The Northwest Asia (NWA) model was developed internally by GEM in collaboration with the Swiss Seismological Service (SED), Swiss Federal Institute of Technology (ETH), Switzerland. The model is based on a distributed seismicity approach. Similar to Northern Africa, we have applied rate redistribution to better represent the spatial variability of seismicity.
Information about the OQ model versions and input files can be found on the Results and Dissemination page.
The viewer below depicts the seismic sources and hazard results in terms of PGA for a return period of 475 years. Click on the menu in the upper right corner to select the layer.
Northwestern Asia is generally tectonically quiescent, though very rare earthquakes may occur throughout the craton. The southern margin of the study area contains the Caucasus Mountains in Georgia, southern Russia, Azerbaijan and Armenia, where the northern deformation front of the Anatolian-Zagros Orogen resulting from the Arabian-Asian convergence zone is located. Major reverse and strike-slip faults in this zone have moderate to high slip rates and pose significant seismic hazard.
For the purpose of having a unique catalogue valid for NWA, GEM created a new Mw-homogenised earthquake catalogue by assembling globally available sources (ISC review bulleting, GCMT, ISG-GEM, GHEC catalogues) with existing homogenized catalogues from previous projects (SHARE, EMME, EMCA). The GEM implementation of the NWA earthquake catalogue, presently consists of 15106 events with 3.5 ≤ Mw ≤ 8.2, covering a period from 10 to 2015 (Figure 1).
Figure 1 - The Mw-homogenized earthquake catalogue prepared by GEM for NWA.
Seismic Source Characterisation
Area Source Zonation
The seismic source model for NWA consists of 98 area source zones (Figure 2). The main constrain for the development of the source model came from literature and from a set of geological and seismotectonics considerations, such as style, geometry and distribution of existing faulting systems and their relation to the local stress and deformation regimes. Local and regional source models from previous hazard studies have also been taken into great consideration as starting point for the proposed zonation and to assure compatibility across the borders, particularly with the Europe, Middle East and Central Asia models.
Figure 2 - The proposed source zonation for NWA. Different colors are used to represent the seismicity level of the region.
Seismicity in each area source is assumed to follow a double truncated Gutenberg-Richter magnitude occurrence relation (or magnitude-frequency distribution, MFD). Lower truncation is arbitrarily assigned to Mw 4.5. Gutenberg-Richter b-values have been calibrated for the whole catalogue and independently for each source group. Conversely, occurrence rates (a-values) have been calculated separately for each source zone by imposing the previously calibrated b-values. A different maximum magnitude (Mw-Max) estimate is derived independently for each source group as the largest observed event plus an arbitrary - although quite conservative - increment of 0.5 magnitude units. Seismicity parameters are summarised in Table 1.
|Id||Name||Region Type||a value||b value||Mmax|
Table 1 - Seismicity parameters used in the NWA model.
To better represent the spatial variability of seismicity across the study area, the annual occurrence rates previously obtained for the homogenous source zones have been redistributed within each polygon using a procedure that accounts for the irregular spatial pattern of the observed events (Figure 3). The procedure shares some similarity with the popular smoothed seismicity approach (e.g. Frankel, 1995), but is more convenient in that a unique fit of the magnitude-frequency distribution is here required for each zone, while the corresponding total earthquake occurrence is only a-posteriori spatially reorganised as a function of the epicentral distance to all neighbouring events. Moreover, the combined use of zones gives the possibility to account for different modelling parameters (b-value, depth distribution, rupture mechanism) in separate regions.
Figure 3 - Example of spatial redistribution of the cumulative annual rates (M>0) using a decay parameter (λ) of 100. Only shallow crust seismicity is here considered. Rates are intended by unit area of 0.1deg2 (about 11km2).
Ground Motion Characterisation
To represent the epistemic variability of the ground motion between regions of different attenuation characteristics, we identified three main tectonic groups (active shallow, stable continental and cratonic) for which a set of ground motion prediction equations have been selected. The selection is consistent with what has been used for the Northeast Asia model.
|Stable Continental Crust||Weight|
|Active Shallow Crust||Weight|
Table 2 - GMPEs used in the NWA model.
Hazard curves were computed with the OQ engine for peak ground acceleration (PGA) and spectral acceleration (SA) at 0.2s, 0.5s, 1.0s, and 2s. The computation was performed on a grid of 67929 sites (spaced at approximately 10 km) with reference soil conditions with shear wave velocity in the upper 30 meters (Vs30) of 760-800 m/s.
The hazard map for PGA corresponding to a 10% probability of exceedance in 50 years (475 year return period), can be seen using the interactive viewer. For a more comprehensive set of hazard and risk results, please see the GEM Visualization Tools.
Frankel, A. (1995). Mapping seismic hazard in the Central and Eastern United States. Seismological Research Letters 66:4, 8-21.