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.