In four chapters various aspects of earthquake source are studied.
Chapter I
Surface displacements that followed the Parkfield, 1966, earthquakes were measured for two years with six small-scale geodetic networks straddling the fault trace.The logarithmic rate and the periodic nature of the creep displacement recorded on a strain meter made it possible to predict creep episodes on the San Andreas fault. Some individual earthquakes were related directly to surface displacement, while in general, slow creep and aftershock activity were found to occur independently. The Parkfield earthquake is interpreted as a buried dislocation.
Chapter II
The source parameters of earthquakes between magnitude 1 and 6 were studied using field observations, fault plane solutions, and surface wave and S-wave spectral analysis. The seismic moment, MO, was found to be related to local magnitude, ML, by log MO = 1.7 ML + 15.1. The source length vs magnitude relation for the San Andreas system found to be: ML = 1.9 log L - 6.7. The surface wave envelope parameter AR gives the moment according to log MO = log AR300 + 30.1, and the stress drop, τ, wasfound to be related to the magnitude by τ = 0.54 M - 2.58. The relation between surface wave magnitude MS and ML is proposedto be MS = 1.7 ML - 4.1. It is proposed to estimate the relative stress level (and possibly the strength) of a source-region bythe amplitude ratio of high-frequency to low-frequency waves. An apparent stress map for Southern California is presented.
Chapter III
Seismic triggering and seismic shaking are proposed as two closely related mechanisms of strain release which explain observations of the character of the P wave generated by the Alaskan earthquakeof 1964, and distant fault slippage observed after the Borrego Mountain, California earthquake of 1968. The Alaska, 1964,earthquake is shown to be adequately described as a series of individual rupture events. The first of these events had a bodywave magnitude of 6.6 and is considered to have initiated or triggered the whole sequence. The propagation velocity of thedisturbance is estimated to be 3.5 km/sec. On the basis of circumstantial evidence it is proposed that the Borrego Mountain,1968, earthquake caused release of tectonic strain along three active faults at distances of 45 to 75 km from the epicenter. Itis suggested that this mechanism of strain release is best described as "seismic shaking."
Chapter IV
The changes of apparent stress with depth are studied in the South American deep seismic zone. For shallow earthquakes theapparent stress is 20 bars on the average, the same as for earthquakes in the Aleutians and on Oceanic Ridges. At depthsbetween 50 and 150 km the apparent stresses are relatively high, approximately 380 bars, and around 600 km depth they are againnear 20 bars. The seismic efficiency is estimated to be 0.1. This suggests that the true stress is obtained by multiplyingthe apparent stress by ten. The variation of apparent stress with depth is explained in terms of the hypothesis of oceanfloor consumption.
