Download Travel Time Tomographic Imaging of Shallow Fore

Travel Time Tomographic Imaging of Shallow Fore-arc Basin Structure at the Cascadia
Subduction Zone Offshore Washington
INSTITUT DE PHYSIQUE
DU GLOBE DE PARIS
R. Azarm, H. Carton.
Institut de Physique du Globe de Paris (IPGP)
[email protected]
Below: Travel time misfit plots (the difference between the observed (picked) travel times and the predicted travel times), and histogram of the distribution of residuals before (left image)
and after (right image) inversion.
Introduction
The Cascadia subduction zone (CSZ) where the very young (late Miocene, ~10 Ma old) oceanic crust of Juan de
Fuca (JdF) plate is being subducted beneath the overlying North American continental plate, extends 1100 km
from the Mendocino Escarpment off northern California to northern Vancouver Island.
4500
3.5
3500
2.5
Misfit counts
3000
2500
2000
2
1.5
1500
1
1000
0.5
500
0
0
The probability of occurrence of a giant earthquake in the future not only
poses major seismic and societal hazards to populations of the Pacific
Northwest but would cause tsunami damages across the Pacific Ocean. It
is therefore of particular interest to gain insight into the structure and
asses the seismo-tectonic behaviour along this margin. The knowledge
about the geometry and properties of the plate boundary, the hydration
state of the descending oceanic plate and the amount of subducting
sediment are thus of fundamental importance. Focused geophysical
studies continue to advance our understanding of Cascadia margin.
0.1
0.2
0.3
0.4
0
300
600
900
1200
1500
1800
2100
2400
2700
0
300
600
900
1200
1500
1800
2100
2400
2700
Depth [km]
Depth [m]
Residuals [s]
0
4
8
12
16
20
24
28
32
36
40
44
48
52
56
60
Distance[km]
1.6
1.8
2
0
4
8
12
16
20
24
28
32
36
40
44
48
52
56
0.5
0.6
0.7
0
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
Residuals [s]
60
Distance [km]
2.2
2.4
2.6
Velocity [km/s]
2.8
3
3.2
3.4
1.4
Above: Laterally homogeneous starting velocity
model and tomographic inversion result (Final
model).
Depth [m]
1.4
Right: Final model
restricted to the ray
covered area.
0
300
600
900
1200
1500
1800
2100
2400
2700
0
1.6
4
8
1.8
12
2
16
2.2
2.4
2.6
Velocity [km/s]
20
24
28
32
36
2.8
40
44
3
48
3.2
52
56
3.4
60
Distance[km]
1.4
1.6
1.8
2
2.2
2.4
2.6
Velocity [km/s]
2.8
3
3.2
Above: The derivative weight sum (DWS) for the
best-fit model. The colour-bar represents the
number of rays per inversion cell. Note a good
coverage in central and shallow parts (reliable
regions for interpretation). Regions associated
with lighter colour (toward the edges and deeper
parts) are not well resolved.
3.4
Results
0
300
600
900
1200
1500
1800
2100
2400
2700
Depth [m]
Depth [m]
Below left: Resolvability of the final model is assessed using checkerboard pattern tests with different anomaly sizes. We then compare our tomographically-derived velocity models to
coincident seismic reflection images post-stack time migrated and converted to depth using our results (below right). On the Washington shelf, where the fore-arc basin is segmented into
three sub-basins, ray coverage mostly extends to ∼1.2–1.5 km below seafloor. Velocities in the shallowmost sediments show, at the large scale, a gradual decrease towards the shelf edge
(from 2.1 to 1.8 km/s). At depth, regions devoid of clear reflections such as an ∼5 km large anticline core are associated with lower velocities than that obtained within mildly deformed
sedimentary layers on either side (2.3 vs 2.7 km/s, measured at 1.2 km depth), suggesting the presence of localized fluid-rich regions within the basin.
