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Precariously Balanced Rock Orientations and Fragilities Compared with Cybershake Waveforms: Implications for Seismic Hazard and Possible Super-Shear Ruptures.
Introduction
Earthquake recurrence forecasting, ERF, has steadily improved in recent years. Similarly the numerical power for calculating seismic ground motion, based on various types of modeling and assumed input parameters, have greatly improved (regression methodologies. tera and peta scale computing). The weak link in proceeding to final estimates of seismic hazard is the validation of the various inputs and modeling procedures. All of the current models have to assume values for source parameters, values which cannot be verified without more data (e.g., rupture rise time, slip weakening distance, rupture velocity, direction of rupture, background stress, frictional stress, dynamic stress history). There are simply not enough instrumental data from large earthquakes near-source, to validate or constrain the various programs and assumed source parameterizations. The study of precariously balanced rocks may be the only way to remedy the situation. Without extensive strong motion recordings of earthquakes for 100’s of years, required to validate seismic hazard estimates for low probabilities (e.g., return periods of thousands of years as needed in the design of sensitive structures) the PBRs are currently the only source of data for constraining ground motions over such long periods (thousands of years, Bell et al. 1998, Rood et al., 2008) and are thus of great importance to seismic hazard studies. Precarious rocks can potentially serve as back checks for all of the steps in seismic hazard estimation,
In 2011 we studied critical rocks in the UNR archive of thousands of PBRs (see Archive Work below), strategically selected to be most useful in constraining the next generation of seismic hazard maps to be developed in the next few years. Analysis of discrepancies, in part presented in 2011 SCEC Annual Meeting abstracts and posters, suggests that the precarious rocks are important in testing some of the inputs and assumptions in producing the maps, e.g., the ergodic assumption, attenuation relationships, random background earthquake assumptions, directions of rupture propagation, relative hanging wall-foot wall ground motions, step-over ground motions, frequency of supersonic ruptures, and various other UCERF assumptions (e.g., fault activity, fault dips). Results were presented at the 2011 SCEC annual Meeting.
For examples, the rocks at Silverwood Lake and Grass valley suggest the 1857 earthquake may have not ruptured from NW to SE as commonly assumed, the Cleghorn fault and Pinto Mountain faults may not be nearly as active as assumed in UCERF2, and the effect on PBRs of the dip of the San Andreas fault NE under San Bernardino, as recently suggested by Fuis et al. (Feb. 2012 Bull. SSA), may lead to important new understanding of the tectonics and seismic hazard in the San Bernardino area.
CYBERSHAKE WAVEFORMS AND EARTHQUAKE HAZARD. Precariously Balanced Rocks (PBRs) typically provide the most sensitive constraints on estimated ground motions in a particular direction, and Cybershake low frequency waveforms yield ground motion time histories for any given direction. This allows for straightforward testing of the Cybershake waveforms for consistency with PBRs, provided an estimate is made of the high frequency waveforms associated with the Cybershake waveforms (broad-band Cybershake waveforms). Cataloged PBRs between the San Jacinto and Elsinore faults are predominantly sensitive to fault perpendicular ground motions over approximately 150 km (FIGURE 1). In fact, 75% of the PBRs are sensitive to motion within ± 30o of fault perpendicular (Brune et al., 2006). These rocks exist nearly equidistant between the two faults. The orientations strongly suggest that larger fault parallel motions (relative to fault perpendicular motions) have contributed to the observed distribution of sensitive rocking directions for PBRs. This is contrary to current thinking of predominantly fault perpendicular velocity pulses for sub-shear ruptures, and very rare predominantly fault parallel ground motions for super shear ruptures. . Numerous kinematic and dynamic models of ruptures confirm the expected ground motions for sub-shear and super-shear ruptures. The advent of Cybershake ground motion calculations offer the possibility of developing a better understanding of possible explanations for the distribution of PBR orientations in this region, and in other critical regions.
