This document summarizes paleoseismology research on extensional tectonic environments. It discusses how large extensional earthquakes produce surface deformation through normal faulting and describes the key geomorphic and stratigraphic features created by faulting, including fault scarps and tilted sedimentary beds. It also outlines the earthquake deformation cycle for normal faults and some methods used for dating paleoearthquakes, such as scarp degradation modeling and exposing dating of fault scarps.

1. PALEOSEISMOLOGY OF EXTENSIONAL TECTONIC
ENVIRONMENT
SANJAY KRISHNA BHUYAN
DEPARTMENT OF EARTH & ENVIRONMENTAL SCIENCE
KSKV KACHCHH UNIVERSITY
BHUJ, KACHCHH, GUJARAT

2. Introduction
Large extensional earthquakes in the upper crust produce
surface deformation recorded by displacements on
normal faults, by growth of folds above these faults and
by fault generated elastic and viscoelastic crustal stress
accumulation and release produce changes in land
elevation over wavelengths of many kilometers.

3. The stratigraphic and geomorphic features formed by
extensional faulting are commonly easier to see than
those formed by compressional or strike-slip faulting,
because brittle faulting of surface materials creates sharp-
edged scarps that crosscut all pre earthquake landforms.

4. Crustal extension is typically accommodated by normal
faults, either singly or in sets of parallel synthetic or
antithetic faults.. The primary normal fault is a crustal-
penetrating fault that may have many kilometers of
cumulative throw, and often separates a linear mountain
range from an adjacent basin. Fault dips in the upper
crust are consistently 50º–70º.
Environments of Extensional
Deformation

5. At both crustal and smaller scales, the primary normal
fault is accompanied by secondary normal faults, either
synthetic (dip in the same direction as the primary faults)
or antithetic (dip opposite to the primary fault).
Distributed normal faulting creates uplifted blocks
(horsts), down faulted blocks (grabens), and rotated
blocks or tilted fault blocks.

6. The largest region areas of continental extension (USA
Basin and Range province, and northeastern China),
followed by narrower Intracontinental rifts (East African,
Rhine, Rio Grande, Baikal).
Another large region of continental extension is high
plateaus behind a collisional orogen, such as the Tibetan
Plateau north of the Himalayas, or the South American
Altiplano.

7. The Earthquake Deformation Cycle in
Extensional Environments
The earthquake deformation cycle for normal faults can be divided
into a coseismic phase, a postseismic phase, an interseismic phase,
and a preseismic phase. For most on-land normal faults, the only
phases that can be measured reliably are the coseismic and
postseismic phases. That is because in historic earthquakes, the
topology of the preceding state is known. For all normal faults that
are presently in the interseismic part of the cycle, the preceding
topology is not known, so geologic evidence of the deformation cycle
is lacking.

9. Geomorphic evidence of
Paleoearthquakes
The primary geomorphic indicator of paleoearthquakes
on normal faults is a fault scarp. The term fault scarp
usually refers to a small escarpment in unconsolidated
deposits created by direct surface faulting.

10. Gilbert’s theory of formation of complex fault scarps. Close
horizontal lines indicate unconsolidated sediments of the hanging
wall; wide diagonal lines indicate consolidated bedrock of the
footwall.

11. Stratigraphic evidence of
Paleoearthquakes
Normal surface faulting results in the instantaneous creation of faults,
fissures, and tilted beds, and in the delayed response of fault-induced
sedimentation. The sequence of paleoearthquakes cannot usually be
reconstructed from tectonic or depositional features alone, instead, a
combined analysis is required. The key to successful interpretation is
to distinguish between tectonic versus depositional features, and to
distinguish depositional units that predate faulting from those that
postdate faulting.

12. Dating Paleoearthquakes
Paleoearthquakes can be dated directly or indirectly. The most
direct techniques are dating a fault scarp via scarp degradation
modeling, cosmogenic surface-exposure dating, or by quantitative
analysis of scarp soils. Indirect dating methods involve bracketing
the age of the paleoearthquake by numerical dating of landforms
or deposits that predate and postdate faulting.

13. References
PALEOSEISMOLOGY by JAMES P. McCALPIN
Wikipedia
Google

14. Thank You