ALBERT

Description: In a recent study of microearthquakes along the Parkfield segment of the San Andreas fault, Nadeau et al. (1995) have found that much of the seismicity in the region is characterized by quasi-periodic repeating sequences of small earthquakes that are essentially identical in waveform, size and, location. Nadeau and Johnson (1998) interpreted these as repeated slip on a given asperity driven by a steady slip rate of 2.3 cm/yr and concluded that the stress drops needed to be extremely high, of the order of 20 kilobars. We propose another explanation for these small repeating events, namely that an inner asperity is surrounded by a larger creeping zone, which in turn is surrounded by a still larger locked zone. This geometry produces a local slip velocity much less than the overall creep velocity observed on a still larger scale (slip velocity shielding). We have constructed a foam rubber model to illustrate the phenomenon. The time sequences of small events at the asperity, punctuated by large events which rupture the whole block, look very similar to the cumulative moment plots of Nadeau and Johnson. The actual dynamic stress drops are of the same order as for the large events. Thus the results of the model correspond to the observations of Nadeau and Johnson and suggest that the model may be appropriate to explain their observations, without requiring super strong asperities.

Description: The overturning fragility of a freestanding block such as a precariously balanced rock (PBR) has been parameterized as a function of a vector of ground-motion intensity measures. Methodologies are outlined to estimate the failure probabilities of such objects given their residence times. For deterministic seismic hazard analyses (DSHAs), a PBR is exposed to the scenario earthquakes that occur during its exposure time providing an estimate of the probability that the PBR survives the ensemble of events. For probabilistic seismic hazard analyses (PSHAs), the PBR overturning fragility is multiplied by the ground-motion occurrence rate from a vector-valued probabilistic seismic hazard analysis (VPSHA), yielding the marginal overturning rate for each ground-motion bin. Summing the marginal rates over all ground-motion bins produces the total overturning rate. For time-independent Poisson-based PSHA estimates, the probability of block failure can be easily calculated as a function of exposure time. This latter method is used to test VPSHA estimates similar to the 2002 U.S. Geological Survey (USGS) National Seismic Hazard Maps via PBR residence times. PBR overturning fragilities are estimated at sites in southern California near the San Andreas fault, between the San Jacinto and Elsinore faults, and near the White Wolf fault. The resulting failure probabilities for several of the PBRs are very high, suggesting that they are inconsistent with the 2002 USGS ground motions. An investigation of the hazard calculated with zero aleatory variability in the ground-motion prediction equations (GMPEs) suggests that the median ground motions or the earthquake rupture rates are too high at certain PBR sites.

Description: New examples of precarious rocks are presented. These rocks provide constraints on ground motion for historical and recent large earthquakes. An approximate field calibration of the precarious-rock methodology was provided by the M7.1 Hector Mine earthquake of 16 October 1999. Previously documented rocks 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 article. 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. The other rocks discussed here 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 possibly recent but prehistoric earthquakes on the Banning and Garlock faults. The ground-motion constraints for these earthquakes are lower than predicted by some recent ground-motion attenuation curves (which are extrapolations to near-fault distances from a data set dominated by data from larger distances) but are generally consistent with peak ground accelerations observed from the recent large Turkey and Taiwan earthquakes and provide important additional information for seismic-hazard analysis. On the other hand, there are areas where precarious rocks would be expected on the basis of previous studies but 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 recent earthquakes on the blind-thrust faults proposed in the area by Grant et al. (1999, 2002) and Rivero et al. (2000).

Description: Precariously balanced rocks and overturned transformers in the vicinity of the White Wolf fault provide constraints on ground motion during the 1952 M (sub s) 7.7 Kern County earthquake, a possible analog for an anticipated large earthquake in the Los Angeles basin (Shaw et al., 2002; Dolan et al., 2003). On the northeast part of the fault preliminary estimates of ground motion on the footwall give peak accelerations considerably lower than predicted by standard regression curves. On the other hand, on the hanging-wall, there is evidence of intense ground shattering and lack of precarious rocks, consistent with the intense hanging-wall accelerations suggested by foam-rubber modeling, numerical modeling, and observations from previous thrust fault earthquakes. There is clear evidence of the effects of rupture directivity in ground motions on the hanging-wall side of the fault (from both precarious rocks and numerical simulations). On the southwest part of the fault, which is covered by sediments, the thrust fault did not reach the surface ("blind" thrust). Overturned and damaged transformers indicate significant transfer of energy from the hanging wall to the footwall, an effect that may not be as effective when the rupture reaches the surface (is not "blind"). Transformers near the up-dip projection of the fault tip have been damaged or over-turned on both the hanging-wall and footwall sides of the fault. The transfer of energy is confirmed in a numerical lattice model and could play an important role in a similar situation in Los Angeles. We suggest that the results of this study can provide important information for estimating the effects of a large thrust fault rupture in the Los Angeles basin, specially given the fact that there is so little instrumental data from large thrust fault earthquakes.

Description: Foam rubber experiments simulating unilaterally propagating strike-slip earthquakes provide a means to explore the sensitivity of near-fault ground motions to rupture geometry. Subsurface accelerometers on the model fault plane show rupture propagation that approaches a limiting velocity close to the Rayleigh velocity. The slip-velocity waveform at depth is cracklike (slip duration of the order of narrower fault dimension W divided by S-wave speed beta ). Surface accelerometers record near-fault ground motion enhanced along strike by rupture-induced directivity. Most experimental features (initiation time, shape, duration and absolute amplitude of acceleration pulses) are successfully reproduced by a 3D spontaneous-rupture numerical model of the experiments. Numerical- and experimental-model acceleration pulses show similar decay with distance away from the fault, and fault-normal components in both models show similar, large amplitude growth with distance along fault strike. This forward directivity effect is also evident in response spectra: the fault-normal spectral response peak (at period approximately W/3beta ) increases approximately sixfold along strike, on average, in the experiments, with similar increase (about fivefold) in the corresponding numerical simulation. The experimental- and numerical-model response spectra agree with an empirical directivity model for natural earthquakes at long periods (near approximately W/beta ), and both overpredict shorter-period empirical directivity effects, with the amount of overprediction increasing systematically with diminishing period. We attribute this difference to rupture- and wavefront incoherence in natural earthquakes, due to fault-zone heterogeneities in stress, frictional resistance, and elastic properties present in the Earth but absent or minimal in the experimental and numerical models. Rupture-front incoherence is an important component of source models for ground-motion prediction, but finding an effective kinematic parameterization may be challenging.

Description: Preliminary interpretations of precarious rock observations suggest relatively low near-fault (〈3 km) footwall ground motions for normal faults relative to strike-slip faults, as reported for a dynamic foam rubber normal fault model by Brune and Anooshehpoor (1999). Since for large normal fault earthquakes there are no instrumental data at such close distances from the fault trace, precarious rocks may provide important data for estimating the ground motion for these earthquakes. Use of empirical curves in the literature, based mainly on strike-slip data, may lead to overestimates of seismic hazard for the near-fault footwall of normal faults. These preliminary results may warrant more comprehensive and more quantitative follow-up studies.