Carbon isotopic thermometry calibrated by dolomite-calcite solvus temperatures

The temperature dependence of carbon isotopic fractionations between calcite and graphite, and between dolomite and graphite are calibrated by the calcite-dolomite solvus geothermometry using marbles collected from the contact metamorphic aureole in the Kasuga area, central Japan. The carbon isotopic fractionations (Δ¹³C_Cc-Gr and Δ¹³C_DoGr) systematically decrease with increasing metamorphic temperature. The concordant relationships between the fractionations and solvus temperatures are approximately linear with T^?2 over the temperature range. 400° to 680°C: Δ¹³C_CcGr (%.) = 5.6 × 10⁶ × T^?2 (K) ? 2.4 Δ¹³C_DoGr (%.) = 5.9 × 10⁶ × T^?2 (K) ? 1.9 These systematic relationships between fractionation and temperature suggest that carbon isotopic equilibria between carbonates and graphite were attained in many cases. The equation for the calcite-graphite system has a slope steeper than Bottinga's (1969) results. It is, however, in good agreement with that of Valley and O'Neil (1981) in the temperature range from 600° to 800°C.Because of the relatively high sensitivity to temperature, these isotopic geothermometers are useful for determining the temperatures in moderate- to high-grade metamorphosed carbonate rocks.     

Magnitude scale and quantification of earthquakes

Despite various shortcomings, the earthquake magnitude scale is one of the most fundamental earthquake source parameters to be used for catalogs. Although use of a uniform scale is desirable, it is not always possible because of changes in instrumentation, the data reduction method and the magnitude formula, the station distribution, etc. As a result, various magnitude scales have been developed and are currently in use. Recent developments in seismometry and earthquake source theories provide more quantitative source parameters than the magnitude. In order to maintain continuity and uniformity of the data, it is important to relate these magnitude scales and the new parameters. In view of this importance, relations between different magnitude scales are examined with an emphasis on the difference in the period of the waves used for the magnitude determination. Use of several magnitude scales determined at different periods provides a convenient method for characterizing earthquakes. The moment magnitude can be used to quantify both shallow and deep earthquakes on the basis of wave energy radiated, and provides a uniform scheme.     

Transient flexural vibrations of a timoshenko column supported by an elastic half-space

Transient flexural vibrations of an elastic column supported by an elastic half-space are investigated analytically under the condition that an arbitrarily shaped free-field lateral acceleration is given as an input. Applying the Timoshenko theory to the column and making use of Laplace transformations with respect to time and numerical inverse Laplace transformations, the time histories of the column free end acceleration are presented. Numerical results obtained from the Timoshenko theory are compared with those of a previous paper¹ (applying the Bernoulli-Euler theory to the column), and the effects of column slenderness and foundation stiffness on the transient flexural vibrations of the column are clarified.     

Complexity of rupture in large strike-slip earthquakes in Turkey

Complexity of rupture propagation has an important bearing on the state of stress along the earthquake fault plane and on the prediction of strong ground motion in the near-field. By studying far-field body waveforms recorded by WWSSN long-period seismograms it has been possible to investigate the degree of complexity of several Turkish earthquakes. The results, which are obtained by matching synthetic P waveforms to observed data indicate that the July 22, 1967 Mudurnu Valley earthquake (M_s = 7.1) is a complex event which can be explained by the superposition of elementary sources with variable amplitudes and source time sequence history. In this regard, it is very similar to the February 4, 1976 Guatemala earthquake (M_s = 7.5). A comparison of these two events indicates that their source-time series ranges from 5 to ca. 20 s and, regardless of the total moment of the earthquake, the moment of the individual events is bounded at around 5 × 10²⁶ dyn cm. The November 24, 1976 E. Turkey earthquake (M_s = 7.3), on the other hand, has a complexity which cannot be explained by such a simple model; in this respect, it may be more similar to the Tangshan, China, earthquake and as such, may involve significant thrust, normal or other complications to its faulting mechanism than the strike-slip mechanism of the P-wave first-motion data. The source time history for the 1967 Mudurnu Valley event is used to illustrate its significance in modeling strong ground motion in the near field. The complex source-time series of the 1967 event predicts greater amplitudes (2.5 larger) in strong ground motion than a uniform model scaled to the same size for a station 20 km from the fault. Such complexity is clearly important in understanding what strong ground motion to expect in the near-field of these and other continental strike-slip faults such as the San Andreas.     

Minerals formed and mineral formation from volcanic ash by weathering

Weathering of volcanic ash and pumice is unique regarding minerals formed and mineral formation. These minerals include allophane, imogolite, opaline silica and some halloysites, and have distinctive structures and properties. Studies on the surface and subsurface weathering of various volcanic ashes in different settings are reviewed. This review illustrates that relatively rapid weathering of finely comminuted volcanic ash well reflects the effects of climate, vegetation and time, and produces various combinations of weathering products. The moisture regime, the nature and amount of organic matter incorporated, the deposition of volcanic ash as overburden, and the chemical and mineralogical composition of parent ash are important in controlling the mineral formation in weathered volcanic ash.     

Magnetic detection of heated soils at paleolithic sites in Japan

We present a methodological approach to detect heated soil on ancient sites, using magnetic measurements. The method is based on changes in magnetic signals of soil by heating. The following three types of soil were used for testing the method: silty soil (SS), weathered volcanic ash (WVA, = loam) and fairly fresh volcanic ash (VA) called Odori tephra. On heating above 250–600°C, the magnetic susceptibility and remanent magnetization intensity increased for the SS and WVA samples, reflecting chemical alteration of magnetic minerals (from goethites to magnetites through hematites). The VA sample showed no susceptibility change suggesting the absence of goethites within it. On heating below 250°C, only the intensities of all the samples increased. This is possibly due to acquisition of thermal remanent magnetization. The largest change of the magnetic signals was identified for the SS sample and the smallest one was seen for the VA sample. Therefore, the in situ susceptibility measurement, which is the nondestructive and indirect method, seems to be effective to detect heated soil for sites of aqueous deposits as the SS. On the other hand, for sites of aeolian deposits as the WVA (loam) and VA, the intensity measurement of collected soils seems to be the most reliable method to detect evidence of heating. The degree of the magnetic stability (coercivity) against progressive alternating-field demagnetization was also an important parameter, indicating whether the investigated soils were heated or unheated. © 1999 John Wiley & Sons, Inc.