Additional Interpretation of the Orientations of Precariously Balanced Rocks in the Band Between the San Jacinto and Elsinore Faults

James N. Brune, John G. Anderson, & Richard J. Brune

Published August 9, 2017, SCEC Contribution #7455, 2017 SCEC Annual Meeting Poster #259

In a previous study Brune et al.(2006), found most of the precariously balanced (PBRs) along the line of PBRs between the San Jacinto and Elsinore faults were most sensitive to strong motion perpendicular to the faults, whereas the directivity pulse from a smooth dynamic rupture would have produced strongest ground motions in this direction. We have since added a number of rock orientations to the archive, and they support this result. Brune et al.(2006) concluded that “fault parallel polarized ground motions may have contributed to the observed distribution of toppling azimuths, e.g., the strongest ground motions may have been in the fault parallel direction for at least one earthquake over the life of the rocks (Purvance et al., 2004). This distribution could have resulted from supershear ruptures, unusually strong mode 3 ruptures, or an unexpected dominance of the seismicity by numerous short mode 2 ruptures”. Here we add a related explanation: - Very large, long, ruptures may occur, but with very incoherent patterns of slip in the fault plane, generating ground motions similar to a rapid sequence of M=6-7 earthquakes spaced erratically, and thus not generating a coherent fault-normal directivity pulse. Anderson et al., (2017) showed that the Composite Fault Model (CFM) of Zeng, Anderson and Yu (1994) with a large maximum sub-event stress drop produced total slip values along the fault which showed patches of very high total slip interspersed with patches of relatively low slip. As a consequence the predominant PGA ground motion near the fault, and out to the distances of the PBRs from the San Jacinto and Elsinore faults (about 17 km), was fault- parallel (Brune et al., 2016 SCEC abstract). Thus the possible existence of such complex large events with high sub-event stress drops might be another explanation of the predominant fault normal sensitivity of the PBRs along the Elsinore- San Jacinto line of PBRs.

Key Words
Seismic hazard, ground motion, Directionality

Citation
Brune, J. N., Anderson, J. G., & Brune, R. J. (2017, 08). Additional Interpretation of the Orientations of Precariously Balanced Rocks in the Band Between the San Jacinto and Elsinore Faults. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions