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Subduction tectonics: Earthquake cycle and long-term deformation. Charles DeMets Dept. of Geology & Geophysics Univ. of Wisconsin-Madison. Acknowledgments to Francisco Correa-Mora and Stuart Schmitt, who developed some of the graphics below as advisees of C. DeMets at UW-Madison.
Dept. of Geology & Geophysics
Univ. of Wisconsin-Madison
Acknowledgments to Francisco Correa-Mora and Stuart Schmitt, who developed some of the graphics below as advisees of C. DeMets at UW-Madison.
Goal: Develop useful spatial and temporal frameworks for students to understand short-term and long-term deformation related to subduction
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.
Zone of interseismic locking
Triggered by earthquake
Spatio-temporal model for subduction earthquake cycleUpper 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.
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. Correa-Mora 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 slipfrom Brudzinski et al. (2006) GJINote that slip is DEEP – well downdip from the seismogenic zone.Reinforces results from previous slide.
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….
Let’s focus on the third of these, which is relevant to parts of the Middle America trench…..
Tectonic settingof Nicaragua/El Salvador segment of Middle America subduction zone. - oblique subduction - strain partitioning yields sinistral trench-parallel 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 – UW-Madison)
Dramatic difference in onshore character of GPS velocity fields along shallow-dipping Mexican segment of the MAT and steeply-dipping Central American MAT.
Oaxaca segmentmoderate to strong frictional coupling (~75%)inferred from measured site motions High EQ hazardSalvador segmentweak or zero frictional coupling inferred from measured GPS site motionsLow EQ hazard ?
Prediction of fully coupled elastic model
Trench-normal GPS site motions
THE END: (A GPS appendix follows, but is provided for GPS novices)
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 24-hour 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
GPS Site motions- Raw GPS processing done at UW-Madison- Continuous GPS station motions relative to ~mantle-fixed reference frame (above)- Motion around and toward best pole of rotation (left)