aBstr aCt. Seamounts are active or extinct undersea volcanoes with heights exceeding ~ 100 m. They represent a small but significant fraction of the volcanic extrusive budget for oceanic seafloor and their distribution gives information about spatial and temporal variations in intraplate volcanic activity. In addition, they sustain important ecological communities, determine habitats for fish, and act as obstacles to currents, thus enhancing tidal energy dissipation and ocean mixing. Mapping the complete global distribution will help constrain models of seamount formation as well as aid in understanding marine habitats and deep ocean circulation. Two approaches have been used to map the global seamount distribution. Depth soundings from single-and multibeam echosounders can provide the most detailed maps with up to 200-m horizontal resolution. However, soundings from the > 5000 publicly available cruises sample only a small fraction of the ocean floor. Satellite altimetry can detect seamounts taller than ~ 1.5 km, and studies using altimetry have produced seamount catalogues holding almost 13,000 seamounts. Based on the size-frequency relationship for larger seamounts, we predict over 100,000 seamounts > 1 km in height remain uncharted, and speculatively 25 million > 100 m in height. Future altimetry missions could improve on resolution and significantly decrease noise levels, allowing for an even larger number of intermediate (1-1.5-km height) seamounts to be detected.Recent retracking of the radar altimeter waveforms to improve the accuracy of the gravity field has resulted in a twofold increase in resolution. Thus, improved analyses of existing altimetry with better calibration from multibeam bathymetry could also increase census estimates.

S U M M A R YRecent revisions to the satellite-derived vertical gravity gradient (VGG) data reveal more detail of the ocean bottom and have allowed us to develop a non-linear inversion method to detect seamounts in VGG data. We approximate VGG anomalies over seamounts as sums of individual, partially overlapping, elliptical polynomial functions, which allows us to form a non-linear inverse problem by fitting the polynomial model to the observations. Model parameters for a potential seamount include geographical location, peak VGG amplitude, major and minor axes of the elliptical base, and the azimuth of the major axis. The non-linear inversion is very sensitive to the initial values for the location and amplitude; hence, they are constrained by the centre and amplitude of the uppermost contours obtained with a 1-Eötvös contour interval. With these initial conditions from contouring, we execute a step-wise and fully automated inversion and obtain optimal model estimates for potential seamounts; these are statistically evaluated for significance using the Akaike Information Criterion and F tests. A logarithmic barrier technique is applied to ensure positivity of all seamount amplitudes. After automatic and manual inspections of the model parameters we estimate actual heights and basal ellipses of the inspected potential seamounts directly from the predicted bathymetry grid. In this study, we find globally 24 643 potential seamounts (h ≥ 0.1 km) that are located away from continental margins; 8458 potential seamounts are taller than 1 km. Although our global estimate is significantly lower than predictions from previous studies, a first-order reconciliation of the size-frequency statistics obtained from those studies reveals that the previous counts are systematically overestimated. Because of the ambiguity of gravity signals due to small seamounts of h < 1 km and the overlap with abyssal hills, we estimate the global seamount census to lie in the 40 000-55 000 range. The seamount data from this study are accessible from http://www.soest.hawaii.edu/PT/SMTS.

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Initiation of subduction is rarely encountered in modern tectonic environments due to its ephemeral and destructive nature. We report the geological and geophysical evidence indicating a transitional phase from buckling to embryonic subduction along the eastern Korean margin. The transition appears to be caused by compressional reactivation of the strike-slip boundary between the continental (Korean Peninsula) and oceanic (Ulleung Basin) crusts since the Early Pliocene. Evidence for compressional reactivation includes (1) a west-dipping major thrust and coincident crustal buckling of the Ulleung Basin; (2) an east-west structural asymmetry inferred from the gravity anomaly and P-wave tomography; and (3) ongoing crustal uplift and high-angle faults along the eastern Korean margin. The juxtaposition of underthrusting and buckling of the crust in the Ulleung Basin, and its associated ubiquitous reverse faulting on the eastern Korean margin, imply the potential development of a new subduction system along the western margin of the East Sea (Japan Sea). We propose that the East Sea comprises two incipient subduction margins (i.e., the Korean and Japanese sides), which are now competing to reach a self-sustaining subduction stage.

[1] We present a new spatial filter designed to separate short-length-scale bathymetric features from regional bathymetry. The directional median (DiM) filter divides a given filter circle into N ''bow tie'' sectors, allocates data points inside the filter circle to each sector based on their relative position within the circle, estimates a median for each sector, and returns the lowest of these N medians. This approach prevents DiM filtering from choosing the biased medians near the features on a sloping regional trend, which is a serious artifact of standard median filtering. As the bathymetry comprises diverse length-scale features, the separated results vary with the choice of filter width. Such variations are spatially distributed. Using a finite range of filter widths, we evaluate the spatial distribution of variations in the DiM-based separation by estimating their median absolute deviation (MAD) at each data point. The distribution of MAD values is indicative of uncertainties inherent in the separation. We demonstrate the effectiveness of DiM filtering with both synthetic and actual bathymetric data.