The initial objective of Part I was to determine the nature of\r\nupper mantle discontinuities, the average velocities through the\r\nmantle, and differences between mantle structure under continents\r\nand oceans by the use of P'dP', the seismic core phase P'P' (PKPPKP)\r\nthat reflects at depth d in the mantle. In order to accomplish this,\r\nit was found necessary to also investigate core phases themselves\r\nand their inferences on core structure. P'dP' at both single stations\r\nand at the LASA array in Montana indicates that the following zones\r\nare candidates for discontinuities with varying degrees of confidence:\r\n800-950 km, weak; 630-670 km, strongest; 500-600 km, strong but\r\ninterpretation in doubt; 350-415 km, fair; 280-300 km, strong, varying\r\nin depth; 100-200 km, strong, varying in depth, may be the bottom of\r\nthe low-velocity zone. It is estimated that a single station cannot\r\neasily discriminate between asymmetric P'P' and P'dP' for lead times\r\nof about 30 sec from the main P'P' phase, but the LASA array reduces\r\nthis uncertainty range to less than 10 sec. The problems of scatter\r\nof P'P' main-phase times, mainly due to asymmetric P'P', incorrect\r\nidentification of the branch, and lack of the proper velocity\r\nstructure at the velocity point, are avoided and the analysis shows\r\nthat one-way travel of P waves through oceanic mantle is delayed\r\nby 0.65 to 0.95 sec relative to United States mid-continental\r\nmantle.

\r\n\r\nA new P-wave velocity core model is constructed from observed\r\ntimes, dt/d\u0394's, and relative amplitudes of P'; the observed times of\r\nSKS, SKKS, and PKiKP; and a new mantle-velocity determination by\r\nJordan and Anderson. The new core model is smooth except for a\r\ndiscontinuity at the inner-core boundary determined to be at a\r\nradius of 1215 km. Short-period amplitude data do not require the\r\ninner core Q to be significantly lower than that of the outer core.\r\nSeveral lines of evidence show that most, if not all, of the arrivals\r\npreceding the DF branch of P' at distances shorter than 143\u00b0 are\r\ndue to scattering as proposed by Haddon and not due to spherically\r\nsymmetric discontinuities just above the inner core as previously\r\nbelieved. Calculation of the travel-time distribution of scattered\r\nphases and comparison with published data show that the strongest\r\nscattering takes place at or near the core-mantle boundary close to\r\nthe seismic station.

\r\n\r\nIn Part II, the largest events in the San Fernando earthquake\r\nseries, initiated by the main shock at 14 00 41.8 GMT on February 9,\r\n1971, were chosen for analysis from the first three months of\r\nactivity, 87 events in all. The initial rupture location coincides\r\nwith the lower, northernmost edge of the main north-dipping thrust\r\nfault and the aftershock distribution. The best focal mechanism\r\nfit to the main shock P-wave first motions constrains the fault\r\nplane parameters to: strike, N 67\u00b0 (\u00b1 6\u00b0) W; dip, 52\u00b0 (\u00b1 3\u00b0) NE;\r\nrake, 72\u00b0 (67\u00b0-95\u00b0) left lateral. Focal mechanisms of the aftershocks\r\nclearly outline a downstep of the western edge of the main thrust\r\nfault surface along a northeast-trending flexure. Faulting on this \r\ndownstep is left-lateral strike-slip and dominates the strain release\r\nof the aftershock series, which indicates that the downstep limited\r\nthe main event rupture on the west. The main thrust fault surface\r\ndips at about 35\u00b0 to the northeast at shallow depths and probably\r\nsteepens to 50\u00b0 below a depth of 8 km. This steep dip at depth is a\r\ncharacteristic of other thrust faults in the Transverse Ranges and\r\nindicates the presence at depth of laterally-varying vertical\r\nforces that are probably due to buckling or overriding that causes\r\nsome upward redirection of a dominant north-south horizontal\r\ncompression. Two sets of events exhibit normal dip-slip motion with\r\nshallow hypocenters and correlate with areas of ground subsidence\r\ndeduced from gravity data. Several lines of evidence indicate that\r\na horizontal compressional stress in a north or north-northwest\r\ndirection was added to the stresses in the aftershock area 12 days\r\nafter the main shock. After this change, events were contained in\r\nbursts along the downstep and sequencing within the bursts provides\r\nevidence for an earthquake-triggering phenomenon that propagates\r\nwith speeds of 5 to 15 km/day. Seismicity before the San Fernando\r\nseries and the mapped structure of the area suggest that the downstep\r\nof the main fault surface is not a localized discontinuity but is\r\npart of a zone of weakness extending from Point Dume, near Malibu, to\r\nPalmdale on the San Andreas fault. This zone is interpreted as a\r\ndecoupling boundary between crustal blocks that permits them to deform\r\nseparately in the prevalent crustal-shortening mode of the Transverse\r\nRanges region.

\r\n", "doi": "10.7907/JDKQ-2Z91", "publication_date": "1973", "thesis_type": "phd", "thesis_year": "1973" } ]