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

Abstract

Little is known about the frequency‐magnitude relationship for volcano‐tectonic events over long sections of fast spreading ridges. A technique that can 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 occasions using multibeam sonar systems. We have performed a detailed comparison of repeated bathymetric surveys of a 300‐km‐long section of the southern East Pacific Rise with the objective of detecting and characterizing any changes in seafloor morphology associated with eruptions and perhaps tectonic deformation. Repeat bathymetry data were collected as part of three cruises that surveyed the axial high between 16°S and 18°40′S latitude in late 1995, in late 1996, and in mid‐1997. During the latter two surveys the way points of the 1995 survey were repeated with the same survey vessel, thus closely approximating the acquisition geometry of the first survey. We have developed new methods for removing systematic errors and suppressing random noise in the bathymetry data (which reduce errors between surveys 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 area from survey to survey. As the ability to resolve a depth anomaly is a function of both its thickness and lateral area, anomalies of small lateral area require a larger depth anomaly than larger area features to be statistically significant. For this 1.5‐year survey period, no significant seafloor changes in the bathymetric data were detected with areas ≤0.2 km 2 and depth differences >8 m or with areas >0.2 km 2 and depth differences >4 m. We estimate that any seafloor depth changes that occurred between the survey times must have dimensions less than these values or they would have been detected. For comparison, lava flows detected along the Juan de Fuca and Gorda ridges using similar techniques are ∼10–75 m in thickness and have estimated total areas of 0.52–2.8 km 2 .