Koji Matsunami

Laboratory tests of excitation and attenuation of coda waves using 2-D models of scattering media

Abstract Effects of the scattering and intrinsic Q measured for direct waves on the decay of seismic coda with time and the level of the coda excitation were investigated by laboratory experiments. Seismograms with long codas were produced using an ultrasonic technique and sufficiently large 2-D models of scattering media. The results are: (1) the level of coda excitation is high when scattering attenuation is large, whereas it is low when scattering attenuation is small; (2) the observed coda decay depends only on the intrinsic Q of media models as predicted by the energy transfer theory of Shang and Gao; and (3) the scattering Q −1 value has a peak around kd = 2–3 ( k is the wave number and d is the scatterer size) and decreases steeply on both sides of the peak.

Laboratory measurements of spatial fluctuation and attenuation of elastic waves by scattering due to random heterogeneities

To study the effects of strong scattering on elastic waves, spatial fluctuation and scattering attenuation ofP waves were examined by laboratory experiments for 2-D models of random media approximately characterized by a triangular correlation function in the range of 2<ka<33, wherek is the wave number anda is the correlation distance of the heterogeneities, i.e., the heterogeneity size. The results obtained are as follows: (1) Forka>10, both the intensity and the correlation distance of the amplitude fluctuation are approximate for any phase of theP-wave train. The correlation distance nearly agrees with the heterogeneity size. These fluctuation properties are quite consistent with the theoretical prediction by the forward-scattering approximation. (2) For 3<ka<6, the fluctuation intensity becomes stronger in later phases of theP-wave train. This shows that scattering is approximately isotropic, and therefore, the scattered energy increases with time within theP-wave train. The correlation distance of the amplitude fluctuation disagrees with the heterogeneity size, and it shows a frequency-dependent property decreasing from 7a to 4a with the increase ofka from 3 to 6. These properties for 3<ka<6 have not yet been predicted theoretically. (3) Forka<3, though the fluctuation is considerably smaller compared with that ofka>10 and 3<ka<6, the fluctuation property is considered similar to that of 3<ka<6. (4) The observed scattering attenuation,Q−1, increases withka forka<3, has a peak aroundka=3∼5, and then decreases withka. (5) When θmin = 15° and σ = 0.075, the theoreticalQ−1 curve, predicted by the approximate theory of Wu, roughly matches the observedQ−1 values, where θmin is the minimum scattering angle measured from the propagation direction of theP waves and σ is the rms of fractional velocity fluctuation. This suggests that the energy scattered in the range of θ>15° is lost from theP waves, while the energy scattered in the range of θ<15° is retained; and that the approximate theory overestimates by about three times the σ value of the model media used owing to the neglect of multiple scattering. (6) When the size of velocity heterogeneities responsible for forward scattering at 3<ka<6 is estimated from the θmin value of 15° on the basis of Wus theory, it nearly agrees with the correlation distance for the initial phase of theP-wave train.

Estimation of Seismic Site Response in the Tangshan Area, China, Using Deep Underground Records

Site amplification in the Tangshan area, China, was analyzed using records of small events ( M L 2.9–4.4) observed by a strong motion array with a deep (–822 m) underground station deployed between 1982 and 1984 following the 1976 Tangshan earthquake. The effect of destructive interference at the underground site due to surface reflections was confirmed to be negligible in this case. The source, propagation path, and site effects were separated simultaneously by linear inversion using the underground reference site. The S -wave quality factor was found to be approximated by the relation 29 f 0.9 in the frequency range 1–10 Hz. The conventional surface rock reference site was found to have a characteristic frequency-dependent site effect, which increased markedly from the theoretical amplification factor of 2 over the frequency range of interest. A reference-independent technique assuming an ω -squared source model was also introduced and produced results that were in good agreement with the results obtained using the underground reference. The reference-independent technique revealed the underground station to have a nearly flat response consistent with the theoretical response, indicating that an underground or borehole bedrock site is a better reference than a surface rock for seismic hazard analysis. The reference-independent technique produced accurate absolute site amplification factors for the area examined in this study, demonstrating the usefulness of the method for site effect analysis.

Review of Monitoring Parameters of the Kostanjek Landslide (Zagreb, Croatia)

Since 2011, in the framework of the Croatian-Japanese SATREPS FY2008 Project, scientists have been working on the establishment of the Kostanjek landslide monitoring system in the City of Zagreb (Croatia). External triggers at Kostanjek landslide are measured with rain gauge and accelerometers. Displacements at the surface are measured by GNSS sensors and extensometers, while subsurface displacement is measured by vertical extensometers and inclinometer. Hydrological measurements consist of groundwater level measurements, discharge measurements, chemical and isotope analysis. Monitoring sensors recorded landslide reactivation due to external triggers in the winter period of 2012/2013. During the period from September 2012 to March 2013 the total cumulative precipitation was 793.7 mm and horizontal displacements were in the range of 9–20 cm. The installed monitoring sensor network proved to provide reliable data for the establishment of relations between landslide causal factors and landslide displacement rates aimed at establishing threshold values for early warning system.

Local site amplification and damage to wooden houses in Shimoenoki, Tottori, Japan, by the 2000 Western Tottori Earthquake

The degree of damage to wooden houses in the vicinity of the source area of the 2000 Western Tottori Earthquake was very low compared to the 1995 Hyogo-ken Nanbu Earthquake. Shimoenoki in Hino Town, however, suffered remarkable damage compared to other villages, particularly to residential wooden houses. Furthermore, although Shimoenoki is a small area of only 300 × 400 m2, the damage varied markedly from the foot of the mountains through central Shimoenoki to the Hino River. From the damage distribution based on a survey of all wooden houses, the local site-amplification characteristics estimated from aftershock records, and the transfer functions of wooden houses evaluated using microtremors, the spatial variation of damage appears to be attributed to the variation in site-amplification factors at frequencies between 2 and 5 Hz, which is close to the first natural frequency of wooden houses.

Soil Condition and Seismic Characteristics in Shiga Prefecture, Japan

The seismic observation network in Shiga prefecture iscomposed of four systems.All the observation data obtained were used to estimatethe dynamic characteristicsof the surface soils around the lake and the seismichazard for Hikone City. Since1995, 21 records have been obtained at the USP site ofwhich those for the 1999Shigaken-Hokubu Earthquake and 1999 Wakayarnaken-HokubuEarthquake wereused in this study. Transfer functions of the surfacesoil in each ward of ShigaPrefecture based on the Mt. Kojin site (KJY), whichhas relatively hard soil, werecalculated from the records of the two earthquakes.Assuming a scenario earthquakeat the Hyakusaiji fault close to Hikone City, maximumground motions were obtainedfor 15 city blocks in order to establish a damageestimation for that city. The numberof collapsed wooden houses and the damage ratiowere calculated based on the distributionof construction of wooden houses by year. Blockswith a larger ratio of old, wooden housesin Hikone showed a close correlation with blockswith a large ratio of aged persons inthem, indicative that weak persons, such as theelderly, may find themselves in a severesituation during a natural disaster.