Shallow structure of the Landers fault zone from explosion-generated trapped waves

Yong-Gang Li, Keiiti Aki, John E. Vidale, & Fei Xu

Published 1999, SCEC Contribution #471

With fault-zone trapped waves generated by near-surface explosions within the fault zone of the 1992 Landers, California, earthquake and recorded by linear seismic arrays deployed along and across the fault, we have delineated the shallow seismic structure near the 30-km-long southern rupture on the Johnson Valley fault. The explosion-excited trapped waves with relatively large amplitude and long-duration wave train after the S waves are similar to those generated by aftershocks [Li et al, 1994a, b] but have lower frequencies and travel more slowly. Coda-normalized amplitude spectra of explosion-excited trapped waves show a maximum at ∼2 Hz, which decreases rapidly with the station offset from the fault trace on the cross-fault profiles. Normalized amplitudes of trapped waves on the along-fault profile also decreased with distance between the explosion and station, giving an apparent Q of ∼18 at 1–2 Hz in the fault zone for the shot near the profile. The dispersion of trapped waves from 0.6 to 2.5 Hz recorded on this along-fault profile implies a shear velocity of ∼1.0 km/s for the fault zone and ∼1.8 km/s for the wall rock, while the data from the farther shot show an increase in velocity and Q with depth. Measured group velocities and Q values were used as constraints in the numerical modeling of trapped waves on cross-fault and along-fault profiles. Results reveal that the shallow Johnson Valley fault is marked by a zone 250 m wide where the shear velocity is 1 km/s and Q is 20. Calculation of finite difference synthetics for a depth-varying fault structure show that these model parameters apply to the depth of ∼1 km, below which the fault zone shear velocity increases to 1.9 km/s and Q increases to 30.

Citation
Li, Y., Aki, K., Vidale, J. E., & Xu, F. (1999). Shallow structure of the Landers fault zone from explosion-generated trapped waves. Journal of Geophysical Research, 104(B9), 20257-20275.