Atsushi Yamaji

The multiple inverse method: a new technique to separate stresses from heterogeneous fault-slip data

Abstract A new inverse technique is presented here to separate stresses from heterogeneous fault-slip data without a priori information on the stresses or on the classification of faults according to the stresses. Four parameters are determined by the inversion: one for the shape of the stress ellipsoid and three for the direction of the stress axes. Accordingly, the inversion is equivalent to the projection of the fault-slip data to a point in four-dimensional parameter space. The data are divided into k -element subsets to which the inverse technique is applied, where k =4 or 5 is the optimal choice for the stability of solutions and for the reduction of computation. Significant solutions are identified as clusters in parameter space. The technique is demonstrated first by simulated fault-slip data. As an example, field data obtained from Miocene fore-arc sediments in western Japan were processed by the method.

Lunar Radar Sounder Observations of Subsurface Layers Under the Nearside Maria of the Moon

Observations of the subsurface geology of the Moon help advance our understanding of lunar origin and evolution. Radar sounding from the Kaguya spacecraft has revealed subsurface layers at an apparent depth of several hundred meters in nearside maria. Comparison with the surface geology in the Serenitatis basin implies that the prominent echoes are probably from buried regolith layers accumulated during the depositional hiatus of mare basalts. The stratification indicates a tectonic quiescence between 3.55 and 2.84 billion years ago; mare ridges were formed subsequently. The basalts that accumulated during this quiet period have a total thickness of only a few hundred meters. These observations suggest that mascon loading did not produce the tectonics in Serenitatis after 3.55 billion years ago. Global cooling probably dominated the tectonics after 2.84 billion years ago.

Long-Lived Volcanism on the Lunar Farside Revealed by SELENE Terrain Camera

We determined model ages of mare deposits on the farside of the Moon on the basis of the crater frequency distributions in 10-meter-resolution images obtained by the Terrain Camera on SELENE (Selenological and Engineering Explorer) (Kaguya). Most mare volcanism that formed mare deposits on the lunar farside ceased at ∼3.0 billion years ago, suggesting that mare volcanism on the Moon was markedly reduced globally during this period. However, several mare deposits at various locations on the lunar farside also show a much younger age, clustering at ∼2.5 billion years ago. These young ages indicate that mare volcanism on the lunar farside lasted longer than was previously considered and may have occurred episodically.

Instrumentation and observation target of the Lunar Radar Sounder (LRS) experiment on-board the SELENE spacecraft

The Lunar Radar Sounder (LRS) on-board the SELENE lunar orbiter is currently being equipped to provide the data of subsurface stratification and tectonic features in the shallow part (several km deep) of the lunar crust, by using an FM/CW radar technique in HF (∼5 MHz) frequency range. Knowledge of the subsurface structure is crucial to better understanding, not only of the geologic history of the Moon, but also of the Moon’s regional and global thermal history of the Moon and of the origin of the Earth-Moon system. In addition to the subsurface radar experiment, LRS will provide the spectrum of plasma waves and solar and planetary radio waves in a wide frequency range from 10 Hz to 30 MHz. This paper provides the basic function parameter of the LRS system based on the final function test and proposes observation targets and data analysis that will provide important information leading to a greater understanding of the tectonics and thermal history of the Moon.

Lack of Exposed Ice Inside Lunar South Pole Shackleton Crater

The inside of Shackleton Crater at the lunar south pole is permanently shadowed; it has been inferred to hold water-ice deposits. The Terrain Camera (TC), a 10-meter-resolution stereo camera onboard the Selenological and Engineering Explorer (SELENE) spacecraft, succeeded in imaging the inside of the crater, which was faintly lit by sunlight scattered from the upper inner wall near the rim. The estimated temperature of the crater floor, based on the crater shape model derived from the TC data, is less than ∼90 kelvin, cold enough to hold water-ice. However, at the TCs spatial resolution, the derived albedo indicates that exposed relatively pure water-ice deposits are not on the crater floor. Water-ice may be disseminated and mixed with soil over a small percentage of the area or may not exist at all.

