PL modes in the earthquake seismogram were discussed again. Theoretical dispersion curves for three watercovered crustal models were computed by OLIVER and MAJOR'S and SU and DORMAN'S methods. The transition from the oceanic PL to the continental PL was discussed. Analyzed records are rather "new" and are supported by the high accuracy of time and the suitable instrumental response. Crustal thicknesses were determined from observed group velocities of PL modes. Some discussions on the dispersion of the PL mode which was represented in HUNKINS and KUO'S paper were also given.
In the present paper ω and ξ are taken as angular frequency and angular wave number in horizontal direction. Leaking modes in characteristic equation M (ω, ξ)=0 for dispersive waves have been solved by many authors respectively with various additional conditions. When the imaginary part of U≡dω/dξ is assumed to be zero, we have approximately the same solution to that where the imaginary part of ξ is made zero. Dispersion curves for various conditions, imaginary part of ξ, U and ω are zero, are compared with each other on the figure which has been obtained from the results in the previous sections. The dispersion curve for γ=0 is also compared with that obtained by OLIVER and MAJOR's approximation, in which γ means an imaginary part of γ=ω/ξ.
Surface waves of short periods were finely found on the records of three Teshikaga earthquakes in 1965. The observation point was Kushiro in Hokkaido and the seismometer used was Type 59 of J. M. A.. The magnitudes of these earthquakes were about 5 and the epicentral distances were between 50 and 60km. The EW-component of the seismometer recorded Love waves and the NS- and the vertical components recorded Rayleigh waves, because the direction propagated by these surface waves was nearly north to south. Three modes, M11, M12 and Love were identified on the records and group velocities of them were obtained from these records. Only three models of the crustal structure are given in the present paper, though much more models were examined numerically. One of the successful models, HOK-5 B, has the sedimentary layer whose thickness is 2.25km.
An extensive seismic refraction experiment with the use of explosive sources was conducted in the Kitakami region, northern Honshu, Japan, on November 1, 1990. The experiment area is divided into two geological units by the Hayachine Tectonic Belt (HTB). The southern terrane consists of pre-Silurian basements and Silurian-lower Cretaceous marine sediments, while the northern one is characterized by a Jurassic accretionary complex. Both of the units were intruded by Cretaceous granitic rocks.An almost N-S seismic refraction profile of 194-km length was extended from Kuji City, Iwate Prefecture to Ishinomaki City, Miyagi Prefecture, on which 4 shots with a charge size of 450-700 kg were fired. The generated seismic signals were recorded at 179 stations, from which a precise crustal structure was determined. The uppermost crust is covered with a very thin (0.5-1 km) surface layer with a velocity of 3.1-5.4 km/s. Our results show a remarkable structural difference between the northern and southern Kitakami terranes. A velocity of the uppermost crust is 5.90 km/s in the northern part of the profile while 6.05-6.15 km/s in the southern part. This lateral velocity change occurs just beneath the HTB. A mid-crustal interface corresponding to well-recorded wide-angle reflections (PiP phase) shows an abrupt southward depth decrease from 25 to 20 km beneath the HTB. The reflections from the Moho boundary (PmP phase) were well observed in all of the record sections. The Moho depth determined from the PmP phase also decreases southward from 34 to 32 km. The velocity of the lower crust is 6.8-7.0 km/s. The refracted waves from the uppermost mantle (Pn phase) were observed with weak amplitudes at offsets greater than 160 km. These data suggest that the Pn velocity in the experiment area is less than 7.7 km/s.The VP/VS ratio within the crust was determined from S-waves recognized in the vertical component seismograms. The VP/VS ratio in the upper crust also shows a significant lateral change at the HTB, namely 1.72-1.73 in the northern part whereas 1.75-1.76 in the southern part. In the lower crust, the VP/VS ratio is 1.73-1.76. In the present experiment, weak refracted waves from the upper mantle (Sn phase) were observed at offsets greater than 170 km, from which the Sn velocity was estimated to be 4.3 km/s.
abstract Group velocities of fundamental-mode Rayleigh and Love waves are analyzed to construct a crustal structure of the Tibetan Plateau. A moving window analysis is employed to compute group velocities in a wide period range of 7 to 100 sec for 17 individual paths. The crustal models derived from these dispersion data indicate that under the Tibetan Plateau the total crustal thickness is about 70 km and that the crustal velocities are generally low. The low velocities are most probably caused by high temperatures. A low-velocity zone located at an intermediate depth within the crust appears to be strongly demanded by the observed dispersion data. The main features of the proposed crustal structure will place stringent constraints on future tectonic models of the Tibetan Plateau which is generally regarded as a region of active deformation due to the continent-continent collision between India and Asia.
Refraction measurements by the two ship method were conducted in July 1969 in the deep sea terrace of Hidaka, Hokkaido. The general features of the crustal structures derived are similar to those in the adjacent oceanic areas. Thick sediments cover the whole area, especially its northernmost part. The layer with a P-wave velocity of 6km/s is found in all profiles. That is, the crustal structure in this area is continental and similar to that off the Sanriku area.
