Subevent analysis for the Tabas earthquake of September 16, 1978, using near field accelerograms

Sarkar, Irene; Sriram, V.; Hamzehloo, H.; Khattri, K. N.

doi:10.1016/j.pepi.2005.01.004

Cited by 16 publications

(1 citation statement)

References 22 publications

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“…Therefore, some differences between this study and geodetic results could be due to the time and magnitude dependence of strain. Sarkar et al, [34] have analysed sub-events of the Tabas earthquake and claimed that the fault plane solutions of earthquakes generally show WNW-ESE strikes, subparallel to the regional strike of the documented main fault systems. In addition, they concluded that the preferred fault planes correspond to the main event and to the following sub-events strike of ESE-WNW and steep dip angles, and exhibit predominantly reverse thrust motion with some left-lateral strike-slip components.…”

Section: Regmentioning

confidence: 99%

State of stress in the northern Tabas block, east-central Iran, as inferred from focal mechanisms of the 1978 Tabas earthquake sequence

Zamani

2011

Open Geosciences

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Abstract:In this paper, the state of stress in the northern Tabas block in east-central Iran is analyzed based on the systematic inversion of aftershock focal mechanisms from the 1978.09.16 Tabas earthquake, to characterise the stress regime that controls most earthquakes in this area. Here, stress inversions of double-couple focal mechanisms of earthquakes recorded during the 30 days following the main shock have been carried out. The calculated average stress regime indicates dominant major 226 • to 237 • trending compression for the Tabas region. The dominating regime in east-central Iran is thrusting with a minimum stress axis, σ 3 , close to vertical. The reconstruction of the main seismotectonic stress in east-central Iran with a NE-SW compression is consistent with independent information of the active plate convergence related to Arabia-Eurasia convergence. Most earthquakes in the mentioned area occur near or around concealed Quaternary thrust faults with their activity being controlled by the NE-SW compression. Where Φ, the ratio of principal stress differences, is 0.5, a small difference between σ 2 σ 3 and σ 1 and small amounts of deviatoric stress is indicated. Therefore, for small deviatoric horizontal σ 1 it is not possible to increase and reactivate small sections of basement thrust faults and create secondary basement aftershocks. Reconstructed stress regimes in this study for sedimentary cover (237) and basement (226) of Tabas are similar. Therefore, it seems that the basement and cover were coupled together, possibly along the 2-4 km of upper Precambrian low-grade metamorphic rocks. Then these segments of the fold-and-thrust belt were involved in similar seismic activity under a similar stress regime. IntroductionIran is located within the convergence domain between the Arabian and Eurasian plates. The central parts of * E-mail: b.zamani@tabrizu.ac.ir, Tel: +98 411 3392700, fax: +98 411 4448914Iran move northwards with respect to western Afghanistan at˜15 mm yr −1 [1]. This northward motion introduces 15 mm yr −1 of north-south right-lateral shear across eastern Iran which is accommodated on right-lateral strike-slip fault systems on both the western (GowakNayband fault system) and eastern (Neh and Zendan fault systems) margins of the aseismic Dashte-e Lut desert [2,3]. The study area is situated in the eastern part of central Iran, and northern part of the Tabas block ( Fig. 1). Tabas is a town with approximately 30,000 inhabitants, 950 kilometers southeast of Tehran. The city was hit by a major destructive earthquake on September 16 th 1978, during which about 22,000 lost their lives. The recorded earthquake magnitude was between 7.5 and 7.9 on the Richter scale. Despite the major tectonic hazard in the Tabas region, relatively few studies have attempted to analyze the local-scale seismotectonic stress regime. Consequently, this research aims to characterise the stress regime which controls most of the earthquakes occurring in this area through an inversion of the earthquake focal mechanisms...

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Section: Regmentioning

confidence: 99%

State of stress in the northern Tabas block, east-central Iran, as inferred from focal mechanisms of the 1978 Tabas earthquake sequence

Zamani

2011

Open Geosciences

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A Study of Source Parameters, Site Amplification Functions and Average Effective Shear Wave Quality Factor Qseff from Analysis of Accelerograms of the 1999 Chamoli Earthquake, Himalaya

Kumar

Ram

Khattri³

2006

Pure appl. geophys.

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The accelerograms of the 1999 Chamoli earthquake and nine of its aftershocks, which occurred in Uttaranchal Himalaya, have been analyzed to investigate their source parameters, the site amplification functions and the average effective shear-wave quality factor Q seff in the region. The fault plane solution of the main shock is obtained using the spectral amplitudes of SH waves (approximated by transverse components of accelerograms) of the high-energy packets observed in the accelerograms of the main shock. It is found to be comparable with the reported solutions in other studies. Similarly the other source parameters (viz., seismic moment = (5.03±1.7) · 10 25 dyne-cm, stress drop = 65 bars, source duration = 5.2 s and moment magnitude = 6.4) estimated for the main shock are consistent with the values obtained in other studies. The stress drops estimated for the aftershocks vary from 23 bars to 153 bars and the seismic moment from 1.4 · 10 23 dyne-cm to 2.9 · 10 23 dyne-cm. The average estimated values of the effective shear-wave quality factor Q seff vary from 655±359 in the Uttaranchal sector of Himalaya and 1475±130 in the Delhi region. In general, the Q seff value increases with an increase in the epicentral distance reflecting the penetration of the waves into deeper layers of the crust as the epicentral distance of the observation point increases. These values of Q seff indicate that in general the curst is at low temperatures that will promote brittle behavior and conditions for episodic failure as compared to creep, under the accumulated strains from plate collision at the Himalaya plate boundary. The site amplification characteristics at sites have been identified from the frequency bands of significant amplification observed in the spectral ratios of the horizontal to the vertical component records. The decay of peak ground acceleration (PGA) values with distance has been investigated using the empirical regression curves vis-a`-vis the site amplification factors.

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Extraction of forward-directivity velocity pulses using S-Transform-based signal decomposition technique

Amiri

Moghaddam

2014

Bull Earthquake Eng

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Subevent analysis for the Tabas earthquake of September 16, 1978, using near field accelerograms

Cited by 16 publications

References 22 publications

State of stress in the northern Tabas block, east-central Iran, as inferred from focal mechanisms of the 1978 Tabas earthquake sequence

State of stress in the northern Tabas block, east-central Iran, as inferred from focal mechanisms of the 1978 Tabas earthquake sequence

A Study of Source Parameters, Site Amplification Functions and Average Effective Shear Wave Quality Factor Qseff from Analysis of Accelerograms of the 1999 Chamoli Earthquake, Himalaya

Extraction of forward-directivity velocity pulses using S-Transform-based signal decomposition technique

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