Nakamura, Y., New identification of deep moonquakes in the Apollo lunar seismic data, Phys. Earth Planet. Int., 139, 197-205, 2003, 15 citations, doi:10.1016/j.pepi.2003.07.017, #1655 
A new computer search of the Apollo lunar seismic data set for identification of deep moonquakes using a combination of waveform cross-correlation and single-link cluster analysis has increased the number of positively identified deep moonquakes by more than a factor of five, from the previous 1360 to 7245. At least 88 new deep moonquake source regions (nests) have been discovered, while some source regions previously identified as separate entities are now determined to be identical, reducing the number of previously known source regions from 108 to 77. The newly identified deep moonquakes and their relationships to each other are likely to revise our understanding of their spatial and temporal distributions.
Pulliam, J., Y. Nakamura, C. I. Huerta-Lopez, and B. Yates, Field test of an inexpensive, small broadband ocean-bottom seismograph, Bull. Seismol. Soc. Amer., 93, 152-171, 2003, 4 citations, doi:10.1785/0120010275, #1586
We conducted tests of a three-component broadband ocean-bottom seismograph (OBS), including side-by-side comparisons with the broadband Global Seismic Network/U.S. National Seismic Network station HKT at Hockley, Texas, and a 28-day deployment in the Gulf of Mexico. Our goals were to evaluate seismometer performance and determine whether our seafloor deployment strategy allows useful earthquake data to be collected. The seismometer generally performed well, but showed unexpectedly high intrinsic noise at frequencies above 1 Hz and produced occasional spikes that were confined to a single component at a given time. We identified 32 earthquakes from the Gulf of Mexico data; only five additional events were observed at the nearby, highly sensitive station HKT on land. While noise levels were higher throughout the 0.01-20 Hz frequency band in the Gulf of Mexico compared with HKT, site amplification effects in the gulf were only significant at frequencies above 1 Hz and power in most earthquake signals peaked at frequencies below 0.1 Hz, nearly coinciding with a minimum in background noise. We found the seismometer and OBS package encouraging for far-regional and teleseismic studies, given the low cost of the OBS and inexpensive means for deployment and recovery, but less encouraging for detecting and locating small-magnitude local and near-regional events.
Trehu, A. M., N. L. Bangs, M. A. Arsenault, G. Bohrmann, C. Goldfinger, J. E. Johnson, Y. Nakamura, and M. E. Torres, Complex subsurface plumbing beneath the southern Hydrate Ridge, Oregon continental margin, from high-resolution 3D seismic reflection and OBS data, Fourth Int. Conf. Gas Hydrates, Yokohama, Japan, 19023, 90-96, 2002, #1805
Christeson, G. L., Y. Nakamura, R. T. Buffler, J. V. Morgan, and M. R. Warner, Deep crustal structure of the Chicxulub impact crater, J. Geophys. Res., 106, 21751-21769, 2001, 27 citations, #1543
We present the results of a wide-angle seismic survey conducted over the Chicxulub impact crater. Profile Chicx-A/Al is a chord across the offshore portion of the crater, and Chicx-B/F is an onshore-offshore profile through the center of the crater. We use travel times recorded by 32 ocean bottom seismograph and 19 land-based receivers to model the crustal structure of the crater. The primary feature in the shallow velocity structure is a low-velocity Tertiary basin, the edge of which correlates with a steep gravity gradient in some regions of our survey. Directly beneath the Tertiary basin at the center of the crater is a region of relatively low velocities and densities that is interpreted as a unit of melt rocks (either suevite breccias with isolated melt pods or suevite breccias overlying a coherent melt sheet). This unit has a thickness of ∼1 km and a diameter of ∼100 km. Central uplift with a diameter of 40â60 km is observed on Chicx-B/F. A lower limit on the vertical extent of uplift is 9 km. Shallow basement is also observed along the northwest portion of Chicx-B/F, associated with a prominent northwest trending high in the gravity field. Superimposed on a regional trend is Moho uplift of ∼1 km near the center of the Chicx-A/Al profile, with adjacent Moho deepening of ∼1.25â1.5 km. The Moho topography may be related to deformation processes associated with the formation of the outer ring or to the excavation and collapse of the transient cavity.
Huerta-Lopez, C. I., J. Pulliam, Y. Nakamura, and K. H. Stokoe, Soft sediment characterization from passive motion measurements on the seafloor, Proc. 2001 Int. Conf. Offshore Tech. Res., Houston, TX, 32-51, 2001, #1567
Huerta-Lopez, C. I., J. Pulliam, Y. Nakamura, and B. Yates, Modeling amplification effects of marine sedimentary layers via horizontal/vertical spectral ratios, Proc. 71st Ann. Mtg. SEG, San Antonio, Texas, MC 2.8, v. 1, 825-828, 2001, #1581
Wang, T.-K., K. D. McIntosh, Y. Nakamura, C.-S. Liu, and H.-W. Chen, Velocity-interface structure of the southwestern Ryukyu subduction zone from EW9509-1 OBS/MCS data, Marine Geophysical Researches, 22, 265-287, 2001, 7 citations, doi:10.1023/A:1014671413264, #1616
A wide-angle seismic survey, combining ocean-bottom seismometers (OBS) and multi-channel seismic (MCS) profiling, was implemented in the southwestern Ryukyu subduction zone during August and September 1995. In this paper, we present the data analysis of eight OBSs and the corresponding MCS line along profile EW9509-1 from this experiment. Seismic data modeling includes identification of refracted and reflected arrivals, initial model building from velocity analysis of the MCS data, and simultaneous and layer-stripping inversions of the OBS and MCS arrivals. The velocity-interface structure constructed along profile EW9509-1 shows that the northward subduction of the Philippine Sea Plate has resulted in a northward thickening of the sediments of the Ryukyu Trench and the Yaeyama accretionary wedge north of the trench. The boundary between the subducting oceanic crust and the overriding continental crust (represented by a velocity contour of 6.75 km/s) and a sudden increase of the subducting angle (from 5 degrees to 25 degrees) are well imaged below the Nanao Basin. Furthermore, velocity undulation and interface variation are found within the upper crust of the Ryukyu Arc. Therefore, the strongest compression due to subduction and a break-off of the slab may have occurred and induced the high seismicity in the forearc region.
