Subduction tectonics:
Earthquake cycle and
long-term deformation
Charles DeMets
Dept. of Geology &
Geophysics
Univ. of WisconsinMadison
Acknowledgments to
Francisco Correa-Mora
and Stuart Schmitt, who
developed some of the
graphics below as advisees
of C. DeMets at UWMadison.
Presentation goal and outline
Goal: Develop useful spatial and temporal frameworks
for students to understand short-term and longterm deformation related to subduction
Outline:
•
2-D & 3-D spatial models for shallow subduction
•
Describe tectonic processes in subduction zones
•
Characterize subduction earthquake cycle
•
Categorize types of long-term deformation
Note to participants: The majority of this presentation focuses on short-term processes
that influence subduction zone tectonics (earthquake cycle). The latter third or so
deals with long-term deformation, but uses GPS measurements (short-term once
again!) to reveal one example of long-term upper plate deformation.
Conceptual model – 2D
Major components of the “system”
1. Upper plate (free to move in any direction - shown as fixed above)
2. Subducting plate – moves downward into mantle
3. Subduction interface – contact zone between upper and subducting
plates. Frictional “surface” along which thrust earthquakes occur.
Interface is subdivided into free-slip, seismogenic, and transitional zones.
These are differentiated by their respective frictional properties.
3-D spatial model – layer properties
1. Upper layers – accumulate and release strain elastically
2. Lower layers – respond to stress “jump” via protracted
viscous flow.
Subduction interface
2D model – Earthquake cycle processes – Seismogenic zone
Zone of interseismic locking
1. Seismogenic zone is locked by friction during long inter-earthquake periods (decades
to centuries) – no motion between upper and subducting plates. Crust accumulates this
“deficit” in slip elastically. Unrelieved slip accumulates at rates of mm to tens of mm per
year.
2. Earthquake eventually ruptures seismogenic zone and recovers most or all of the
interseismic slip deficit – meters of slip occur in just seconds to tens of seconds
2D model (continued) – After an earthquake
Triggered by earthquake
Earthquakes “trigger” three transient processes in the crust and mantle.
•
Fault afterslip – a logarithmically decaying dynamic frictional response DOWNDIP from the
seismogenic zone – manifested at surface as log-decay elastic response. Can equal 50-100% of the
coseismic elastic deformation! Requires weeks to years to decay away.
•
Viscoelastic flow – coseismic stress jump in the viscoelastic lower crust and upper mantle triggers
protracted FLOW in these regions, measured at surface as exponentially-decaying deformation.
Continues for decades after large events.
•
Poroelastic deformation – coseismic volumetric changes in crust alters pore volumes and forces
fluid flow, which in turn causes measurable surface deformation. Weeks to months.
Spatio-temporal model for
subduction earthquake cycle
Upper diagram shows movement of a
hypothetical GPS site through the seismic
cycle.
- Interseismic – superposition of steady elastic
strain accumulation across locked seismogenic
zone due to steady plate convergence and
occasional short-duration aseismic strain release
across frictionally-transitional zone downdip from
locked region. Free slip areas contribute nothing
to surface deformation
- Coseismic – Rapid opposite-direction release
of accumulated elastic strain with slip dominantly
along seismogenic zone
- Postseismic – Superposition of triggered
afterslip in transitional region and viscoelastic flow
in mantle wedge and lower crust and poroelastic
response in fluid-bearing regions of crust. Decays
through time back to steady strain accumulation.
Continuous GPS along
Mexican Pacific coast
illustrates different phases of
the seismic cycle
Conceptual model
Oaxaca CGPS
Jalisco CGPS
Dense GPS network
samples seismic cycle
deformation in
southern Mexico
The next 5 slides show
our imaging of the
spatial relationship
between interseismic
locking and transient
strain beneath Mexico
from Cocos plate
subduction.
Ph.D. research of F. CorreaMora advised by C. DeMets
Shallow regions of most subduction interfaces are characterized by
occurrence of large shallow-dipping thrust earthquakes that define the
seismogenic zone.
3-D modeling of a
subduction zone
permits different
material properties to
be assigned to different
layers and zones, e.g.
oceanic crust is
“stiffer” than
continental crust and
hence has a diminished
elastic response. Here,
a dense 3-D mesh
simulates the geometry
of the Middle America
subduction zone in the
study area of southern
Mexico.
Continuous and annual measurements of ~30 bedrock geodetic pins in
the region with GPS are used to establish their motions through time.
In the following two slides, I show results from inverting these GPS
motions to estimate the location and magnitude of frictional locking along
the subduction interface.
continuous site example
‘01-’07 velocity field
Left – (A) location
and magnitude of
TRANSIENT slip in
2004. White dashed
lines indicate areas
of earthquake
rupture in 1968 and
1978 mega-thrust
earthquakes
(defines
seismogenic zone).
(B) Location and
magnitude of
INTER-SEISMIC
frictional LOCKING.
