Constituents of Vertical-component Coda Waves at Long Periods

Aki (1969) first modeled coda waves of a local earthquake as a superposition of scattered surface waves. This paper attempts to clarify the constituents of surface-wave coda at long periods at very long lapse times. For a large earthquake of magnitude 7 or larger, vertical component oscillation in periods from 90 to 180 s persists for more than 20 hours from the earthquake origin time. Although the early portion of the coda envelope is successfully modeled by assuming incoherent scattered Rayleigh waves by heterogeneities distributed all over the Earth, the later potion of the observed coda envelope (roughly later than 35,000 s) has systematically larger amplitude than theoretical prediction. To clarify the cause of this discrepancy, we studied the constituents of vertical-component seismograms of three large earthquakes recorded by the F-net in Japan using the f-k power spectral analysis. We found that the direct and scattered fundamental-mode Rayleigh waves of velocity about 3.7 km/s are dominant in the earlier part of each envelope. It justifies the use of a scattering model of the fundamental Rayleigh waves for synthesizing the envelope. At lapse times later than 20,000 s–35,000 s, higher modes with phase velocities around 20 km/s become dominant. The transition time to the dominance of higher modes is found to become earlier for a deeper focus earthquake. The small coda attenuation factor from (1.90±0.23) × 10⁻³ to (2.38±0.32) × 10⁻³ estimated from later coda envelopes recorded at IRIS stations distributed worldwide also agrees with the attenuation factor of spheroidal modes according to PREM. We may interpret that higher mode waves are uniformly distributed at large lapse time due to large velocity dispersion and/or scattering and they dominate over the fundamental mode waves because of smaller attenuation in the lower mantle. The coda attenuation measurement proposed by Aki is found to be useful even for long periods and at very large lapse times.     

Expansion of the Nishinomiya Built Environment Database

The Nishinomiya Built Environment Database, which can be used to analyze the disaster process of the 1995 Hanshin-Awaji Earthquake Disaster in Nishinomiya City, has been expanded with new data entries. The database contains the following very detailed datasets: (1) the urbanization area base map, (2) casualty data, (3) three sets of building damage data surveyed by the Nishinomiya City, the Architectural Institute of Japan and the City Planning Institute of Japan, and the Kobe University, (4) building property data based on the real estate tax roll, (5) photographs of the damaged buildings with the information on the place and orientation of the picture, and (6) the estimated distribution of the seismic ground motion. The seismic ground motion was simulated for the southern part of Nishinomiya City and two verification sites in Kobe City and Amagasaki City. In the simulation, the borehole data of public facilities were used to model the surface soils as one-dimensional layers, taking into consideration the fact that the spatial distribution of the sediment/basement interface forms a slope. The model of the fault rupture process simulated the characteristics of the seismic motion at basement level, and amplification effects of the surface layers were evaluated based on multiple reflection theory. The distribution of peak ground acceleration and peak ground velocity was estimated from acceleration response spectra at each borehole point. In addition, the relationship between simulated seismic ground motion and building damage was studied based on newly proposed band-passed spectrum intensity using the expanded database. This confirmed that detailed categorization is necessary in order to evaluate the fragility functions, especially for reinforced concrete structures. The database should provide fundamental information for identifying the relationship between the ground motions and the extent and pattern of building damage, or the pattern of the occurrence of casualties.Presently     

Deformation structures in shale bed indicate flow direction of overlying Miocene subaqueous pyroclastic flow

Thrusts, pinch-and-swell structures and undulations are present within a 2-m-thick layered shale bed of Miocene age that is overlain by a rhyolite subaqueous pyroclastic flow deposit 2–3 m thick. The deformation structures were caused by loading and lateral compression by the subaqueous pyroclastic flow, probably analogous to those observed in layered muds deformed by a sand mass advancing across them. Prominent thrusts strike east-west and dip south, and the crests of undulations strike east-west, indicating that the subaqeuous pyroclastic flow moved northward.     

Raman spectroscopic study of clinopyroxenes in the system CaScAlSiO6 - CaAl2SiO6

Seven clinopyroxenes in the system CaScAlSiO₆- CaAl₂SiO₆ synthesized at 1 atm and under high pressure have been studied by Raman spectroscopy. The T-O-T stretching band of CaScAlSiO₆ pyroxene can be deconvoluted into three bands corresponding to Al-O-Al, Al-O-Si, and Si-O-Si stretching vibrations, although that of CaAl₂SiO₆ can be deconvoluted into the two bands (Al-O-Al+Al-O-Si) and Si-O-Si. The Al-O-Si Raman shifts of CaScAlSiO₆ and CaAl₂SiO₆ pyroxenes are found to fall on the linear plot of the relationship between T-T distance and Raman shifts in ABSi₂O₆-type pyroxenes, suggesting that the Al-O-Si chains are relatively long. Variation of areal fractions of the Raman bands demonstrates that the partial disordering of Al/Si depends on the ionic radius and electronegativity of the octahedral ion.     

Imbrication, flow direction and possible source areas of the pumice-flow tuffs near Bend, Oregon, U.S.A.

Imbrication, indicating flow and source direction, occurs in three Pleistocene or upper Pliocene pumice-flow tuffs exposed in a 700-km² area on the east flank of the Cascade Range near Bend, Oregon, and shows the location of previously unknown source vents of these tuffs. The imbrication is formed by inclined elongate and/or flat pumice or lithic fragments and locally by elongate plagioclase crystals. Imbrication is best developed within the lower zones of individual flow units; the pumiceous top zones also locally show imbrication directions parallel to that in the lower zones. Moreover, the areal pattern of size distribution of lithic and pumice fragments in the flows is concordant with the flow direction pattern indicated by imbrication.The upper pumice flow shows a fan-shaped pattern of flow directions indicated by imbrication which points to a western source. A possible vent, about 20 km west of Bend in the highland near Broken Top Volcano, is marked by many silicic domes and basaltic cinder cones where there is a 6–8 mgal negative Bouguer gravity anomaly. In contrast, imbrication in the middle and lower pumice flows indicates flow from a source southwest of Bend. Vents in this direction are not obvious. Possible buried vents are located about 30 km and 45 km southwest of Bend near Sitkum Butte and Lookout Mountain, respectively.     

Effect of occurrence rate of acoustic emissions on their statistical behavior

The parameterm in Ishimoto-Iida's relation was investigated for acoustic emissions (AEs) occurring in rock samples under uniaxial compression. In the experiment, we found: 1) The large AEs are counted without serious error but the number of small AEs is systematically underestimated at high AE rates, 2) the frequency distribution of maximum AE amplitudes becomes nonlinear in logarithmic scale with increasing AE rate, and 3) there exists a strong negative correlation betweenm-value and AE rate. The miscount of small AEs was interpreted as due to overlap of the large and small AEs. We call the miscount masking effect. A statistical analysis based on the masking effect showed that them-value decreases more effectively as the AE rate increases, and thus the masking effect is a possible origin both for the nonlinear frequency distribution of maximum AE amplitudes and for the negative correlation ofm-value with AE rate. We emphasize that one should be careful of the masking effect to examine correctly the change, ofm-value. In order to eliminate the masking effect, AEs should be measured by a measurement system with low sensitivity. Even if the masking effect is eliminated, them-value decreases before the main fracture of a rock sample. Them-value is a key parameter to predict the main fracture.