Kim, S. D., S. Nagihara, and Y. Nakamura, P- and S-wave velocity structures of the Sigsbee abyssal plain of the Gulf of Mexico from ocean bottom seismometer data, Trans., Gulf Coast Assn. Geol. Socs., 50, 475-483, 2000, #1766
McIntosh, K. D., F. E. Akbar, C. Calderon-Macias, P. L. Stoffa, S. Operto, G. L. Christeson, Y. Nakamura, T. H. Shipley, E. R. Flueh, A. U. Stavenhagen, and G. Leandro, Large aperture seismic imaging at a convergent margin: Techniques and results from the Costa Rica seismogenic zone, Marine Geophysical Researches, 21, 451-474, 2000, 5 citations, doi:10.1023/A:1026597927732, #1514
In March and April 1995 a cooperative German, Costa Rican, and United States research team recorded onshore-offshore seismic data sets along the Pacific margin of Costa Rica using the R/V Ewing. Off the Nicoya Peninsula we used a linear array of ocean bottom seismometers and hydrophones (OBS/H) with onshore seismometers extending across much of the isthmus. In the central area we deployed an OBS/H areal array consisting of 30 instruments over a 9 km by 35-km area and had land stations on the Nicoya Peninsula adjacent to this marine array and also extending northeast on the main Costa Rican landmass. Our goal in these experiments was to determine the crustal velocity structure along different portions of this convergent margin and to use the dense instrument deployments to create migrated reflection images of the plate boundary zone and the subducting Cocos Plate. Our specific goal in the central area was to determine whether a subducted seamount is present at the location of the 1990, M 7 earthquake off the Nicoya Peninsula and can thus be linked to its nucleation. Subsequently we have processed the data to improve reflection signals, used the data to calculate crustal velocity models, and developed several wide-aperture migration techniques, based on a Kirchhoff algorithm, to produce reflection images. Along the northern transect we used the ocean bottom data to construct a detailed crustal velocity model, but reflections from the plate boundary and top and bottom of the subducting Cocos plate are difficult to identify and have so far produced poor images. In contrast, the land stations along this same transect recorded clear reflections from the top of the subducting plate or plate boundary, within the seismogenic zone, and we have constructed a clear image from this reflector beneath the Nicoya shelf. Data from the 3-D seismic experiment suffer from high-amplitude, coherent noise (arrivals other than reflections), and we have tried many techniques to enhance the signal to noise ratio of reflected arrivals. Due to the noise, an apparent lack of strong reflections from the plate boundary zone, and probable structural complexity, the resulting 3-D images only poorly resolve the top of the subducting Cocos Plate. The images are not able to provide compelling evidence of whether there is a subducting seamount at the 1990 earthquake hypocenter. Our results do show that OBS surveys are capable of creating images of the plate boundary zone and the subducting plate well into the seismogenic zone if coherent reflections are recorded at 1.8 km instrument spacing (2-D) and 5 km inline by 1 km crossline spacing for 3-D acquisition. However, due to typical high amplitude coherent noise, imaging results may be poorer than expected, especially in unfavorable geologic settings such as our 3-D survey area. More effective noise reduction in acquisition, possibly with the use of vertical hydrophone arrays, and in processing, with advanced multiple removal and possibly depth filtering, is required to achieve the desired detailed images of the seismogenic plate boundary zone.
Christeson, G. L., R. T. Buffler, and Y. Nakamura, Upper crustal structure of the Chicxulub impact crater from wide-angle ocean bottom seismograph data, in Large Meteorite Impacts and Planetary Evolution II, edited by Dressler, B. O. and V. L. Sharpton, Boulder, CO, Geol. Soc. Amer. Spec. Paper, 339, 291-298, 1999, 2 citations, #1393
Nakamura, Y., Preface to the special issue commemorating the 30th anniversary of Apollo 11's landing on the Moon, (in Japanese), Yuseijin (Planetary People); J. Jap. Soc. Planet. Sci., 8 (2), 75-76, 1999, #1438
Chen, A. T., and Y. Nakamura, Velocity structure beneath the eastern offshore region of southern Taiwan based on OBS data, TAO: Terrestrial, Atmospheric and Oceanic Sciences, 9, 409-424, 1998, 14 citations, #1415
McIntosh, K. D., and Y. Nakamura, Crustal structure beneath the Nanao forearc basin from TAICRUST MCS/OBS line 14, TAO: Terrestrial, Atmospheric and Oceanic Sciences, 9, 345-362, 1998, 22 citations, #1407
Nakamura, Y., K. D. McIntosh, and A. T. Chen, Preliminary results of a large offset seismic survey west of Hengchun Peninsula, Southern Taiwan, TAO: Terrestrial, Atmospheric and Oceanic Sciences, 9, 395-408, 1998, 26 citations, #1408
Morgan, J. V., M. R. Warner, the Chicxulub Working Group, J. Brittan, R. T. Buffler, A. Camargo, G. L. Christeson, P. Denton, A. Hildebrand, R. W. Hobbs, H. Macintyre, G. MacKenzie, P. Maguire, L. Marin, Y. Nakamura, M. Pilkington, V. L. Sharpton, D. B. Snyder, G. Suarez, and A. Trejo, Size and morphology of the Chicxulub impact crater, Nature, 390, 472-476, 1997, 148 citations, doi:10.1038/37291, #1330
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.
