Stewart W. Smith

Seismicity of the Gorda Plate, structure of the continental margin, and an eastward jump of the Mendocino Triple Junction

Analysis of 9 years of data from the Humboldt Bay seismic network sheds new light on the structure and evolution of the Gorda plate and Mendocino triple junction. Significant findings include the buttressing effect of the Pacific plate which demonstrates that there is no underthrusting of the Gorda plate along the Mendocino fault, and the pattern of left-lateral northeast-trending faults, which demonstrates how the Gorda plate accommodates N-S shortening without such underthrusting. Focal mechanisms are consistent with northward compression of the Gorda plate by the Pacific plate until the Gorda plate passes the triple junction, beyond which the N-S compressive stress is effectively removed, and the focal mechanisms show a change from strike slip to normal faulting with downslab tension. Another significant feature is the shallow Benioff zone (10–40 km) which shows a double seismogenic layer. Unlike most other double seismic layers in deep subduction zones, this one appears to be due to underplating, with a new subduction zone being developed east of the former one. A corner of continental margin material has been partially subducted. We attribute this double seismic zone to reactivation of both the old and new subduction boundaries under N-S compression produced by Pacific-Gorda plate interaction at the triple junction. Because the geometry of this triple junction indicates that it is unstable and because there is evidence that as part of its evolution the San Andreas fault has migrated eastward, we have constructed a model that accounts for both the eastward migration of the San Andreas fault and the doubling of the seismic zone in the Gorda plate. This can be done by assuming that at the time of the last eastward jump of the fault, the overriding continental margin north of the triple junction was broken and underthrust producing a new subduction zone that is collinear with the San Andreas fault. Other scenarios for the evolution of the Mendocino triple junction do not require that the subduction zone migrate eastward. However, the position and dip of the double zone indicate that an eastward jump of 100 km some 5 my ago could have taken place. This is consistent with other evidence for an eastward jump of the San Andreas fault.

The anelasticity of the mantle

Abstract Data on attenuation of body waves, surface waves and free oscillations are summarized. Virtually all of the seismic attenuation in the mantle occurs in the low-Q zone that corresponds to the low-velocity zone for seismic waves. Regardless of the data source, all experiments show an order of magnitude increase in Q in the region between 400 and 800 km depth. Data compiled here indicate the rate of increase of Q is more rapid than has been suggested in the past, that is, lower boundary of the low-Q zone is very sharp. An intercomparison of the various results shows that a larger difference between Q-values exists than would be expected from looking at the internal consistency of each experiment. These discrepancies cannot be explained on the basis of path differences or frequency dependence alone, but may also be due in part to amplitude dependence. The current uncertainties in data on attenuation of surface waves and free oscillations arc so large that it does not appear to call for the same level of effort now being applied in the inversion of velocities of surface waves and frequencies of free oscillations. Improved long-period instrumentation and the larger number of long-period stations that now exist can be expected to provide a substantial improvement in the quality of attenuation data in the near future.

Regional secular strain fields in southern Nevada

Abstract Data from a network of six strainmeters in southern Nevada were analyzed to determine how much of the observed strain at each station was common to the entire region. Using a least-square technique, a regional strain field was detected in data covering a two-month time interval. Several changes in the field occurred, one of which, beginning on January 18, 1971, appeared to be a major change in the rate of strain accumulation. The regional field grows and decays, with maximum variation of 10 over the two months studied. The principal axes show northwest extension, and their directions remain fixed as the magnitude of the field varies.

Reduction of territorial uncertainty of earthquake forecasting

Abstract After the increase of probability of a strong earthquake is diagnosed for a certain territory, this territory can be narrowed down by additional analysis of recent seismicity, at the cost of some increase of the rate of failure to predict. Specifically, a strong earthquake has to be expected within the area, marked by a short-term quiescence on the evidence of sufficiently high seismicity. Such areas were outlined retrospectively for 18 earthquakes in California, Nevada, Kuril arc, Japan, Taiwan, Mexico and Caucasus. In 17 cases the epicenters lie within these areas and the territorial uncertainty of prediction was reduced by a factor of 4 to 14. In one case (Japan) the actual epicenter was well outside the area outlined.