A detailed comparison of repeated bathymetric surveys along a 300-km-long section of the southern East Pacific Rise

Little is known about the frequency-magnitude relationship for volcano-tect onic events over long sections of fast spreading ridges. A technique that c an both detect and characterize the size, shape, and approximate time of a volcanic or tectonic event is to survey an area of seafloor on multiple occ asions using multibeam sonar systems. We have performed a detailed comparis on of repeated bathymetric surveys of a 300-km-long section of the southern East Pacific Rise with the objective of detecting and characterizing any c hanges in seafloor morphology associated with eruptions and perhaps tectoni c deformation. Repeat bathymetry data were collected as part of three cruis es that surveyed the axial high between 16 degreesS and 18 degrees 40'S lat itude in late 1995, in late 1996, and in mid-1997. During the latter:two su rveys the way points of the 1995 survey were repeated with the same survey vessel, thus closely approximating the acquisition geometry of the first su rvey. We have developed new methods for removing systematic errors and supp ressing-random noise in the bathymetry data (which reduce errors between su rveys to a root-mean-square value of <2 m) and for selecting regions of the seafloor that surpass threshold values in depth difference and lateral are a from survey to survey. As the ability to resolve a depth anomaly is a fun ction of both its thickness and lateral area, anomalies of small lateral ar ea require a larger depth anomaly than larger area features to be statistic ally significant. For this 1.5-year survey period, no significant seafloor changes in the bathymetric data were detected with areas <less than or equa l to>0.2 km(2) and depth differences >8 m or with areas >0.2 km(2) and dept h differences >4 m. We estimate that any seafloor depth changes that occurr ed between the survey times must have dimensions less than these values or they would have been detected. For comparison, lava flows detected along th e Juan de Fuca and Gorda ridges using similar techniques are similar to 10- 75 m in thickness and have estimated total areas of 0.52-2.8 km(2).