Nakamura, Y., Moon: Seismicity, Encyclopedia of Planetary Sciences, Chapman & Hall, London, 513-516, 1997, #942
Banardt, B., R. Abercrombie, S. Keddie, H. Mizutani, S. Nagihara, Y. Nakamura, and W. T. Pike, Planetary interiors, in Planetary Surface Instruments Workshop,, edited by C. Meyer, A. H. Treiman, and T. Kostiuk, Tech. Rep. LPI/TR-95-05, Lunar and Planetary Inst., 41-50, 1996, #1306
Christeson, G. L., Y. Nakamura, K. D. McIntosh, and P. L. Stoffa, Effect of shot interval on ocean bottom seismograph and hydrophone data, Geophys. Res. Lett., 23, 3783-3786, 1996, 10 citations, #1246
Data collected by 18 ocean bottom receivers for a seismic line shot at both 50‐m (∼24 s shot interval) and 125‐m (∼58 s shot interval) shot spacing provide a direct field comparison of the effect of shot interval on marine wide‐angle seismic data. Our results indicate that both shot spacings produce high‐quality refraction data in shallow water (<1000 m) on hydrophone and vertical channel data. In deeper water, the data quality of the 50‐m line is adequate for the vertical channel, but it is often poor at large offsets for the hydrophone channel in comparison to the 125‐m shot spacing data. A theoretical model to explain these observations provides further information useful for designing an experiment using ocean‐bottom receivers.
Nagihara, S., J. G. Sclater, J. D. Phillips, E. W. Behrens, T. Lewis, L. A. Lawver, Y. Nakamura, J. Garcia-Abdeslem, and A. E. Maxwell, Heat flow in the western abyssal plain of the Gulf of Mexico: Implications for thermal evolution of the old oceanic lithosphere, J. Geophys. Res., 101, 2895-2913, 1996, 8 citations, #1182
The seafloor depth of an oceanic basin reflects the average temperature of the lithosphere. Thus the western abyssal plain of the Gulf of Mexico, which has technically subsided much (> 1 km) deeper than other basins of comparable ages (late Jurassic), should be underlain by an anomalously cold lithosphere. In order to examine this hypothesis, we made suites of high-accuracy heat flow measurements at 10 sites along a line connecting Deep Sea Drilling Project (DSDP) sites 90 and 91 in the Sigsbee abyssal plain. The new heat flow sites were initially surveyed by 3.5-kHz echo sounding, 4-channel seismic reflection, seismic refraction with eight ocean bottom seismometers, and nine piston cores. We occupied a total of 48 heat flow stations along the seismic survey line (3 to 6 at each site), including 28 where we measured in situ thermal conductivities over the practical depth interval (4 m) of the new multioutrigger bow heat flow probe. We determined the heat flow associated with the lithosphere by correcting the values measured at the seafloor (41 to 45 mW/m2) for (1) the thermal effect of the sedimentation and (2) the additional heat from the radioactive elements within the sediments. The sedimentation history, required for the first, was reconstructed at each heat flow site based on ages and thicknesses of the major seismic stratigraphical sequences, age data from the DSDP cores, 3.5-kHz subbottom reflectors, and correlation of turbidite units found in the piston cores. Radiogenic heat production was measured for 55 sediment samples from four DSDP holes in the gulf, whose age ranged from present to Early Cretaceous (0.83 μW/m3 on the average). This provided the correction for the second. The effects of these two secondary factors approximately cancel one another. The lithospheric heat flow under the abyssal plain thus estimated ranges from 40 to 47 mW/m2. These heat flow values are among the lowest in the Mesozoic ocean basins where highly reliable data (45 to 55 mW/m2) have been reported. Therefore the lithosphere under the gulf seems indeed colder than that under other old ocean basins. However, it is not as cold as expected from the large tectonic subsidence. The inconsistency between the depth and heat flow may imply an anomaly in the regional thermal isostasy.
Chen, A. T., Y. Nakamura, and L.-W. Wu, Ocean bottom seismograph: Instrumentation and experimental technique, TAO: Terrestrial, Atmospheric and Oceanic Sciences, 5, 109-119, 1994, #1068
Trehu, A. M., I. Asudeh, T. M. Brocher, J. H. Luetgert, W. D. Mooney, J. L. Nabelek, and Y. Nakamura, Crustal architecture of the Cascadia forearc, Science, 266, 237-243, 1994, 70 citations, doi:10.1126/science.266.5183.237, #1097
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.
