ALBERT

Notes: Abstract The 9 March 1957 Aleutian earthquake has been estimated as the third largest earthquake this century and has the longest aftershock zone of any earthquake ever recorded—1200 km. However, due to a lack of high-quality seismic data, the actual source parameters for this earthquake have been poorly determined. We have examined all the available waveform data to determine the seismic moment, rupture area, and slip distribution. These data include body, surface and tsunami waves. Using body waves, we have estimated the duration of significant moment release as 4 min. From surface wave analysis, we have determined that significant moment release occurred only in the western half of the aftershock zone and that the best estimate for the seismic moment is 50–100×1020 Nm. Using the tsunami waveforms, we estimated the source area of the 1957 tsunami by backward propagation. The tsunami source area is smaller than the aftershock zone and is about 850 km long. This does not include the Unalaska Island area in the eastern end of the aftershock zone, making this area a possible seismic gap and a possible site of a future large or great earthquake. We also inverted the tsunami waveforms for the slip distribution. Slip on the 1957 rupture zone was highest in the western half near the epicenter. Little slip occurred in the eastern half. The moment is estimated as 88×1020 Nm, orM w =8.6, making it the seventh largest earthquake during the period 1900 to 1993. We also compare the 1957 earthquake to the 1986 Andreanof Islands earthquake, which occurred within a segment of the 1957 rupture area. The 1986 earthquake represents a rerupturing of the major 1957 asperity.

Notes: Abstract The Cocos plate subducts beneath North America at the Mexico trench. The northernmost segment of this trench, between the Orozco and Rivera fracture zones, has ruptured in a sequence of five large earthquakes from 1973 to 1985; the Jan. 30, 1973 Colima event (M s 7.5) at the northern end of the segment near Rivera fracture zone; the Mar. 14, 1979 Petatlan event (M s 7.6) at the southern end of the segment on the Orozco fracture zone; the Oct. 25, 1981 Playa Azul event (M s 7.3) in the middle of the Michoacan “gap”; the Sept. 19, 1985 Michoacan mainshock (M s 8.1); and the Sept. 21, 1985 Michoacan aftershock (M s 7.6) that reruptured part of the Petatlan zone. Body wave inversion for the rupture process of these earthquakes finds the best: earthquake depth; focal mechanism; overall source time function; and seismic moment, for each earthquake. In addition, we have determined spatial concentrations of seismic moment release for the Colima earthquake, and the Michoacan mainshock and aftershock. These spatial concentrations of slip are interpreted as asperities; and the resultant asperity distribution for Mexico is compared to other subduction zones. The body wave inversion technique also determines theMoment Tensor Rate Functions; but there is no evidence for statistically significant changes in the moment tensor during rupture for any of the five earthquakes. An appendix describes theMoment Tensor Rate Functions methodology in detail. The systematic bias between global and regional determinations of epicentral locations in Mexico must be resolved to enable plotting of asperities with aftershocks and geographic features. We have spatially “shifted” all of our results to regional determinations of epicenters. The best point source depths for the five earthquakes are all above 30 km, consistent with the idea that the down-dip edge of the seismogenic plate interface in Mexico is shallow compared to other subduction zones. Consideration of uncertainties in the focal mechanisms allows us to state that all five earthquakes occurred on fault planes with the same strike (N65°W to N70°W) and dip (15±3°), except for the smaller Playa Azul event at the down-dip edge which has a steeper dip angle of 20 to 25°. However, the Petatlan earthquake does “prefer” a fault plane that is rotated to a more east-west orientation—one explanation may be that this earthquake is located near the crest of the subducting Orozco fracture zone. The slip vectors of all five earthquakes are similar and generally consistent with the NUVEL-predicted Cocos-North America convergence direction of N33°E for this segment. The most important deviation is the more northerly slip direction for the Petatlan earthquake. Also, the slip vectors from the Harvard CMT solutions for large and small events in this segment prefer an overall convergence direction of about N20°E to N25°E. All five earthquakes share a common feature in the rupture process: each earthquake has a small initial precursory arrival followed by a large pulse of moment release with a distinct onset. The delay time varies from 4 s for the Playa Azul event to 8 s for the Colima event. While there is some evidence of spatial concentration of moment release for each event, our overall asperity distribution for the northern Mexico segment consists of one clear asperity, in the epicentral region of the 1973 Colima earthquake, and then a scattering of diffuse and overlapping regions of high moment release for the remainder of the segment. This character is directly displayed in the overlapping of rupture zones between the 1979 Petatlan event and the 1985 Michoacan aftershock. This character of the asperity distribution is in contrast to the widely spaced distinct asperities in the northern Japan-Kuriles Islands subduction zone, but is somewhat similar to the asperity distributions found in the central Peru and Santa Cruz Islands subduction zones. Subduction of the Orozco fracture zone may strongly affect the seismogenic character as the overlapping rupture zones are located on the crest of the subducted fracture zone. There is also a distinct change in the physiography of the upper plate that coincides with the subducting fracture zone, and the Guerrero seismic gap to the south of the Petatlan earthquake is in the “wake” of the Orozco fracture zone. At the northern end, the Rivera fracture zone in the subducting plate and the Colima graben in the upper plate coincide with the northernmost extent of the Colima rupture zone.

