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Seismicity and tectonic deformation rate in Soviet Armenia: Implications for local earthquake hazard and evolution of adjacent regions
Author(s) -
Westaway Rob
Publication year - 1990
Publication title -
tectonics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.465
H-Index - 134
eISSN - 1944-9194
pISSN - 0278-7407
DOI - 10.1029/tc009i003p00477
Subject(s) - geology , seismology , induced seismicity , slip (aerodynamics) , tectonics , quaternary , seismic hazard , fault (geology) , plate tectonics , paleontology , physics , thermodynamics
The Spitak, Armenia, earthquake of December 7, 1988 (M s 6.7; M o ∼10×10 18 Nm), was the largest this century in Soviet Transcaucasia, in the northern part of the zone where the Arabian and Eurasian plates converge at ∼30 mm yr −1 . Field observations in its epicentral area and information from historical and recent earthquakes in and around Armenia are used to determine shortening rates and hence to investigate how shortening is partitioned between the Great Caucasus, Armenia and other parts of Soviet Transcaucasia, and eastern Turkey and NW Iran farther south. This earthquake produced more than 20 km of surface faulting with up to 1.5 m reverse slip and 0.4 m right‐lateral slip on a fault with WNW strike and southward slip vector azimuth. Much of the local topography has formed by slip on faults with similar orientation. There is no evidence of substantial recent folding elsewhere in Armenia, suggesting that shortening in Armenia occurs mainly by slip on these faults. Shortening rate across Armenia is ∼1 mm yr −1 at strain rate ∼2 × 10 −16 s −1 . With likely slip rate on each fault ∼0.2 mm yr −1 , the local topography has formed in ∼1 Myr. Shortening rate in the eastern Great Caucasus farther north can be estimated using Quaternary uplift rate, assuming uplift is caused by crustal thickening and occurs in isostatic equilibrium. Although Quaternary uplift rates from 0.05 to ∼10 mm yr −1 have been previously suggested, any rate above ∼1 mm yr −1 is too rapid to be explained by this process. The remaining rates range from 0.05 to 0.2 mm yr −1 and imply ∼1 to 5 mm yr −1 local shortening rate, suggesting overall shortening rate across the Soviet part of the convergent zone is only ∼2 to 6 mm yr −1 . Historical seismicity suggests that at least ∼1 mm yr −1 of this shortening is coseismic. Most of the convergence between Arabia and Eurasia is thus taken up at present farther south in eastern Turkey and NW Iran along with probable substantial westward expulsion of crustal material, and most of the topography and crustal thickness in the Great Caucasus probably already existed at least 10 Myr ago.