Hello

I have read with interest the many e-mails regarding potential
surface rupture along the Kutch (Kachchh) Mainland Fault in the area
northwest of Bachau. The area described in these e-mails, in
particular the e-mails from the Oyo/RMS group, is an area of
extensive lateral spreading. As we reported last week, the lateral
spreading was observed first during aerial reconnaissance and then
checked in the field. The lateral spreading extends for a distance
of at least 16 kilometers and is locally up to 1/2 to 1 km wide. The
lateral spread extends roughly from 23 degrees 30', 70 degrees 10' on
the west to 23 degrees 25', 70 degrees 17' on the east.

The lateral spread is elongate along and directly north of the
Kachchh mainland fault. The photos shown by the Oyo group are from
the exact location visited by the EERI team the previous week. In
this area, the lateral spread is approximately 350 to 500 meters
wide. We estimated cumulative lateral spreading of up to 1 meter
distributed across the zone, with areas of locally intense spreading
associated with liquefaction sand boils. The lateral spreading
locally forms "stair-case" type extensional deformation and very nice
compressional toe bulges. In some areas, the lateral spreading has
completely closed meter-wide irrigation ditches. Locally, the
lateral spread occurs within a small graben formed by the expulsion
of sand during liquefaction.

Along trend, the lateral spread occurs to form large arcs about 1 km
long or longer. The trend of individual ground fractures changes
systematically from east-northesast to east-west to west-northwest
across the arc. The direction of lateral spreading is almost due
north. Thus, given the geometry of the arc, the lateral spread
appears to have slight right-lateral and left-lateral displacements
along the margins.

The team of Rockwell, Wesnousky and Seeber et al also have visited
this location and agree that the deformation is probably the result
of lateral spreading, not tectonic fault rupture. Rockwell et al,
however, also report a possible fault offset in this same general
location but slightly farther east than the lateral spread described
above. In this area (roughly 23 27.644 and 70 22.954), Rockwell's
group observed apparent right lateral offset of a few tens of
centimeters. They believe that this offset may be tectonic secondary
tear faulting. I did not see this location, and thus cannot offer an
opinion.

In summary, Jim Hengesh and I did not observe ANY evidence of
significant fault rupture. We performed aerial and field
reconnaissance of the Kachchh Mainland Fault, Katrol Hills Fault,
Island Belt Fault, and other unnamed faults in the epicentral region.
In a moment of brazen uncontrolled speculation, we believe that the
large elongate zone of lateral spreading along and directly north of
the Kachchh Mainland fault may have been localized in this area by
broad monoclinal warping of the alluvial fan. The monoclinal warping
provided the gradient for the lateral spread to occur. In this
interpretation, rupture on the Kachchh Mainland Fault did not reach
the surface but produced warping of the ground surface in a zone up
to a km wide or more. This speculation obviously would require
careful field surveying and profiling of terrace and alluvial fan
gradients to verify.

Regardless, it appears almost certain that the January 26th
earthquake did not produce significant surface fault rupture or any
other form of easily detectable ground deformation. This undoubtedly
will provide an important lesson for assessing seismic hazards - we
just need to know which classroom we are in. The critical question
is to characterize the tectonic setting of this earthquake - are we
in an active fold and thrust belt, or are we in a "stable" intraplate
region. We need to better understand the tectonic context of this
earthquake before the lessons can be properly learned. The recent
reconnaissance by Rockwell et al (and this group will provide an
update on this shortly) seems to suggest that there is little if any
evidence for active folding in the late Quaternary at least. Thus,
the Bhuj earthquake may indeed have occurred in an intraplate setting
similar to the New Madrid area. Rockwell's group will shed more
light on this in the near future after they return home from India
later today.

Bill Lettis

20 Feb 2001