PRECARIOUS ROCK TOPPLING ACCELERATIONS IN SOUTHERN NEVADA AND THE EASTERN MOJAVE: IMPLICATIONS FOR SEISMIC HAZARD

Precariously balanced rocks (PBRs) exist at various sites near Las Vegas.  Constraints on seismic hazard in the region around Las Vegas can estimated by comparison with PBRs in two extensively documented areas within 200 km of Las Vegas,--the eastern Mojave and Yucca Mountain.  In the Mojave PBRs are usually not found closer than about 15 km from the San Andreas fault.   However with increasing distance the toppling accelerations (TAs,--about 30% greater than the aspect ratios, alphas) of the most easily toppled rocks decreases, roughly from about 0.4 g at 15 km to about 0.25 g at 30 km, a result of the attenuation of ground motion with distance from the fault.  TAs of about .25 g are found at Granite Pediment in the eastern Mojave Desert, about 140 km SW of Las Vegas.  Table 1 shows the raw data for alpha, the approximate area covered in each case, the numbers normalized to the number with alpha of 0.5, and the number normalized to area. 

Raw Data				Normalized to 100				Distribution per Km ²
α	Lovejoy		Granite Pediment	α	Lovejoy		Granite Pediment	α	Lovejoy 0.46km²		Granite Pediment 0.23km²
0.1	0		6	0.1	0		6.82	0.1	0.00		26.09
0.2	0		37	0.2	0		42.05	0.2	0.00		160.87
0.3	22		68	0.3	21		77.27	0.3	47.83		295.65
0.4	69		83	0.4	66		94.32	0.4	150.00		360.87
0.5	105		88	0.5	100		100	0.5	228.26		382.61

The obvious rough interpretation of these data is that rocks with alphas of   0.1, 0.2, and some with 0.3, have been knocked down at Lovejoy Buttes near the San Andreas fault, but many remain at Granite Pediment, closer to Las Vegas.  The distribution of alphas at Granite Pediment is approximately flat for alphas between 0.5 and 0.3, suggesting that the numbers of naturally evolved alphas is approximately independent of alpha, consistent with the fact that there is no known reason why there should be a drastically different number of joint blocks of any particular shape.  At Yucca Mountain, about 140 km north of Las Vegas, there are numerous PBRs with TAs of about 0.25 g.  Although only limited surveys of PBRs have been carried out nearer Las Vegas, comparison with rocks at Granite Pediment and Yucca Mountain can give a rough estimate of the relative hazards.  The inferred hazard around Las Vegas is consistent with the hazard maps. inferred hazard at Yucca Mountain and Granite Pediment, and with the recently released 2007 draft hazard maps.