HR: 1340h
AN: G43B-1199    [Abstracts]
TI: Use of Coral Microatolls and a Tide Model to Measure Coseismic Vertical Deformation: Potential Utility and Common Mistakes
AU: * Meltzner, A J
EM: meltzner@gps.caltech.edu
AF: Tectonics Observatory, California Institute of Technology, MC 100-23, 1200 E. California Blvd., Pasadena, CA 91125, United States
AB: In the past few years, several great ( M>8) subduction megathrust ruptures have occurred beneath tropical seas and their fringing coral reefs. As predicted by elastic-dislocation theory, coastlines above the rupture patch rose, and adjacent regions subsided. Several investigators have used emerged or submerged coastal features to document land-level changes associated with these events. Unfortunately, when referencing these measurements to their high- or low-tide datums, some have overlooked the fact that both high and low tide levels can vary by more than a meter in some regions. In locations where tides are semidiurnal, a measured local high- tide value may be either the lower or higher high tide of the day, and a measured low-tide value may be either the higher or lower low tide. Furthermore, a measured high or low tide may be anywhere between the fortnightly spring and neap tides. Finally, even the elevations of spring and neap tides vary from month to month. One must know these variations to properly reference geological measurements. Some researchers have also made questionable assumptions about how the geological features they measured relate to tidal levels. As a result, published uplift or subsidence values in some studies may have errors of a meter or more, despite stated uncertainties of a few centimeters or less. A new approach, highlighted below, couples geological observations with a tide model to dramatically reduce uncertainties and produce more accurate estimates of uplift or subsidence. The upward growth of coral microatolls is controlled by low tide. Off the west coast of northern Sumatra, Porites microatolls' highest level of survival (HLS) is typically ~5 cm above annual low tide, but this is different for other genera and may be different in other regions. A comparison of pre- and post-earthquake HLS on a microatoll is the best method for documenting coseismic uplift; however, in cases where an entire reef was killed (and post-earthquake HLS cannot be found) or in cases of subsidence, a comparison of the pre-earthquake HLS to the post-earthquake annual low tide can provide an accurate and precise measurement of elevation change. Using a satellite- altimetry based tide model, which can determine tidal heights at any arbitrary time at any coastal location in the world, to properly tie any water level measurement into the tidal cycle (and hence to the annual low tide) for a site, it is possible to measure uplift or subsidence with realistic, demonstrable 2-sigma uncertainties of about a decimeter for sites with an overall tidal range of ~1m. While not as precise as continuous or even most campaign GPS measurements, this new approach allows for the determination of elevation changes with much greater spatial resolution than is normally achievable with GPS networks. With suitable adaptations, this approach can be used anywhere in the world.
DE: 1209 Tectonic deformation (6924)
DE: 1211 Non-tectonic deformation
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 1294 Instruments and techniques
SC: Geodesy [G]
MN: 2007 Fall Meeting