Effects of multiple scattering on coda waves in three-dimensional medium

The contribution of multiple scattering to the coda waves for three-dimensional elastic medium is investigated by extending the computational procedures developed earlier for the two-dimensional medium. It is shown that the effects of multiple scattering start to become important at a shorter lapse time,t _c = 0.65(n ₀σu)^?1, than in the two-dimensional case (t _c = 0.8(n ₀σu)^?1).     

Effect of slip rate on stress drop

Recent observations made by Kanamori and Allen about earthquake recurrence time and average stress drop revealed a very interesting relation: earthquakes with longer recurrence times have higher average stress drops. They attributed the difference in stress drop to the difference in long-term average slip rate. To interpret their result in terms of the healing effect, we simulated earthquake recurrence with a one-dimensional mass-spring model, incorporating a recently developed rate-and-state dependent friction law for different loading rates and heterogeneous strength distributions. We first calculated the stress drop and recurrence time as functions of loading rate for a homogenous fault model. We found that the stress drop increases up to 30% when the loading rate decreases from 10 cm/yr to 0.01 mm/yr. Thus, the observed great variability of stress drop, from a few bars to a few hundred bars, which is obtained by replotting the data of Kanamori and Allen in the form of stress drop versus long-term slip rate, may not be attributable to the healing effect alone. Our numerical simulation shows that the variability may be due primarily to the spatial heterogeneity of strength on the fault. Our simulation also suggests that of the two empirical laws that were inferred from the same laboratory friction data, called the power law and the logarithmic law by Shimamoto and Logan, the former can explain the observed relation between stress drop and slip rate better than can the latter, at least for strike-slip fault. The logarithmic law is an earlier and simpler version of the rate-state-dependent friction law.     

Multiple scattering and energy transfer of seismic waves—Separation of scattering effect from intrinsic attenuation II. Application of the theory to Hindu Kush region

In order to separate the scattering effect from intrinsic attenuation, we need a multiple scattering model for seismic wave propagation in random heterogeneous media. In paper I (Wu, 1985), radiative transfer theory is applied to seismic wave propagation and the energy density distribution (or the average intensity) in space for a point source is formulated in the frequency domain. It is possible to separate the scattering effect and the absorption based on the measured energy density distribution curves. In this paper, the data from digital recordings in the Hindu Kush region are used as an example of application of the theory. We also discuss two approximate solutions of coda envelope in the time domain: the single scattering approximation and the diffusion approximation and discuss the relation with the frequency domain solution. We point out that in only two cases can the apparent attenuation be expressed as an exponential decay form. One is thedark medium case, i.e., whenB ₀0.5, whereB ₀ = _s /( _s + _a ) is the seismic albedo, _s is the scattering coefficient, _a is the absorption coefficient. In this case the absorption is dominant, the apparent attenuationb can be approximated by the coherent wave attenuationb = _s + _a . The other case is thediffuse scattering regime, i.e., whenB ₀0.5 (bright medium) andRL _s ,t _s , whereR andt are the propagation distance and lapse time,L _s and _s are the scattering lengths (mean free path) and scattering time (mean free time), respectively. However, in this case the envelope decays with a rate close to the intrinsic attenuation, while the intensity decreases with distance with a coefficientb d ₀( _s + _a ) d _{s s} , whered ₀ andd _s are the diffusion multipliers (0<d ₀,d _s <1).For the Hindu Kush region, by comparing the theory with data from two digital stations of 53 events distributed up to depths of 350 km, we find that the scattering is not the dominant factor for the measured apparent attenuation ofS waves in the frequency range 2–20 Hz. From the observation on high frequency (f>20 Hz) seismograms, we suggest the existence of a stron-scattering surface layer with fine scale heterogeneities in the crust, at least for this region.     

Microstructural Characterization of Granular Cast Iron for Permeable Reactive Barriers

Although intensive research on Fe(0) permeable reactive barriers (PRB) for in situ groundwater remediation has been conducted and multiple applications have been installed in the past two decades, some properties of reactive materials in use have not been fully considered and discussed yet. In the present investigation, a typical granular cast iron has been characterized with different techniques. The grain size distribution not only has an influence on the resulting pore geometry and the surface area but material properties significantly differ between fine and coarse grains. Metallographic analyses revealed large differences in both graphite inclusions and microstructures that likely influence the reactivity. Both graphite and cementite proved to be more resistant toward acidic dissolution compared to Fe⁰. The intrinsic material characteristics described here have not been covered in the existing PRB literature.     

