Precarious Rocks Methodology and Preliminary Results

Transcription

1 FINAL TECHNICAL REPORT Precarious Rocks Methodology and Preliminary Results James N. Brune, Rasool Anooshehpoor, Yuehua Zeng, and John G. Anderson Seismological Laboratory University of Nevada, Reno Reno, NV 89557

2 SUMMARY This report describes the results of the use of precariously balanced rocks to provide constraints on strong ground motions in southern California. It constitutes the final report for several tasks in the last few years, and specifically for the last year of SCEC funding. Precarious rocks in seismically active regions are effectively strong-motion seismoscopes that have been in place for thousands of years. Thus estimates of the dynamic toppling acceleration of these rocks can provide constraints on the peak ground accelerations experienced during previous earthquakes. Numerous precarious rocks exist near the San Andreas Fault system and associated thrust. Estimates of toppling accelerations of the surveyed precarious rocks in this study have led to the following conclusions: Attenuation relations based on toppling accelerations of precarious rocks in Mojave Desert and San Gabriel Mountains, on a profile perpendicular to the San Andreas fault, agree with the 10% in 50 year hazard maps published by USGS-CDMG, but predict lower ground motions than those for the 2% in 50 yr hazard maps. Precarious rocks provide constraints on ground motion for earthquakes such as 1812 and 1857 earthquakes on San Andreas fault, 1899 and 1912 events on San Jacinto fault, the 1952 Kern County earthquake, and recent, but prehistoric earthquakes on Banning and Garlock faults, that are lower than ground motions predicted by recent attenuation curves. Strong motion data from the Izmit, Turkey, and Chi Chi, Taiwan, earthquakes have pointed out the uncertainties in current strong motion attenuation curves for large earthquakes. Near-source strong motion data from those two earthquakes was considerably below the median for current attenuation curves. However the data from these two earthquakes is consistent with constraints estimated form precarious rocks. Precarious rock evidence suggests relatively low ground accelerations during earthquakes on trans-tensional sections of strike-slip faults such as the San Jacinto earthquakes of 1899 and These events occurred along branches of the San Jacinto fault where a right-step trans-tensional basin, the Hemet basin, occurs. Toppling of a previously documented rock at Granite Pass near Kelso, California, by the M7.1 Hector Mine earthquake of 16 October 1999, has provided an approximate field calibration of the precarious rock methodology. The time of the last earthquake for the section of the fault nearest to the precarious rocks has recently been determined to be more than 10 ka, consistent with previously estimated ages for precarious rocks. 2

3 PRINCIPAL ACCOMPLISHMENTS Introduction and History Since there are no near-source strong-motion recordings from great earthquakes (magnitude 7.5 and above) along the San Andreas fault, nor from large thrust faults in the Los Angeles Basin, estimates of source parameters and strong motion for such future earthquakes have to rely on questionable extrapolations from smaller earthquakes at larger distances, and from earthquakes in other regions which may have differing tectonic and geologic environments. Precarious rocks, on the other hand, which have been in place thousands of years in Southern California, may provide the only existing local constraints on the source parameters and strong motion. Surveying to discover and obtain the locations of these precarious rocks, and their distances from nearby faults, may be essential to provide data to constrain models. Our original proposal to Southern California Earthquake Center in 1998 was to study the toppling accelerations of precarious rocks along a profile perpendicular to the San Andreas fault from the Mojave desert to the San Gabriel mountains (Figs. 1, 2), with the ultimate objective of constraining strong ground motion attenuation relationships for great earthquakes on the San Andreas fault. Preliminary estimates of toppling accelerations in this region (Table 1) had suggested rough agreement with mean values predicted by various ground motion attenuation relationships, and with 10% in 50 yr hazard maps published by USGS-CDMG (Fig. 3). However there was an apparent discrepancy with the USGS-CDMG hazard maps for 2% in 50 yrs. Therefore we proposed to perform quantitative field, laboratory, and numerical tests to check the preliminary estimates. These tests confirmed the preliminary estimates. Publications reporting these results included: Brune et al., 1998; Anooshehpoor and Brune, 2000; Brune, 2001, 2002) During reconnaissance surveys for precarious rocks in southern California it was noticed that some precarious rocks seemed to exist closer to the fault trace on the footwall of thrusts than would be predicted by standard attenuation relationships. Since physical and numerical modeling of thrust faults being carried out at University of Nevada, Reno, suggested good physical reasons why this might be the case, we proposed a more systematic study of precarious rocks on the footwall of the Banning and White Wolf faults. In the process of field surveys it was also noticed that the hanging wall rocks of some thrust faults in southern California exhibited an intensely fractured character (Fig. 4). Further surveys verified that this was true of the hanging wall of all major thrusts. This indicated very strong ground motion on the hanging walls of thrusts, and along with the precarious rock evidence for low ground motions on the footwall of thrusts, indicated a very strong asymmetry in ground motions that was not reflected in current empirical curves. These observations resulted in Brune (2001) with SCEC Contribution number 526. The results for toppling accelerations along the profile perpendicular to the San Andreas fault in the Mojave desert/san Gabriel Mountains region can eventually provide a quantitative upper bound on ground motions and source parameters for the many great earthquakes which have occurred along that section of the San Andreas fault in the last few thousand years. Therefore, we proposed to study source models which might be consistent with the data. We investigated very smooth rupture models and very irregular rupture models, as well as models corresponding to the response spectra for the USGS-CDMG 2% in 50 yr probabilities. The smooth models predicted 3

