Several large shallow earthquakes (Ms > 7.0) have occurred along the Middle American Trench since the installation of the WWSSN network. Included are the 1965, 1968, and 1978 Oaxaca events, the 1970 Chiapas event, the 1973 Colima event, and the 1979 Petatlan event. These earthquakes have been studied in an attempt to identify similarities and differences between them that may lead to a better understanding of fracture and subduction processes. The events have seismic moments ranging from 1.0×1027 dyne cm for the 1968 event to 3.2×1027 dyne cm for the 1978 event. All events are of predominantly thrust type, consistent with subduction to the northeast of the Cocos plate. Body waves for the 1965, 1968, 1978, and 1979 events along the trench indicate rather simple faulting processes. These events all had focal depths of 15 to 20 km and stress drops of the order of 10 bars. The 1970 and 1973 events, the easternmost and westernmost, respectively, of the events studied here, are located close to triple junctions for the Cocos–North America–Caribbean plates (1970) and the Cocos–North America–Rivera plates (1973). These two events generated more complex body waves than the Oaxaca and Petatlan quakes. This source complexity may have been related to the more complicated tectonics near the 1970 and 1973 epicenters. This may offer a new perspective to viewing source complexity in relation to subduction zones.

Seismological simulations show how energy from a large impact can be coupled to the interior of the Earth. The radially diverging shock wave generated by the impact decays to linearly elastic seismic waves. These waves reconverge (minus attenuation) along the axis of symmetry between the impact and its antipode. The locations that experience the most strain cycles with the largest amplitudes will dissipate the most energy and have the largest increases in temperature at a given depth (for a given attenuation efficiency). We have shown that the locus of maximum energy deposition in the mantle lies along the impact-antipodal axis. Moreover, the most intense focusing is within the asthenosphere at the antipode, over the range of depths where mechanical energy is most readily converted to heat. We suggest that if large impacts on the Earth leave geological evidence anywhere other than the impact site itself, it will be at the antipode. Strong axial focusing also occurs directly beneath the impact site and should be considered in models of impact-induced volcanism.

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We used a deep (1500 m) cased borehole near the town of Datil in west-central New Mexico to study high-frequency (>1 Hz) seismic noise characteristics. The remote site had very low levels of cultural noise, but strong winds (winter and spring) made the site an excellent candidate to study the effects of wind noise on seismograms. Along with a three-component set of surface sensors (Teledyne Geotech GS-13), a vertical borehole seismometer (GS-28) was deployed at a variety of depths (5, 43, and 85 m) to investigate signal and noise variations. Wind speed was measured with an anemometer. Event-triggered and time-triggered data streams were recorded on a RefTek 72-02 data acquisition system located at the site. Our data show little cultural noise and a strong correlation between wind speed and seismic background noise. The minimum wind speed at which the seismic background noise appears to be influenced varies with depth: 3 m/sec at the surface, 3.5 m/sec at 43 m in depth, and 4 m/sec at 85 m in depth. For wind speed below 3 to 4 m/sec, we observe omni-directional background noise that is coherent at frequencies below 15 Hz. This coherence is destroyed when wind speeds exceed 3 to 4 m/sec. We use a test event (Md ∼ 1.6) and superimposed noise to investigate signal-to-noise ratio (SNR) improvement with sensor depth. For the low Q valley fill of the Datil borehole (DBH) site, we have found that SNR can be improved by as much as 20 to 40 dB between 23 and 55 Hz and 10 to 20 dB between 10 and 20 Hz, by deploying at a 43-m depth rather than at the surface. At the surface, there is little signal above noise in the 23- to 55-Hz frequency band for wind speeds greater than 8 m/sec. Thus, high-frequency signal information that is lost at the surface can be recorded by deploying at the relatively shallow depth of 40 m. Because we observe only minor further reductions in seismic background noise (SBN) at deeper depths, 40 m is likely to be a reasonable deployment depth for other high-frequency-monitoring sites in similar environmental and geologic conditions.

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