Redistribution and export of contaminated sediment within eastern Fukushima Prefecture due to typhoon flooding

Tropical cyclones expose river basins to heavy rainfall and flooding, and cause substantial soil erosion and sediment transport. There is heightened interest in the effects of typhoon floods on river basins in northeast Japan, as the migration of radiocaesium‐bearing soils contaminated by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident will affect future radiation levels. The five main catchments surrounding FDNPP are the Odaka, Ukedo, Maeda, Kuma and Tomioka basins, but little quantitative modelling has been undertaken to identify the sediment redistribution patterns and controlling processes across these basins. Here we address this issue and report catchment‐scale modelling of the five basins using the GETFLOWS simulation code. The three‐dimensional (3D) models of the basins incorporated details of the geology, soil type, land cover, and used data from meteorological records as inputs. The simulation results were checked against field monitoring data for water flow rates, suspended sediment concentrations and accumulated sediment erosion and deposition. The results show that the majority of annual sediment migration in the basins occurs over storm periods, thus making typhoons the main vectors for redistribution. The Ukedo and Tomioka basins are the most important basins in the region in terms of overall sediment transport, followed by the other three basins each with similar discharge amounts. Erosion is strongly correlated with the underlying geology and the surface topography in the study area. A low permeability Pliocene Dainenji formation in the coastal area causes high surface water flow rates and soil erosion. Conversely, erosion is lower in an area with high permeability granite basement rocks between the Hatagawa and Futaba faults in the centre of the study area. Land cover is also a factor controlling differences in erosion and transport rates between forested areas in the west of the study area and predominantly agricultural areas towards the east. The largest sediment depositions occur in the Ogaki and Takigawa Dams, at the confluence of the Takase and Ukedo Rivers, and at the Ukedo River mouth. Having clarified the sediment redistribution patterns and controlling processes, these results can assist the ongoing task of monitoring radioactive caesium redistribution within Fukushima Prefecture, and contribute to the design and implementation of measures to protect health and the environment. Copyright © 2016 John Wiley & Sons, Ltd.     

A shallow attenuating anomaly inside the ring fracture of the Valles Caldera, New Mexico

Spectral ratios of teleseismic direct and scattered P waves observed in the Valles Caldera, New Mexico, show a systematic pattern of low amplitudes at sites inside the caldera relative to sites on or outside the ring fracture. Waveforms recorded at caldera stations are considerably more complex than those recorded outside the caldera. The data used in this study were collected during a passive seismic monitoring experiment conducted in 1987. Twenty-four teleseismic events were recorded on two linear arrays spanning the caldera. To first order, the pattern of low amplitudes did not vary with source incidence angle or azimuth of approach, and could not be explained by anomalous amplification at the ring fracture. This observation suggests the presence of a shallow, attenuating zone associated with the caldera fill material inside of the ring fracture. We estimated the general features of the caldera's near-surface structure for the two-dimensional vertical cross section beneath the array, using a modification of the Aki-Larner discrete-wavenumber method to forward model the observed amplitude variations. Our results indicate that the caldera fill material must be subdivided into at least two distinct zones: a strongly attenuating lower zone, extending to depths in excess of 4 km, and a mildly attenuating surface layer. To fit the data we had to assign an unrealistically low value to seismic Q in the deeper attenuating anomaly. We attribute this to the inability of the Aki-Larner method to account for strong re-direction of energy away from the caldera due to local heterogeneity that we could not include within the low-Q anomaly. This interpretation is consistent with the pervasive, fractured hydrothermal system that is known to exist in the caldera fill material.     

Preliminary Results on the 3-dimensional Seismic Structure of the Lithosphere under the USGS Central California Seismic Array

About 1500 readings of teleseismic P -time residuals obtained from the US Geological Survey seismograph network in central California have been used to obtain a three-dimensional image of seismic velocity anomalies for this area by the method of Aki, Christoffersson & Husebye We found that the California network is less suitable than the LASA and NORSAR arrays for this kind of studies because of its greater proportion of peripheral blocks in which the resolution is very poor for the stochastic inverse solution and the random error effect is severe for the generalized inverse solution. Nevertheless, the resultant velocity anomalies show a remarkable correlation with the San Andreas fault zone to a depth of 75 km. The anomaly pattern changes drastically as the depth exceeds 75 km, suggesting that the asthenosphere has been reached.     

Seismicity simulation with a mass-spring model and a displacement hardening-softening friction law

Dieterich simulated aftershocks numerically, using a one-dimensional mass-spring model with a time-dependent friction law. But an important precursory phenomenon called quiescence cannot be produced by this model unless, as Mikumo and Miyatake showed with a three-dimensional continuum model, a somewhat arbitrary bimodal distribution of frictional strength is assumed. Here we used the friction law proposed by Stuart, which is a displacement hardening-softening model, and simulated the quiescence. By varying the parameters of the friction law in our mass-spring model, we found a variety of seismicity patterns. When we choose extremely large critical displacement we get a recurrent sequence of creep followed by mainshock without small earthquakes. But when we choose a critical displacement in the same order of magnitude as the slip-weakening critical displacement estimated by Papageorgiou and Aki from strong motion data, we get a normal seismicity pattern, including quiescence before large events. This simple model points to a promising approach for the interpretation of the rupture process during an earthquake by the same physical model.