Nagihara, S., J. G. Sclater, J. D. Phillips, E. W. Behrens, L. A. Lawver, Y. Nakamura, A. E. Maxwell, T. Lewis, and J. Garcia, Heat flow in the western abyssal plain of the Gulf of Mexico: Implications for thermal evolution of the old oceanic crust, SIO Tech. Rep., Scripps Inst. of Oceanography, 45 pp., 1993, #1305
Trehu, A. M., and Y. Nakamura, Onshore-offshore Wide-angle Seismic Recordings from Central Oregon: The Ocean bottom Seismometer Data, Open File Report USGS-OFR-93-317, U.S. Geological Survey, 30 pp., 1993, #1304
Chen, A. T., Y. Nakamura, and L.-W. Wu, Ocean bottom seismograph and its experimental technique, (in Chinese), Ti-Chih, 12 (1), 47-54, 1992, #944
Chen, A. T., H.-L. Shi, and Y. Nakamura, The sedimentary structure of Nanjintao Basin - a preliminary interpretation based on the OBS data, Proc., Taiwan Symp. on Geophys., 4, 497-506, 1992, #957
Chen, A. T., Y. Nakamura, and L.-W. Wu, The first Chinese ocean bottom seismograph: Instrumentation and field experiment, Proc., 4th Taiwan Symp. Geophys., 4, 477-486, 1992, #958
Oberst, J., and Y. Nakamura, A search for clustering among the meteoroid impacts detected by the Apollo lunar seismic network, Icarus, 91, 315-325, 1991, 32 citations, doi:10.1016/0019-1035(91)90027-Q, #861
We examined temporal clustering of meteoroid impacts detected by the Apollo lunar seismic network and found a distinct difference between âsmallâ meteoroids (masses smaller than about 1 kg) and âlargeâ 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 âmeteorite streams.â
Frohlich, C., R. Louat, and Y. Nakamura, Earthquake activity in the southern Vanuatu arc recorded by the Texas digital OBS, Marine Geophysical Researches, 12, 253-267, 1990, 1 citation, doi:10.1007/BF02428197, #812
We have reconfigured the Texas digital ocean bottom seismograph (OBS) to operate in a triggered mode and record regional earthquake signals. This paper reports the results of a deployment program designed to test these digital OBS, by moni toring earthquake activity in and near the trench in southern Vanuatu (formerly, the New Hebrides). We successfully recorded hundreds of earthquakes, including 133 located regional earth quakes recorded by three or more stations. We also report J-B residuals for 21 earthquakes reported and located by the ISC. Fourier analysis of seismograms from regional earthquakes suggest that the frequencies of spectral peaks at any station were nearly the same for large, small, nearby, and distant events. However, we obtained very disparate frequencies when we analyzed seismograms for the same earthquake recorded at different stations. The most plausible interpretation is that spectral peaks do not depend on the characteristics of the earthquake source, but instead on site characteristics, or, more specifically, on the coupling of the instrument to the seafloor. To record reliable spectral data, we need to overcome this problem.
Solomon, S. C., D. L. Anderson, W. B. Banerdt, R. G. Butler, P. M. Davis, F. K. Duennebier, Y. Nakamura, E. A. Okal, and R. J. Phillips, Scientific Rationale and Requirements for a Global Seismic Network on Mars, Tech. Rep. LPI/TR-91-02, Lunar and Planetary Science Inst., 51 pp., 1990, #1315
Louat, R., C. Frohlich, P. Charvis, Y. Hello, P. M. McPherson, Y. Nakamura, and B. Pontoise, Etude d'un essaim de seismes dans le sud du Vanuatu (SO-Pacifique) par un reseau de stations sismologiques sous-marines (OBS), C. R. Acad. Sci. Paris, 309, ser. II, 213-218, 1989, 1 citation, #809
Oberst, J., and Y. Nakamura, Monte Carlo simulations of the diurnal variation in seismic detection rate of sporadic meteoroid Impacts on the Moon, Proc. 19th Lunar Planet. Sci. Conf., 615-625, 1989, #753
Ebeniro, J. O., Y. Nakamura, D. S. Sawyer, and W. P. O'Brien, Sedimentary and crustal structure of the northwestern Gulf of Mexico, J. Geophys. Res., 93, 9075-9092, 1988, 8 citations, #744
We conducted a large-offset seismic experiment in the northwestern Gulf of Mexico using largecapacity air guns and digital ocean-bottom seismographs to determine the velocity structure of the sediments, crust, and upper mantle. Five multiple-detector split-profile lines were shot over an area extending from the shallow midshelf south of Galveston to the continental rise just south of the Sigsbee escarpment. The data were of adequate spatial density and quality to allow combined use of near-vertical reflections, wide-angle reflections, and refractions for interpretation. Several techniques were used to obtain velocity-depth functions: (1) conventional constant-velocity-layer analysis, (2) interval velocities from moveouts of the precritical reflections, (3) analysis in the tau-p domain to determine extremal depth bounds, (4) estimation of the thickness of the allochthonous salt from the limiting distance of salt refractions, and (5) forward modeling using two-dimensional (2-D) ray tracing. A sedimentary sequence of nearly constant thickness (13 to 15 km) covers the study area. Various mobilized salt features exist within the sediments in the slope area, ranging from deeply buried layers and diapirs under the inner slope to shallow, thin, allochthonous bodies under the outer slope. In contrast to the sediments, the crust shows considerable variation in thickness, from normal oceanic crustal thickness beneath the continental rise to nearly normal continental thickness beneath the shelf. The transition under the slope, however, is not monotonie but includes thinning to nearly oceanic thickness under the midslope, possibly suggesting an incipient rift that failed to materialize during the opening of the gulf.
