Jerry P. Eaton

New Evidence on the State of Stress of the San Andreas Fault System

Contemporary in situ tectonic stress indicators along the San Andreas fault system in central California show northeast-directed horizontal compression that is nearly perpendicular to the strike of the fault. Such compression explains recent uplift of the Coast Ranges and the numerous active reverse faults and folds that trend nearly parallel to the San Andreas and that are otherwise unexplainable in terms of strike-slip deformation. Fault-normal crustal compression in central California is proposed to result from the extremely low shear strength of the San Andreas and the slightly convergent relative motion between the Pacific and North American plates. Preliminary in situ stress data from the Cajon Pass scientific drill hole (located 3.6 kilometers northeast of the San Andreas in southern California near San Bernardino, California) are also consistent with a weak fault, as they show no right-lateral shear stress at ∼2-kilometer depth on planes parallel to the San Andreas fault.

Sierra Nevada Batholith The batholith was generated within a synclinorium

The Sierra Nevada batholith is localized in the axial region of a complex faulted synclinorium that coincides with a downfold in the Mohorovicic discontinuity and in P-wave velocity boundaries within the crust. Observed P-wave velocities are compatible with downward increase in the proportion of diorite, quartz diorite, and calcic granodiorite relative to quartz monzonite and granite in the upper crust, with amphibolite or gabbro-basalt in the lower crust, and with periodotite in the upper mantle. The synclinorium was formed in Paleozoic and Mesozoic strata during early and middle Mesozoic time in a geosyncline marginal to the continent. Granitic magmas are believed to have formed in the lower half of the crust at depths of 25 to 45 kilometers or more, primarily as a result of high radiogenic heat production in the thickened prism of crustal rocks. Magma was generated at different times in different places as the locus of down-folding shifted. It rose into the upper crust because it was less dense than rock of the same composition or residual refractory rocks. Refractory rocks and crystals that were not melted and early crystallized mafic minerals that settled from the rising magma thickened the lower crust. Wall and roof rocks settled around, and perhaps through, the rising magma and provided space for its continued rise. Erosion followed each magmatic episode, and 10 to 12 kilometers of rock may have been eroded away since the Jurassic and 7 to 10 kilometers since the early Late Cretaceous.

THE CAPE MENDOCINO, CALIFORNIA, EARTHQUAKES OF APRIL 1992 : SUBDUCTION AT THE TRIPLE JUNCTION

The 25 April 1992 magnitude 7.1 Cape Mendocino thrust earthquake demonstrated that the North America—Gorda plate boundary is seismogenic and illustrated hazards that could result from much larger earthquakes forecast for the Cascadia region. The shock occurred just north of the Mendocino Triple Junction and caused strong ground motion and moderate damage in the immediate area. Rupture initiated onshore at a depth of 10.5 kilometers and propagated up-dip and seaward. Slip on steep faults in the Gorda plate generated two magnitude 6.6 aftershocks on 26 April. The main shock did not produce surface rupture on land but caused coastal uplift and a tsunami. The emerging picture of seismicity and faulting at the triple junction suggests that the region is likely to continue experiencing significant seismicity.

Earthquake refraction profiles of the root of the Sierra Nevada

We examine the seismic structure of the Sierra Nevada using records of nine earthquakes and one explosion in and near the Sierra, recorded on stations in the Sierra. We first interpret travel times from these paths, which are confined to a single tectonic block, in terms of one-dimensional structures. The most nearly reversed pair of earthquakes, the 1966 Truckee and 1983 Durrwood Meadows earthquakes, share refracted (Pn) arrival times (corrected to surface focus) along a line t=8.75±0.25+Δ/8.0, suggesting that a nearly flat layer of 8.0 km/s mantle material lies at depths of 46–48 km. First arrivals from these events do not constrain velocities from ≈30 to 45 km depth. Secondary arrivals and some first arrivals from other earthquakes suggest that velocities in part of this region range between 6.9 and 7.8 km/s. The presence of this “7.x-km/s” layer can help to explain previous contradictory observations. The 7.x-km/s layer could be interpreted as either the mafic bottom of a silicic, Mesozoic magmatic arc or as accreted mafic underplating or rejuvenated mantle related to Cenozoic arc volcanism or Basin and Range spreading. Arrivals at stations in the foothills and the crest of the Sierra cannot be fit with a single longitudinal structure, indicating a lateral variation of velocity structure. These variations support previously inferred variations of lithospheric structure, with higher-velocity, thinner crust to the west beneath the Sierran foothills and slower-velocity crust (or possibly upper mantle) beneath the high mountains in the eastern Sierra. Rapid changes in arrival times between stations separated by short distances in the eastern Sierra suggest that a sharp boundary exists between the Sierra and the Basin and Range at Moho depths. We also present fresh evidence of the asymmetry of the root of the Sierra, wherein arrivals from earthquakes on the west of the Sierra are delayed within the Sierra and return to original values in the Basin and Range, while arrivals from earthquakes and explosions from the Sierra into the Great Valley. We suggest that if the 7.x-km/s material occurs in a wedge above the Moho, then the asymmetry can be explained by arrivals from the west being delayed by the dipping 8.0-km/s Moho, while those from the east may be entering the root along a 7.x-km/s layer that is near the depth of the Basin and Range Moho.