UTIG RESEARCH PROJECTS ARCHIVE

Publications

Principal Investigator: Frederick W. Taylor

Funded by: National Science Foundation

The plate tectonic process of subduction was first recognized and described at the Tonga arc by Isacks et al. (1968). This region remains the world's greatest laboratory for studies of subduction tectonics and seismicity (Fig. 1). Our research in this region has evolved to include multi-disciplinary studies of "active tectonics" in addition to traditional seismology. Active tectonics is the study of ongoing or recent deformation of the earth of which seismology is an integral part. Extremely rapid rates of tectonic plate motion produce numerous earthquakes and volcanoes which, in turn, result in some of the most rapid crustal deformation rates to be found on the globe.

The first active tectonics studies in this region exploited the unique record of vertical tectonic motions represented in the morphology and age structure of living and uplifted coral reefs. This concept dates back to Charles Darwin's book "The Structure and Distribution of Coral Reefs" in which he recognized that barrier reefs and atolls form by subsidence as coral reefs grow upward to keep up with rising sea level. In contrast, tectonic uplift raises coral reefs above sea level and creates islands capped by a series of coral reef terraces. Because isotopic dating of fossil corals allows us to determine when the reefs were at sea level, we can determine uplift and subsidence rates and infer a detailed tectonic history. We can even use individual coral heads as natural tide gauges that record the increments of crustal motion that occur during large earthquakes. Thus, from emerged coral reefs we are able to obtain a record not only of the background vertical deformation history, but also a history of the times and amounts of individual earthquake uplifts. This approach is currently being applied in Indonesia under NSF funding in collaboration with American and Indonesian scientists.

In 1988 we introduced a new Active Tectonics technology to the SW Pacific by beginning to measure crustal motions using the Global Positioning System (GPS). This method of satellite-based geodesy is so precise that we can determine the distance between sites on islands hundreds of km apart to less than 1 cm. By repeating these measurements of distance we can calculate the horizontal motions of tectonic plates and the deformation within crustal blocks. This technique has tremendous potential for observing the relationships between earthquakes, plate motions, and the ongoing interseismic accumulation and release of elastic strain that causes earthquakes . By understanding these relationships, we not only gain insights into the processes of growth and modifications of island arcs, we also better understand how earthquakes are generated and the factors that influence their size, distribution, and timing.

One of the rewarding aspects of the GPS work involved introducing a useful technology to local surveyors. We held annual training classes and collaborated with the local governments of seven island nations. On many occasions we performed service work such as precisely locating the international airport at Nandi, Fiji, by GPS.

Publications
Calmant, Stéphane; Pelletier, Bernard; Lebellegard, Pierre; Bevis, Michael; Taylor, Frederick W.; Phillips, David A. New insights on the tectonics along the New Hebrides subduction zone based on GPS results, Journal of Geophysical Research, Vol. 108, No. B6, 2319

Calmant, S., Pelletier, B., Pillet, R., Regnier, M., Lebellegard, P., Maillard, D., Taylor, F., Bevis, M., and Recy, J., 1997, Interseismic and coseismic motions in GPS series related to the M (sub s) 7.3 July 13, 1994, Malekula earthquake, central New Hebrides subduction zone, Geophysical Research Letters, vol. 24, Issue 23, pp. 3077-3080

Taylor, F. W., Bevis, M. G., Schutz, B. E., Kuang, D., Recy, J., Calmant, S., Charley, D., Regnier, M., Perin, B., Jackson, M., and Reichenfeld, C., 1995, Geodetic measurements of convergence at the New Hebrides island arc indicate arc fragmentation caused by an impinging aseismic ridge, Geology Boulder, vol. 23, Issue 11, pp. 1011-1014.