Interseismic and Coseismic Deformation and the role of the Upper Plate in the Maule Segment
Abstract
The 2010 Maule earthquake was preceded by several decades of intense geological study of the Central and Southern Andes and 15 years of geodetic GPS monitoring of the South America-Nazca Plate boundary. With an excellent record of interseismic and coseismic deformation, and comprehensive description of upper plate regimes of shortening, vertical axis rotation, and along strike segmentation, the Maule event is more completely recorded than most other earthquakes of its size. The interseismic strain calculated from CAP GPS velocity vectors preceding the event shows the typical pattern of maximum shortening at a high angle to the margin but is distinguished by a maximum shear strain rate about twice as large as that observed north of 33°S in the segment adjoining the Maule rupture segment. The high shear strain rate continues south of the Maule segment as well to the region of the 1960 Chile earthquake, indicating the the entire boundary S of 33°S has higher interseismic strain rates than north of 33. This observation, in combination with greater intracontinental shortening north of 33, greater elastic thickness of South America S of 33°, significantly more sediment in the trench south of 33°, and progressively younger plate southward toward the triple junction all suggest that the plate boundary is weaker and the South American continent is more rigid south of 33°. Thus, to the north, plate convergence is partitioned between subduction and upper plate shortening whereas, to the south, all convergence occurs on the plate boundary, itself. The northern end of the Maule rupture coincides with the intersection of the Juan Fernandez ridge with the trench and, in the upper plate, the long-lived, NW-trending Melipilla-Maipo-Diamante transverse structural zone. To the south, another zone of NW-striking faults was the site of a M6.9 crustal normal fault aftershock with NW-striking nodal planes. The calculation of coseismic GPS strain yields principal horizontal extension axes that are parallel to the T-axes of crustal normal fault aftershocks near both the northern and southern ends of the Maule rupture. Coseismic upper plate extension was also accomplished by open tension cracks. Upper plate forearc extension via cracking and normal faulting is common throughout the western margin of South America and their activation during the Maule event demonstrates that they are coseismic features. In the region of the Arauco peninsula, 50 years of interseismic clockwise vertical axis rotation following the 1960 earthquake was almost exactly recouped in a few minutes of coseismic counterclockwise vertical axis rotation. Evidence from northern Chile indicates that, on a million year time scale, plate boundary ruptures tend to occur in the same segments. As in northern Chile, we suggest that the Maule event’s lateral extent is controlled by long-lived, pre-existing geological features in the South America.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.G33A0842A
- Keywords:
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- 1209 GEODESY AND GRAVITY / Tectonic deformation;
- 8104 TECTONOPHYSICS / Continental margins: convergent