Note that LOCKING
occurs across
seismogenic AND
downdip zones
(C) Slip magnitude
and location during
2006 transient
event. Note
similarity to 2004
result!
Inversion of GPS site motions during a transient slip event in 2004
to define location and magnitude of the transient slip
from Brudzinski et al. (2006) GJI
Note that slip is DEEP – well downdip from the seismogenic zone.
Reinforces results from previous slide.
Synoposis of seismic cycle (short-term)
deformation
1.
2.
Period of steady interseismic strain accumulation that
ends with major thrust earthquake also includes
occasional aseismic releases of elastic strain that has
accumulated downdip from seismogenic zone.
Earthquakes trigger three post-seismic responses.
Separating the three decaying processes (afterslip,
viscoelastic flow, and poroelastic deformation) from
each others is a “challenging” modeling exercise and is
frequently non-unique.
What about long-term deformation ?
Thus far, we have focused largely on elastic and
thus recoverable deformation, which leaves little
or no long-term permanent record. But UPPER
plates clearly deform in a permanent manner
(faulting, folding, uplift, subsidence) inboard from
subduction zones. Geologists are more
frequently interested in the long-term
deformation record, as I imagine many of you
may be…..
Let’s quickly review the three end-member types of upper-plate
deformation and their causes….
1. Upper plate shortening (mountain building) - Possible causes: Rapid
trenchward motion of upper plate, overrides subducting plate,
associated with shallow subduction, deformation far inboard from
trench. Possible other cause – collision of seamount, oceanic plateau,
or continental fragment traveling on subducting plate with the trench.
2. Upper plate extension (back-arc spreading) – Possible cause: Upper
plate motion AWAY from trench induces upper plate extension.
Possibly trench roll-back, but any lateral motion of a subducting slab
must PUSH a great deal of mantle out of its path. Is roll-back possible?
3. Coast-parallel sliver transport – Possible cause: Oblique subduction
and partitioning of obliquity into trench-normal subduction and trenchparallel upper-plate shear. Other cause – coastwise lateral escape
from collision zone between a buoyant subducting feature with
trench/upper plate.
Let’s focus on the third of these, which is
relevant to parts of the Middle America
trench…..
Tectonic setting
of Nicaragua/El
Salvador segment of
Middle America
subduction zone.
- oblique subduction
- strain partitioning
yields sinistral trenchparallel forearc shear
- Basin and Range-like
extension in Honduras,
Guatemala, El Salvador
- strike-slip tectonics
along Motagua-Polochic
faults (CA-NA plate
boundary)
2000-2005 GPS velocity field – CA plate fixed.
(1) cent/eastern Hond/Nic sites are on CA interior. (2) forearc slip obvious, (3) E-W
stretching obvious
(Proprietary results from ongoing M.S. research by D. Alvarado and M. Rodriguez – UWMadison)
Dramatic difference in
onshore character of GPS
velocity fields along
shallow-dipping Mexican
segment of the MAT and
steeply-dipping Central
American MAT.
Oaxaca
El Salvador
Oaxaca segment
Prediction of fully coupled elastic model
moderate to strong
frictional coupling
(~75%)
inferred from measured
site motions
High EQ hazard
50% coupling
Trench-normal
GPS site motions
El Salvador/Honduras
Salvador segment
weak or zero frictional
coupling inferred from
measured GPS site
motions
Low EQ hazard ?
Nicaraguan GPS sites also move parallel to coast – no inland
component of motion - indicates weak coupling across
subduction interface
Absence of subduction “overprint” on onshore velocities affords
clear view of upper plate LONG-TERM deformation!
Conclusions: Long-term deformation
• Deformation of upper plate is dictated in part by upper
plate motion relative to the trench. The subducting plate
and hence trench cannot migrate laterally with ease and
thus stays in place no matter how upper plate moves.
• Deformation also depends on geometry of trench
(bends), buoyant oceanic or continental fragments that
are being subducted (or not), and RELATIVE direction of
upper and subducting plate convergence (obliquity).
THE END: (A GPS appendix follows, but is provided for GPS
novices)
2-slide GPS appendix for skeptics – proof of
technique
If one or more of you are unfamiliar with the application of
GPS geodesy to crustal deformation research, the
following two slides show results from processing 24hour continuous GPS station data to high precision in
order to measure changes through time in the absolute
coordinates and height of the fixed GPS monument.
Graphic 1 shows that the daily site coordinates show
random scatter superimposed on linear motion (well
behaved).
Graphic 2 illustrates the remarkable velocity pattern defined
by numerous GPS sites, representing a powerful proof of
the concept that GPS can be used to map plate motions
and other forms of crustal deformation
Using GPS tracking to
monitor plate
movements
Technical background
GPS Site motions
Technical background
- Raw GPS processing done at UWMadison
- Continuous GPS station motions
relative to ~mantle-fixed reference
frame (above)
- Motion around and toward best pole
of rotation (left)