TWO-DIMENSIONAL KINEMATIC FAULT MODELING OF THE PACOIMA DAM STRONG-MOTION RECORDINGS OF THE FEBRUARY 9, 1971, SAN FERNANDO EARTHQUAKE

A simple, uncontrived model of the rupture during the San Feruando earthquake can explain the main features of the particle velocity traces derived from the accelerograms recorded at Pacoima Dam. This result, combined with the probable small effect of surface topography on the velocity traces, strengthens the case for acceptance of the peak particle velocity at Pacoima Dam (115 em/sec) as a valid ground-motion parameter for design purposes ill earthquake engineering. Most of the conspicuous motion on the velocity traces during the first 4 sec after triggering seems to result from thrust faulting, starting at a focal depth within several kilometers of 14 km, on a fault surface dipping at least 50 ° and extending only part way to the surface at a velocity near 2.5 km/sec. The data also indicate that this faulting continued to the surface at a slower rupture velocity (less than 2 km/sec) along a less steeply dipping surface. The amount of relative offset across the fault surface is difficult to determine, both because of inherent limitations in the two-dimensional model and because of nouuniqueness in the fitting of the data. The estimates of this dislocation, however, are consistent with the wide range of values reported by other authors in studies using various types of data. The data are also consistent with a model suggested by Alewine and Jordan (1973) and Trifunac (1974) in which the total dislocation has a minimum near the center of the fault surface, with approximately equal amounts of total offset on the fault near the hypocenter and near the Earth's surface.

[1]  T. Mikumo.  Faulting process of the San Fernando earthquake of February 9, 1971 inferred from static and dynamic near-field displacements , 1973, Bulletin of the Seismological Society of America.

[2]  J. Brune.  Tectonic stress and the spectra of seismic shear waves from earthquakes , 1970 .

[3]  Y. Tsai,et al.  Near Field Small Earthquakes - Dislocation Motion , 1972 .

[4]  M. Wyss,et al.  The Alaska earthquake of 28 March 1964: A complex multiple rupture , 1967 .

[5]  David M. Boore,et al.  The effect of simple topography on seismic waves: Implications for the accelerations recorded at Pacoima Dam, San Fernando Valley, California , 1973, Bulletin of the Seismological Society of America.

[6]  Mihailo D. Trifunac,et al.  A three dimensional dislocation model for the San Fernando, California, earthquake of February 9, 1971 10F, 3T, 42R Bull. Seism. Soc. Am. V64, N1, Feb. 1974, P149–172 , 1974 .

[7]  M. Toksöz,et al.  Static and dynamic study of earthquake source mechanism: San Fernando Earthquake , 1972 .

[8]  Thomas C. Hanks,et al.  The faulting mechanism of the San Fernando Earthquake , 1974 .

[9]  Mihailo D. Trifunac,et al.  Complexity of energy release during the Imperial Valley, California, earthquake of 1940 , 1970 .

[10]  David M. Boore,et al.  Near-field motions from kinematic models of propagating faults , 1974 .

[11]  N. Christensen.  Shear wave velocities in metamorphic rocks at pressures to 10 kilobars , 1966 .

[12]  T. Jordan,et al.  Generalized Inversion of Earthquake Static Displacement Fields , 1973 .

[13]  Mihailo D. Trifunac,et al.  Analysis of the Pacoima dam accelerogram—San Fernando, California, earthquake of 1971 , 1971 .