27 Feb 2001

>Topic: Re: very preliminary Gujarat fault plane from aftershocks
>Author: Arch Johnston johnston@ceri.memphis.edu
>
>The Mid-America Earthquake Center reconnaissance team* for the Republic Day
>earthquake deployed a portable array of K-2 seismometers for aftershock
>monitoring. Very preliminary locations of 40 aftershocks from the first two
>days of monitoring suggest a number of important characteristics of the
>fault rupture. DISCLAIMER: the sample size of the full data set (~1000
>events) is very small and the crustal velocity model used was untested and
>certainly can be significantly improved. Nevertheless, even with some gross
>and systematic errors (+/- 10-15 km in depth) possible, the general
>conclusions below would still stand. Since some researchers are still in
>the field in Gujarat and others are planning to go soon, I believe it's
>important to get these conclusions out to aid field reconnaissance for
>possible surface rupture. After examining the data I have about 80%
>confidence that
>
>1) The fault rupture was unexpectedly deep. Of the 40 located aftershocks,
>16 had hypocentral depths of greater than 30 km; another 17 had depths
>between 20 and 30 km. The shallowest event was 12.5 km. Even if these
>depths are systematically ~10 km too deep, the aftershocks still indicate
>that significant, perhaps predominant, fault rupture occurred downdip of
>the rupture nucleation depth. This could help explain the lack of obvious
>surface expression of the rupture.
>
>2) The fault rupture plane dips south. The aftershock epicenters cluster
>fairly tightly in the area 23.10-23.60 deg. north by 70.00-70.60 deg. east,
>just north of Bhachau. In north-south profile the hypocenters are diffuse
>but with a distinct southerly dipping trend, suggesting a dip of 45-50 deg.
>south for a ENE-WSW fault plane.
>
>3) If the fault dip remains constant updip of the aftershocks (a big if),
>the surface projection of the fault plane would be approximately 23.70 deg
>north +/- 0.10 deg. and between 70.00-70.60 deg. east. This projected zone
>lies almost entirely in the Great Rann, just to the south of Khadir Island.
>I'm not clear whether Bill Lettis's aerial recon was able to get that far
>north or not, but I believe this is deeper into the Rann than most of the
>ground surveys have been able go. It would certainly be worth a look.
>
>Arch Johnston