Migration of June 28, 1991 M5.8 Sierra Madre event data

Seismological Lab (174), University of Nevada, Reno
This page is accessible electronically at the location: http://www.seismo.unr.edu/ftp/pub/louie/nr/smmig.html

We have been imaging crustal reflective structures in the area below the Jan. 17, 1994 Northridge earthquake, using data from Northridge aftershocks recorded on the Southern California Seismic Network (click here for a summary and list of presentations). However, no other geophysical data corroborate the location of reflective structures below 4 km depth in San Fernando Valley, leaving the accuracy of our imaging methods in question. Therefore, we undertook the imaging of a section below the San Gabriel Mountains near LARSE line 1 from seismic network recordings of the June 28, 1991 M5.8 Sierra Madre event and its aftershocks in an attempt to reproduce the bright spot seen by Fuis et al. (1996).
(Click on the image above for a viewable Adobe Acrobat PDF file.)

The two sections above show the same 50 km long by 40 km deep image of crustal structure along the section on the map above, plotted with different color scales for the amplitude of reflectivity. Negative reflectivities are white at left or yellow at right, and positive reflectivities are black at left and brown at right. The image shows the pre-stack Kirchhoff-sum 3-d time migration of records from 18 events, including the Sierra Madre mainshock (click here for a list of events). The data set included as few as 18 and as many as 102 seismograms from each event, all having impulsive P-wave picks. Data processing and imaging methods are identical to those used for the Northridge section.

Among the usual near-vertical Kirchhoff migration artifacts, the images clearly show a strong north-dipping reflective zone at 15-20 km depth below the northern side of the San Gabriel Mountains, extending north below the San Andreas fault into the Mojave Desert.

This image overlays our migration on the LARSE line 1 stack of Fuis et al. (1996) at the same scale, assuming the stack is plotted with approximately no vertical exaggeration, and at corresponding locations. The locations on the two sections of the bright spot reflector correspond well, and there are hints that both sections also are imaging a diffractor at about 10 km depth below the southern side of the San Gabriel Mountains.
(Click on the image above for a viewable Adobe Acrobat PDF file.)

It is remarkable that the narrow-angle LARSE line 1 explosion data and the very wide-angle aftershock network records can show exactly the same reflective structure. Further, this fact suggests not only the veracity of the mid-crustal reflective zone, but also the accuracy of both data sets and imaging methods.

The animation below, given in three formats, shows the successive migration imaging of the bright spot as data from each event is added. The first frame of the animation is the migration of just the mainshock data, and the final frame is the sum of all 18 events as shown above.

Animated GIF, QuickTime, or MPEG.