Geodetic studies of Mars in Hokkaido University

Abstract During the last decade, several explorers visited Mars, and produced enormous amount of geodetic data, such as gravity field or topography. They have largely become open to public on line by now (visit, e.g. the Planetary Data System Geosciences Node, http://pds-geosciences.wustl.edu/). Here we introduce several geodetic studies of Mars performed in Hokkaido University using such data. First we will compare static part of gravity and topography, and discuss isostatic compensation and thermal history of Mars. We then study true polar wander episodes that may have occurred several times and look for records of crustal deformation possibly caused by them. We finally present a recent topic on the time-variable part of the Martian gravity and topography caused by seasonal growth and decay of polar CO 2 snow caps. We infer seasonal and inter-annual changes of the density of snow, and discuss the seasonal compaction by sintering of CO 2 snow particles and influence of dust storms on the snow density

Preface to the Special Issue on “Geophysical and Climate Change Studies in Tibet, Xinjiang, and Siberia (TibXS) from Satellite Geodesy”

This special issue publishes papers on recent results in geophysical and climate change studies over Tibet, Xinjiang and Siberia (TibXS) based upon some of the key sensors used in satellite geodesy, including satellite gravimetric sensors (GRACE and GOCE), satellite altimeters (TOPEX, Jason-1 and -2, and ENVISAT), and Global Positioning System satellites. Results from ground- and airborne-based geodetic observations, notably those based on airborne gravimeter, superconducting gravimeter (SG) and seismometers are also included in the special issue. In all, 22 papers were submitted for this special issue; 17 papers were accepted

Time-Resolved Studies of Stick-Slip Friction in Sheared Granular Layers

Sensitive and fast force measurements are performed on sheared granular layers undergoing stick-slip motion, along with simultaneous imaging. A full study has been done for spherical particles with a +-20% size distribution. Stick-slip motion due to repetitive fluidization of the layer occurs for low driving velocities. Between major slip events, slight creep occurs that is variable from one event to the next. The effects of changing the stiffness k and velocity V of the driving system are studied in detail. The stick-slip motion is almost periodic for spherical particles over a wide range of parameters, but becomes irregular when k is large and V is relatively small. At larger V, the motion becomes smoother and is affected by the inertia of the upper plate bounding the layer. Measurements of the period T and amplitude A of the relative motion are presented as a function of V. At a critical value Vc, a transition to continuous sliding motion occurs that is discontinuous for k not too large. The time dependence of the instantaneous velocity of the upper plate and the frictional force produced by the granular layer are determined within individual slipping events. The force is a multi-valued function of the instantaneous velocity, with pronounced hysteresis and a sudden drop prior to resticking. Measurements of vertical displacement reveal a small dilation of the material (about one tenth of the mean particle size in a layer 20 particles deep) associated with each slip event. Finally, optical imaging reveals that localized microscopic rearrangements precede (and follow) each slip event. The behavior of smooth particles is contrasted with that of rough particles.Comment: 20, pages, 17 figures, to appear in Phys. Rev.

Heterogeneous coupling of the Sumatran megathrust constrained by geodetic and paleogeodetic measurements

Geodetic and paleogeodetic measurements of interseismic strain above the Sumatran portion of the Sunda subduction zone reveal a heterogeneous pattern of coupling. Annual banding in corals provides vertical rates of deformation spanning the last half of the 20th century, and repeated GPS surveys between 1991 and 2001 and continuous measurements at GPS stations operated since 2002 provide horizontal velocities. Near the equator, the megathrust is locked over a narrow width of only a few tens of kilometers. In contrast, the locked fault zone is up to about 175 km wide in areas where great interplate earthquakes have occurred in the past. Formal inversion of the data reveals that these strongly coupled patches are roughly coincident with asperities that ruptured during these events. The correlation is most spectacular for rupture of the M_w 8.7 Nias-Simeulue earthquake of 2005, which released half of the moment deficit that had accumulated since its previous rupture in 1861, suggesting that this earthquake was overdue. Beneath the Mentawai islands, strong coupling is observed within the overlapping rupture areas of the great earthquakes of 1797 and 1833. The accumulated slip deficit since these events is slowly reaching the amount of slip that occurred during the 1833 earthquake but already exceeds the slip that occurred during the 1797 earthquake. Thus, rerupture of part of the Mentawai patch in September 2007 was not a surprise. In contrast, coupling is low below the Batu islands near the equator and around Enggano island at about 5°S, where only moderate earthquakes (M_w < 8.0) have occurred in the past two centuries. The correlation of large seismic asperities with patches that are locked during the interseismic period suggests that they are persistent features. This interpretation is reinforced by the fact that the large locked patches and great ruptures occur beneath persistent geomorphologic features, the largest outer arc islands. Depth- and convergence-rate-dependent temperature might influence the pattern of coupling, through its effect on the rheology of the plate interface, but other influences are required to account for the observed along-strike heterogeneity of coupling. In particular, subduction of the Investigator Fracture Zone could be the cause for the low coupling near the equator