4 ground motions low enough to be consistent with the precarious rock data in all cases. Rough models, such as the composite model of Zeng et al. (1994) could be made consistent with the data if the dynamic stress drops were low enough, and attenuation high enough. However, seismograms corresponding to the USGS-CDMG 2% in 50 yr probabilities were inconsistent with the data (i.e., would have knocked the rocks down). In our last year s research for SCEC I we focused on further quantifying the constraints on ground motions and source parameters for great earthquakes on the San Andreas fault, and comparing the results with ground motion recordings from the two M7.6 and M7.2 Turkey earthquakes and the M7.6 Taiwan earthquake. The ground motions for these large earthquakes were considerably below the mean predictions from standard attenuation relationships, but consistent with the precarious rock data. We have presented these results in the AGU abstracts, but have not finished a final publication. We have a Ph.D. student using the data for part of his thesis. The 1999 Hector Mine earthquake fortuitously provided the first actual field calibration of the precarious rock methodology, since it toppled two rocks at Granite Pass, California, which had been documented earlier by Brune (1996). A nearby strong motion record from Amboy, CA, indicated a ground acceleration of about 0.2 g, consistent with the rocks being classified as precarious. This report covers all precarious rock studies funded over the last few years ( ) and, in particular, two tasks during the last year of SCEC I. Major Research Accomplishments The main scientific results of our SCEC I research on precarious rocks are described in more detail below: 1. An approximate field calibration of the precarious rock methodology was provided by the M7.1 Hector Mine earthquake of 16 October Previously documented rocks (Fig. 5) at Granite Pass near Kelso, California (Brune, 1996), were overturned by the earthquake, and a nearby strong motion station indicated ground motions of about 0.2 g., consistent with the toppling accelerations estimated in the published paper. The time of the last earthquake for the section of the fault nearest the precarious rocks has recently been determined to be more than 10 ka, consistent with previously estimated ages for precarious rocks. This first example of an actual earthquake field test lends strong support to the precarious rock methodology. More quantitative analyses of this calibration are being prepared in a paper in revision for publication in the Bulletin of the Seismological Society of America (Anooshehpoor and Brune, in revision, 2002) 2. Recently documented precarious rocks give constraints on ground motions for historic earthquakes such as the 1812 and 1857 San Andreas fault earthquakes, the 1899 and 1912 San Jacinto Fault earthquakes, the 1952 Kern County Earthquake, and recent, but prehistoric, earthquakes on the Banning and Garlock faults. The ground motion constraints 4