Methods
0
4
8
12
16
20
24
28
32
36
40
44
48
52
56
60
0
300
600
900
1200
1500
1800
2100
2400
2700
0
4
8
12
16
20
24
0
300
600
900
1200
1500
1800
2100
2400
2700
0
4
8
12
16
20
24
28
32
36
40
44
48
52
56
60
0
300
600
900
1200
1500
1800
2100
2400
2700
0
4
8
12
16
20
24
0
4
8
12
16
20
24
28
32
36
40
44
48
52
56
60
0
300
600
900
1200
1500
1800
2100
2400
2700
0
4
8
12
16
20
Distance [km]
-0.1
-0.08
-0.06
-0.04
32
36
40
44
48
52
56
60
28
32
36
40
44
48
52
56
60
36
40
44
48
52
56
60
Distance [km]
Depth [m]
Distance [km]
0
300
600
900
1200
1500
1800
2100
2400
2700
28
Distance [km]
Depth [m]
Depth [m]
Distance [km]
Depth [m]
We conduct a P-wave tomography study of shallow
fore-arc basin structure at the Cascadia subduction
zone using first-arrival travel times from two multichannel seismic (MCS) profiles acquired with an 8-km
long streamer in the frame of the 2012 Cascadia Ridge
to trench program. The first profile extends offshore
Gray’s Harbor in Washington and the second extends
offshore Oregon at the latitude of Hydrate ridge, with
the fore-arc basin imaged below ∼60 and ∼70-km long
shallow water (< 500 m) portions of these profiles,
respectively. We use the travel time tomography
method of VanAvendonk et al. [2004], which is based
on the shortest path method for ray tracing, and
iterative inversions driven by gradual reduction of the
chi-square misfit (root mean square value of the
difference between predicted and observed travel
times normalized by pick uncertainty). We construct
our starting model by hanging to the seafloor a 1D
velocity profile based on interval velocities derived
from semblance analysis of MCS data.
4
3
The convergence rate varies from 30 mm/y at the southern end of the plate boundary to 45 mm/y at its
northern end. Even though the relatively short documentary history (late eighteenth century) dose not provide
any written record to estimate the size and recurrence interval of
earthquakes at Cascadia, geological evidences suggest the potential
capability of generating Magnitude ~ 9.0 earthquakes.
The Cascadia Ridge to Trench study
was designed to characterise the
structure and evolution of the JdF
plate and to test the hypothesis that
the JdF plate is significantly
hydrated prior to subduction,
transporting water into the
subduction zone, and contributing to
along-strike variations in structure
and seismicity along the Cascadia
margin. The experiment included
two East-West transects extending from the JdF ridge to the accretionary wedge and shelf of the Cascadia
margin and a long trench-parallel line outboard of the Cascadia accretionary wedge.
×10
4000
Misfit counts
IPGP
[email protected]
-0.02
0
0.02
Velocity [km/s]
24
28
32
Distance [km]
0.04
0.06
0.08
0.1
-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
Velocity [km/s]
0.04
0.06
0.08
0.1
Refrences
AH. J. Van Avendonk, D. J. Shillington, W. S. Holbrook, and M. J. Hornbach. Inferring crustal structure in the Aleutian island arc from a sparse wide-angle seismic data set. Geochemistry, Geophysics, Geosystems, 5(8), 2004.
T. J. Crone, M. Tolstoy, and H. Carton. Estimating shallow water sound power levels and mitigation radii for the R/V Marcus G. Langseth using an 8 km long MCS streamer. Journal of Geochemistry, Geophysics, Geosystems,
15(10):15252027, 2014.
S. Han, (2015) Accretion and subduction of oceanic lithosphere: 2D and 3D seismic studies of off-axis magma lenses at East Pacific Rise 9 3740N area and downgoing Juan de Fuca Plate at Cascadia Subduction Zone, PhD
thesis, Columbia Univ.
T. Parsons, R. E. Wells, M. A. Fisher. Three-dimensional velocity structure of Siletzia and other accreted terranes in the Cascade forearm of Washington, J. of Geophysical research 104(B8), 1999