Observed Peak Ground Acceleration Research Articles

やや深発地震の短周期地震動の励起強さとその地域性 : 最大加速度値に基づく検討

やや深発地震の短周期地震動の励起強さとその地域性 : 最大加速度値に基づく検討

1996年8月秋田・宮城県境付近の地震群のK-NET強震記録から推定した震源・伝播・サイト特性

We estimated source spectra, an attenuation function, and site amplification factors using the K-NET records for six earthquakes (MJ=4.4-5.9) that occurred in the border of Akita and Miyagi Prefectures in August, 1996. These earthquakes occurred inland at the focal depths from 6 to 11km. The hypocentral distances of the stations used in this study range from 5 to 200km. We analyzed 212 source-station pairs×3 components, that is, radial, transverse, and vertical components. The Q-1 estimated by assuming the geometrical spreading to be the reciprocal of the hypocentral distance showed strong decay with frequency. It was modeled by Q-1=0.005f-1.5 in the frequency range from 0.3 to 1Hz in which Q-1 had almost the same value among the three components. We also found that the Q-1 has no clear difference between the eastern and western regions to the volcanic front. The site amplification factor at IWT09 (Daito station) in the Kitakami region whose geology is categorized into pre-Tertiary rock is smallest in the frequency range from 0.1 to 4Hz. Therefore we examined site amplification factors and source spectra in the frequency range from 0.1 to 4Hz using IWT09 as a reference site. The site amplification factors relative to IWT09 vary from site to site very strongly and their standard deviation for all the used 59 sites is the largest, about a factor of 2.5 of the average, for the horizontal components in the frequency range from 0.5 to 2Hz. For the vertical components that is a factor of 2.2 of the average in the similar frequency range. The source spectra for all the six earthquakes have predominant peaks at the frequency about 0.3Hz for all the three components. We found that the remarkable dominance of the horizontal components in the frequency of 0.3Hz at MYG05 (Naruko, the nearest station with an epicentral distance of about 10km) are caused by the source spectra as well as its site amplification. The Brune's stress drops were estimated to be 5 bar, much smaller than the average stress drop of 200 bar estimated for subduction-zone earthquakes in the Tohoku district. Acceleration magnitude Mpga, which is determined to match the observed peak ground acceleration with an empirical attenuation relation, is 0.3 to 0.7 smaller than the JMA magnitude MJ. The relation between the Mpga and the seismic moment M0 of the Harvard CMT solution, also suggested low static stress drops from 5 to 15 bar. However, the relation between the MJ and M0 is consistent with the SATO's empirical relation which is equivalent to the static stress drops of 50 bar. We interpreted this discrepancy to be caused by the prominent generation of waves in the frequency range around 0.3Hz at which MJ is determined.

Modelling the Roermond earthquake of 1992 April 13 by stochastic simulation of its high-frequency strong ground motion

SUMMARY The Roermond earthquake of 1992 April 13 is one of the most important seismic events for assessing potentially destructive strong ground-motion parameters in central Europe. Owing to the lack of unclipped near-source records, however, the source depth as well as the high-frequency signal contents are only known to within large uncertainties. Using a stochastic simulation technique, site-dependent synthetic acceleration seismograms and simulated Wood-Anderson records for the Roermond Earthquake were calculated for the epicentral region and a site in 60 km distance. For the distant site, three different damping models were modelled while for the epicentral region, a tertiary and quaternary coverage of 1500 m was assumed. Both constant- and frequency-dependent Q were considered. the resulting simulations were tested for their consistency with (a) epicentral acceleration between 1.0 and 2.0m sec−2 as indicated by the low observed epicentral intensities of slightly above VII, (b) the observed peak ground accelerations at a distance of 50-60 km which are reported to be 0.3 ± 0.1 m sec−2, and (c) the observed Wood-Anderson magnitude of MWA≈ 5.9±0.2. While the stochastic simulation does not result in a single unique model, it allows the comparison of certain models (including their uncertainties) in terms of their consistency with the observations. the most pronounced result shows that of the three different source depths (13, 21 and 25 km) which were investigated, the shallow source is by a wide margin the least consistent with the observations, independent of the remaining model assumptions. On the other hand, peak accelerations of 0.3 ± 0.1 m sec−2 in 50-60km distance can be explained by reasonable combinations of seismic moments and corner frequencies for all the site-damping models. While under the constant-Q assumption, the most consistent M0/fc combinations show a clear trade-off between fc and M0 for M0≈ (5-10) × 1016 Nm, this trade-off is strongly reduced if Q is assumed frequency dependent as Q=Q0×fα. For an average seismic moment of M0= 7.5 × 1016 Nm, a corner frequency of 1.0–1.25 Hz provides the maximum joint consistency scores independent of the other model assumptions.

Spatial coherence of the ground motion produced by the 1979 Imperial Valley earthquake across El Centro Differential Array

Spatial coherence of digitally recorded acceleration time histories across five elements of the El Centro Differential Array recorded during the October 15, 1979 Imperial Valley, California, earthquake is studied. Recorded within a total spacing of 214 m, the three-component time histories at five stations of this linear array provide a unique set of free-field data at sites of similar geology in the near field of a strong earthquake. The observed peak ground accelerations vary by as much as 43, 13 and 40% on the E-W, N-S and vertical components. It is shown that the incoherence of high frequency waves brought about by wave scattering near the receiver is partially responsible for the observed variation. This is evident from the frequency dependent reduction of the peak cross-correlation amplitudes of the strong phase of ground shaking. The observed reductions increase from ∼ 1% below 1 Hz to 20–30% in the 1–10 Hz band and to nearly 80% above 10 Hz. Investigation of the polarization angle of P-wave coda indicates that high frequency waves (> 10 Hz) approach the recording station randomly from nearly all directions. The peak cross-correlation of the polarization angle in general is lower than that of amplitudes. The successive sampling of this angle in the low frequency band suggest a northwestward propagation of the source during the faulting process.