Featured Seismograms

Mapping Fault Zones

Yong-Gang Li, University of Southern California

Two days after the Hector Mine earthquake, we deployed a 60-channel Geometrics Strata View 24-bit exploration Seismograph with 20 L-22 2 Hz three-component velocity sensors on a 100-m-long line across the surface rupture. Outboard of the dense line, we placed two 6-channel REFTEK recorders, each with 2 three-component sensors deployed at 75 m and 175 m away from the mainfault trace on each side of the rupture zone.

We recorded more than 800 events in 10 days. Most of the events were aftershocks with P-to-S time less than 3s. Preliminary examination of the data revealed 4-6 Hz fault-zone trapped waves with long duration wavetrains after the S arrival at stations located close to the mainfault trace, but a much more brief S wave at stations farther away from the fault zone. The trapped waves are similar to those observed at the Landers rupture zone but show higher frequencies, implying a narrower low-velocity waveguide for the Hector Mine rupture zone.Shown above is an example of vertical component displacement seismograms recorded for a M2.1 aftershock occurring on Oct. 24 19:53 (GMT) at the depth of 5.8 km within the rupture zone about 12 km NNW of the seismic array. Fault-zone trapped waves with relatively large amplitudes and long duration (marked by brackets) are prominent at stations located in the rupture zone. Station ST06 was located at the mainfault trace on the east edge of the rupture zone while stations ST10, ST14 and ST16 were located at the sub-fault traces in the rupture zone. Two arrows denote the width (~60-80 m) of the rupture zone at the surface, where guided waves were recorded clearly. Coda-normalized amplitude spectra of trapped waves for this event show a maximum peak at 4-5 Hz, which decreases with distance from the fault zone. Preliminary results from observations and modeling of these trapped waves suggest that the rupture zone on the Lavic Lake fault is around 60-100m wide. The average shear velocity is ~2 km/s, reduced by about 40 to 50 percent from the surrounding rocks. Q ranges between 20 and 50 in the upper 10km.

Fault-zone trapped waves arise from coherent multiple reflections at the boundaries between the low-velocity fault zone and high-velocity surrounding rock. Accordingly, the waves can be used as a high-precision probe to study internal structure and physical properties of the fault zone.

Additional Information

For additional information, see Li, Y. G., J. E. Vidale, K. Aki, and F. Xu, Depth-dependent structure of the Landers fault zone from trapped waves generated by aftershocks, J. Geophys. Res., 105, 6237-6254, 2000.