Geology Reference
In-Depth Information
chronology of earthquakes (Fig. 6.14), and their
regular spacing and apparently steady uplift rate
suggests a rather regular (characteristic?) rupture
behavior on the underlying fault.
A
Chilean Beach Berms
Stratigraphic evidence
A key question in many paleoseismic studies of
subduction zones concerns the lateral extent of
previous ruptures. The largest subduction-zone
quakes have rupture lengths of 1000 km or
more. The annual detail of some coral records
in tropical settings permits earthquake-by-
earthquake correlation of multiple sites along a
subduction zone, thereby enabling reconstruc-
tion of rupture lengths and magnitudes (Sieh
et al.
, 2008). But, even this record only goes
back a few hundred years.
In efforts to reconstruct the magnitude and
lateral extent of subduction-zone earthquakes
over Holocene time scales, the stratigraphic
record has been used with considerable success
outside of the tropics. As shown so clearly by the
coral records (Fig. 6.12), a predictable sequence
of submergence and emergence characterizes
the seismic cycle. Whether interseismic emer-
gence or submergence prevails at a given site
depends on its position with respect to the
locked patch of the subduction zone. Sites farther
from the trench (or closer to the down-dip limit of
the locked patch) tend to slowly rise and then
abruptly sink during interseismic and seismic
intervals, respectively. At sensitive coastal sites,
we might expect that these ups and downs with
respect to sea level would be reflected in the
stratigraphy.
Indeed, this sort of record has been discovered
along the Cascadia subduction zone of Washington
and Oregon, where the record is being used to
Santa
María Is.
0
0.5 1.0
kilometers
1.5
B
Berm Surveys
7
9
10
7
1
3
berm number
survey point
6
5
11
12
13
4
14
3
0.5
1.0
1.5
2.0
2.5
distance along beach berm (km)
C
time (ka)
0
1
2
3
mean uplift
rate:
2.3±0.2 m/ky
2.6±0.2 m/yr
2.1±0.2 m/yr
2.1±0.2 m/yr
7
6
5
2.4±0.2 ka
2.4±0.2 ka
2.3±0.2 ka
1.8±0.2 ka
4
2.1±0.3 m/yr
3
2
1
Uplift Rates
0.2
0.6
1.0
1.4
1.8
distance from active beach (km)
Fig. 6.14
Raised Chilean berms recording 20 late
Holocene earthquakes.
A. At least 20 late Holocene beach berms are preserved
on Santa María Island along Chile's west coast. These
rather low ridges (up to 1 m high) are protected in a
northeast-facing embayment lying within a few meters of
modern sea level (see inset). White lines depict berm
crests. Stars show the locations of OSL dates ranging to
2.4 ka. B. Surveys along berm crests reveal both their
continuity and their subtle tilt toward the northwest that
is interpreted to result from differential uplift above a
westward-dipping thrust fault. C. OSL ages indicate the
middle suite of berms range from about 1.8 to 2.4 ka.
Given the number of berms, a recurrence interval of
∼
180
years with
∼
0.5 m of uplift per earthquake can be defined.
In the context of the OSL ages, berm crest heights above
sea level define an average uplift rate of 2.3
±
0.2 m/kyr.
