S-wave Attenuation Research Articles

Ray path analysis of sediment shear wave effects on bottom reflection loss

The effect of sediment shear wave propagation on bottom reflection loss (RL) from a typical deep sea sediment layer is analyzed by means of a ray path decomposition of the acoustic field. The amplitude of each shear (S) and the compressional (P) ray is obtained from the interface reflection and transmission coefficients. These coefficients are analyzed by means of an expansion in the parameter ε which is proportional to the relatively small S-wave velocity of marine sediments. Ray paths with amplitudes up to order ε2 are identified and used to develop a qualitative ray path model of RL. P-wave conversion at the sediment–substrate interface is the dominant mechanism for sediment S-wave excitation. To order ε2, RL is the result of the interference of three waves in the water whose amplitudes are proportional to ε0, ε1, and ε2. Comparison with RL obtained from a computational model shows that the ray path model correctly predicts the dependence of RL on frequency, S-wave speed, S-wave attenuation, and P-wave attenuation.

Seismological studies at Cerro Prieto

During the period 14 February — 11 March 1978, a radio-telemetered 12-station seismographic network was operated in a dense-array configuration over the Cerro Prieto field as shown in Fig. 1. Horizontal and vertical 4–5-Hz geophones were deployed with interstation distances of order 1 km in a multi-purpose survey for microearthquake occurrence and possible anomalies in P-and S-wave velocities and attenuation associated with the field. Seventeen sites were occupied on NE-SW and NW-SE lines crossing the production zone. Seventy-four earthquakes having S-P interval times less than 5 s (distance less than about 40 km) were detected. Six of these earthquakes were located within the field in a single swarm on February 18 and 19, at a depth of 2–3 km. Radiation patterns are consistent with right lateral strike-slip faulting on a north-west — south-east trending fault. P- and S-wave times for the events within the field reveal a high Poisson's ratio of 0.4 in the upper 2 km of sediments at the field. The field appears to be characterized by high attenuation near stations 5 and 6. There is some indication of slightly higher P-wave velocity throughout the field area, with a locally anomalous low velocity near the center of the production zone around station 6. Although data obtained thus far are limited, attenuation and velocity studies at Cerro Prieto show anomalies related to the production area. This suggests that these data might be useful for delineating the field boundaries. A five-station semi-permanent network of 150 m deep, three-component digitally recorded seismographs will be installed over the field in the fall of 1978 for more continuous monitoring of the field characteristics.

Short-period S-wave attenuation under the United States

A study of short-period S -wave spectra from deep events recorded at LRSM stations indicates severe attenuation of S waves under the Western United States. The average differential in ts * between the Eastern and Western United States was found to be ∼0.67 ± 0.19 sec (95 per cent confidence interval), which is several times greater than Δ tp * ∼ 0.2 - 0.25 for short-period P . This indicates that most, if not all, energy loss occurs in shear deformation. The numerical value of the ts * differential is also smaller than that derived from long-period spectra indicating a frequency dependence of Q .

S-wave shadows in the Krafla Caldera in NE-Iceland, evidence for a magma chamber in the crust

During the present tectonic activity in the volcanic rift zone in NE-Iceland it has become apparent that the attenuation of seismic waves is highly variable in the central region of the Krafla volcano. Earthquakes associated with the inflation of the volcano have been used to delineate two regions of high attenuation of S-waves within the caldera. These areas are located near the center of inflation have horizontal dimensions of 1–2 km and are interpreted as the expression of a magma chamber. The top of the chamber is constrained by hypocentral locations and ray paths to be at about 3 km depth. Small pockets of magma may exist at shallower levels. The bottom of the chamber is not well constrained, but appears to be above 7 km depth. Generally S-waves propagate without any anomalous aftenuation through laver 3 (vp=0.5 km sec−1) across the volcanic rift zone in NE-Iceland. The rift zone therefore does not appear to be underlain by an estensive magma chamber at crustal levels. The Krafla magma chamber is a localized feature of the Krafla central volcano.

The evolution of the upper mantle beneath mid-ocean ridges

Several recent measurements of Rayleigh wave dispersion in the Pacific and Atlantic oceans have demonstrated conclusively that phase and group velocities in the period range 20–200 seconds increase systematically with increasing age of the sea floor. The increase in velocity can be explained by the growth and cooling of the oceanic lithosphere as it moves away from the mid-ocean ridges. If the base of the lithosphere is defined as the top of the low-velocity zone, the average lithospheric thickness can be no more than 25 km for a region from the ridge axis to an isochron of 5 million years. Within 10 m.y. after formation, the lithosphere reaches a thickness of about 60 km. As the mantle continues to cool, the thickness gradually increases, reaching a maximum of about 90 km in old ocean basins. In addition to the slow growth after 10 m.y., there is a gradual increase in shear velocity within the lithosphere from about 4.4 km s −1 at 10 m.y. to 4.6 to 4.7 km s −1 beneath old sea floor. The increase has also been found in regional studies of S n velocity in the Atlantic. Beneath the lithosphere extending to a depth of about 175 km, there is a zone of very low shear velocities apparently associated with partial melting. There is a significant increase in the shear velocity within this low-velocity zone with increasing age. A narrow zone of low Q and increased partial melting has been outlined directly beneath the ridge axis by observations of lateral variation in attenuation of longperiod S-waves and Rayleigh waves. This zone is roughly coincident with the elevated region of low compressional and shear velocities under sea floor which is less than 10 m.y. old. The seismic data are consistent with a model of the oceanic lithosphere as a thermal boundary layer growing at the expense of a laterally homogeneous asthenosphere. Substantial partial melting occurs at shallow depths beneath the ridge axis where the asthenospheric mantle material rises near the surface. The boundary-layer model is consistent with the change in elevation of the sea floor with age and with recently revised estimates of oceanic heat flow.

New evidences about the deep structure of the lipari arc

Deep-focus earthquakes in the region of the Lipari Islands are studied from the point of view of high-frequency S-wave attenuation. The results are interpreted on the basis of plate-tectonics theory, and allow the definition of a three-dimensional model of the deep structure of this area. The model is the ensemble of planes obtained for each station by considering only the events that show good propagation of high-frequency S-waves. The geometry of this model shows a rigid structure dipping with an angle of approximately 60° with a curvature that follows the northern coast of Sicily and southern coast of Calabria and is concave to the WNW direction. The possiblity of the division of the Mediterranean Sea into two basins, and the possibility of an oceanization process in the western part are also discussed.

S波の減衰よ琴見た日本のマントル上部機造

i)S波吸収の強さの地域的分布を深発地震のloog(A/TB)-△ 曲線によって調査し,日本の地下における地震波の吸収係数を求めた。それらは平均値として,西日本に対しては 4.6×10-3/km,東日本では10-4/km程度である。ii)震源分布から地震活動帯(SA-zone)の構造を明らかにした。SA-zoneは浅発地震活動帯(SSA-zone)と深発地震活動帯(DSA-zone)とに大別され,その分岐線が,ほぼ盛岡一白河線(MS線)および別府一鹿児島線(BK線)に沿っていることを明らかにした。これらの線は活火山帯と位置的に密接な関係がある。iii)地震波吸収の強さは一般にSA-zoneにおいて小さく,その他の地域で大きい。特に,SSA-,DSA-zoneにはさまれた地域(SD-Gap)においては分岐線に近い部分ほど大きく,それより遠ざかるにしたがって小さくなる。これらの事実と異常震域との関連について述べた。