The Gutenberg-Richter (G-R) relationship describes a relationship between the number of earthquakes in a region greater than a certain magnitude and that magnitude (Gutenberg & Richter, 1956). This relationship states:where a represents the number of earthquakes when the moment magnitude (M W ) = 0, and b is the slope of the scaling relationship. For instance, when b = 1, there are 10 1 times more seismic events at a given magnitude than at the next lower magnitude value. On average, the b-value of global earthquakes is ∼1 (Lay & Wallace, 1995), but for slow slip events and active volcanic regions, the b-value is close to 1.
Fractures within ice shelves are zones of weakness, which can deform on timescales from seconds to decades. Icequakes produced during the fracturing process show a higher b-value in the Gutenberg-Richter scaling relationship than continental earthquakes. We investigate icequakes on the east side of rift WR4 in the Ross Ice Shelf, Antarctica. Our model suggests a maximum icequake slip depth that is ˜7.8 m below rift surface, where the slip area can only grow laterally along the fracture planes. We propose ductile deformation below this depth, potentially due to saturation of unfrozen water. We use remote sensing and geodetic tools to quantify surface movement on different time scales and find that the majority of icequakes occurred during falling tides. The total seismic moment is < 1% of the estimated geodetic moment during a tidal cycle. This study demonstrates the feasibility of using seismology and geodesy to investigate ice rift zone rheology.
The NASA Dawn mission revealed that the floor of Occator crater on the dwarf planet Ceres (in the main asteroid belt between Mars and Jupiter) is populated with small quasi-conical hills. Many of these features exhibit morphometric properties that are like those of ice-cored periglacial hills called pingos. Alternatively, some of these Cerean hills have also been hypothesized to be cryovolcanic in origin. If these hills are analogous to pingos, they represent ice-rich environments that are attractive targets for future exploration. We report new constraints on the morphologies of the Occator hills that aid in determining their origin. We also directly test how morphologically similar the hills in Occator are to pingos and volcanic cones on Earth using comparative statistical analyses. Using a novel application of kernel density estimation and Markov chain Monte Carlo methods we show that the morphologies of terrestrial pingos and volcanic cones are quantifiably distinct, and that the Cerean hills share significant morphometric similarities with pingos on Earth. Our findings indicate that a statistical treatment of morphometry alone can be a powerful tool for classifying and comparing planetary surface features, and that the majority of the resolved Cerean hills are morphometrically more similar to pingos than to small terrestrial volcanic cones.
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