Nakamura, Y., D. S. Sawyer, F. J. Shaub, K. R. MacKenzie, and J. Oberst, Deep crustal structure of the northwestern Gulf of Mexico, Trans., Gulf Coast Assn. Geol. Socs., 38, 207-215, 1988, #743
Oberst, J., and Y. Nakamura, A seismic risk for the lunar base, Proc., 2nd Conf. on Lunar Bases & Space Activities of the 21st Century, NASA Conf. Pub., 3166, 231-233, 1988, #769
Ebeniro, J. O., Y. Nakamura, and D. S. Sawyer, Estimating thickness of shallow salt from seismic refractions, Geophysics, 52, 1708-1714, 1987, 1 citation, doi:10:1190/1.1442287, #702
The presence of shallow, tectonized salt is a major impediment to exploration efforts in many sedimentary basins, including the northern Gulf of Mexico. The salt there forms a shallow tongue of high-velocity material emplaced between lower-velocity Tertiary and Quaternary sediments. Using conventional seismic reflection techniques, explorationists often have difficulty identifying the base of the salt tongue. Only in a very few instances (e.g., Buffler, 1983; Buffler et al., 1978; Watkins et al., 1978) have they been able to identify the base of the shallow salt. Interfaces below the salt are even more rarely observed.
Nakamura, Y., P. L. Donoho, P. H. Roper, and P. M. McPherson, Large-offset seismic surveying using ocean-bottom seismographs and air guns: Instrumentation and field technique, Geophysics, 52, 1601-1611, 1987, 36 citations, doi:10:1190/1.1442277, #700
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.
Oberst, J., and Y. Nakamura, Distinct meteoroid families identified on the lunar seismograms , Proc. 17th Lunar Planet. Sci. Conf., J. Geophys. Res., 92, E769-E773, 1987, 16 citations, #686
We reexamined the seismic events originating from impacts of meteoroids on the lunar surface, as recorded by the Apollo lunar seismic network, in terms of (1) difference in temporal distribution between large and small impacts, (2) clustering of impacts in a two-dimensional space of the time of the year and the time of the month (lunar day), and (3) their relationship with terrestrial observations. The study led us to identify several distinct families of meteoroids impacting the moon. Most meteoroids producing small impact-seismic events appear to approach from retrograde heliocentric orbits, including many that are associated with well-known cometary showers. In contrast, most meteoroids associated with large impact-seismic events appear to approach from prograde orbits. Although some of them may also be cometary, the observation is consistent with a hypothesis that many of them represent stony asteroidal material. A unique cluster of large-amplitude impact events observed in June, 1975, appears to be associated with the Taurid cometary complex, suggesting that the Taurids contain more massive or even denser objects than other meteor showers. The previously reported discrepancy between lunar and terrestrial meteoroid-flux estimates may be due to the differences in lunar and terrestrial detection efficiency among various families of meteoroids.
Nakamura, Y., J. Oberst, S. M. Clifford, and B. Bills, Comment on the letter "On the influx of small comets into the Earth's upper atmosphere, II. Interpretation" , Geophys. Res. Lett., 13, 1184-1185, 1986, 5 citations, #676
The interpretation of the âatmospheric holesâ in terms of an influx of comet‐like objects is difficult to reconcile with the fact that the lunar seismic network, set up during the Apollo project, did not detect the postulated influx of such objects.
Nakamura, Y., Seismic velocity structure of the lunar mantle, J. Geophys. Res., 88, 677-686, 1983, 103 citations, #541
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 V p = 7.74 km/s and V s = 4.49 km/s in the section above 270-km depth to V p = 7.46 km/s and V s = 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 p = 8.26 km/s and V s = 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.
Nakamura, Y., and J. Koyama, Seismic Q of the lunar upper mantle, J. Geophys. Res., 87, 4855-4861, 1982, 25 citations, #455
We have determined the frequency dependence of anelastic attenuation for both P and S waves in the upper mantle of the moon in the frequency range of roughly 3 to 8 Hz. The method used is a combination of a single-station method, in which Q −1 is obtained as an integral with an unknown constant, and a multiple-station method, the result of which is used to determine the integration constant. The Q determined for P waves (Qp ) appears to decrease with increasing frequency, though it is not significantly different from a constant Q, while the Q for S waves (Qs ) increases with frequency, becoming significantly greater than Qp at high frequencies. All Q values remain greater than 4000 within this frequency range. Qs above 5 Hz is approximately proportional to the 0.7th power of frequency. The high Q for shear waves at high frequencies suggests that compressional heat loss may be a dominant dissipation mechanism. Some uncertainties in the absolute Q values remain because the detailed velocity structure of the lunar upper mantle is largely unknown.
Nakamura, Y., G. V. Latham, and H. J. Dorman, Apollo lunar seismic experiment - Final summary, Proc.13th Lunar Planet. Sci. Conf., J. Geophys. Res., 87, A117-A123, 1982, 46 citations, #528
Processing and initial analysis of the entire set of Apollo lunar seismic data collected continuously from 1969 through 1977 have now been completed. Recent results include: 1) better defined deep moonquake locations, which appear to be bounded rather sharply between about 800 km and 1000 km depths with concentrations near both boundaries; and 2) middle mantle (∼500 to 1000 km depth) seismic velocities of V p , = 8.3 ± 0.4 km/sec and V s = 4.6 ± 0.2 km/sec, which are significantly higher than previous estimates and represent an increase of velocities from the upper mantle as opposed to a decrease in previous estimates.
