Surface Wave Propagation and Source Studies in the Gulf of California Region

Citation

Thatcher, Wayne Raymond (1972) Surface Wave Propagation and Source Studies in the Gulf of California Region. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/0981-B217. https://resolver.caltech.edu/CaltechTHESIS:03282018-134558094

Abstract

A number of aspects of seismic surface wave propagation and earthquake mechanism in the Gulf of California region are investigated in this thesis. In addition, several associated problems raised by this study are also explored in some detail.

Surface wave dispersion and P-wave travel time delays are measured to delineate the crust and upper mantle structure in the Imperial Valley-Gulf of California region. Crustal thicknesses beneath Baja California and Sonora are comparable and near 25 km, while within the Gulf crustal structure varies laterally from nearly oceanic on the western side to continental shelf thicknesses (~20 km) towards the north and east. Love wave group velocities for Baja California paths are unusually high and were not used to determine structure. P_n and teleseismic P-wave delays are used in a reconnaissance survey of crustal structure in the Imperial Valley and across the Peninsular Range batholith. The data are consistent with an increase in crustal thickness of 12 km from flank to crest in the Peninsular Ranges, and a decrease of 8 km across the Imperial Valley.

The high Love wave group velocities measured across Baja California are shown to be similar to velocities of the first higher mode. It is also demonstrated that higher Love modes can have group velocities very close to fundamental mode velocities for a range of wave periods and realistic earth models. The mode interference which is a consequence of this intertwining of group velocity curves has a significant effect on measured phase velocities, and this problem is investigated in detail. An important conclusion of this study is that anomalously high Love wave phase velocities reported for the United States midcontinent and Japan are straightforwardly explained by mode interference without appealing to complex or anisotropic models, as had been done previously.

Seismic processes associated with actively spreading oceanic rises are examined in the study of a strong swarm of earthquakes located near an inferred spreading center in the Northern Gulf of California. Close-in travel time data constrain the origin times of swarm events and demonstrate that the epicenters are confined to the upper crust. Teleseismic P-delays suggest unusually low seis1nic velocities beneath the source. The previously suspected normal faulting nature of swarm earthquakes is also confirmed. Seismic coupling across 200 km between adjacent spreading centers in the Northern Gulf is indicated by a survey of recent seismicity.

It is noted in the study of the Gulf swarm that these sources have significantly higher surface wave amplitudes than events with similar assigned magnitudes in Northern Baja California. In the final chapter of this thesis a detailed analysis is made of the Baja earthquakes and it is concluded that as a group they have distinctly smaller source dimensions and larger stress drops than events within the Gulf of California. These differences are quite marked and are often very clearly seen even on records from band-limited seismographs. Several examples exist where propagation paths are very similar but the visual appearance of records differs considerably, suggesting that near-source or path effects are not likely explanations of the observed differences.

For small magnitude North Baja earthquakes, both source dimensions and long period surface excitation average only about a factor or two larger than corresponding quantities previously measured for underground nuclear explosions of similar magnitude.

Thesis Files