PBRs, Cybershake1.0 , and USGS Hazard Maps
In Purvance et al. (2008, Bull. SSA), the overturning fragilities of precariously balanced rocks (PBRs) were parameterized as a function of a vector of the ground motion intensity measures peak ground acceleration (PGA) and response spectra at 1 sec (Sa1, closely correlated with peak ground velocity, PGV). The resulting overturning probabilities (OPs) for many of the PBRs were very high, suggesting they were inconsistent with the 2002 USGS ground motions(including rocks between the Elsinore and San Jacinto faults, and rocks in the Mojave Desert near the San Andreas fault. A similar comparison with the 2008 hazard maps indicated that there was somewhat less but still considerable inconsistency (Brune et al., 2010, SCEC abstracts). Graves et al. (2010) indicate that the Cybershake1.0 Sa3 hazard map values and the 2008 USGS Sa3 hazard map values are about the same for rock sites, suggesting that the Cybershake Sa3 hazard maps are also inconsistent with the PBRs. Since the Cybershake results presumably take care of most of the path and site effects, this suggests that the earthquake source representation is the primary cause for the discrepancy between the PBRs and both the USGS and Cybershake hazard maps.
Cybershake Ground Motion Orientations
The Cybershake 1,0 earthquake Sa3 hazard curve for the Perris site, about halfway between the Elsinore and San Jacinto faults, illustrates the problem ( Fig 3). The SA3 values for the fault perpendicular ground motion are considerably higher than the values for fault parallel, as expected for the sub-sonic ruptures assumed in the Cybershake models. Also, the SA3 values at 10-6 annual probability are very large, and possibly unphysical. Understanding this situation may be crucial for both the Earthquake Recurrence Forecast and attenuation relationships Use of Cybershake waveforms will be critical for this. Critical to this process will be having accurate estimates of fragilities for critical rocks.
Although approximate estimates of fragilities will be useful for preliminary studies, we eventually need digital Photomodel shapes of rocks for accurate comparisons. The following is a list of areas where rocks have been Photographed for Photomodeler:
Santa Ynez Mountains-Santa Barbara, Wilson Canyon near Anza. CA, Jacumba area near southern Elsinore fault, Enchanted Canyon- Motte Rimrock Reserve near Perris, Lake Perris North-near the San Jacinto Fault, Pioneertown and Yucca Valley near the Pinto Mountain fault, Pacifico Mountain-San Gabrel Mountains, Grass Valley rocks-San Bernardino Mts., Lovejoy Buttes-Mojave Desert, Beaumont South near San Jacinto fault, Lake Isabella-Southern Sierras, Round Top rock near Vail Lake-Peninsular Ranges.
Of the rocks involved, approximately 20 have already been modeled to produce digital 3-D shape models. Of these, about 10 are in the present Cybershake broad band coverage area, and are being investigated by Jessica Donavan, Tom Jordan at USC, and James Brune at UNR. Preliminary result are very encouraging, and indicate that future studies will greatly improve our understanding of earthquake hazard.
CONCLUSION
We continued testing of precarious rocks in areas important for constraining earthquake hazard. We used our improved methods of surveying to maximize the efficiency in terms of time and money. We improved and updated our archive for Southern California PBRs, including approximate fragilities for many important PBRs (see next page). We are in the process of comparing the PBR data with Cybershake waveforms and hazard maps. Initial results indicate a major step forward will result from further research in SCEC4.
FIGURE 1. a) DEM of So Cal with PBRs and primary toppling directions. b) Rosette overlain with the percentage of PBR toppling directions within 10o bins. The proportion of PBRs in each bin are included when > 10%. 75% of the PBRs topple within ± 30o of the fault perpendicular direction.
(Fig. 2) (Fig. 3)
FIGURE 2. Rosette of the orientations of all digitized short axes in images from 4 PBR sites between the San Jacinto and Elsinore faults. The numerical values are the percentage of the total number of short axes within the corresponding 10o bin. The probability of randomly selecting 75% or more rocks with short axes within ± 30o of fault perpendicular is vanishingly small ( -7e-10) %.
Figure 3. Cybershake hazard curve for location PERRIS half-way between the Elsinore and San Jacinto faults
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