Large numbers of small earthquakes recorded over 2 decades and analyzed with advanced techniques are used to characterize the detailed kinematics, structure and recurrence interval scaling properties of micro‐seismicity in a 4 × 4 km lateral and 6 km deep crustal volume encompassing the region of the SAFOD deep drilling experiment. The characterization reveals that the seismically active San Andreas fault in the vicinity of SAFOD's repeating magnitude 2 target earthquakes is composed of two sub‐parallel fault strands that are creeping at comparable rates and that one of the strands lies between the SAFOD drilling platform and SAFOD's target events. In the region, ∼55% of the earthquakes are members of 52 characteristically repeating earthquake sequences. The recurrence intervals of the repeating target events are consistent with the interval scaling of the other sequences. However this scaling is contrary to that expected from standard constant stress‐drop theory.
Large rock slope failures from near-vertical cliffs are an important geomorphic process driving the evolution of mountainous landscapes, particularly glacially steepened cliffs. The morphology and age of a 2·19 × 10 6 m 3 rock avalanche deposit beneath El Capitan in Yosemite Valley indicates a massive prehistoric failure of a large expanse of the southeast face. Geologic mapping of the deposit and the cliff face constrains the rock avalanche source to an area near the summit of ~8·5 × 10 4 m 2 . The rock mass free fell ~650 m, reaching a maximum velocity of 100 m s, impacted the talus slope and spread across the valley fl oor, extending 670 m from the base of the cliff. Cosmogenic beryllium-10 exposure ages from boulders in the deposit yield a mean age of 3·6 ± 0·2 ka. The ~13 kyr time lag between deglaciation and failure suggests that the rock avalanche did not occur as a direct result of glacial debuttressing. The ~3·6 ka age for the rock avalanche does coincide with estimated late Holocene rupture of the Owens Valley fault and/or White Mountain fault between 3·3 and 3·8 ka. The coincidence of ages, combined with the fact that the most recent (AD 1872) Owens Valley fault rupture triggered numerous large rock falls in Yosemite Valley, suggest that a large magnitude earthquake (≥M7.0) centered in the south-eastern Sierra Nevada may have triggered the rock avalanche. If correct, the extreme hazard posed by rock avalanches in Yosemite Valley remains present and depends on local earthquake recurrence intervals. Published in
Systematic cataloguing of seismic moment, depth and mechanism of regional earthquakes down to magnitude M∼4 can now be achieved in close to real‐time using data from sparse networks of digital broadband stations. The procedure we have developed relies on 3 independent methods for the determination of moment tensor, providing confidence limits on the results. We show the results of application to the seismicity in central and northern California for a one year period starting in March 1992 and illustrate how patterns of stress and strain release can now be monitored systematically and reliably in a timely fashion.
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