Notes: Abstract Twenty-one different caprine and 13 ovine MHC-DRB exon 2 sequences were determined including part of the adjacent introns containing simple repetitive (gt)n(ga)m elements. The positions for highly polymorphic DRB amino acids vary slightly among ungulates and other mammals. From man and mouse to ungulates the basic (gt)n(ga)m structure is fixed in evolution for 7 × 107 years whereas ample variations exist in the tandem (gt)n and (ga)m dinucleotides and especially their “degenerated” derivatives. Phylogenetic trees for the α-helices and β-pleated sheets of the ungulate DRB sequences suggest different evolutionary histories. In hoofed animals as well as in humans DRB β-sheet encoding sequences and adjacent intronic repeats can be assembled into virtually identical groups suggesting coevolution of noncoding as well as coding DNA. In contrast a-helices and C-terminal parts of the first DRB domain evolve distinctly. In the absence of a defined mechanism causing specific, site-directed mutations, double-recombination or gene-conversion-like events would readily explain this fact. The role of the intronic simple (gt)n(ga)m repeat is discussed with respect to these genetic exchange mechanisms during evolution.

Notes: Summary The greater hip fracture rate among elderly women is generally ascribed to differences in femoral neck strength between the sexes. Strength of a given bone is a function of both its material properties and the magnitudes of mechanical stresses within it. This study examined the hypothesis that these apparent strength differences between the sexes are due to dissimilarities in the restructuring of the femoral neck with age, which result in higher stresses in elderly women. Using Hip Strength Analysis, a computer program developed by the authors, femoral neck cross-sectional geometric properties for stress analyses were derived from bone mineral image data of 409 community living, white subjects ranging from 19 to 93 years of age. Though both sexes show declines in femoral neck bone mineral density (BMD) and cross-sectional area with age, only females show a decline in the cross-sectional moment of inertia (CSMI, a geometric index of bone rigidity). The lack of decline in male CSMI appears to be a result of a small but significant increase in femoral neck girth. Similar age-related changes have been observed in the femoral shaft by others. The net effect of these observed changes is that mechanical stresses in the femoral neck of females appear to increase at three times the rate per decade of those of males. These results lend support to the hypothesis that the higher fracture rate in elderly women is due, at least in part, to elevated levels of mechanical stress, resulting from a combination of greater bone loss and less compensatory geometric restructuring with age.

Notes: Summary Bone strength is a function of both bone mass and its geometric distribution, a factor that is obscured in the conventional bone mineral analysis. Structural geometry is particularly important in areas such as the femoral neck that are exposed to bending loadsin vivo. Here we present results of a study examining age changes in the structural geometry of the female femoral neck derived from dual photon absorptiometry (DPA) data. In a previous study, differences in the aging patterns of males and females over the entire adult age range were demonstrated. In that study, only males showed “compensatory” geometric restructuring of the femoral neck which tended to offset loss of bone mineral with age. In the present study, femoral neck structural properties from 1044 women were examined for aging trends before and after the approximate age of menopause (50 years). Women in the premenopausal age range showed a 4% decline per decade in femoral neck BMD, but no change in the femoral neck cross-sectional moment of inertia (CSMI). This aging pattern is similar to that of males in our earlier study, and in both cases resulted in little or no increase in femoral neck bending stresses. After age 50, however, women show a more rapid decline in femoral neck BMD (7% per decade) accompanied by a decline in CSMI of 5% per decade. These changes result in increases in femoral neck stresses of 4–12% per decade due to the apparent lack of compensatory restructuring to offset the loss of bone mineral. These results shed further light on the age-related mechanisms underlying sex differences in fracture incidence among the elderly. They also argue for the routine use of such structural analyses in any study of age-related osteopenia or the effects of therapeutic intervention on this condition.

Notes: Abstract The phototrophic growth ofChloroflexus aurantiacus under anoxic conditions was determined as a function of continuous UV irradiance. Cultures grown under an irradiance of 0.01 Wm−2 exhibited a slightly depressed yield over the nonirradiated control. Yields decreased further with increasing irradiance. Inhibition was severe at an irradiance of 0.66 Wm−2. Growth ofE. coli cultures was severely depressed at UV-C irradiances that permitted good growth ofC. aurantiacus. Low levels of Fe3+ provided a very effective UV absorbing screen. The apparent UV resistance ofChloroflexus and the effectiveness of iron as a UV-absorbing screen in sediments and microbial mats are suggested to be likely mechanisms of survival of early phototrophs in the Precambrian in the absence of an ozone shield.