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Mechanism of tsunami earthquakes 总被引：1，自引：0，他引：1
Hiroo Kanamori 《Physics of the Earth and Planetary Interiors》1972,6(5):346-359
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We study the evolution (expansion or oscillation) of cylindrically symmetric magnetic flux ropes when the energy dissipation is due to a drag force proportional to the product of the plasma density and the radial speed of expansion. The problem is reduced to a single, second-order, ordinary differential equation for a damped, non-linear oscillator. Motivated by recent work on the interplanetary medium and the solar corona, we consider polytropes whose index, , may be less than unity. Numerical analysis shows that, in contrast to the small-amplitude case, large-amplitude oscillations are quasi-periodic with frequencies substantially higher than those of undamped oscillators. The asymptotic behaviour described by the momentum equation is determined by a balance between the drag force and the gradient of the gas pressure, leading to a velocity of expansion of the flux rope which may be expressed as (1/2)r/t, where r is the radial coordinate and t is the time. In the absence of a drag force, we found in earlier work that the evolution depends both on the polytropic index and on a dimensionless parameter, K. Parameter K was found to have a critical value above which oscillations are impossible, and below which they can exist only for energies less than a certain energy threshold. In the presence of a drag force, the concept of a critical K remains valid, and when K is above critical, the oscillatory mode disappears altogether. Furthermore, critical K remains dependent only on and is, in particular, independent of the normalized drag coefficient, v*. Below critical K, however, the energy required for the flux rope to escape to infinity depends not only on K (as in the conservative force case) but also on v*. This work indicates how under certain conditions a small change in the viscous drag coefficient or the initial energy may alter the evolution drastically. It is thus important to determine v* and K from observations. 相似文献
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Yu Nishihara Toru Shinmei Shun-ichiro Karato 《Physics of the Earth and Planetary Interiors》2006,154(1):30-43
Grain-growth kinetics in wadsleyite was investigated using a multianvil high-pressure apparatus. Fine-grained wadsleyite aggregates were synthesized by isostatic hot-pressing and were subsequently annealed under high pressure and temperature in a controlled chemical environment. Wadsleyite samples show normal grain-growth characterized by a log-normal grain-size distribution following the relation, where n is a constant, L the grain-size at time t, L0 the grain-size at time t = 0 and k is a rate constant that depends on temperature T and chemical environments (fO2: oxygen fugacity in Pa, COH: water content in H/106Si) as:
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Source parameters and scaling relationships in the Friuli-Venezia Giulia (Northeastern Italy) region
Gianlorenzo Franceschina Stefano Kravanja 《Physics of the Earth and Planetary Interiors》2006,154(2):148-167
We estimated the source parameters of 53 local earthquakes (2.0<ML<5.7) of the Friuli-Venezia Giulia (Northeastern Italy) area, recorded by the short-period local seismic network of the Istituto Nazionale di Oceanografia e Geofisica Sperimentale (OGS), in the period 1995-2003. Data were selected on the basis of high quality locations and focal mechanisms. Standard H/V spectral ratios (HVRS) of the three-component stations of the network were performed in order to assess local amplifications, and only stations showing HVRS not exceeding two were considered for the source parameters estimation. Both velocity and acceleration data were used to compute the SH-wave spectra. Observed spectra were corrected for attenuation effects using an independent regional estimate of the quality factor Q and a station dependent estimate of the spectral decay parameter k. Only earthquakes with ML>3.0 recorded with a sampling rate of 125 cps were used to compute k, thus allowing to visualize a linear trend of the high frequency acceleration spectrum up to 40-50 Hz. SH-wave spectra, corrected for attenuation, showed an ω−2 shape allowing a good fit with the Brune model. Seismic moments and Brune radii ranged between 1.5×1012 and 1.1×1017 N m and between 0.1 and 2.7 km respectively. We obtained Mo=1.1×1017 N m for the seismic moment of the Kobarid (SLO) main shock, in good agreement with the Harvard CMT solution (Mo=3.5×1017 N m). Brune stress drops were confined to the range from 0.07 to 5.31 MPa, with an average value of 0.73 MPa and seem to be approximately constant over five orders of magnitude of seismic moment. Radiated seismic energy computed from two nearby stations scales with seismic moment according to , and apparent stress values are between 0.02 and 4.26 MPa. The observed scatter of Brune stress drop data allowed to hypothesize a scaling relation between seismic moment and corner frequency in order to accommodate both Brune stress drop and apparent stress scalings. No systematic differences are evidenced between stress parameters of earthquakes with different focal mechanisms. As a consequence, a relation of the seismic stress release with the strength of rocks can be hypothesized. A high correlation (r>0.9) of Brune stress drop is found with both apparent stress and RMS stress drop, according to and respectively. 相似文献