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

Geology

Whitcomb, James Hall ; Anderson, Donald L.

University:California Institute of Technology

Department:Geological and Planetary Sciences

关键词: Geology;   

Others  :  https://thesis.library.caltech.edu/8090/1/Whitcomb%201973.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

The initial objective of Part I was to determine the nature ofupper mantle discontinuities, the average velocities through themantle, and differences between mantle structure under continentsand 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 themselvesand their inferences on core structure. P'dP' at both single stationsand at the LASA array in Montana indicates that the following zonesare candidates for discontinuities with varying degrees of confidence:800-950 km, weak; 630-670 km, strongest; 500-600 km, strong butinterpretation in doubt; 350-415 km, fair; 280-300 km, strong, varyingin depth; 100-200 km, strong, varying in depth, may be the bottom ofthe low-velocity zone. It is estimated that a single station cannoteasily discriminate between asymmetric P'P' and P'dP' for lead timesof about 30 sec from the main P'P' phase, but the LASA array reducesthis uncertainty range to less than 10 sec. The problems of scatterof P'P' main-phase times, mainly due to asymmetric P'P', incorrectidentification of the branch, and lack of the proper velocitystructure at the velocity point, are avoided and the analysis showsthat one-way travel of P waves through oceanic mantle is delayedby 0.65 to 0.95 sec relative to United States mid-continentalmantle.

A new P-wave velocity core model is constructed from observedtimes, dt/dΔ's, and relative amplitudes of P'; the observed times ofSKS, SKKS, and PKiKP; and a new mantle-velocity determination byJordan and Anderson. The new core model is smooth except for adiscontinuity at the inner-core boundary determined to be at aradius of 1215 km. Short-period amplitude data do not require theinner core Q to be significantly lower than that of the outer core.Several lines of evidence show that most, if not all, of the arrivalspreceding the DF branch of P' at distances shorter than 143° aredue to scattering as proposed by Haddon and not due to sphericallysymmetric discontinuities just above the inner core as previouslybelieved. Calculation of the travel-time distribution of scatteredphases and comparison with published data show that the strongestscattering takes place at or near the core-mantle boundary close tothe seismic station.

In Part II, the largest events in the San Fernando earthquakeseries, initiated by the main shock at 14 00 41.8 GMT on February 9,1971, were chosen for analysis from the first three months ofactivity, 87 events in all. The initial rupture location coincideswith the lower, northernmost edge of the main north-dipping thrustfault and the aftershock distribution. The best focal mechanismfit to the main shock P-wave first motions constrains the faultplane parameters to: strike, N 67° (± 6°) W; dip, 52° (± 3°) NE;rake, 72° (67°-95°) left lateral. Focal mechanisms of the aftershocksclearly outline a downstep of the western edge of the main thrustfault surface along a northeast-trending flexure. Faulting on this downstep is left-lateral strike-slip and dominates the strain releaseof the aftershock series, which indicates that the downstep limitedthe main event rupture on the west. The main thrust fault surfacedips at about 35° to the northeast at shallow depths and probablysteepens to 50° below a depth of 8 km. This steep dip at depth is acharacteristic of other thrust faults in the Transverse Ranges andindicates the presence at depth of laterally-varying verticalforces that are probably due to buckling or overriding that causessome upward redirection of a dominant north-south horizontalcompression. Two sets of events exhibit normal dip-slip motion withshallow hypocenters and correlate with areas of ground subsidencededuced from gravity data. Several lines of evidence indicate thata horizontal compressional stress in a north or north-northwestdirection was added to the stresses in the aftershock area 12 daysafter the main shock. After this change, events were contained inbursts along the downstep and sequencing within the bursts providesevidence for an earthquake-triggering phenomenon that propagateswith speeds of 5 to 15 km/day. Seismicity before the San Fernandoseries and the mapped structure of the area suggest that the downstepof the main fault surface is not a localized discontinuity but ispart of a zone of weakness extending from Point Dume, near Malibu, toPalmdale on the San Andreas fault. This zone is interpreted as adecoupling boundary between crustal blocks that permits them to deformseparately in the prevalent crustal-shortening mode of the TransverseRanges region.