Acoustic imaging using exploration seismology tools and earthquake sources allows us to produce reflection views of the crust to depths where other data are commonly not available. For instance, Revenaugh (1995a, b) has recently proven that analyses of scattered waves using stacking and Kirchhoff migration are rather useful for the evaluation of generalized site effects and earthquake hazard.
The close spatial distribution of Northridge aftershocks illuminates structures not previously mapped at depth (Hauksson et al., 1995). In this case a reliable velocity structure over the three-dimensional (3-d) array of sources (aftershocks) and receivers (network stations) provides the information needed to locate crustal reflectors in cross sections or 3-d volumes.
The main purpose of this work is to image the structure beneath San Fernando Valley using Northridge aftershocks and a combination of simple data editing and imaging techniques commonly used for oil exploration.
Fig. 1 provides an example of the short-period data we use for our imaging purposes. As in oil industry seismic-reflection surveying, imaging of structure through high-frequency reflectivity demands high-multiplicity data, or a large number of overlapping sources and receivers. Despite the intrinsic limitations of close source spacing (because of multiple events) and wide station spacing, our initial work (Chávez-Pérez and Louie, 1995) showed that a small cluster of tens of aftershocks has the spatial sampling needed to image crustal reflectors in this area.
Clipped and saturated records are quite common in short-period data. We regard them as sign-bit recordings (O'Brien et al., 1982). Thus, with sufficient data redundancy, stacking and migration allow us to recover geometric information. Record sections include 200 km in epicentral distance and 30 s duration to include wide-angle reflections between first compressional, Pg, and first shear, Sg, arrivals. We mute outside the window between Pg and Sg traveltime branches to extract only compressional arrivals, mostly Pg, PmP and S-P converted energy. Preprocessing includes bandpass filtering and trace equalization for intersource amplitude balancing (i.e., the amplitudes are normalized so that the mean-squared amplitude over the whole trace is the same for all traces). This is roughly equivalent to energy normalization for varying magnitudes.
Kirchhoff depth migration is an imaging technique that produces an estimate of subsurface reflectors. One fundamental assumption used in this process is that the primary reflected energy, when treated as a P-P scattering problem, is isotropic (Wu and Aki, 1985). Thus, we obtain images by summing the data at traveltimes computed through a background velocity model.
The Kirchhoff depth migration process we use is similar to that utilized by Louie et al. (1988). The depth migration is a mapping or back projection of assumed primary reflection amplitudes into a depth section. It has been identified by Le Bras and Clayton (1988) as the tomographic inverse of the acoustic wave equation under the Born approximation in the far field, utilizing WKBJ rays for downward continuation and two-way reflection travel time for the imaging condition. We compute traveltimes with Vidale's (1988) finite-difference solution to the eikonal equation.
Depth sections depict reflectivity along a 50 km south-north migration profile (Fig. 2) by using 3-d Kirchhoff depth migration with Hadley and Kanamori's (1977) one-dimensional (1-d) P wave velocity model shown in Table 1.
|Velocity, km/s||Depth, km|
DISCUSSION AND SYNOPSIS
Figure 4a shows a crustal reflectivity depth section of data from 27 A-quality aftershocks (823 seismograms) lying within 3 km depth. Datum is at sea level. Black depicts positive reflectivity, white depicts negative reflectivity and gray depicts no reflectivity.
Artifacts due to poor reflection coverage show kinks in the nearly vertical trajectories they define. These are due to propagation through the 1-d velocity model and we can see them at the deepest interface, the Moho, at about 32 km depth. Strong dipping reflectors appear that do not follow the trajectories defined by the artifacts. They seem to correlate closely with the position of the Pico and Elysian Park thrusts, but extend below the projected depth of the proposed mid-crustal detachment. Note that there is a dipping reflector almost parallel to the Elysian Park thrust. We can also distinguish some nearly horizontal reflectors at depths between 20-30 km.
Figure 4b shows the depth section of Fig. 4a with Northridge main shock, A-quality aftershocks (magnitude 3 and greater) at all depths, and the thrust faults and mid-crustal detachment, proposed by Davis and Namson (1994), superimposed. Note how the Pico and Elysian Park thrusts correlate with some of the main dipping reflectors. This is how we can devise tests for current geological models of the area.
Existing aftershock data for the Northridge hypocentral area offers a cost effective alternative for an integration of seismic reflection data in this and similar areas. Our ongoing work focuses on improving structural definition and detail.
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, 12335-12355.
Le Bras, R.J. and R.W. Clayton (1988). An iterative inversion of backscattered acoustic waves. Geophysics, 53, 501-508.
Louie, J.N., R.W. Clayton, and R.J. Le Bras (1988). 3-d imaging of steeply dipping structure near the San Andreas fault, Parkfield, California. Geophysics, 53, 176-185.
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.
Revenaugh, J. (1995a). A scattered-wave image of subduction beneath the Transverse Ranges, California. Science, 268, 1888-1892.
Revenaugh, J. (1995b). Relation of the 1992 Landers, California, earthquake sequence to seismic scattering. Science, 270, 1344-1347.
Vidale, J.E. (1988). Finite-difference calculation of travel times. Bull. Seism. Soc. Am., 78, 2062-2076.
Wu, R. and K. Aki (1985). Scattering characteristics of elastic waves by an elastic heterogeneity. Geophysics, 50, 582-595.