Repeated length measurements of several geodetic baselines near Parkfield, California, have revealed significant variations in the local rates of shallow fault shp and strain. This network of baselines, surveyed several times eadt week, straddles the San Andreas fault in the transition zone between the creeping section to the northwest and the locked section to the southeast. The length measurements, characterized by a precision approaching 0.1 ppm, reveal large fluctuations in the rates of baseline extension. Principal mode analysis of the length change data indicates that the two largest components of the signal are (1) secular extensions and contractions consistent with surface slip on the main strand of the San Andreas fault, and (2) a large seasonal oscillation with no obvious spatial coherence. On most of these basehues, the second component appears to be in phase with seasonal rainfall. When data from the baselines with the largest asnplitude of the seasonal signal are excluded, the remaining data can be modeled in terms of both spatial and temporal variations in surface slip, variations in the components of the changes in uniform strain, and the possible displacement of the central monument in this radial network In parameterizing this model, the spatial variation of shp beneath the near surface is reflected by changes in shear strain. Although the computed secular shear is highly dependent upon the specified parameterization of surface shp, the data are consistent with the hypothesis that shp at intermediate depths lags behind the surface shp rate. However, the range in models that fit the data does not necessarily imply that there is a deficit in slip at depth relative to the surface. Comparison of the inferred values of surface shp from the model with the observed fault shp measured by very short baseline creep meters indicates close agreement in secular rates, but the short-term variations observed with the creep meters are either highly attenuated or nonexistent in the modeled shp since the modeled shp is a spatial average which smooths out possible shortwavelength variations in the surface slip for which the creep instruments are most sensitive. An interesting conclusion from the two-color data is that surface slip on the San Andreas fault appears to be spread over a 2-km-wide zone on the south flank of Middle Mountain but is confined to a very narrow zone to the south as the fault passes through the center of the network. This conclusion is dependent upon the assumption that a few critical monuments are stable and track tectonic displacements in the long term. Finally, the largest observed strain change is an extensional strain coincident with the Kettleman Hills earthquake/i//5.5 in August 1985. INTRODUCTIONSince mid-1984, the U.S. Geological Survey (USGS) has been monitoring length changes on a number of geodetic baselines near Parkfield, California. These measurements are made several times weekly and are one component in a program of measurements that were initiated soon after a prediction of a...

Spectral analysis of the biweekly measurements of line lengths from the 12-baseline network at Pearblossom, California, reveals that the precision of the prototype two-color Geodimeter is a---[a 2 q-D2b2] 1/2, where a -0.3 mm and b = 0.12 ppm and D is the length of the baseline. In contrast, the computed precision is overestimated by 15-20% when a secular trend is fit to 15 months of measurements. By using spectral analysis, the variance a 2 can be separated into two components: the frequency independent instrumental precision and a frequency dependent part. Evaluation of the same data with regard to their dependence upon fluctuations in weather reveal that at most, weather makes a secondorder contribution of 0.01 ppm to the observed strain changes. This contribution is a factor of 5-10 times smaller than the instrumental precision. Thus the observed strain changes at Pearblossom reported by Langbein et al. (1982) during 1981 are not due to systematic errors resulting from an improper model of refraction in the atmosphere. 1. baselines near Hollister, California, were found to be unsuitable for assessment of instrumental precision since tectonic strain changes were large and episodic. The episodic nature of crustal deformation due to fault creep near Hollister was reflected in the power density spectra computed by Lan•bein [1979] of residual line length changes which resulted from a fit of linear trend. The computed power density decreased with frequency f according to f-2. It was not until the frequency exceeded 0.1 cycles/d that the power spectra became independent of frequency. By integrating under the power spectra where the spectral amplitude was independent of frequency, Langbein computed the variance to be (0.1 ppm) 2. The data, however, are unsuitable to test whether the instrument is indeed stable for periods in excess of 10 days since the large tectonic signal masks any instrumental drift. Some of the shortcomings of the previous studies are overcome in the analysis reported here in that we have a large number of two-color measurements of the distances for a network undergoing a reasonably low and uneventful rate of tectonic strain. This study involves 15 months of measurements on 11-13 baselines near Pearblossom, California, where the strain rate is much lower than near Hollister. The ideal study would consist of daily measurements on several baselines in an aseismic area where strain does not significantly accumulate. This requirement is almost met in the area near Pearblossom which exhibits a secular rate of tensor shear strain of 0.2 ppm/yr and similarly low rates for the orthogonal components of strain parallel and normal to the San Andreas fault [Langbein et al., 1982]. Measurements here were made approximately twice per week on an irregular schedule. We have applied spectral analysis techniques to the data for determining the variance associated with random errors. It became clear that the amount of random error could be overestimated if the length of the time sampled was too long for the segment...

A multiwavelength distance-measuring (MWDM) instrument that utilizes the dispersive nature of the atmosphere has been developed and tested. With this instrument there is no need for the usual meteorological observations and corrections. This MWDM instrument simultaneously measures the optical path length at three wavek/ngths, two in the optical region of the spectrum and one in the microwave region. The instrument calculates correction terms from the optical path length differences and then computes the corrected base line distance, the first-order effects of temperature, pressure, and water vapor fluctuations along the line thus being eliminated. Field testing demonstrated that the instrument was capable of making consecutive distance determinations with a standard deviation from the mean as small as 4 parts in l0 s. The long-term stability of the instrument was investigated from June 1974 to November 1974. The results showed a standard deviation of 1.3 parts in 107 when the data were fit to a sine wave of annual periodicity. The amplitude of this sine wave was 3 parts in 107, which was in good agreement with the predicted results of a two-dimensional t, hermoelastic model. o d c• i I ß ß ß ß ß ß ß 'l ß ß ß ß ß l' ß ß I. ß ß ß e e ß ß ß '.l ß ß ß ß ß .I t (•u •u) 30NV.L$10 SLATER AND HUGGETT: A MWDM INSTRUMENT

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