Jafar Shoja-Taheri

The 1978 Tabas, Iran, earthquake: An interpretation of the strong motion records

On 26 December 2003, a destructive earthquake occurred in southeastern Iran, demolishing the city of Bam and vicinity. The highest intensity of shaking (VIII–IX) was observed in the city of Bam. The source of this shock was reported to have had a right-lateral strike-slip mechanism initiated in a blind fault in the north-south direction. A regional network consisting of 23 strong motion stations (SSA-2 Accelerograph), located within 1–290 km from the epicenter, registered the earthquake. The compact and pulse-shape arrivals of strong signals recorded at the Bam station strongly suggest that the rupture was initiated south of the city and propagated toward Bam. Based on the relative arrival times of the rupture front and the arrivals of P and S waves at this station, the velocity of rupture was estimated as 2.5±0.2 km/sec. Comparisons made between the attenuation curves constructed for this earthquake and those of the regional curves show that the effects of directivity caused significant deviations at near distances from the fault. This strong motion data yields estimates of source parameter values of 8.3×025 (dyne-cm), 7.5 km, and 90 bars, respectively, for seismic moment, source radius, and stress drop.

ML and MW Scales in the Iranian Plateau Based on the Strong-Motion Records

The availability of a large amount of strong-motion data recorded by the National Strong Motion Network of Iran (nsmni) has motivated this study to develop relations for routine determination of M L and M W from digital horizontal components of the strong-motion records. The dataset comprises 861 two-component horizontal acceleration time series recorded for 125 earthquakes with magnitudes of 4.5 and larger. The M L scale is based on the horizontal synthesized Wood–Anderson seismograms. We have applied the Monte Carlo technique to evaluate distance correction curves for use in determining the local magnitude, M L , in Iran and in its northern, eastern, and Zagros subregions. Results indicate that the distance correction curves show trilinear behavior for geometrical spreading. The resulting coefficients evaluated for Iran as a whole are: R 1 = 96 ± 5 km; R 2 = 131 ± 5 km; n 1 = 1.01 ± 0.02; n 2 = −0.14 ± 0.1; n 3 = 0.14 ± 0.03; k = 0.00020 ± 0.00008. The distances less than R 1 correspond to attenuation of the direct waves. Between R 1 and R 2 is the distance where the multiply reflected and refracted shear waves from Moho dominate the arrivals. n 1 , n 2 , and n 3 are the coefficients of geometrical spreading for distances from the source to R 1 , R 1 to R 2 , and beyond R 2 . k is the coefficient of inelastic attenuation. For estimating the M W scale from the strong-motion data, we used the method proposed by Andrews (1986). To find the best correlation between the moment magnitudes measured from the strong-motion data and those measured from teleseismic data, we examined several time windows (e.g., whole trace, S -wave coda, and source time durations). The regressions show that the M W estimates from different time windows are all equally well correlated with the corresponding reported values with nearly identical standard deviations. Finally, relations between the estimates of local and moment magnitudes for the regions show that for earthquakes with magnitudes larger than about 6.0, the M L scale gradually becomes saturated and, therefore, it gives smaller values than those obtained by the M W scale. However, for smaller earthquakes, the M L scale overestimates the M W scale. This discrepancy occurs mainly because the frequency contents of the waveforms employed in these scales are different.

Source geometry and mechanism of 1978 Tabas, Iran, earthquake from well located aftershocks

Abstract Aftershocks of the September 16, 1978 Tabas earthquake located from close-in observations made during a four-week fielding of temporary stations have been analyzed for the purpose of delineating detailed source geometry of the 1978 earthquake. Spatial distribution of aftershocks and their composite focal mechanism suggest that the geometry of faulting is far from planar. Aftershocks define two prominent alignment. The southern alignment strikes E-W to WNW-ESE, whereas the northern alignment strikes in a N-S to NNW-SSE direction with an abrupt change of nearly 55–60 degrees near 33.4°N latitude. Both field observations of surface faulting pattern and systematic variation of principal directions of stress axes computed from aftershock focal mechanisms are consistent with the upthrusting and imbrication of a wedge shaped crustal block with the wedge angle of about 120 degrees. Both geological and seismological evidence suggest that the deformed zone is truncated at the southern edge by preexisting E-W fault structures. New observations may provide a partial answer to the unexplained farfield asymmetry of the long period Rayleigh wave radiation pattern recently observed for the mainshock across IDA network.

Stochastic Finite-Fault Modeling of Strong Ground Motions from the 26 December 2003 Bam, Iran, Earthquake

Equation (6) of Shoja-Taheri and Ghofrani (2007) was inadvertently deleted from page 1956. BSSA regrets the error. The equation is included here, along with surrounding text. Corrected text: To compare the attenuation of the synthetic ground motions with the attenuation relation for the region (Shoja-Taheri et al., 2005), we simulated accelerograms for nearly 5000 nodes in a mesh covering the area shaken by this earthquake. We assumed that all the stations in the area are located on rock. We fitted the simulated PGAs using

An ML Scale in Northeastern Iran

Local-magnitude scales are derived for northeastern Iran (Khorasan province) from waveform data recorded at six stations from 205 local earthquakes, ranging in distance from 10 to 600 km. By averaging the horizontal components in a single measure, we used 1506 zero-to-peak amplitudes from synthetic Wood- Anderson seismograms to determine, in a least-squares sense, the appropriate logA0 attenuation functions, the event local magnitude, and the station corrections. Both a parametric and a nonparametric description of logA0 is considered while performing the inversions. In both cases, the constraint of 1-mm motion recorded at 100 km for M 3:0 earthquakes was used. To evaluate the distance correction curves in determining the local magnitude, ML, in northeastern Iran we applied both linear and trilinear inversions to our datasets. The result of the linear inversion for dis- tance correction is given by: logA0 1:370 0:050logR=100 0:0020 0:0001R 100 3. For trilinear inversion we have applied the Monte Carlo technique. The resulting coefficients evaluated for the area are R1 106 5 km; R2 347 49 km; n1 1:380 0:045; n2 0:597 0:132; n3 0:415 0:236; k 0:0033 0:0003, where n1, n2, and n3 are the coefficients of geometrical spreading for distances from the source to R1, R1 to R2, and beyond R2. k is the coef- ficient of inelastic attenuation. The remarkable agreement between the parametric and nonparametric results confirms that both linear and trilinear attenuation functions that we made for deriving the parametric distance correction are equally reasonable. More- over, inversion of bootstrap replications of our dataset furnished stable solutions. Sta- tion magnitude corrections range between 0:17 and 0.27, suggesting a variable and noticeable effect of station-site properties on recorded amplitudes.