Seismic Reflection Views of the 1994 Northridge Earthquake Hypocentral Region Using Aftershock Data

Sergio Chávez-Pérez and John N. Louie
Seismological Laboratory/174, Mackay School of Mines, University of Nevada, Reno, NV 89557-0141; USA

sergio@seismo.unr.edu http://www.seismo.unr.edu/

Poster Presented at the Fall Annual Meeting of the American Geophysical Union, San Francisco, CA, December 11-15, 1995.

Abstract

Crustal-scale imaging can be done using data from a regional network of short-period vertical seismometers given an appropriate combination of advanced summation and imaging techniques and simple data editing. Hence, we can use exploration seismology tools like stacking and migration to provide the critical link between shallow and deep structures. We apply these ideas to aftershocks of the 17 January 1994 Northridge, California, earthquake to image the structure of the proposed blind thrust fault system in the Northridge earthquake region. Our previous results, equivalent to two-dimensional common-midpoint (CMP) stacked sections, have depicted major reflective structures and hinted at prominent structure beneath and north of the Northridge epicentral area. Further processing of highest quality aftershocks lying within 3 km depth identifies prominent shallowly-dipping features between 5-10 s (two-way time). Data preprocessing and signal conditioning includes muting, trace balancing and gain control, bandpass filtering and stacking methods other than simple summation (e.g., an N-th root process) to eliminate phase incoherence and remove effects of different focal mechanisms. We limit the data set to 200 km distances and 30 s duration to include wide-angle reflections well-separated from Pg and Sg but between the two arrivals. Muting outside the window between Pg and Sg arrivals permits us roughly to include only compressional direct and reflected arrivals (mostly Pg, PmP and S to P converted energy). In addition to the CMP stacked sections, we will show results of three-dimensional prestack Kirchhoff depth migration trials with existing P-wave velocity models, as well as statistical analyses to help validate imaged reflectors and depict possible fault locations and other major structures.

INTRODUCTION

Assessing seismic potential in the San Fernando Valley requires knowledge of the existence and geometry of hidden faults. Towards that end, we use seismic imaging to identify potential faults.

Crustal-scale imaging can be done using data from a regional network of short-period vertical seismometers given an appropriate combination of simple data editing and Kirchhoff summation.

We can use exploration seismology tools like stacking and migration to provide the critical link between shallow (less than 5 km) and deep structures. Thus, acoustic imaging using earthquake sources allows us to produce reflection views of the crust where other data are not available.

We apply these ideas to aftershocks of the 17 Jan 1994 Northridge, California earthquake to image the structure of the proposed blind thrust fault system in the Northridge earthquake region.

The close spatial distribution of Northridge aftershocks illuminates structures not previously mapped at depth (Hauksson et al., 1995).

We use data of the Southern California Seismic Network (SCSN) provided through the SCEC Data Center.

DATA SELECTION AND PREPROCESSING

We use A-quality aftershocks (magnitude 3 and larger) lying within 3 km depth.

We regard clipped and saturated records as sign-bit recordings (O'Brien et al., 1982). Thus, with sufficient data redundancy, summation techniques like stacking and migration are viable tools for recovering geometric information.

We limit our record sections to 200 km distances and 30 s duration to include wide-angle reflections well-separated from Pg and Sg but between the two arrivals.

Muting outside the window between Pg and Sg traveltime branches permits us roughly to include only compressional arrivals, mostly Pg, PmP and S to P converted energy.

Preprocessing includes bandpass filtering (1-40 Hz) and trace equalization for inter-source amplitude balancing, equivalent to energy normalization for varying magnitudes.

Page 5 - Map

Example of a Record Section. Raw Data - CUSPID 3140939, Mag = 3.6, Depth ~ 2 km, Origin Time -> 94/01/17 20:17:38.5. Note the Pg and Sg traveltime branches.

DEPTH MIGRATION

Davis and Namson (1994) interpreted the Santa Susana Mountains and Santa Monica mountains anticlinoria as crustal-scale fault-propagation folds above the Pico and Elysian Park thrusts, respectively. If this is the case, we would expect to see reflections coming from the main thrust faults and the mid-crustal detachment where the Elysian Park thrust supposedly roots (at 20-22 km).

Picking fault locations and interpreting the core of a fault-propagation fold are problematic because of large lateral velocity changes (Morse et al., 1991). Most of the reflected events arise from the basal detachment, and depth migration is required to obtain a reasonable subsurface image.

Our depth sections depict reflectivity along a 50 km south-north line. We use three-dimensional Kirchhoff prestack migration with Hadley and Kanamori's (1977) flat-layer P wave velocity model:

Trial images include both incoherent data (without correction for varying focal mechanisms; 33 events) and coherent data (using high-quality impulsive picks to correct for sign reversals; 27 events).

SUMMARY

Northridge aftershock migrations image fault structures beneath San Fernando Valley, some near the positions proposed on balanced cross sections.
Initial crustal reflectivity sections provide estimates of image resolution and migration artifacts, and ray coverage for the region of interest.
Further work will focus on:
- improving structural definition and detail;
- determining, through statistical tests, what migrated structures are real and which are artifacts due to noisy data and poor reflection coverage;
- including more data and amplitude corrections for varying radiation patterns (for those events with known focal mechanisms);
- using existing velocity models and relocations (Pujol, 1994; Mori et al., 1995; Zhao and Kanamori, 1995).

REFERENCES

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

Hadley, D. and H. Kanamori (1977). Seismic structure of the Transverse Ranges, California, Geol. Soc. Am. Bull. 88, 1469-1478.

Hauksson, E., L.M. Jones, and K. Hutton (1995). The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects, J. Geophys. Res. 100, 12,335-12,355.

Mori, J., D.J. Wald, and R.L. Wesson (1995). Overlapping fault planes of the 1971 San Fernando and 1994 Northridge, California earthquakes, Geophys. Res. Lett. 22, 1033-1036.

Morse, P.F., G.W. Purnell, and D.A. Medwedeff (1991). Seismic modeling of fault-related folds, in: Fagin, S.W. (ed), Seismic Modeling of Geologic Structures, Applications to Exploration Problems, Society of Exploration Geophysicists, Tulsa, OK.

O'Brien, J.T., W.P. Kamp, and G.M. Hoover (1982). Sign-bit amplitude recovery with applications to seismic data, Geophysics 47, 1527-1539.

Pujol, J. (1994). An integrated 3D-velocity inversion -- JHD relocation analysis of events in the Northridge area, Bull. Seism. Soc. Am., submitted for publication.

Zhao, D. and H. Kanamori (1995). The 1994 Northridge earthquake: 3-D crustal structure in the rupture zone and its relation to the aftershock locations and mechanisms, Geophys. Res. Lett. 22, 763-766.