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.
Our main purpose was to investigate the relationship between cycles of temporary elastic deformation making great earthquakes at convergent plate boundaries and permanent deformation resulting in mountain building. The megathrust, the fault zone where one tectonic plate thrusts beneath another at a trench, generates high topography and the strongest earthquakes. Temporary earthquake deformation and longer-term mountain building near megathrusts are distinct, but related processes that occur in tandem. Both are processes central to understanding both fundamental Earth processes and hazards from earthquakes and tsunamis. To distinguish between temporary and permanent deformation we must measure vertical motions that have occurred both during and between earthquakes. Measurements must be done on land, but land overlying convergent plate megathrust earthquakes is rare. However, where the Australian plate thrusts beneath the Western Solomon Islands, much of the zone overlying megathrust earthquakes is occupied by islands to within 5 km of the trench. In 2007, a large megathrust earthquake occurred here and uplifted the area by up to 2.6 m. This uplifted area next to the trench also undergoes rapid longer-term tectonic uplift. During this earthquake an area adjacent to the earthquake rupture, but farther from the trench, subsided. This subsided area also uplifts over the long term. Thus, another objective is to understand the nature of that adjacent uplift and its relationship to nearby megathrust earthquakes. We used living and fossil reef corals along shorelines as natural tide gauges that record the times and amounts of up and down motions relative to sea level both preceding and during the 2007 earthquake. Some of the age information is from counting the annual growth bands in coral heads (similar to tree rings). But corals also incorporate uranium in their limestone skeletons that allows isotopic dating accurate to within a few years. For example, two corals at 5.4 m above sea level on Ranongga Island gave ages for their death by uplift of 756 ± 5 and 766 ± 4 yr ago. Other corals were raised above sea level here at about 601, 761, 838, 1285, 2081, and 3873 yr ago. Thus we have examples of 2007 earthquake uplift, subsidence of islands over the preceding years, and amounts of net permanent uplift left over from older earthquakes. Adding all uplifts and subsidences together resulted in raising islands by hundreds of meters. Does this mean that earthquakes cause the uplift that contributes to the rise of the islands over time? Earthquakes are an essential part of the process, but much of the permanent uplift may occur between rather than during earthquakes. The coral record for Ranongga Island before the 2007 earthquake revealed that some places where 2007 uplift was large had little or no total uplift over longer times. But other places where 2007 uplift was large had large amounts of total long-term uplift. The difference between these sites is that where total uplift is small, subsidence in the decades before 2007 was large. Where total uplift has been large, the subsidence was small. Simultaneous uplift and subsidence make the subsidence appear to be smaller than it really is. The subsidence is due primarily to elastic deformation that accumulates to power the earthquakes. Therefore, it appears that much of the tectonic deformation causing permanent uplift actually occurs during the times between earthquakes and the total vertical motion is the addition of all the uplift and subsidence over the entire cycle before, during and after an earthquake. With regard to areas of 2007 earthquake subsidence farther from the trench than the 2007 uplift zone, we discovered that area also has uplifted abruptly in the past, but not when islands overlying the megathrust earthquake zone uplifted. This suggests that earthquakes that are not on the main megathrust earthquake zone may be responsible for this second zone of uplift farther from the trench. Even more surprising is that this farther region underwent a period of extremely rapid uplift from about 10,000 to 7,000 years ago. Uplift then became very slow with only smaller one-meter uplifts at about 565 and 725 yr ago. At this time we do not understand what type of earthquake caused uplift of this zone that does not not overlie the earthquake rupture zone. In summary, we have assembled coral-based measurements from shorelines overlying the Western Solomons earthquake megathrust within which long-term tectonic deformation and the elastic earthquake cycle can be distinguished. The elastic and long-term uplift in this area are related, but distinct, processes whose deformation signals are combined. Based on our discoveries we plan to develop and test hypotheses to explain our observations. This work also provides the first compelling information on which to base forecasts of future large earthquakes in the Western Solomon Islands. Our aims include advising the Solomon Islands Disaster Management Office in using this information for risk management.
