Geology Reference
In-Depth Information
Numerous unresolved questions motivate
geodetic studies. The fundamental question of
whether deformation rates that can be meas-
ured at decadal (geodetic) time scales are also
persistent through time underpins many studies
(Fig. 5.1). If rates were steady, this persistence
would provide a potent basis for predictive
deformation models based on presently meas-
ured rates. Plate-motion rates based on sea-
floor magnetic anomalies commonly indicate
steady rates across spans of several million
years (DeMets
et al.
, 1990, 1994). But, are they
truly steady, and, if not, how much do they vary
and at what time scales? When H. F. Reid (1910)
propounded the
elastic rebound theory
, he
postulated a gradient in crustal deformation
across an active fault (Fig. 4.2). Then, as now,
the steadiness of strain accumulation during
the interseismic interval remains uncertain. Do
rates accelerate or decelerate at predictable
intervals within the seismic cycle? If so, could
these changes be used to predict when an
earthquake was going to occur? Many
paleoseismic studies use observed displacements
of stratigraphic or structural markers across
faults to calculate coseismic slip. But, did all
that slip accumulate during earthquakes, or
were there two phases of slip: coseismic slip
during the earthquake followed by afterslip in
the weeks or months following the quake? If
afterslip is important, how much of the total
observed slip does it represent? Reconstructions
of fault slip for many earthquakes suggest that
coseismic slip at some depth in the crust is
greater than at the surface (Delouis
et al
., 2002;
Simons
et al
., 2002). If so, how does the surface
catch up to the subsurface displacement? Does
afterslip or creep occur on the shallow fraction
of a fault or is strain distributed or absorbed in
off-fault strain?
For phenomena such as afterslip that occur at
time scales of months to decades, geodetic
studies of similar or longer duration can poten-
tially encompass the entire process ( Johanson
et al
., 2006) and, hence, provide durable
insights on short-term crustal deformation. For
problems spanning geological time scales that
extend well beyond those of geodetic observa-
tion (Fig. 5.1), geodesy can never provide a
10
A
rate change
5
Plate-Motion
Rates
0
5
10
15
Time (Ma)
100
B
intercluster
rate
clustered-slip
rate
50
long-term
slip rate
Fault-Slip
Rates
0
0
50
100
150
Time (ka)
C
100
pre-seismic
rate
interseismic
rate
post-
seismic
slip
50
Co- and Inter-
Seismic Rates
0
0
500
1000
1500
Time (yr)
Fig. 5.1
Conceptual models of temporal variations
in displacement rates at different time scales.
Panels illustrate potential limitations to the extrapolation
to longer time scales of geodetic rates measured at
decadal time scales. A. To the extent that plate-motion
rates are steady for millions of years, regionally
measured geodetic rates may be reliably extrapolated.
B. If earthquakes occur in clusters, geodetic
measurements are likely to yield rates that are either
faster or slower than the long-term slip rate. C. Geodetic
sites that are primarily affected by individual faults can
record a wide range of rates (including a reversal of
direction) depending on the interval during the seismic
cycle in which measurements are made. Modified after
Friedrich
et al.
(2003).
complete answer. But, geodetic studies yield a
critical current context of modern rates, as well
as revealing their steadiness or variability in
time and space.
