RADON ANOMALIES ON 3 KINDS OF FAULTS IN CALIFORNIA

Citation
Cy. King et al., RADON ANOMALIES ON 3 KINDS OF FAULTS IN CALIFORNIA, Pure and Applied Geophysics, 141(1), 1993, pp. 111-124
Citations number
NO
Categorie Soggetti
Geosciences, Interdisciplinary
Journal title
ISSN journal
00334553
Volume
141
Issue
1
Year of publication
1993
Pages
111 - 124
Database
ISI
SICI code
0033-4553(1993)141:1<111:RAO3KO>2.0.ZU;2-0
Abstract
Radon emanation is known to be anomalously high along active faults in many parts of the world. We tested this relationship in California du ring July and early August 1992, using a portable radonmeter to conduc t soil-air radon surveys at 5 sites across three kinds of faults: Cree ping, locked, and freshly broken. Along a 350-m long survey line acros s a creeping segment of the San Andreas fault at Nyland Ranch in San J uan Bautista, we found anomalous radon concentrations not in the creep zone itself as determined by a creepmeter, but on the adjacent sides, 10 and 30 meters from the center line of the fault. The anomalous val ues were 5 times higher than the background values measured farther aw ay from the fault. A similar radon anomaly was observed along a 420-m long survey line across a creeping segment of the Calaveras fault near 7th Street in Hollister. There, the anomalous values were about 6 to 11 times the background values and about 40 and 50 m from the center l ine of the fault. The double-peaked feature of the anomalies may be in dicative of a relatively low gas permeability of the fault-gouge mater ials in the creeping zones and high permeability of fractured rocks in the adjacent shear zones. Along a 144-m survey line across the curren tly locked segment of the San Andreas fault at the Earthquake Trail ne ar Olema, the radon concentration was indeed anomalously high in the f ault zone, by a factor of two above background values. However, the ma ximum values (3 to 6 times background) again were recorded about 10 me ters from the center line. Three weeks after the magnitude 7.5 Landers earthquake of 28 June 1992, we conducted a survey along a 300-m line across the earthquake fault alongside Encantado Road in the epicenter area. The radon values measured at the two main fault breaks were an o rder of magnitude higher than the background values. A similar result was found along a 420-m line alongside Reche Road about 1.7 km south o f Encantado Road.