Reflectivity Structure Below San Fernando Valley, California, from Northridge Aftershock Recordings

John N. Louie and Sergio Chavez-Perez
Seismological Laboratory, Mackay School of Mines, University of Nevada, Reno, NV 89557; URL: http://www.seismo.unr.edu

Presented as a poster to the American Geophysical Union Fall Meeting, San Francisco, Calif., December 9, 1994.

Introduction

Acoustic imaging using earthquake sources allows us to produce reflection views of the crust where other data are not available (see, e.g., James et. al., 1987; Spudich and Bostwick, 1987; Morales and McMechan, 1990; Rietbrock and Scherbaum, 1994).

Aftershocks of the 17 Jan 1994 Northridge earthquake have the spatial sampling needed to image local structures at a resolution comparable to the average distance between the events.

Preliminary results show the potential of aftershock stacked sections in defining major reflectors in two and three dimensions below San Fernando Valley.

Map showing the Northridge main shock and aftershocks having M>3.0, and the ten lines of overlapping common midpoints we stacked into.
(PDF file available.)

Method

We randomly selected 50 aftershocks of magnitude 3 and greater, and stacked the records for each event into South-North and West-East lines of common midpoints (CMPs).

Preprocessing of the data prior to stacking consisted of bandpass filtering (10-40 Hz) and trace equalization.

Stacks assume a constant crustal velocity of 6 km/s using the first 15 s of all records within 200 km distance.

Midpoints are projected up to 110 km perpendicular to each line, and stacked into one trace.

Datum is at sea level.

The following figures show two S-N and one W-E stack. While one could not identify likely structures by looking at just one stack, the Moho and crustal structures near the base of the seismogenic zone persist from stack to stack.

South-North Aftershock Stack Nos. 1 and 2

South-North Aftershock Stack Nos. 3 and 4

South-North Aftershock Stack No. 5

East-West Aftershock Stack Nos. 6 and 7

East-West Aftershock Stack Nos. 8 and 9

East-West Aftershock Stack No. 10

Interpretation

Processing of data from a few events identifies reflective structures by hinting at prominent shallowly-dipping features.

A reflective portion of the crust, between 5-7 s, hints at the location of what we call the 18 km Reflector. Also, strong reflections at about 10 s hint at the location of Moho near 30 km.

Aftershock stacked sections enable us to test the existence of major structures defined by balanced cross-sections. For example, compare the locations of the 18 km Reflector and of the Elysian Park Thrust on the recent cross-section of Davis and Namson (1994), above.

N-S Stack No. 3 and Projected Seismicity

Davis and Namson's "Pico Thrust" follows the south-dipping trend of aftershocks toward the main shock in this section above. Below, the Elysian Park Thrust may control many subsidiary blind thrusts. Our section shows persistent crustal reflectivity well below the the depth of the main shock, suggesting that the Elysian Park Thrust may be deeper than projected by Davis and Namson.
(PDF file available.)

Conclusions

1) A small cluster of tens of aftershocks has the spatial sampling needed to image crustal reflectors below the Northridge aftershock zone.
2) CMP aftershock stacks depict major structures and show the potential of this approach to define the existence and location of possible blind thrust faults.
3) CMP aftershock stacks will yield structural information to test balanced cross-sections and complement tomographic results.
4) Further work will focus on statistical validity of the structures, signal conditioning, velocity analysis, and pre-stack migration.

5) We will also obtain portable-station data from San Fernando Valley, at least doubling our data volume and including near-vertical reflections.

References

Davis, T.L. and J.S. Namson (1994). A balanced cross-section of the 1994 Northridge earthquake, southern California, Nature 372, 167-169.

James, D.E., T.J. Clarke, and R.P. Meyer (1987). A study of seismic reflection imaging using microearthquake sources, Tectonophysics 140, 65-79.

Morales, J. and G.A. McMechan (1990). Imaging of earthquake sources, Internat. J. Imaging Systems Tech. 2, 231-238.

Rietbrock, A. and F. Scherbaum (1994). Acoustic imaging of earthquake sources from the Chalfant Valley, 1986, aftershock series, Geophys. J. Internat. 119, 260-268.

Spudich, P. and T. Bostwick (1987). Studies of the seismic coda using an earthquake cluster as a deeply buried seismograph array, J. Geophys. Res. 92, 10526-10546.