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: 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: 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.

Description: Recent thrust faulting models, both physical and numerical, have indicated very high ground motions on the hanging wall and relatively low motions on the footwall of thrust faults. In this article, recent field evidence supporting this strong asymmetry is described. Earlier field observations of objects thrown in the air also suggest that ground motions must have exceeded 1g and 100 cm/sec. New shattered rock evidence on the hanging wall of thrust faults in Southern California supports the occurrence of intense ground motions. On the other hand, the lack of shattered rock and the presence of precariously balanced rocks on the footwall of two thrust faults in Southern California indicate relatively low ground motions, thus indicating strong asymmetry in ground motions near the fault trace for thrust faults. Some current ground-motion attenuation curves and hazard maps may give too low values on the hanging wall and too high values on the footwall. This could be of critical importance to estimating earthquake hazard from potentially large thrust fault earthquakes in the Los Angeles Basin and elsewhere in the world. The new evidence also supports the idea that dynamic inertial detachment of the hanging wall from the footwall occurs in nature in the manner suggested by some recent physical and numerical models. If such detachment occurs, it could also have important implications for understanding thrust fault rupture mechanics, for example, the paradox of large overthrusts and the heat flow paradox for thrusts.