Part I. A study of the velocity structure of the earth by the use of core phases. Part II. The 1971 San Fernando earthquake series focal mechanisms and tectonics
<p>The initial objective of Part I was to determine the nature of upper mantle discontinuities, the average velocities through the mantle, and differences between mantle structure under continents and oceans by the use of P'dP', the seismic core phase P'P' (PKPPKP) that...
| Summary: | <p>The initial objective of Part I was to determine the nature of
upper mantle discontinuities, the average velocities through the
mantle, and differences between mantle structure under continents
and oceans by the use of P'dP', the seismic core phase P'P' (PKPPKP)
that reflects at depth d in the mantle. In order to accomplish this,
it was found necessary to also investigate core phases themselves
and their inferences on core structure. P'dP' at both single stations
and at the LASA array in Montana indicates that the following zones
are candidates for discontinuities with varying degrees of confidence:
800-950 km, weak; 630-670 km, strongest; 500-600 km, strong but
interpretation in doubt; 350-415 km, fair; 280-300 km, strong, varying
in depth; 100-200 km, strong, varying in depth, may be the bottom of
the low-velocity zone. It is estimated that a single station cannot
easily discriminate between asymmetric P'P' and P'dP' for lead times
of about 30 sec from the main P'P' phase, but the LASA array reduces
this uncertainty range to less than 10 sec. The problems of scatter
of P'P' main-phase times, mainly due to asymmetric P'P', incorrect
identification of the branch, and lack of the proper velocity
structure at the velocity point, are avoided and the analysis shows
that one-way travel of P waves through oceanic mantle is delayed
by 0.65 to 0.95 sec relative to United States mid-continental
mantle. </p>
<p>A new P-wave velocity core model is constructed from observed
times, dt/dΔ's, and relative amplitudes of P'; the observed times of
SKS, SKKS, and PKiKP; and a new mantle-velocity determination by
Jordan and Anderson. The new core model is smooth except for a
discontinuity at the inner-core boundary determined to be at a
radius of 1215 km. Short-period amplitude data do not require the
inner core Q to be significantly lower than that of the outer core.
Several lines of evidence show that most, if not all, of the arrivals
preceding the DF branch of P' at distances shorter than 143° are
due to scattering as proposed by Haddon and not due to spherically
symmetric discontinuities just above the inner core as previously
believed. Calculation of the travel-time distribution of scattered
phases and comparison with published data show that the strongest
scattering takes place at or near the core-mantle boundary close to
the seismic station. </p>
<p>In Part II, the largest events in the San Fernando earthquake
series, initiated by the main shock at 14 00 41.8 GMT on February 9,
1971, were chosen for analysis from the first three months of
activity, 87 events in all. 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, N 67° (± 6°) W; dip, 52° (± 3°) NE;
rake, 72° (67°-95°) left lateral. Focal mechanisms of the aftershocks
clearly outline a downstep of the western edge of the main thrust
fault surface along a northeast-trending flexure. Faulting on this
downstep is left-lateral strike-slip and dominates the strain release
of the aftershock series, which indicates that the downstep limited
the main event rupture on the west. The main thrust fault surface
dips at about 35° to the northeast at shallow depths and probably
steepens to 50° below a depth of 8 km. This steep dip at depth is a
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 upward 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. 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 downstep and sequencing within the bursts provides
evidence for an earthquake-triggering phenomenon that propagates
with speeds of 5 to 15 km/day. Seismicity before the San Fernando
series and the mapped structure of the area suggest that the downstep
of the main fault surface is not a localized discontinuity but is
part of a zone of weakness extending from Point Dume, near Malibu, to
Palmdale on the San Andreas fault. This zone is interpreted as a
decoupling boundary between crustal blocks that permits them to deform
separately in the prevalent crustal-shortening mode of the Transverse
Ranges region.</p>
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