5 for these earthquakes are lower than predicted by some recent ground motion attenuation curves, but are generally consistent with ground motions observed from the recent large Turkey and Taiwan earthquakes, and provide important additional information for understanding source physics and for seismic hazard analysis. These results are presented in an article accepted for publication in the BSSA for summer of 2002 (Brune, 2002; SCEC contribution number 621). 3. In some areas where precarious rocks would be expected on the basis of previous studies, they are apparently not found. This suggests possible earthquakes on previously unrecognized, or only recently recognized, faults. One such area is in northwestern San Diego and southwestern Orange County between the Elsinore and Newport-Inglewood faults. The lack of precarious rocks in this area might be attributed to earthquakes on the recent blind-thrust faults proposed in the area by Grant et al. (2001) and Rivero et al. (2000). This result is being published in Brune (2002; SCEC contribution number 621). 4. Recent strong motion data from the Izmit, Turkey, and Chi Chi, Taiwan, earthquakes have pointed out the uncertainties in current strong motion attenuation curves for large earthquakes. Near-source strong motion data from those two earthquakes was considerably below the median for current attenuation curves, almost one standard deviation (Anderson et al., 2000; Ni et al., 2000, Boore, 2001). However the data from these two earthquakes is consistent with constraints estimated form precarious rocks (Brune, 1999; Anderson and Brune, 1999; Ni et al., 2000; Anderson et al., 2000). 5. Precarious rock evidence has suggested that trans-tensional sections of strike-slip faults produce relatively low ground accelerations. For example, the San Jacinto earthquakes of 1899 (M~7.1) and 1918 (M~6.9) occurred along branches of the San Jacinto fault where a right-step trans-tensional basin, the Hemet basin, occurs. An area of nicely exposed granitic boulders extends several kilometers northeast of the San Jacinto fault toward Beaumont. The preliminary estimates of ground acceleration are somewhat lower than the median estimates of ground motion for M6.9 and 7.1 earthquakes. A factor which may play a role along this section of the San Jacinto fault, and possibly not along the San Andreas fault, is the fact that this section of the fault is obviously trans-tensional, as evidenced by the existence of the Hemet right-step tans-tensional basin. Evidence from physical and numerical modeling (Brune and Anooshehpoor, 1998; Ely, 2001; Day and Ely, 2002) and seismic evidence (McGarr et al., 2000) Indicate that because the shallow fault cannot be locked by tectonic forces in trans-tensional areas, the energy released from the shallow part of the fault may be expected to be less, with correspondingly lower nearfault ground motions. The recent Izmit, Turkey, earthquakes may also have occurred on trans-tensional parts of the Anatolian Fault. This might explain the low ground motion values observed for this earthquake at distances comparable to those reported here (Anderson et al., 2000). Similar evidence for low ground motions from trans-tensional sections of strike slip faults is provided by precariously balanced rocks in Red Rock Canyon, along the western Garlock fault, where there are numerous precariously balanced rocks within a few km of the fault. These rocks may provide constraints on ground motion from nearby recent or historic earthquakes (for example, relatively recent 5

6 earthquakes documented by trenching on the fault further to the east, McGill and Sieh, 1991, 1993; Dawson, personal communication, 2001). These results are discussed in a preliminary fashion in Brune (2002), and in an article submitted for publication in the special issue of PEPI (Brune, 2003). 6. In study to assess the site conditions of the precariously balanced rocks in Mojave Desert, measurements of peak ground velocities at 0.5-1, 1-2, 2-4, 4-8, 8-16 Hz bandwidths for 56 earthquakes (M L = ) at two sites of precarious rocks were compared to velocities recorded for the same earthquakes by three TRINET stations located on engineering rock (NEHRP site class B). The residuals (the logarithm of the ratio of the amplitudes recorded at the precarious rocks to the TRINET amplitudes for the same earthquake and epicentral distance) at frequencies less than about 4 Hz were negative (i.e. deamplifications of %), whereas the residuals were slightly positive at the higher frequencies (i.e. amplifications of about 25-50%). High-frequency ground motions (e.g. peak ground accelerations) may therefore be slightly amplified at the precarious rock sites, which means that site conditions do not appear to explain the existence of the precarious rocks in areas where high peak ground accelerations are predicted in recent probabilistic seismic hazard (PSH) models (Stirling et al., 2002; SCEC contribution number 638). REFERENCES Anderson, J.G. (coordinator) (2000). Implications for seismic hazard analysis in Kocaeli, Turkey, Earthquake of August 17, 1999 Reconnaissance Report, TIL.Youd, J.P. Bardet and J.D. Bray, editors, Earthquake Spectra 16, Supplement A, Others coordinated in writing this article include: H. Sucuoglu, A. Erberik, T. Yilmaz, E. Inan, E. Durukal, M. Erdik, A. Anooshehpoor, J.N. Brune, and S. D. Ni. Anderson, J.G. and J.N. Brune (1999). Probabilistic seismic hazard analysis without the Ergodic assumption, Seism. Res. Lett. 70, Anderson, J.G. and J.N. Brune (1998). Methodology for using precarious rocks in Nevada to test seismic hazard models, Bull. Seism. Soc. Am. Anderson, J.G., J.N. Brune, A. Anooshehpoor, and S.-D. Ni (2000). New ground motion data and concepts in seismic hazard analysis, Current Science 79, Anooshehpoor, R., and Brune, J. N. (2000). Constraints on ground motions using precariously balanced rocks, proceedings of the 14th Engineering Mechanics Conference, May21-24, 2000, Austin, Texas. Anooshehpoor, A., Brune, J. N. (2002), Methodology for obtaining constraints on ground motion from field tests of precariously balanced rocks, submitted to Bull. Seism. Soc. Am. (in revision). Bell, John W., J.N. Brune, Tanzhuo Liu, Marek Zreda, and James C. Yount (1998). Dating precariously balanced rocks in seismically active parts of California and Nevada, Geology 26(6), Boore, D. (2001). Comparisons of Ground Motions from the 1999 Chi-Chi Earthquake with Empirical Predictions Largely Based on Data from California, Bull. Seism. Soc. Am. 91, Brune, J.N. (1996). Precariously balanced rocks and ground motion maps for southern California, Bull. Seism. Soc. Am. 86, Brune, J.N. (1999). Precarious rocks along the Mojave Section of the San Andreas Fault, California: 6

