An effort to find farside deep moonquakes among the recently discovered nests of deep moonquakes has identified about 30 nests that are likely to be on the farside. Although only a few of them are locatable with currently available data and those few provide little new information about the deep interior of the Moon, the inferred distribution of the rest of these nests indicates that either the region of the Moon's deep interior within about 40 degrees from the antipode of the Moon is nearly aseismic or, alternatively, the very deep interior of the Moon severely attenuates or deflects seismic waves. Since this has important bearing on the origin and evolution of the Moon, future missions to the Moon need to be oriented toward resolving this uncertainty. Some limited data favor the attenuation/deflection hypotheses.

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I have inverted the recently completed set of seismic arrival times from the Apollo lunar seismic network to estimate the average seismic velocities in three sections of the lunar mantle: two for the upper mantle and one for the middle mantle. The method used is a variation of the linearized least squares inversion where the inversion is accomplished in steps. The estimated average velocities in the upper mantle decrease from Vp = 7.74 km/s and V, = 4.49 km/s in the section above 270-km depth to Vp = 7.46 km/s and V, = 4.25 km/s in the section between 270-and 500-km depth, confirming the earlier finding of negative gradients based on seismic amplitude variations. The average velocities in the middle mantle between the depths of 500 km and 1000 km of V• = 8.26 km/s and V, -4.65 km/s are significantly higher than those in the upper mantle, contradicting earlier estimates based on more limited data. The higher velocities may suggest initial melting of the moon down to at least 1000-km depth. INTRODUCTION The moon is the only planetary body beside the earth for which we have seismic data to deduce its internal structure. The data analysis, however, has been difficult because of the paucity of seismic stations, the limited number of usable seismic events, and the complexity of seismic signals caused by intense scattering. Despite these problems, earlier attempts based on more limited data than now available were remarkably successful in determining major features of the lunar interior [e.g., Nakamura et al., 1974, 1976a; Toksdz et al., 1974; Goins et al., 1981].It is now widely known that the moon has a well-defined crust and a mantle and that the lower portion of the mantle appears to be partially molten. Details, however, are not so well defined, and there remain questions about seismic velocity variations and possible discontinuities in the lunar mantle.With the completion of processing of all the lunar seismic data collected during the 8 years of the Apollo lunar seismic network operation, we now have a complete set of seismic arrival times from which to infer the velocity structure of the lunar interior. This data base, unfortunately, is not likely to expand in the near future, because the receiving of the transmitted data has been terminated and no more lunar landings are being planned at present. In this paper, I present an inversion of this arrival time data set to estimate the seismic velocity structure of the lunar mantle. The present result generally agrees with previous results, but some significant differences are found especially for the lower part of the lunar mantle, for which a greatly expanded data base is now available using deep moonquake signals.This paper deals only with the seismic velocity structure of the moon, and therefore it is assumed that the reader has some familiarity with the experiment itself. For this reason, such frequently published figures as typical seismograms and station and source location maps are not included in this paper. A recent summary paper by Nakamura et al. [1982] co...

Seismic profiling data indicate that the thickness of an accreted oceanic terrane of Paleocene and early Eocene age, which forms the basement of much of the forearc beneath western Oregon and Washington, varies by approximately a factor of 4 along the strike of the Cascadia subduction zone. Beneath the Oregon Coast Range, the accreted terrane is 25 to 35 kilometers thick, whereas offshore Vancouver Island it is about 6 kilometers thick. These variations are correlated with variations in arc magmatism, forearc seismicity, and long-term forearc deformation. It is suggested that the strength of the forearc crust increases as the thickness of the accreted terrane increases and that the geometry of the seaward edge of this terrane influences deformation within the subduction complex and controls the amount of sediment that is deeply subducted.

A three‐axis short‐period seismometer has been operating on the surface of Mars in the Utopia Planitia region since September 4, 1976. During the first 5 months of operation, approximately 640 hours of high‐quality data, uncontaminated by lander or wind noise, have been obtained. The detection threshold is estimated to be magnitude 3 to about 200 km and about 6.5 for the planet as a whole. No large events have been seen during this period, a result indicating that Mars is less seismically active than earth. Wind is the major source of noise during the day, although the noise level was at or below the sensitivity threshold of the seismometer for most of the night during the early part of the mission. Winds and therefore the seismic background started to intrude into the nighttime hours starting on sol 119 (a sol is a Martian day). The seismic background correlates well with wind velocity and is proportional to the square of the wind velocity, as is appropriate for turbulent flow. The seismic envelope power spectral density is proportional to frequency to the −0.66 to −0.90 power during windy periods. A possible local seismic event was detected on sol 80. No wind data were obtained at the time, so a wind disturbance cannot be ruled out. However, this event has some unusual characteristics and is similar to local events recorded on earth through a Viking seismometer system. If it is interpreted as a natural seismic event, it has a magnitude of 3 and a distance of 110 km. Preliminary interpretation of later arrivals in the signal suggest a crustal thickness of 15 km at the Utopia Planitia site which is within the range of crustal models derived from the gravity field. More events must be recorded before a firm interpretation can be made of seismicity or crustal structure. One firm conclusion is that the natural background noise on Mars is low and that the wind is the prime noise source. It will be possible to reduce this noise by a factor of 103 on future missions by removing the seismometer from the lander, operation of an extremely sensitive seismometer thus being possible on the surface.