Are the solutions of stress inversion correct? Visualization of their reliability and the separation of stresses from heterogeneous fault-slip data

It is now a conventional technique to determine the optimal stress from fault-slip data by inversion. However, the method is weak when applied to heterogeneous data. A new technique is presented here to visualize the reliability of the solution. It is also shown that the technique allows us to separate stresses from those data. The technique is simple: it is the visualization of the object function of the inversion. The present method is compared with the conventional inverse method and the multi-inverse method using artificial and natural heterogeneous data sets. The conventional method can determine one of the stresses, if the orientation of the faults has a large variation. It is shown that the solutions of the method are non-unique and unstable for some data sets, indicating that they are not reliable. The present graphical method and multi-inverse method are more robust than the conventional one for heterogeneity. The multi-inverse method seems to have better resolution than the present method. However, unlike the multi-inverse method, the time of computation of the present method does not increase with the number of faults, so that the method becomes favorable for processing hundreds of faults.

Slab rollback suggested by latest Miocene to Pliocene forearc stress and migration of volcanic front in southern Kyushu, northern Ryukyu Arc

Abstract The northern Ryukyu Arc has active backarc rift, neutral-stress forearc, and active accretionary prism. The Okinawa Trough has been shaped by the episodic rifting in the backarc. Paleostresses were inferred in this study from mesoscale faults in Neogene forearc sediments called the Miyazaki Group, southeast Kyushu in the northern Ryukyu Arc. The forearc stress changed from compressional to extensional from the latest Miocene through Early Pliocene time. The stress history is concordant with the transition in tectonic regime from folding to rifting in the backarc. The transition in the stress state occurred simultaneously also with trenchward movement of the volcanic front. These phenomena suggest that the subducting slab under southern Kyushu became steeper in the Early Pliocene. Extensional tectonics ceased sometime in the late Pliocene or early to mid-Pleistocene, concordant with the counterclockwise change of subducting direction of the Philippine Sea Plate.

The multiple inverse method applied to meso-scale faults in mid-Quaternary fore-arc sediments near the triple trench junction off central Japan

The multiple inverse method for fault-slip data is applied to meso-scale faults observed in mid-Quaternary fore-arc sediments near the triple trench junction oA the Boso Peninsula, Japan. Data from the Otadai, Umegase and Kokumoto Formations were processed, and three stresses were obtained as significant solutions: vertical, axial compression, and triaxial stresses with the s3 axis in WNW‐ESE and NNE‐SSW directions. The triaxial stresses were determined from the Otadai and Umegase data. However, the WNW‐ESE tensile stress is not detected from the youngest, Kokumoto, suggesting that the stress is older than the formation. The area was subject to a WNW‐ESE tensile stress 01.2‐1.0 Ma, but tensile direction changed to a NE‐SW trend thereafter. The succession was simultaneous with a tectonic event in the landward slope of the Sagami Trough, suggesting that the subduction of the Philippine Sea plate aAected the stresses in the overriding plate. The older tensile stress was probably a manifestation of the gravitational collapse of the Hayama‐Mineoka ridge, which was growing parallel to the trough. The inferred stress history is concordant with the variation of plate convergence at the trough. # 2000 Elsevier Science Ltd. All rights reserved.

Periodic hotspot distribution and small-scale convection in the upper mantle

Hotspots are commonly attributed to mantle plumes and to a first order, the hotspot distribution is correlated with long-wavelength geoid anomalies and with higher temperatures in the lower mantle, suggesting that the plumes are generated in the lower mantle. I show that the second-order distribution is periodic with a typical spacing of 800–1000 km. Such a spacing would be too narrow for plumes to ascend directly from the lower mantle, but is close to the thickness of the upper mantle. The alignment of convection cells in the upper mantle seems to control the periodicity. To reconcile the first-a and second-order distributions, assuming limited mass fluxes across the 670-km boundary, a lateral deflection of plumes as they breach the transition zone from below is required.

Improved resolution of the multiple inverse method by eliminating erroneous solutions

Abstract The multiple inverse method is a numerical technique designed to separate stresses from heterogeneous fault–slip data. The method is one of the resampling methods based on the pattern recognition. Plotting solutions determined from k -fault subset into the parameter space, we have clusters representing significant stresses for the dataset. This paper presents a technique to improve the resolution of stress for the method. The regularity/singularity test of the subsets taken from the fault–slip data is the key for this purpose. The resolution and accuracy of the method are improved by eliminating erroneous stresses or artifacts that were yielded by the method. The performances of the method are demonstrated with a variety of artificial datasets.