Malin, P. E., and Y. Nakamura, Progress in modeling the distribution of elastic inhomogeneities in the lunar crust, Phys. Earth Planet. Int., 26, 261-263, 1981, 1 citation, doi:10.1016/0031-9201(81)90030-3, #518
Nakamura, Y., Earthquakes... on the Moon, Discovery, 6 (1), 26-29, 1981, #523
Nakamura, Y., Geophysical data on structure and tectonics of the Apollo 16 landing site, in Workshop on Apollo 16, edited by O. B. James and F. Horz, LPI Tech. Rep. 81-01, 87-94, 1981, #525
Horvath, P., G. V. Latham, Y. Nakamura, and H. J. Dorman, Lunar near-surface shear wave velocities at the Apollo landing sites as inferred from spectral amplitude ratios, J. Geophys. Res., 85, 6572-6578, 1980, 15 citations, #435
We reexamined the horizontal-to-vertical amplitude ratios of the long-period seismograms to determine the shear wave velocity distributions at the Apollo 12, 14, 15, and 16 lunar landing sites. Average spectral ratios, computed from a number of impact signals, were compared with spectral ratios calculated for the fundamental mode Rayleigh waves in media consisting of homogeneous, isotropic, horizontal layers. The shear velocities of the best fitting models at the different sites resemble each other and differ from the average for all sites by not more than 20% except for the bottom layer at station 14. The shear velocities increase from 40 m/s at the surface to about 400 m/s at depths between 95 and 160 m at the various sites. Within this depth range the velocity-depth functions are well represented by two piecewise linear segments, although the presence of first-order discontinuities cannot be ruled out.
Koyama, J., and Y. Nakamura, Focal mechanism of deep moonquakes, Proc. Lunar Planet. Sci. Conf., 11, 1855-1865, 1980, #437
Nakamura, Y., G. V. Latham, and H. J. Dorman, How we processed Apollo lunar seismic data, Phys. Earth Planet. Int., 21, 218-224, 1980, 9 citations, doi:10.1016/0031-9201(80)90071-0, #371
Abstract
The Apollo lunar seismic station network gathered data continuously at a rate of 3 Ã 108 bits per day for nearly eight years until the termination in September, 1977. The data were processed and analyzed using a PDP-15 mini-computer. On average, 1500 long-period seismic events were detected yearly. Automatic event detection and identification schemes proved unsuccessful because of occasional high noise levels and, above all, the risk of overlooking unusual natural events. The processing procedures which were finally chosen consist of plotting all the data on a compressed time scale, visually picking events from the plots, transferring event data to separate sets of tapes and performing detailed analyses using the latter. Many problems remain, especially in the automatic processing of extra-terrestrial seismic signals.
Nakamura, Y., Shallow moonquakes: How they compare with earthquakes, Proc. Lunar Planet. Sci. Conf., 11, 1847-1853, 1980, #436
Nakamura, Y., G. V. Latham, C. Frohlich, M. B. Blanchard, and J. P. Murphy, Field Measurement of Penetrator Seismic Coupling in Sediments and Volcanic Rocks, NASA Tech. Memo. 78572, Ames Res. Center, Moffett Field, 57 pp., 1979, #307
Nakamura, Y., and D. L. Anderson, Martian wind activity detected by a seismometer at Viking Lander 2 site, Geophys. Res. Lett., 6, 499-502, 1979, 4 citations, #338
The seismic âbackground noiseâ detected by the seismometer on the Viking lander 2 has an extremely high correlation with the measured wind speed at the landing site. When displayed in a compressed form, the nearly continuous seismic data clearly exhibit the diurnal as well as seasonal variations of the Martian wind activity. A preliminary spectral analysis of the long‐term variation of the background noise indicates persistent spectral peaks at periods near 1.5, 3, 7 and 10‐20 sols for the first 560 sols (0.84 Martian year) of observation.
Nakamura, Y., G. V. Latham, H. J. Dorman, A.-B. K. Ibrahim, J. Koyama, and P. Horvath, Shallow moonquakes: Depth, distribution and implications as to the present state of the lunar interior, Proc. Lunar Planet. Sci. Conf., 10, 2299-2309, 1979, #349
Dorman, H. J., S. Evans, Y. Nakamura, and G. V. Latham, On the time-varying properties of the lunar seismic meteoroid population, Proc. Lunar Planet. Sci. Conf., 9, 3615-3626, 1978, #269
Latham, G. V., H. J. Dorman, P. Horvath, A.-B. K. Ibrahim, J. Koyama, and Y. Nakamura, Passive seismic experiment: A summary of current status, Proc. Lunar Planet. Sci. Conf., 9, 3609-3613, 1978, #268
Nakamura, Y., A1 moonquakes: Source distribution and mechanism, Proc. Lunar Planet. Sci. Conf., 9, 3589-3607, 1978, #267
Anderson, D. L., W. F. Miller, G. V. Latham, Y. Nakamura, M. N. Toksoz, A. M. Dainty, F. K. Duennebier, A. R. Lazarewicz, R. L. Kovach, and T. C. D. Knight, Seismology on Mars, J. Geophys. Res., 82, 4524-4546, 1977, #62
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.
