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At least 883 records · Page 49

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NASA NTRS1977

The deep seismic structure of the moon

Data from 24 deep moonquakes are used to investigate the seismic structure of the lunar interior below the 300-500 km level. The deep moonquakes provide an uninterrupted ray-path coverage of the lunar mantle. Lower mantle seismic velocities are determined; the data suggest that moonquakes are confined to the near-side lower mantle. A compositional change rather than temperature effects is assumed to explain the upper mantle-lower mantle transition.

Goins, N. R.↗

NASA NTRS1975

Seismically induced modification of lunar surface features

The formation of large impact craters and basins produced not only large volumes of ejecta but also catastrophic seismic waves. Theoretical models, extrapolation of terrestrial explosion data, and extrapolation of lunar impact data suggest surface displacements of 1-10 m at four crater radii from a Copernicus-size impact (4 x 10 to the 30th power ergs). Independent estimates of impact-generated stresses indicate that regions within 4 crater radii of an impact will receive shock waves with pressures exceeding 1 kbar. Greater areas also may receive shock effects as elastic waves following deep ray paths transform into shock waves near the surface. Additional seismic energy will be generated by impacts from secondary ejecta. The resulting large and long-lasting surface movement may contribute to the subdued appearance of the continuous ejecta blanket and secondary craters around large impact craters and to the degradation of small craters outside these zones.

Schultz, P. H.↗

NASA NTRS1978

On the time-varying properties of the lunar seismic meteoroid population

Strong short-term fluctuations of meteroid impact rates are evident in a list of 1557 events derived from Apollo lunar seismic data. Times of fall and seismic signal amplitudes are considered in relation to the possible orbits and identification of the impacting objects. The entire lunar surface is the effective collector and the Apollo network data, gathered between 1970 and 1977, reflect the recognized abundance of large meteorite falls from early April through July. There is also some indication that briefly increased counts represent fragments of about 100 g or greater belonging to several meteor streams: Quadrantids, Aquarids, Perseids, Orionids, Leonids, Geminids, and possibly others as well.

Dorman, J.↗

NASA NTRS1979

The double seismic zone in downgoing slabs and the viscosity of the mesosphere

The seismic zone beneath several island arcs between about 100 and 200 km depth consists of an upper zone having down-dip compression and a lower zone having down-dip tension. Several numerical models of the Aleutian arc were computed to test the hypothesis that these double seismic zones are due to sagging of the slab under its own weight. This sagging occurs because the asthenosphere (between about 100 and 200 km) provides little support or resistance to the slab, which is supported from below by the more viscous mesosphere and from above by the lithosphere. The viscosity of the mesosphere was constrained to the interval between 0.25 x 10 to the 22nd and 0.5 x 10 to the 22nd P by noting that the slab would have mainly down-dip compression at higher viscosities and mainly down-dip tension at lower viscosities. The deviatoric stress in the slab and the fault plane between the slab and the island arc is about 200-300 bars (expressed as shear stress). The models were calibrated to the observed depth and gravity anomalies in the trench.

Sleep, N. H.↗

NASA NTRS1979

Seismic Q and velocity at depth

Measurements of the p-wave and s-wave internal friction quality factors (Q) and velocities of samples of a moderately outgassed terrestrial analog of lunar basalt exposed simultaneously to temperatures and hydrostatic confining pressures in accordance with the best available estimates of the lunar selenotherm are presented. Q values and velocities are found to increase with simulated depth, and an extrapolation of the Q value to a thoroughly outgassed states provides a Q value in reasonable agreement with those derived from lunar seismic data, suggesting a very dry lunar crust. Results also imply that similar seismic determinations for Mars would be able to distinguish between a dry crust and a crust containing water trapped beneath a layer of ice. Results of thermal cracking tests which demonstrate that high degrees of cracking associated with thermal cycling, as during the lunar day, are not inconsistent with high Q in a dry environment are presented, and it is shown that volatiles with diple moments comparable to H2O can greatly affect Q. Possible attenuation mechanisms are then considered, and velocity measurements on a synthetic anorthosite are presented.

