Yukitoshi Fukahata

The Climatic Signature of Incised River Meanders

Messy Mountain Meandering Predicting the influence of climate on landscapes is sometimes straightforward; for example, river deposits might grow with increased rainfall because erosion rates and sediment transport increase. However, long-term tectonic processes complicate the geomorphic signatures of more gradual climate-related phenomena that reconfigure landscapes. By correlating a decades-long record of typhoon rainfall in Japan with digital elevation models, Stark et al. (p. 1497) show that climate directly influences the extent of river meandering. When expanded to a larger region of the western North Pacific, this analysis revealed a strong climatic imprint on the landscape of humid mountainous areas. The region-wide analysis also revealed that underlying bedrock strength, as opposed to tectonic uplift, acts as a secondary control. Typhoon frequency and bedrock strength influence river meandering in mountain environments. Climate controls landscape evolution, but quantitative signatures of climatic drivers have yet to be found in topography on a broad scale. Here we describe how a topographic signature of typhoon rainfall is recorded in the meandering of incising mountain rivers in the western North Pacific. Spatially averaged river sinuosity generated from digital elevation data peaks in the typhoon-dominated subtropics, where extreme rainfall and flood events are common, and decreases toward the equatorial tropics and mid-latitudes, where such extremes are rare. Once climatic trends are removed, the primary control on sinuosity is rock weakness. Our results indicate that the weakness of bedrock channel walls and their weakening by heavy rainfall together modulate rates of meander propagation and sinuosity development in incising rivers.

Afterslip and viscoelastic relaxation following the 2011 Tohoku‐oki earthquake (Mw9.0) inferred from inland GPS and seafloor GPS/Acoustic data

We simultaneously estimate 2.5 years of afterslip and viscoelastic relaxation, as well as coseismic slip, for the 2011 Tohoku-oki earthquake. Displacements at inland GPS and seafloor GPS/Acoustic stations are inverted using viscoelastic Greens functions for a model with an upper elastic layer and lower viscoelastic substrate. The result shows that afterslip is isolated from the rupture area and possibly asperities of historical earthquakes and has almost decayed by 10 September 2013, 2.5 years after the main shock. The inversion result also suggests that observed landward postseismic displacements at the seafloor GPS/Acoustic stations are caused by the viscoelastic relaxation, whereas trenchward displacements at inland stations are mainly an elastic response to afterslip.

A scenario for the generation process of the 2011 Tohoku earthquake based on dynamic rupture simulation : Role of stress concentration and thermal fluid pressurization

We perform dynamic rupture simulations to improve the understanding of the generation process of the 2011 Tohoku earthquake. We assume a dynamic weakening mechanism (dynamic thermal pressurization of pore fluid, hereinafter called TP) on the fault plane to represent nonlinear weakening friction, and take into account the shear stress changes before the Tohoku earthquake, due to the four M 7-class earthquakes that occurred during 2003–2011. To constrain the dynamic rupture simulation, the moment release rate obtained by seismic slip inversions is referred. The simulation result implies the following about the 2011 Tohoku earthquake: (1) The rupture around the hypocenter was enhanced by the stress accumulation due to the preceding M 7-class earthquakes. (2) The enhanced rupture triggered the TP mechanism in the near-trench area causing large slip, which promoted propagation of the rupture over a wide region including the source areas of the M 7-class earthquakes and surrounding conditionally-stable areas. (3) Without sufficient stress accumulation, the moment release of the Tohoku earthquake ended as an M 8-class earthquake. (4) TP in the near-trench area should be effective but moderate. The occurrence time of the next megaquake would be strongly affected by the nonlinear effects of TP and the stress conditions. Thus, our model may contradict the concept of the (quasi-)cyclic occurrence of M 9 earthquakes.

Crustal movements on Shikoku, southwestern Japan, inferred from inversion analysis of levelling data using ABIC

Abstract With an inversion technique based on Akaikes Bayesian Information Criterion (ABIC) we analyzed levelling data for 1893–1983 on Shikoku, southwestern Japan, where large interplate earthquakes have periodically occurred at intervals of about 120 years, releasing tectonic stress produced by steady relative motion of the Philippine Sea and the Eurasian plates. Through the inversion analysis we reconstructed the pattern of crustal movements on Shikoku over the last 120 years, including the occurrence of the 1946 Nankaido earthquake (M 8.1). The result clearly shows that the crustal movements on Shikoku include significant secular vertical motion (uplift in the south and subsidence in the north) in addition to cyclic motion related to the periodic occurrence of interplate earthquakes at the Nankai trough. Contrary to the widely accepted theory, we could not find any correlation between the secular vertical motion and the coseismic vertical displacement. The secular uplift motion on southern Shikoku estimated from the levelling data completely agrees with that inferred from the present heights of marine terraces formed by eustatic sea level changes and crustal uplift for the last 105 yr. This suggests that the fundamental causes of the short-term (102 yr) and long-term (105 yr) movements on southern Shikoku are the same, the steady subduction of the Philippine Sea plate at the Nankai trough. On northern Shikoku, on the other hand, the pattern of secular crustal motion estimated from the levelling data is quite different from that of the Quaternary uplift inferred from the present heights of eroded flat surfaces, indicating the existence of some unknown tectonic process controlling the very long-term (106 yr) crustal movements.

Quasi-Static Strain and Stress Fields due to a Moment Tensor in Elastic–Viscoelastic Layered Half-Space

We derived explicit expressions in the time domain for 3-D quasi-static strain and stress fields, due to a point moment tensor source in an elastic surface layer overlying viscoelastic half-space under gravity. The expressions of strain in the elastic surface layer were directly obtained from the expressions of displacement in our previous paper. The conversion of strain into stress is easy, because the stress–strain relation of elastic material is linear. In the viscoelastic substratum, the expressions of strain were obtained by applying the correspondence principle of linear viscoelasticity to the associated elastic solution. The strain–stress conversion is not straightforward, as the stress–strain relation of viscoelastic material is usually given in a differential form. To convert strain into stress, we used an integral form of the stress–strain relation instead of the usual differential form. The expressions give the responses of elastic half-space at

Correlation between surface heat flow and elevation and its geophysical implication

t = 0

Long‐term changes in the Coulomb failure function on inland active faults in southwest Japan due to east‐west compression and interplate earthquakes

t=0, and the responses of an elastic plate floating on non-viscous liquid at