The Earthquake Cycle and its Role in Permanent Vertical Deformation in the Western Solomons Arc from Coral Paleogeodesy of the Past Few Centuries

University Of Texas At Austin, Austin TX

Investigators

Abstract

Convergent plate margins are notorious for generating the largest historic volcanic eruptions and the largest magnitude earthquakes. However, many fundamental questions remain about the relationships between plate tectonic convergence, the earthquake cycle of elastic strain accumulation and coseismic rupture, and construction of the topography and structures characteristic of convergent margins. This project focuses on the question of how the earthquake cycle of elastic strain accumulation and release translates into longer-term net tectonic deformation. The traditional view is that the forearc is somehow ratcheted up and down during the coseismic phase of the earthquake cycle. However, if interplate slip produces longer-term net deformation, then it is unlikely that the mechanism is limited to coseismic slip on the seismogenic part of the interplate thrust. Perhaps slip between asperities, slip on the interplate thrust zone deeper than the stick-slip seismogenic zone, or aseismic slip within the seismogenic zone may lead to permanent tectonic deformation of the upper plate. The primary goal of this project is to determine the history of vertical movements across a segment of the Western Solomons forearc from the time of the last full-scale megathrust rupture until present and to infer the geography and timing of components of vertical deformation that are permanent versus those that are temporary elastic strain phenomena. The idea is being tested in the Western Solomon Islands where coral-fringed islands occupy much of the overriding plate from near the trench to approximately 100 km arc-ward. Shallow living reef corals are used as living tide gauges that can record the vertical changes that raise and lower corals through sea level either gradually or abruptly. Corals killed by uplift are dated to within one per cent of their calendar ages by uranium-series isotope analysis so that older vertical deformation can be dated and mapped. Coral morphology, annual growth bands in corals, and stable isotopic measurements will provide the details of vertical motions. Besides having a geological history of vertical motions, a suitable site has reef-fringed coasts overlying the area of a convergent margin for which the history of vertical tectonic movements would address the idea that vertical uplift in forearcs occurs at time-scales between seismic and orogenic cycles. Convergent plate margins at island arcs, such as the Solomon Islands, or at the edges of continents, as along the Pacific margin of South America, have produced the largest earthquakes and some of the most violent volcanic eruptions on record. Recent examples in Indonesia and Japan demonstrate that scientists can be taken by surprise when convergent margin earthquakes are even larger and more destructive than known for a particular area based on the relatively sparse paleoseismic studies. Thus is it important to understand more about the generation of such earthquakes. The crustal motions that occur between and during large earthquakes at convergent margins represent a window on the earthquake generation process that can help us understand how tectonic plate convergence is accommodated by permanent deformation of the margins vs. that which goes to generate large earthquakes, including unusually large earthquakes produced when several adjacent seismogenic units rupture together at the same time. This work has implications for interpretation of crustal motions presently being monitored in great detail at seismically hazardous areas such as the Cascadia margin of the northwestern US. Furthermore, the temporary elastically powered earthquake cycle and permanent crustal motions associated with plate convergence are a matter of fundamental significance to the tectonic evolution of convergent plate margins and the development of continental lithosphere.