7 Constraints on ground motion from great earthquakes, Seism. Res. Lett. 70, Brune, J. N. (2001), Shattered rock and precarious rock evidence for strong asymmetry in ground motions during thrust faulting, in press, Bull. Seism. Soc. Am., 91, (SCEC contribution number 526). Brune, J. N. (2002), Precarious rock constraints on ground motion from historic and recent earthquakes in southern California, Bull. Seism. Soc. Am., in press (SCEC Contribution number 621). Brune, J. N. (2003). Precarious rock evidence for low near-source accelerations or trans-tensional strikeslip earthquakes, special issue of PEPI. Brune, J.N., Anooshehpoor, A., Stirling, M.W., Anderson, J.G. (1998), Precarious rocks site effects and seismic hazard in southern California: proceedings of the 12th Engineering Mechanics Conference, May18-20, 1998, San Diego, California. Brune, J.N. and A. Anooshehpoor (1998). A physical model of the effect of a shallow weak layer on strong ground motion for strike-slip ruptures, Bull. Seism. Soc. Am. 88, Day, S. M. and G. P. Ely (2002). Effect of a shallow weak zone on fault rupture: numerical simulation of scale-model experiments, Bull. Seism. Soc. Am, in press. Ely G. (2001). Simulating the effect of a shallow weak zone on near-source ground motion, A Thesis Presented to the Faculty of San Diego State University, Spring Grant, L.B., and L.J. Ballenger, and E.E. Runnerstrom (2001). Coastal uplift of the San Joaquin Hills, Southern Los Angeles Basin, California, by a large earthquake since 1635 A.D., Bull. Seis. Soc. Am., in press September 10, McGarr, A., J.B. Fletcher, R.A. Harris (2000). Some observations regarding the seismic hazard associated with shallow rupture, Seism. Res. Lett., 71, 265. McGill, S., and K. Sieh (1993). Holocene slip rate of the Central Garlock Fault in southeastern Searles Valley, California, J. Geophys. Res., B, Solid Earth and Planets, 98, McGill, S.F., and K.E. Sieh (1991). Surficial offsets on the central and eastern Garlock fault associated with prehistoric earthquakes, J. Geophys. Res. 96, Ni, S., J.N. Brune, and J.G. Anderson (2000). Comparison of ground motion from the Chi-Chi, Taiwan, Earthquake with precarious rock estimates of bounds on ground motion for the 1952 M=7.6 Kern County, California, earthquake, Seism. Res. Lett., 71, 225. Rivero, C., J.H. Shaw, and K. Mueller (2000). Oceanside and Thirty-mile Bank blind thrusts: Implications for earthquake hazards in coastal Southern California, Geology, 28 (10), Stirling, M. W., A. Anooshehpoor, J. N. Brune, G. Biasi and S. G. wesnousky (2002). Assessment of the site conditions of precariously balanced rocks in the Mojave Desert, southern California, Bull. Seism. Soc. Am. 92, in press (SCEC Contribution number 638). Zeng, Y., J. G. Anderson, and G. Yu (1994). A composite source model for computing realistic synthetic strong motion simulation, Geophys. Res. Lett., 21,

8 Table 1:Quasi-static toppling acceleration of selected rocks measured in the field. Rock I.D. Location F (N) Azimuth Mass (kg) (g) Victorville VOG , VOG , VOG , VOG , VOG , VOG , VOG , Lovejoy Buttes LB , LB2a , LB2b , LB5a , LB5b , LB , LB15a , LB15b , Pacifico Mountains SG , SG , SG , SG , SG , A qs 8

9 Figure 1: Examples of precariously balanced rocks found at Lovejoy Buttes, about 15 kilometers from the Mojave section of San Andreas Fault. 9

10 Figure 2: Locations and the quasi-static toppling acceleration of selected precarious rocks at Lovejoy Buttes, Victorville and San Gabriel Mountains are shown here. The vectors show the direction and magnitude of the measured toppling accelerations (Table 1). 10

11 Figure 3: Estimates of PGA provided by this study are compared with different attenuation curves. Estimates of PGA with distance from fault for the 17 August 1999 Kocaeli, Turkey earthquake are shown with solid triangles. The open circles above each triangle represent approximate values corrected for the difference in magnitude. 11

12 Figure 4: An example of thrust fault hanging-wall shattered rock within a few to several kilometers of the fault outcrop. Figure 5: The precarious rock at Granite Pass, California (Fig. 2e in Brune, 1996), that was apparently toppled by the Hector Mine earthquake. 12