Tittmann, B. R.↗

NASA NTRS1979

Brief note for consideration of active seismic exploration on Mars

Considerable controversy has arisen regarding the existence of water and water ice below the surface of Mars. Results of laboratory internal friction measurements on strongly outgassed rocks, recently made as part of an effort to interpret the high seismic Q values observed in the lunar crust, show that moisture absorbed in pores and cracks causes the low Q values. Unfortunately, the Viking seismology investigation found a surprisingly inactive planet. This note makes the case for active seismic exploration in order to ascertain the existence of liquid water in the crust from Q determinations.

Tittmann, B. R.↗

NASA NTRS1980

The craton - Its effect on the distribution of seismicity and stress in North America

The gross distribution of seismicity in North America suggests the interior platform and shield provinces are relatively stable and significantly affect the intraplate stress field. Focal mechanisms and in-situ stress measurements indicate the cratonic areas of North America must be relatively immobile with respect to the eastern and western portions of the continent. Simple kinematic models are presented to illustrate the importance of the craton in any attempt to explain the general pattern of intraplate stress and seismicity in North America.

Wesnousky, S. G.↗

NASA NTRS1982

Scanning Seismic Intrusion Detector

Scanning seismic intrusion detector employs array of automatically or manually scanned sensors to determine approximate location of intruder. Automatic-scanning feature enables one operator to tend system of many sensors. Typical sensors used with new system are moving-coil seismic pickups. Detector finds uses in industrial security systems.

Lee, R. D.↗

NASA NTRS1981

Seismic codas on the earth and the moon - A comparison

The seismic codas, representing that part of the seismogram following the arrival of the surface waves or the direct S waves, characteristic of the earth and moon are compared with regard to the implications of coda characteristics for scattering and attenuation. Scattering models based on single S-S scatter theory, in which seismic energy in the coda is assumed to be S waves that have undergone only one scattering without conversion, and on diffusion theory, which assumed that energy in the coda has been scattered many times, are examined, and situations where the single-scattering and multiple-scattering theories are applicable are distinguished by the ratio of attenuation distance to the scattering mean free path. Values of the attenuation distance and the mean free path derived from coda studies for the earth and moon are compared, and it is found that for the frequency range 0.5-10 Hz, diffusion scattering is important in lunar codas, while at frequencies from 1 to 25 Hz single scattering is important in terrestrial codas. It is pointed out that attenuation acts to eliminate scattering paths much longer than the attenuation length. The observed differences between terrestrial and lunar codas are thus attributed to lesser attenuation and greater scattering on the moon.

Dainty, A. M.↗

NASA NTRS1983

Scanning seismic intrusion detection method and apparatus

An intrusion monitoring system includes an array of seismic sensors, such as geophones, arranged along a perimeter to be monitored for unauthorized intrusion as by surface movement or tunneling. Two wires lead from each sensor to a central monitoring station. The central monitoring station has three modes of operation. In a first mode of operation, the output of all of the seismic sensors is summed into a receiver for amplification and detection. When the amplitude of the summed signals exceeds a certain predetermined threshold value an alarm is sounded. In a second mode of operation, the individual output signals from the sensors are multiplexed into the receiver for sequentially interrogating each of the sensors.

Lee, R. D.↗

NASA NTRS1982

Apollo lunar seismic experiment - Final summary

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 (approximately 500 to 1000 km depth) seismic velocities of V sub p = 8.3 + or - 0.4 km/sec and V sub s = 4.6 + or - 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.

Nakamura, Y.↗

NASA NTRS1983

Geodynamics of the Baikal-Stanovoy seismic belt

The convection generated tensional stress field in the earth, as inferred from satellite gravity data, reveals an anomalous lens of upwelling mantle rocks under the Baikal rift zone. The point of no strain at 56 deg N 116 deg E forms a seismic gap along the Baikal-Stanovoy seismic belt. East of this point, the stress field changes from extension to compression. Therefore, the position of no strain at the eastern termination of the rift accounts for the dying-out of the rift zone and for the appearance of a compressive structure in the Stanovoy Range.

Liu, H.-S.↗

NASA NTRS1984

Coherent tilt signals observed in the Shumagin seismic gap Detection of time-dependent subduction at depth?