Anderson, D. L., W. F. Miller, G. V. Latham, Y. Nakamura, M. N. Toksoz, A. M. Dainty, F. K. Duennebier, A. R. Lazarewicz, R. L. Kovach, and T. C. D. Knight, Seismology on Mars, Calif. Inst. Tech., Div. Geological and Planetary Sci., Contribution No., 2910, 1977, #2179
Nakamura, Y., Detection and analysis of acoustic emission signals, in Proc., First Conf. on Acoustic Emission/Microseismic Activity in Geologic Structures and Materials, edited by H. R. Hardy and F. W. Leighton, Series on Rock and Soil Mechanics , 2, 445-457, 1977, #75
Nakamura, Y., HFT events: Shallow moonquakes?, Phys. Earth Planet. Int., 14, 217-223, 1977, 21 citations, doi:10.1016/0031-9201(77)90174-1, #145
A few large distant seismic events of distinctly high signal frequency, designated HFT (high-frequency teleseismic) events, are observed yearly by the Apollo lunar seismic network. Their sources are located on or near the surface of the moon, leaving a large gap in seismic activity between the zones of HFT sources and deep moonquakes. No strong regularities are found in either their spatial or temporal distributions. Several working hypotheses for the identity of these sources have been advanced, but many characteristics of the events seem to favor a hypothesis that they are shallow moonquakes. Simultaneous observations of other lunar phenomena may eventually enable the determination of their true identity.
Nakamura, Y., Seismic energy transmission in an intensively scattering environment, J. Geophysics - Zeitschrift fur Geophysik, 43, 389-399, 1977, 29 citations, #195
Nakamura, Y., G. V. Latham, H. J. Dorman, P. Horvath, and A.-B. K. Ibrahim, Seismic indication of broad-scale lateral heterogeneities in the lunar interior: A preliminary report, Proc. Lunar Sci. Conf., 8, 487-498, 1977, #214
Anderson, D. L., F. K. Duennebier, G. V. Latham, M. N. Toksoz, R. L. Kovach, T. C. D. Knight, A. R. Lazarewicz, W. F. Miller, Y. Nakamura, and G. Sutton, The Viking seismic experiment, Science, 194, 1318-1321, 1976, 7 citations, doi:10.1126/science.194.4271.1318, #193
A three-axis short-period seismometer is now operating on Mars in the Utopia Planitia region. The noise background correlates well with wind gusts. Although no quakes have been detected in the first 60 days of observation, it is premature to draw any conclusions about the seismicity of Mars. The instrument is expected to return data for at least 2 years.
Duennebier, F. K., Y. Nakamura, G. V. Latham, and H. J. Dorman, Meteoroid storms detected on the Moon, Science, 192, 1000-1002, 1976, 11 citations, doi:10.1126/science.192.4243.1000, #86
eismometers on the moon have detected several brief periods of enhanc ed meteoroid-impact activity, believed to represent encounters of the moon with "c louds" of objec ts in the kilogram range. The latest and most active encounter, in June 1975, is interpreted as a meteoroid c(loud of diameter 0.1 astronomical unit and total mass 10l3 to 1014 grams.
Nakamura, Y., Seismic energy transmission in the lunar surface zone determined from signals generated by movement of lunar rovers, Bull. Seismol. Soc. Amer., 66, 593-606, 1976, 18 citations, #77
Nakamura, Y., Lunar seismology, (in Japanese), Kagaku (Science), 46 (3), 135-144, 1976, #81
Nakamura, Y., F. K. Duennebier, G. V. Latham, and H. J. Dorman, Structure of the lunar mantle, J. Geophys. Res., 81, 4818-4824, 1976, 36 citations, #94
We have reexamined the structure of the lunar mantle by using data on the decay of shear wave amplitude with distance and the relative arrival times of P and S waves. The new analysis confirms our previous lunar model based primarily upon travel times and more closely defines certain properties of the lunar mantle. A negative shear wave velocity gradient of 0.0013 (km/s)/km, shear wave Q of 4000, and Poisson's ratio of 0.250 ± 0.025 are found for the upper mantle, which lies between the depths of 60 and 300 km. A rapid decrease of shear wave velocity with increasing depth is observed starting at around a 300-km depth, associated with a lower Q for shear waves. Poisson's ratio in the lower part of the middle mantle, which extends to a depth of about 1000 km, is estimated to be 0.36 ± 0.02.
Nakamura, Y., G. V. Latham, H. J. Dorman, and F. K. Duennebier, Seismic structure of the Moon: A summary of current status, Proc. Lunar Sci. Conf. (Geochimica et cosmochimica acta, Suppl.), 7, 3113-3122, 1976, #95
Duennebier, F. K., H. J. Dorman, D. R. Lammlein, G. V. Latham, and Y. Nakamura, Meteoroid flux from passive seismic experiment data, Proc. Lunar Sci. Conf. (Geochimica et cosmochimica acta, Suppl.), 6, 2417-2426, 1975, #74
Latham, G. V., Y. Nakamura, H. J. Dorman, F. K. Duennebier, M. Ewing, and D. R. Lammlein, Results from the Apollo passive seismic experiment, (in Russian), in Cosmic Chemistry, Moon and Planets, edited by A. P. Veetogrador, Acad. Sci., Moscow, USSR, 299-310, 1975, #44
Nakamura, Y., H. J. Dorman, F. K. Duennebier, D. R. Lammlein, and G. V. Latham, Shallow lunar structure determined from the passive seismic experiment, Moon, 13, 57-66, 1975, 20 citations, #63
Nakamura, Y., Spatial filtration in detection of acoustic emission, Hihakai Kensa (J. Non-Destructive Inspection), 24, 400-404, 1975, #64
Dainty, A. M., M. N. Toksoz, K. R. Anderson, P. J. Pines, Y. Nakamura, and G. V. Latham, Seismic scattering and shallow structure of the Moon in Oceanus Procellarum, Moon, 9, 11-29, 1974, #65
