Yosio Nakamura

Size and morphology of the Chicxulub impact crater

The Chicxulub impact in Mexico has been linked to the mass extinction of species at the end of the Cretaceous period. From seismic data collected across the offshore portion of the impact crater, the diameter of the transient cavity is determined to be about 100 km. This parameter is critical for constraining impact-related effects on the Cretaceous environment, with previous estimates of the cavity diameter spanning an order of magnitude in impact energy. The offshore seismic data indicate that the Chicxulub crater has a multi-ring basin morphology, similar to large impact structures observed on other planets, such as Venus.

Crustal architecture of the cascadia forearc.

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.

Passive seismic experiment.

Seismometer operation for 21 days at Tranquillity Base revealed, among strong signals produced by the Apollo 11 lunar module descent stage, a small proportion of probable natural seismic signals. The latter are long-duration, emergent oscillations which lack the discrete phases and coherence of earthquake signals. From similarity with the impact signal of the Apollo 12 ascent stage, they are thought to be produced by meteoroid impacts or shallow moonquakes. This signal character may imply transmission with high Q and intense wave scattering, conditions which are mutually exclusive on earth. Natural background noise is very much smaller than on earth, and lunar tectonism may be very low.

Seismic data from man-made impacts on the moon

Unusually long reverberations were recorded from two lunar impacts by a seismic station installed on the lunar surface by the Apollo 12 astronauts. Seismic data from these impacts suggest that the lunar mare in the region of the Apollo 12 landing site consists of material with very low seismic velocities near the surface, with velocity increasing with depth to 5 to 6 kilometers per second (for compressional waves) at a depth of 20 kilometers. Absorption of seismic waves in this structure is extremely low relative to typical continental crustal materials on earth. It is unlikely that a major boundary similar to the crustmantle interface on earth exists in the outer 20 kilometers of the moon. A combination of dispersion and scattering of surface waves probably explains the lunar seismic reverberation. Scattering of these waves implies the presence of heterogeneity within the outer zone of the mare on a scale of from several hundred meters (or less) to several kilometers. Seismic signals from 160 events of natural origin have been recorded during the first 7 months of operation of the Apollo 12 seismic station. At least 26 of the natural events are small moonquakes. Many of the natural events are thought to be meteoroid impacts.

New Seismic Data on the State of the Deep Lunar Interior

Direct shear-wave arrivals from seismtic events originating on the far side of the moon are not observed at some of the stations of the Apollo seismic network. These data suggest that the material in the lunar interior at a depth of 1000 to 1100 kilometers is more dissipative for seismic shear waves than the lithosphere above, and possibly exists in a partially molten state akin to the earths asthenosphere.

Lunar subsurface investigated from correlation of seismic noise

By correlating seismic noise recorded by four sensors placed on the Moon during the Apollo 17 mission, we have retrieved a well-defined dispersed Rayleigh wave pulse. Inversion of its group velocity provides new constraints on the lunar subsurface structure. The estimated ”signal-to-noise” ratio (SNR) of the retrieved Rayleigh wavetrain is strongly dependent on solar illumination, effectively making solar heating a source of seismic noise on the Moon. This result suggests that in future planetary missions it is feasible to extract information on the internal structure of extraterrestrial objects by correlating seismic noise even when natural quakes are absent.

Seismic scattering and shallow structure of the moon in oceanus procellarum

Long, reverberating trains of seismic waves produced by impacts and moonquakes may be interpreted in terms of scattering in a surface layer overlying a non-scattering elastic medium. Model seismic experiments are used to qualitatively demonstrate the correctness of the interpretation. Three types of seismograms are found, near impact, far impact and moonquake. Only near impact and moonquake seismograms contain independent information. Details are given in the paper of the modelling of the scattering processes by the theory of diffusion.Interpretation of moonquake and artificial impact seismograms in two frequency bands from the Apollo 12 site indicates that the scattering layer is 25 km thick, with a Q of 5000. The mean distance between scatterers is approximately 5 km at 25 km depth and approximately 2 km at 14 km depth; the density of scatterers appears to be high near the surface, decreasing with depth. This may indicate that the scatterers are associated with cratering, or are cracks that anneal with depth. Most of the scattered energy is in the form of scattered surface waves.

Large-offset seismic surveying using ocean-bottom seismographs and air guns; instrumentation and field technique

Repeatable, closely spaced signal sources from large‐capacity air guns and detection and recording of signals using highly flexible, microprocessor‐controlled, digital ocean‐bottom seismographs allow us to acquire high‐quality, large‐offset, marine seismic refraction and reflection data. The acquired data are readily adaptable to various processing techniques originally developed for seismic reflection data. There are several requirements and problems specific to the technique. For example, bubbly signals from one or two large‐capacity air guns are often preferable to bubble‐suppressed signals from tuned arrays in identifying weak arrivals at large offset distances. Recorded water‐wave signals at near ranges provide precise locations of detectors relative to shots.

Shallow lunar structure determined from the passive seismic experiment

Data relevant to the shallow structure of the Moon obtained at the Apollo seismic stations are compared with previously published results of the active seismic experiments. It is concluded that the lunar surface is covered by a layer of low seismic velocity (Vp ≃ 100 m s−1), which appears to be equivalent to the lunar regolith defined previously by geological observations. This layer is underlain by a zone of distinctly higher seismic velocity at all of the Apollo landing sites. The regolith thicknesses at the Apollo 11, 12, and 15 sites are estimated from the shear-wave resonance to be 4.4, 3.7, and 4.4 m, respectively. These thicknesses and those determined at the other Apollo sites by the active seismic experiments appear to be correlated with the age determinations and the abundances of extralunar components at the sites.

A search for clustering among the meteoroid impacts detected by the Apollo lunar seismic network

Abstract We examined temporal clustering of meteoroid impacts detected by the Apollo lunar seismic network and found a distinct difference between “small” meteoroids (masses smaller than about 1 kg) and “large” meteoroids (masses larger than about 1 kg). Small meteoroids show strong clustering, many of which are identified with showers known from terrestrial meteor studies. In contrast, little clustering is found for large meteoroids, suggesting that they represent meteoroids of type and origin different from those of the small meteoroids. Overall, 28% of the small events and 15% of the large events occur as clusters. The small meteoroids appear to be mostly cometary, while the large meteoroids may be derived from near-Earth asteroids and short-period comets. Two swarms of large meteoroids detected in June 1975 and January 1977 possibly contain high-density meteoritic objects, and thus may represent “meteorite streams.”