Repeated surveys of short level lines in the Shumagin Islands, Alaska, reveal coherent tilt signals associated with subduction of the Pacific plate beneath the North American plate in the Shumagin seismic gap. Ten years of steady tilt down toward the trench is interrupted during 1978-1980 by a rapid episode of reverse tilt. The 'normal' tilt represents surface deformation as subduction occurs, with the plate boundary locked to at least 60 km depth. Using all available tilt, sea level, and seismic data, the tilt reversal is interpreted as due to an episodic reverse slip of about 80 cm magnitude on the plate boundary between about 70 km and 20 km depth, downdip from the main seismogenic zone, which remains locked. This event causes an increase of stress on the locked main thrust zone. It is speculated that such events may be a regular process at subduction zones, that great plate boundary earthquakes may be more common during their occurrence, and that their onset may be detectable early enough to give warning of an increase in probability for the occurrence of a great earthquake.

Beavan, J.↗

NASA NTRS1985

Seismic analysis of the large 70-meter antenna, part 1: Earthquake response spectra versus full transient analysis

As a check on structure safety aspects, two approaches in seismic analysis for the large 70-m antennas are presented. The first approach, commonly used by civil engineers, utilizes known recommended design response spectra. The second approach, which is the full transient analysis, is versatile and applicable not only to earthquake loading but also to other dynamic forcing functions. The results obtained at the fundamental structural frequency show that the two approaches are in good agreement with each other and both approaches show a safe design. The results also confirm past 64-m antenna seismic studies done by the Caltech Seismology Staff.

Kiedron, K.↗

NASA NTRS1985

Investigation of seismicity and related effects at NASA Ames-Dryden Flight Research Facility, Computer Center, Edwards, California

This report discusses a geological and seismological investigation of the NASA Ames-Dryden Flight Research Facility site at Edwards, California. Results are presented as seismic design criteria, with design values of the pertinent ground motion parameters, probability of recurrence, and recommended analogous time-history accelerograms with their corresponding spectra. The recommendations apply specifically to the Dryden site and should not be extrapolated to other sites with varying foundation and geologic conditions or different seismic environments.

Cousineau, R. D.↗

NASA NTRS1986

Historical seismicity near Chagos - A complex deformation zone in the equatorial Indian Ocean

The historical seismicity of the Chagos region of the Indian Ocean is analyzed, using earthquake relocation methods and a moment variance technique to determine the focal mechanisms of quakes occurring before 1964. Moment variance analysis showed a thrust faulting mechanism associated with the earthquake of 1944 near the Chagos-Laccadive Ridge; a strike-slip mechanism was associated with a smaller 1957 event occurring west of the Chagos Bank. The location of the 1944 event, one of the largest intraplate earthquakes known (1.4 x 10 to the 27th dyne/cm), would imply that the Chagos seismicity is due to a zone of tectonic deformation stretching across the equatorial Indian Ocean. The possibility of a slow diffuse boundary extending west of the Central Indian Ridge is also discussed. This boundary is confirmed by recent plate motion studies which suggest that it separates the Australian plate from a single Indo-Arabian plate.

Wiens, D. A.↗

NASA NTRS1987

Why does near ridge extensional seismicity occur primarily in the Indian Ocean?

It is argued that though thermoelastic stresses provide a low level background in all plates, the data favoring their contributing significantly to the stress field and seismicity in the young oceanic lithosphere may be interpreted in terms of stresses resulting from individual plate geometry and local boundary effects. The dramatic concentration of extensional seismicity in the Central Indian Ocean region is shown to be consistent with finite element results for the intraplate stress incorporating the effects of the Himalayan collision and the various subduction zones. Most of the data for both ridge-parallel extension and depth stratification are provided by earthquakes in this area, and it is suggested that these effects may be due more to the regional stress.

Stein, Seth↗

NASA NTRS1988

Deep seismic reflection profiling and continental growth curves

The results of deep seismic reflection profiling is discussed which shows that the lower crust is prominently layered, in many continental areas, regardless of the age of the surface rocks. The seismic Moho is commonly shallower than the petrological Moho, leading to the question of the nature and origin of this prominent reflector in the deep crust. The lower crust is much less well defined in Phanerozoic and Proterozoic accreted terranes, suggesting possible differences in types of lower crusts.

Klemperer, Simon L.↗