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Abstract. Mathematical models are developed to study the propagation of excess pore pressure in heterogeneous and fractured rocks. For a homogeneous rock, the time needed for a pore pressure front to migrate downward is directly proportional to the square of the depth, and inversely proportional to the permeability of the rock. Variational studies show that pore pressure propagation is highly influenced by the permeability heterogeneity of the crust. The results argue for the importance of accounting for geologic complexity when using mathematical models to estimate the extent of downward transmission of increases in hydraulic heads. The models presented in the paper are used to estimate a range of possible hypocentral depths of periodic seismicity observed near Mt. Ogden on the Alaska-British Columbia border. The time lag between these earthquakes and hydrologic loading is on the order of days or weeks, indicating a quick response of seismicity to the increased surface water input from rainfall or glacial discharge, if such a causal relationship exists. Our models estimate that the hypocentral depths of these earthquakes could be on the order of several kilometers, if a high degree of vertical interconnectivity of fractures exists.

We have constructed a model for the crust and uppermost mantle beneath the Colorado Plateau by combining newly acquired seismic refraction/wide-angle reflection data from the 1989 Pacific to Arizona Crustal Experiment (PACE) cross profile with data collected by Roller (1965). A combination of forward modeling methods was used to model travel times and relative amplitudes of crustal and upper mantle phases. The salient features of the model for the Colorado Plateau (hereafter referred to as the plateau) are (1) an upper crust having an average velocity of 6.1 km/s with a low vertical velocity gradient, (2) midcrustal discontinuities at depths of 30 to 38 km beneath the plateau, (3) a lower crust that varies in thickness (5 to 16 km) and has an average velocity of 6.8 km/s, (4) a transitional lower crust/upper mantle boundary, and (5) a crustal thickness of 45 _+ 3 km. A comparison of our new model for the Colorado Plateau with recent models for the Basin and Range shows that the upper crusts of the two provinces are similar but that the lower crust of the plateau is thicker, particularly in the plateau's central region. These observations are consistent with the idea that the Basin and Range could have formed from crust like that of the Colorado Plateau. INTRODUCTION The Colorado Plateau (hereinafter referred to as the plateau) has long aroused scientific curiosity because of differences between it and neighboring geologic provinces. Since Proterozoic time, the plateau has remained relatively stable tectonically (except for cumulative subsidence represented by the deposition of sedimentary rocks over a period of about 600 Ma) amid the active lithospheric deformation of surrounding areas. The Pacific to Arizona Crustal Experiment (PACE), a multiyear, multi-institutional effort, was designed to investigate the structure of the crust from offshore California to Arizona and to elucidate factors that may control the tectonic behavior of these distinct geologic regions [Mc-Carthy et al., 1990]. The 1989 PACE seismic refraction/wideangle reflection experiment was a part of this larger effort. It expanded upon the work of previous years with the addition of two profiles: the 150-km SW-NE main line, which extended through the Transition Zone to the western portion of the Colorado Plateau, and the 150-km SE-NW cross line, which was located exclusively within the Colorado Plateau (Figure 1). The main profile better images the lower crust of the Transition Zone and its boundary with the westernmost portion of the Colorado Plateau; the cross profile, which images the crust of the plateau, is the focus of this paper. GEOLOGIC AND TECTONIC SETTING Numerous publications address the geology and tectonic history of the Colorado Plateau. General features of this tectonic province along with extensive references are presented by Lucchitta [1989], Hinze [1988], and Smith et al. [1989]. A concise tectonic description of the structural evolution of the Colorado Plateau and surrounding areas can 1Now at be presented in three stages: (1) ...