthquakes and Their Measurement
in Chapter 1).
Earthquake magnitude is correlated to the area of the rupture surface. Earthquakes with
large magnitudes always involve large parts of the Earth's crust, because the large ener-
gies being released can only be stored in large volumes of rock, and large rupture areas are
necessary to produce large fault displacements. Correlations between M and rupture area
from observations of historical earthquakes indicate that an increase of 1 magnitude unit
implies, on average, an increase by a factor of about 8 in fault rupture area, and a concurrent
increase by a factor of about 4½ in rupture displacement (Wells and Coppersmith, 1994).
The following examples are typical fault rupture areas and rupture displacements associated
with earthquakes of M 4 and M 5:

M 4 M 5

Fault rupture area: 1.4 km 2 (~0.5 mi 2 ) 11 km 2 (~4.2 mi 2 )

Fault displacement: 1 cm (~0.4 in) 4.5 cm (~1.8 in)

A larger-magnitude earthquake implies both a larger area over which crustal stress is
released and a larger displacement on the fault. From the definition of M , we can expect
that a 1-unit increase in magnitude will be associated with a factor of about 32 larger release