| 1. |
- Heidari, Reza, et al.
(författare)
-
Magnitude-scaling relations using period parameters tau(c) and tau(max)(p), for Tehran region, Iran
- 2013
-
Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 192:1, s. 275-284
-
Tidskriftsartikel (refereegranskat)abstract
- In this study, the first step towards establishing an onsite earthquake early warning system (EWS) in the Tehran region is presented. The system uses the period parameters tau(max)(p) and tau(c) from the first 3 s of the vertical and horizontal components of a P wave, separately and combined. Various regression relations between the magnitude and period parameters were determined for different seismic networks operating in the study area. The data set used in this study contains small ground motions including 194 events with magnitudes between M-L 2.5 and 4.6 located within approximately 80 km from the epicentre in the Tehran region. The SDs of the magnitude-scaling relations for all the component categories (vertical, horizontal and total components) based on the tau(max)(p) and tau(c) approaches were estimated to be on the order of +/- 1.0 and +/- 1.1 unit of magnitude, respectively. These relations were determined from the small magnitude range of the velocity records (M-L 2.5-4.6) as input seismograms. Additional tests were conducted to verify the reliability and robustness of the determined magnitude-scaling relations using acceleration records from the 2002 June 22, M-L 6.5 Changureh-Avaj earthquake; 2004 May 28, M-L 6.1 Firoozabad-Koojour earthquake; 2009 October 17, M-L 3.9 Shahre-Rey earthquake and 2011 February 20, M-L 4.1 Sharif-Abad earthquake; the first two events (Changureh-Avaj and Firoozabad-Koojour) occurred outside the study area. Among the various regression scaling relations obtained, the estimated magnitude based on the tau(max)(p) approach using the vertical components yielded the most stable and reliable results of 6.4 (+/- 0.4), 5.9 (+/- 0.4), 3.3 (+/- 0.5) and 3.8 (+/- 0.3) for the Changureh-Avaj, Firoozabad-Koojour, Shahre-Rey and Sharif-Abad earthquakes, respectively. The magnitudes estimated using the tau(c) method exhibited more scatter with higher SDs than those using the vertical components using the tau(max)(p) approach. Our results also indicate that using the horizontal components produces larger SDs, which may be attributed to the larger site effects; however, the horizontal components can be used as auxiliary available data to provide more constrained information for a multilevels pilot alarm system and to reduce the number of missed or false alarms. The main uncertainties in the proposed magnitude-scaling relations result from the absence of any large earthquakes and poor station distributions in the study area. However, the results presented in this study can be used as a pilot onsite earthquake EWS in the Tehran region.
|
|
| 2. |
- Riahi, Ali, et al.
(författare)
-
Simultaneous retrieval of body and surface waves in the Dehdasht area, Iran, from the seismic ambient field and the observation of spurious artefacts
- 2021
-
Ingår i: Geophysical Journal International. - : Oxford University Press. - 0956-540X .- 1365-246X. ; 227:2, s. 1193-1203
-
Tidskriftsartikel (refereegranskat)abstract
- Recent developments in sensor technology have allowed for the low-cost deployment of dense seismic arrays which continuously record the seismic ambient noise field. Generally, the extraction of body waves from the seismic ambient noise field is more challenging than for surface waves, due to the dominant amplitude of the latter. In this study, we work with data from a dense seismic array deployment in the Dehdasht area, southwestern Iran. We show that by using the polarization properties of seismic waves, we can simultaneously retrieve both high frequency body waves and surface waves from the cross-correlated noise field. As a by-product of this study, we also observe high energy spurious artefacts, particularly those associated with the direct P-phases. Numerical simulations show that these artefacts are a consequence of art uneven distribution of noise sources, and are difficult to suppress during the simultaneous retrieval of body and surface waves.
|
|
| 3. |
- Shirzad, Taghi, et al.
(författare)
-
Shallow crustal structures of the Tehran basin in Iran resolved by ambient noise tomography
- 2014
-
Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 196:2, s. 1162-1176
-
Tidskriftsartikel (refereegranskat)abstract
- In this study, we present an application of the ambient noise tomography (ANT) to study the near-surface geological structures of the metropolitan Tehran/Iran region. Short-period fundamental mode Rayleigh wave Green's functions were estimated using cross-correlations of the vertical component of the ambient noise from 2009 October to 2011 May using a variety of seismic sensors, for example, accelerometers and seismometers, deployed in the Tehran area. Standard common low frequency processing procedures were applied to the cross-correlations, and shorter time-windows comprising 10-min segments were used in the processing step to enhance the time resolution of the signal in the frequency range of interest (1-10 s). Stacking was also conducted using the rms of the estimated empirical Green's functions. Our results demonstrate that ambient seismic noise tomography is a viable technique at periods of 1-10 s in length, even when different sensor types are present. Analysis of the empirical Green's functions indicates that the dominant sources of ambient seismic noise originated from the same origin, and no significant seasonal or spatial variations in the ambient noise sources were observed. Multiple-filter analysis was used to extract the group velocities from the estimated empirical Green's functions, which were then inverted to image the spatially varying dispersion at periods of lengths between 1 and 7 s using tomographic inversion of the traveltimes estimated for each frequency. The resulting group velocity maps show high correlations with known geological and tectonic features of the study region. In general, most of the Tehran basin, with certain exceptions, could be clearly resolved with low group velocities, whereas the mountain ranges were found to be correlated with high group velocities. In the Tehran basin, for 2 and 3 s periods, the low-velocity zone deepens towards the south-southwest, which reflects thicker sediments in the southern part of the basin than in the north. This feature has also been observed in other geological studies. The Vs models also show that bedrock depth varies between 400 and 1400 m from north to south within the Tehran basin. At longer periods main faults are associated with abrupt transitions between regions of high- to low-velocity anomalies. In general, our results indicate that ANT can be a flexible and effective approach for studying near-surface heterogeneity using short-period surface wave data.
|
|
| 4. |
- Shomali, Zaher Hossein, et al.
(författare)
-
Lithospheric structure beneath the Zagros collision zone resolved by non-linear teleseismic tomography
- 2011
-
Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 187:1, s. 394-406
-
Tidskriftsartikel (refereegranskat)abstract
- The upper-mantle structure across the Zagros collision zone, in southwest Iran, is investigated using a non-linear weighted damped least-squares teleseismic tomography approach. The resolution of the structures/transitions in the upper mantle is enhanced significantly by correcting the teleseismic relative arrival time residuals for an a priori crustal velocity model and then performing the inversion with fixed crustal blocks. To investigate whether or not the lithospheric blocks and major transitions in the resulting model are required by the data or are artefacts of the inversion, the data were inverted using two different inverse methods (singular value decomposition and a quadratic programming method). New high-quality seismic velocity models show apparent correlation between surface geological features and seismic velocity structures at lithospheric depth across the Zagros collision zone. The image shows a sharp lithospheric boundary at the Main Zagros Thrust between 100 km and 250 km depth with P-wave velocity about 3 per cent faster within the Arabian Shield to the south. A step-like increase in lithospheric thickness across the Zagros collision zone is assumed to separate two different mantle structures namely the Arabian (to the south) and the Eurasian (to the north) domains. The most striking feature resolved is a north-dipping slab-like positive velocity anomaly.
|
|
