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Poster Abstracts | Group 1 – GMP
1-061
DISTRIBUTION OF PRECARIOUS ROCK TOPPLING ACCELERATIONS WITH
DISTANCE FROM THE SAN ANDREAS FAULT: LOVEJOY BUTTES AND GRANITE
PEDIMENT Brune JN
For precariously balanced rocks (PBRs) in the Mojave, the toppling acceleration (TAs) of the most
easily toppled rocks decreases with distance from the San Andreas fault, about 0.4 g at 15 km to
about 0.25 g at 30 km. The approximate frequency of occurrence of rocks of different alphas (about
30% less than TAs in g’s, assuming a waveform for an earthquake on the San Andreas fault) as a
function of distance is estimated at Lovejoy Buttes (about 15-17 km) and Granite Pediment
(hundreds of km). At Lovejoy Buttes there are no rocks with alphas of 0.2 or 0.1, whereas at Granite
Pediment there are a number. Preliminary conclusions are:
(1) At Granite Pediment, far from known active faults, hundreds of km from the San Andreas fault,
there is no indication of a strong dependence of numbers of rocks as a function of fragility.
(2) For alphas of 0.5 (toppling accelerations of about 0.65 g) the areal density of rocks is about
380/km2 at Granite Pediment, and about 230/km2 at Lovejoy Buttes, 15-17 km from the San
Andreas Fault. The relatively small difference can be easily explained by the difference in
characteristic joint in spacing, rather than ground accelerations from earthquakes.
(3) If we normalize the number of rocks to 100 at both sites, for the least fragile rocks, alphas of 0.5,
then at Lovejoy Buttes there are obviously less rocks with alphas of .3 (21 vs. 77), and none with of
0.2 and 0.1 (vs. 42 and 7 at Granite Pediment, respectively), a clear indication that these more
fragile rocks have been toppled by earthquakes on the nearby San Andreas fault. There are not a
great deal more rocks with alphas of 0.5 than rocks with alphas of 0.4 at any sites.
(4) For alphas of 0.3, 0.4 and 0.5, when compared to Lovejoy Buttes, which has been shaken by
dozens of M~8 earthquakes, there are no indications of very much larger numbers of such rocks
(multiplicative factors of over 1000 suggested by some) at any site east of Lovejoy Buttes.
(5) Considering the above, there is no indication that the rocks at Lovejoy Buttes with alphas of 0.3
or 0.4 (toppling accelerations of 0.4 g and 0.5 g) are only remnants of a much larger population,
most of which have hypothetically been knocked down by previous earthquakes.
102 | Southern California Earthquake Center