A large precursory change in seismic body-wave velocities occurred before the earthquake in San Fernando, California. The discovery that this change is mainly in the P-wave velocity clearly relates the effect to the phenomenon of dilatancy in fluid-filled rocks. This interpretation is supported by the time-volume relation obtained by combining the present data with the data from previous studies. The duration of the precursor period is proportional to the square of an effective fault dimension, which indicates that a diffusive or fluid-flow phenomenon controls the time interval between the initiation of dilatancy and the return to a fully saturated condition which is required for rupture.

The largest events in the San Fernando earthquake series, initiated by the main shock at 14h 00m 41.8s UT on February 9, 1971, were chosen for analysis from the first three months of activity, 87 events in all. C. R. Allen and his co-workers assigned the main shock parameters: 34ø24.7'N, 118ø24.0'W, focal depth h _--8.4 km, and local magnitude ML --6.4. The initial rupture location coincides with the lower, northernmost edge of the main north-dipping thrust fault and the aftershock distribution. The best, focal mechanism fit to the main shock P wave first motions constrains the fault plane parameters to: strike, N67ø(_+6ø)W; dip, 52ø(+__3ø)NE; rake, 72 ø (670-95 ø) left lateral. Focal mechanisms of the aftershocks clearly outline a down step of the western edge of the main thrust fault surface along a northeast-trending flexure. Faulting on this down step is left lateral strike slip and dominates the strain release of the aftershock series, which indicates that the down step limited the main event rupture on the west. The main thrust fault surface dips at about 35 ø to the northeast. at shallow depths and probably steepens to 50 ø below a depth of 8 kin. This steep dip at. depth is • characteristic of other thrust faults in the Transverse ranges and indicates the presence at depth of laterally varying vertical forces that are probably due to buckling or overriding that causes some upwa.rd redirection of a dominant north-south horizontal compression. Two sets of events exhibit. normal dip slip motion with shallow hypocenters and correlate with areas of ground subsidence deduced from gravity data.One set in the northeastern aftershock area is related to shallow extensional stresses caused by the stcepening of the main fault plane. The other set is probably caused by a deviation of displacements along the down step of the main fault. surface that resulted in localized ground subsidence near the western •'nd of the main fault break. Several lines of evidence indicate that a horizontal compressional stress in a north or north-northwest direction was added to the stresses in the aftershock area 12 days after the main shock. After this change, events were contained in bursts along the down step, and sequencing within the bursts provides evidence for an earthquake-triggering phenomenon that propagates with speeds of 5-15 kin/day. Seismicity before the San Fernando series and the mapped structure of the area suggest that the down step of the main fault surface is no• a localized discontinuity but is part of a zone of weakness extending fi'om Point Dume, near Malibu. to Palmdale on the San Andreas fault. This zone is interpreted as a alecoupling boundary between crustal blocks that permits them to deform separately in the prevalent crustal shortening mode of the Transverse ranges region. CONTENTS

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A geometric formulation of the seismic travel time problem is given based upon the use of slowness as an independent variable. Many of the difficulties in the conventional treatment (e.g., singular kernels) are thereby, avoided. Furthermore, it is shown that the inverse problem possesses an inherently linear formulation. In this formalism we are able to provide extremal solutions giving upper and lower depth bounds using linear programing. This approach has been compared with two well‐known nonlinear extremal inversions. We find our technique to be easier to implement and find that it often generates superior results.