Gary V. Latham

Seismicity Gap near Oaxaca, Southern Mexico as a Probable Precursor to a Large Earthquake

An area of significant seismic quiescence is found near Oaxaca, southern Mexico. The anomalous area may be the site of a future large earthquake as many cases so far reported were. This conjecture is justified by study of past seismicity changes in the Oaxaca region. An interval of reduced seismicity, followed by a renewal of activity, preceded both the recent large events of 1965 and 1968. Those past earthquakes have ruptured the eastern and western portions of the present seismicity gap, respectively, so that the central part remaining is considered to be of the highest risk of the pending earthquake.

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.

Lunar structure and dynamics - results from the apollo passive seismic experiment

Analysis of seismic signals from man-made impacts, moonquakes, and meteoroid impacts has established the presence of a lunar crust, approximately 60 km thick in the region of the Apollo seismic network; an underlying zone of nearly constant seismic velocity extending to a depth of about 1000 km, referred to as the mantle; and a lunar core, beginning at a depth of about 1000 km, in which shear waves are highly attenuated suggesting the presence of appreciable melting. Seismic velocitites in the crust reach 7 km s−1 beneath the lower-velocity surface zone. This velocity corresponds to that expected for the gabbroic anorthosites found to predominate in the highlands, suggesting that rock of this composition is the major constituent of the lunar crust. The upper mantle velocity of about 8 km s−1 for compressional waves corresponds to those of terrestrial olivines, pyroxenites and peridotites. The deep zone of melting may simply represent the depth at which solidus temperatures are exceeded in the lower mantle. If a silicate interior is assumed, as seems most plausible, minimum temperatures of between 1450°C and 1600°C at a depth of 1000 km are implied. The generation of deep moonquakes, which appear to be concentrated in a zone between 600 km and 1000 km deep, may now be explained as a consequence of the presence of fluids which facilitate dislocation. The preliminary estimate of meteoroid flux, based upon the statistics of seismic signals recorded from lunar impacts, is between one and three orders of magnitude lower than previous estimates from Earth-based measurements.

Evidence that biological activity affects Ocean Bottom Seismograph recordings

Brief and impulsive signals of uncertain origin appear regularly on records from Ocean Bottom Seismographs (OBS) of several institutions. These signals have been recorded on nearly all deployments of the Texas OBS, including sites at depths greater than 7000 m. At some sites, they account for over 90% of the events recorded. They are of short duration (usually 0.5–4.0 s) and have a characteristic frequency (usually in the range of 4–18 Hz) that differs from site to site. When networks of OBS instruments are deployed, the signals are not recorded simultaneously by different instruments. Neither the frequency content nor the distribution of durations of these signals is similar to what is observed for known earthquake events.We present evidence suggesting that the signals are of biological origin, perhaps caused by animals touching the OBS units. (1) The distribution of these signals on instruments deployed at depths shallower than 1000 m shows a 24 h periodicity, while there is a 24 h periodic pattern on instruments deployed at sites deeper than 1000 m (where there is no visible light). (2) The frequency of occurrence of signals is similar to the vertical distribution of biomass in the oceans, i.e., they appear most frequently on OBS instruments deployed at very shallow depths. (3) Biological material has been found attached to several OBS units upon recovery.

Passive Seismic Experiment, Long Period Event Catalog, Final Version (1969 Day 202 - 1977 Day 273, ALSEP Stations 11, 12, 13, 14, 15, and 16)

The purpose of this catalog is to present users of the Apollo Passive Seismic Experiment data with a listing of all seismic events observed on the long period components of the lunar seismic network. The all-inclusive version of the Apollo Passive Seismic Experiment long period event catalog, combining all the earlier periodic versions of the event catalogs, was originally published in 1981 by the Galveston Geophysics Laboratory of the University of Texas at Austin, Marine Science Institute. It has since been revised several times as new identifications of earlier unidentified events become available. The current version, updated on October 2, 2008, is available as a text file named levent.1008.dat

An Intermediate Term Strategy for Deployment of Mobile Laser Stations

It is expected that an initial program of field testing within the U.S. will follow the completion of the first mobile lunar laser ranging station (MLLRS). This program will be designed to test the operational readiness of the station and the accuracy of the technique by direct comparison with measurements obtained by other methods (VLBI and LGEOS) over common baselines. If the McDonald-Goldstone, and McDonald-Haystack baselines are chosen for this purpose, the MLLRS will operate first at McDonald Observatory, West Texas, followed by Goldstone, California, and Westford, Massachusetts. Following these measurements, the authors suggest that the highest priority be given to (1) measurement of plate movements relevant to major earthquakes that may affect the United States; and (2) intraplate distortion of the North American plate. Toward these objectives, additional sites suggested are Baja, California; California-Oregon border; Fairbanks, Alaska; and a mid-continent (U.S. or Canadian) site. Finally, the intermediate term strategy should include sites spanning zones across which the most rapid convergence and divergence are inferred, to provide the earliest possible test of current geodynamical models. Two groups of sites are suggested: (1) Santiago, Easter Islands, and Tahiti; and (2) Galapagos Is., Cocos Is., and Guatemala City or Tegucigalpa. The first group spans the Peru-Chile trench (convergence zone) and the East Pacific rise (spreading center), where the highest ratios of relative movement are inferred. The second group of sites would provide data on the movement of the Cocos plate relative to the Nazca, Caribbean, and North American plates.

Maurice Ewing and the Exploration of the Oceans

Appropriately, this meeting is dedicated to Professor Maurice Ewing whose career in geophysics began and ended in Texas, and to whom we are indebted for his efforts in fostering new links between astronomy and geophysics. Short-term measurements of the horizontal motions of the earth’s lithospheric plates promise to be among the most important applications of lunar laser ranging. Our present understanding of plate tectonics has been reached largely as a synthesis of relatively new information from marine geology and geophysics, a field where Ewing did much of his best-known work. Therefore, before a group representing diverse physical disciplines, it is appropriate that we should briefly review this important development and Ewing’s substantial role in it.