Lammlein, D. R., G. V. Latham, H. J. Dorman, Y. Nakamura, and M. Ewing, Lunar seismicity, structure and tectonics, Rev. Geophys. Space Phys., 12, 1-21, 1974, 63 citations, #34
Natural seismic events have been detected by the long-period seismometers at Apollo stations 16, 14, 15, and 12 at annual rates of 3300, 1700, 800, and 700, respectively, with peak activity at 13- to 14-day intervals. Repetitive moonquakes from 41 hypocenters produce seismograms characteristic of each. About 90% of the long-period signals are from these and other numerous, less active hypocenters, and meteoroid impact signals account for the remainder. At each hypocenter, moonquakes occur only within an active period of a few days during a characteristic phase of the monthly lunar tidal cycle. An episode of activity may contain up to four quakes from one hypocenter. Nearly equal numbers of hypocenters are active at opposite phases of the monthly cycle, accounting for the 14-day peaks in total lunar seismic activity. A period of about 206 days in the seismic activity of several of the hypocenters is superimposed on a strong one-to two-year trend where the signal amplitudes decrease to the instrumental detection threshold. A 206-day period with no secular decrease in amplitude is also observed in the total lunar seismic activity, suggesting that the total number of active hypocenters does not vary appreciably with time. Moonquake magnitudes range between 0.5 and 1.3 on the Richter scale with a total energy release estimated to be about 1011 ergs annually. With several possible exceptions, the moonquake foci located to date occur in two narrow belts on the near side of the moon. Both belts are 100-300 km wide, about 2000 km long, and 800-1000 km deep, and they lie along great-circle arcs. Seismic data from a far-side focus and a large far-side meteoroid impact define the base of the lunar lithosphere at a depth of about 1000 km. In our present model the rigid lithosphere overlies an asthenosphere of reduced rigidity in which present-day partial melting is probable. Tidal deformation presumably leads to critical stress concentrations at the base of the lithosphere, where moonquakes are found to occur. The striking tidal periodicities in the pattern of moonquake occurrence and energy release suggest that tidal energy is the dominant source of energy released as moonquakes. Thus, tidal energy is dissipated by moonquakes in the lithosphere and probably by inelastic processes in the asthenosphere. The low level of seismicity and the absence of shallow seismicity implies that the moon is neither expanding nor contracting at an appreciable rate. The secular accumulation of strain implied by the uniform polarities of moonquake signals may result from weak convection in the asthenosphere or from secular recession of the moon from the earth.
Nakamura, Y., H. J. Dorman, F. K. Duennebier, M. Ewing, D. R. Lammlein, and G. V. Latham, High-frequency lunar teleseismic events, Proc. Lunar Sci. Conf. (Geochimica et cosmochimica acta, Suppl.), 5, 2883-2890, 1974, #45
Nakamura, Y., G. V. Latham, D. R. Lammlein, M. Ewing, F. K. Duennebier, and H. J. Dorman, Deep lunar interior inferred from recent seismic data, Geophys. Res. Lett., 1, 137-140, 1974, #46
Analysis of recent lunar seismic data from distant meteoroid impacts, high‐frequency teleseismic events and deep moonquakes shows several significant deviations of P‐ and S‐wave travel times from those expected if the lunar interior were homogeneous below the crust. These data are interpreted resulting in a lunar model consisting of at least four and possibly five distinguishable zones: (I) the 50 to 60 km thick crust characterized by seismic velocities appropriate for plagioclase‐rich materials, (II) the 250 km thick upper mantle characterized by seismic velocities consistent with an olivine‐pyroxene composition, (III) the 500 km thick middle mantle characterized by a high (0.33 ‐ 0.36) Poisson's ratio, (IV) the lower mantle characterized by high shear‐wave attenuation and possibly (V) a core of radius between 170 and 360 km characterized by a greatly reduced compressional wave velocity.
Nakamura, Y., Spatial filtration in AE detection, Proc., 2nd Acoustic Emission Symp., Japan Industrial Planning Assoc., 19-41, 1974, #47
Latham, G. V., H. J. Dorman, F. K. Duennebier, M. Ewing, D. R. Lammlein, and Y. Nakamura, Moonquakes, meteoroids, and the state of the lunar interior, Proc. Fourth Lunar Sci. Conf., (Supplement 4, Geochimica et Cosmochimica Acta, Vol., 3, 2515-2527, 1973, #24
Latham, G. V., M. Ewing, H. J. Dorman, Y. Nakamura, F. Press, M. N. Toksoz, G. Sutton, F. K. Duennebier, and D. R. Lammlein, Lunar structure and dynamics - Results from the Apollo passive seismic experiment, Moon, 7, 396-420, 1973, #25
Latham, G. V., M. Ewing, F. Press, H. J. Dorman, Y. Nakamura, M. N. Toksoz, D. R. Lammlein, F. K. Duennebier, and A. M. Dainty, Passive seismic experiment, Apollo 17 Preliminary Science Report, NASA SP-330, 11-1 - 11-9, 1973, #31
Nakamura, Y., D. R. Lammlein, G. V. Latham, M. Ewing, H. J. Dorman, F. Press, and M. N. Toksoz, New seismic data on the state of the deep lunar interior, Science, 181, 49-51, 1973, 46 citations, doi:10.1126/science.181.4094.49, #17
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 earth's asthenosphere.
Latham, G. V., M. Ewing, F. Press, G. Sutton, H. J. Dorman, Y. Nakamura, M. N. Toksoz, D. R. Lammlein, and F. K. Duennebier, Passive seismic experiment, Apollo l6 Prelim. Sci. Rept., NASA SP-315, 9-1 -9-29, 1972, #20