Unified model for intraplate earthquakes - Intraplate Earthquakes - page 296

is of recent origin (e.g., Grollimund and Zoback, 2001 ) . This interpretation is based on the
assumption of repeated earthquakes on a single fault system and ignores the presence of
other LSCs within the rift and the effect of erosion. In the presence of several LSCs within
a rift system, seismicity is not restricted to any one particular fault, but can jump from one
LSC to another (see, e.g., Section 11.9.2 and Figure 11. 4 ) . The effect of erosion can also
be pronounced. If an earthquake is associated with a scarp, as was the case in 1819, when
the M 7.8 earthquake in the Kutch Rift Basin produced a −

the Allah Bund, evidence of it begins to fade due to erosion, and may eventually disappear.
Commenting on the flat terrain south of the Allah Bund, where major upheaval had taken
place during the earthquake, Frere ( 1870 ) wrote: “In some places we were told of small
islets of raised ground, which formerly existed on the Runn, and which had been swallowed
up, and we were taken to the spot, near Vingur, where such an islet used to stand out from
the surface of the Runn. But in no such case did any hollow or chasm remain visible.
The islet seemed to have melted down to the general level, and the description given by
eye-witnesses of what they saw indicated the action one would expect, from the continued
agitation of a mass of wet sand [from earthquakes] surrounded by water.” The presence
of multiple LSCs within the Reelfoot Rift (Section 11.9.2), and the possible obliteration
of minor scarps by erosion over the past several millennia, suggest that tectonism within
the Reelfoot Rift may not have been restricted to the Holocene period, but may have had a
longer history.

4 m high, 90 km long scarp

11.10.2 Temporal growth of S L as a predictor of earthquakes?
According to the unified model, for optimally oriented LSCs, and for any given horizontal
stress difference, a temporal increase in S L manifests itself as an increase in the associated
rotation, γ . Conversely, after a major earthquake and a release of the accumulated stress
there will be a decrease in both S L and γ . Currently, with uncertainties in focal mechanisms,
the accuracy with which γ can be detected is . However, in the future, if the direction of

the stress field at any location is continuously monitored, it may be theoretically possible to
detect temporal changes in γ , and infer a potential growth in S L as an earthquake precursor.

15 °

11.10.3 Apparent absence of strain accumulation

According to the two-dimensional thermo-mechanical continuum model (Hansen and
Nielsen, 2003 ) , as well as the unified model, the presence of local stress build-up in
the vicinity of LSCs should be associated with local pockets of elevated strain. However,
two decades of GPS measurements in the NMSZ have failed to provide a consensus that
such pockets of elevated strain rate exist (see, e.g., Newman et al ., 1999 ) . A possible reason
may be that the instrument placement and “our geodetic approach implicitly focuses on
motions due to plate wide rather than locally derived stresses” (Newman et al ., 1999 ) . How-
ever, appreciable local pockets of horizontal strain were detected in Kutch, where a dense
network of stations was deployed over the aftershock region of the 2001 earthquake and