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

A systematic study of the travel times and apparent velocities of precursors of the seismic core phase PKPPKP indicate that these phases are reflections from the mantle. The strongest reflection is from a depth of 630 km. In order of confidence, other reflectors were found at depths of 280, 520, 940, 410 (very weak), and 1250 km (tentative). The weakne• of the 410-km reflection was surprising in view of the large velocity increase at this depth indicated by refraction and Love-wave studies. This transition region must be broader than the others or must involve a smaller density jump. Reflections were observed that were possibly from the top and bottom of the low-velocity zone at depths of 50 and 130 km, respectively. The above reflections are interpreted in terms of the following solid-solid phase changes, in order of increasing depth: pyroxene-garnet solid solution, olivine --) B spinel, B spinel --) spinel and pyroxene --) spinel -[-stishovite, spinel --) post-spinel, and garnet --) ilmenite or oxides. A spin-spin transition in Fe 2' may be responsible for one of the deeper discontinuities found by others. Recent models of the structure of the upper mantle, such as those of Anderson and ToksSz [1963], Niazi and Anderson [1965], Archambeau et al. [1969], Johnson [1967], Ibrahim and Nuttli [1967], Green and Hales [1968], Julian and Anderson [1968], and Anderson and Julian [1969] include regions of high velocity gradient in the upper mantle. The sharpness of these discontinuities has led to new interest in looking for reflections of seismic waves from structure in the upper mantle Whircomb and Anderson [1968], and Niazi [1969] found reflections from the upper surfaces of discontinuities in the upper mantle. Another reflection of interest is the reflection of P'P' (PKPPKP) from the underneath surface of discontinuities.This reflection has the decided advantage of being in a quiet part of the seismic record before the main P'P' phase. Gutenberg [1960] noted arrivals up to 30 sec before P'P', and Adams [1968] interpreted P'P' precursors as reflections arriving up to 70 sec before the main phase. stations for reading the precursors, he could not recognize that some of his readings for the deep earthquakes were probably SKKKP, which, as E•gdahl and Flinn [1969a] have pointed out, has a different apparent slowness. However, Adams' remaining readings indicated discontinuities near 65 to 70 km and 160 to 180 km in depth. Engdahl and Flinn [1969b] found a precursor with the proper apparent slowness corresponding to a reflection depth of 650 km. Our notation is similar to that used by Bolt et al. [1969], who designated a reflection of the PP phase at a depth d in kilometers as PdP, in that P'dP' is used to indicate a reflection of the P'P' phase at depth d in kilometers. Thus, a reflection at 650 km would be P'650P', and P'OP' is equivalent to P'P'. Seismic records were searched systematically for P'P' precursors up to five minutes before the main phase and were analyzed for reflectors at depth. Two geographic regions of...

The time variation of crustal velocities in tectonic regions is most reasonably attributed to stressinduced variations in crack porosity. The decrease in V•,/Vs before earthquakes is due primarily to a large decrease in Vp. This supports the Nur dilatancy hypothesis but not the effective stress hypothesis. New data from the San Fernando region verify the Vp drop, show that this drop cannot be entirely due to source depth effects, and give strong support to the explanation of material property, or path effect, rather than source effect variations. Calculations show that the crack-widening model works even for midcrustal depths in saturated rock. Narrow cracks of low aspect ratio are required to satisfy the velocity and uplift constraints. The recovery of velocity prior to fracture can be due to fluid flow or crack closure. The t • L •' relation does not require diffusion. Diffusion of groundwater or crack closure leads to increased pore pressure and rock weakening. Observations of gravity, conductivity, and crustal distortions along with velocities should narrow the choice of models. The crust in regions of thrust tectonics is probably always dilatant to some degree. The aftershock region is smaller than tl•e anomalous velocity region, which in turn must be smaller than the dilatant region. A simple relationship is derived for the relative sizes of the anomalous and aftershock regions.

Geodetic data collected in Long Valley, California, from 1975 through 1983 define a pattern of uplift and strain which is evidently associated with a sequence of earthquakes occurring in May 1980 and subsequent swarm activity continuing until the present. We have constructed a model to explain the deformation observed since May 1980 in terms of inflation of two subsurface magma chambers, faulting in the south moat region of the caldera, and slip on the Hilton Creek fault. The most significant new feature of the model is the shallow magma chamber at 5 km depth, located a few hundred meters to the east of the Casa Diablo hot spring area. Inflation of this chamber causes stresses which show consistency with various aspects of the seismicity in the south moat of the caldera. Calculations of stress across vertical planes over the magma chamber can be used together with failure criteria to estimate the inflation volume at which the rock layers intervening between the chamber and the surface will fail by extensional fracture.