The absence of a heat flow anomaly greater than ∼0.3 µcal/cm2/sec associated with the San Andreas fault is used to estimate the upper limit for the steady state or initial shear stress. Under the assumption that the long‐term rate of motion along the fault is 5 cm/yr and occurs primarily in the form of creep, this upper limit is about 100 bars. If the motion is primarily accomplished by faulting during large earthquakes and if the frictional stress is equal to the final stress as suggested by E. Orowan (1960), the upper limit is estimated to be about 200 bars. Without Orowan's assumption, the estimation of the upper limit is about 250 bars, based on earthquake energy‐magnitude‐moment relations. If the long‐term rate of motion along the San Andreas fault is only ∼2 cm/yr, these results are increased to 250, 350, and 400 bars, respectively.

No abstract

No abstract

A new paleomagnetic secular variation (PSV) record from the late Quaternary Wilson Creek beds of Mono Lake, California contains a distinctive periodic vector waveform that follows and is almost certainly related to the Mono Lake excursion. A comparison of all published PSV results from the Wilson Creek beds suggests that the magnetic field at Mono Lake went through an interval (36,000‐28,000 ybp) of very low amplitude PSV followed by the Mono Lake excursion (28,000‐27,000 ybp) and four subsequent recurrences (27,000‐12,500 ybp) of the excursion waveform with relatively diminished amplitudes (lower than the excursion amplitude but higher than PSV amplitudes at Mono Lake since 12,500 ybp). This pattern clearly suggests that the Mono Lake excursion is related to ‘typical’ PSV; it also suggests that the core dynamo process responsible for PSV is capable of near impulse onset, persistence over a time scale of 15,000 years, and quasi‐periodic behavior that is non‐wave‐length dispersive in time.

An experiment to measure heat flow in the vicinity of major strike-slip faults was begun in 1965, since near seismically active faults a significant amount of strain energy might be converted to heat by means of dissipative processes operating during fault slippage. Calculations based on the Gutenberg-Richter seismic energyearthquake magnitude relation and the probable occurrence of two 8-magnitude earthquakes per century along the length of the San Andreas fault suggested that if the amount of energy converted to heat was at least as great as that appearing as seismic waves [Bullard, 1954], and depending on fault geometry, a measurable heatflow anomaly could exist near the trace of a major fault. The existence or nonexistence of a measurable heat-flow anomaly associated with large faults permits limits to be set on the average frictional stresses acting across fault planes 7924

Initial results from a seismic experiment in the central South Island, New Zealand, have imaged a 40 ± 5°s outheast-dipping zone at a depth of c. 22 km beneath the Mt Cook village. It is speculated that this reflector represents the down-dip extension of the Alpine Fault Zone.

Seismic and gravity data taken along line 1 of the 1982 Consortium for Continental Reflection Profiling (COCORP) Mojave Desert Survey (N‐S profile, ∼30 km long) have been used to characterize the upper crust north of the San Andreas fault in the western Mojave block of southern California. Consortium for Continental Reflection Profiling seismic reflection data were reprocessed to emphasize the upper 5 seconds (two‐way travel time). The resultant common depth point (CDP) sections provided starting models for generating a refined geologic cross‐section using a combination of ray tracing (forward modeling) and gravity interpretation. The forward modeling was used to validate the existence of faults and constrain their dips. The gravity data were used to refine the overall model, particularly in poor data areas on the CDP sections. Gravity data, taken along three nearby profiles parallel to primary line of section, were also used to determine the structural trend. Results from the first two seconds indicate the presence of a series of ENE striking reverse faults beneath the late Tertiary and Quaternary sedimentary cover of the western Mojave. The faults dip northward and offset the sediment‐basement interface. The largest such feature has an apparent throw of ∼1.8 km and exhibits a subtle scarp at the Earth's surface suggesting Holocene displacement. The orientation of these faults, although not an instantaneous representation of the present‐day stress field, is consistent with NNW compression across the western Mojave block and WNW striking San Andreas fault, as determined from nearby focal mechanisms and in situ stress measurements. The faults also appear to be closing small sedimentary basins in the Mojave block, which may have formed during an earlier extensional phase, similar to what is happening on a much larger scale in the Los Angeles basin to the south of the San Andreas fault. Reflections between 2 and 5 s, coupled with the local geology and gravity modeling, are consistent with the presence of the Pelona/Rand schist in the subsurface beneath the western Mojave. The upper surface of the schist (i.e., Vincent/Rand thrust equivalent) rises southward toward the San Andreas fault where it is displaced vertically (up to the south) at least 5 km along the E‐W trending Hitchbrook fault, such that the schist crops out between the Hitchbrook and subparallel San Andreas to the south. The same structure may exist beneath the Tehachapi mountains, with the roles of the Hitchbrook and San Andreas faults played by the north and south branches of the Garlock fault, respectively. The rising or arching of the basement toward the San Andreas fault (and toward the Garlock) is not only reflected in the geology and topography local to these faults in many places but is also generally observed on seismic reflection profiles in the vicinity of these faults in the western Mojave. Furthermore, the arching is also consistent with a strong component of fault normal compression.

No abstract