Northwest Asia (NWA)

Authors:

V. Poggi, L. Danciu

Summary

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.

Interactive Viewer

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.

viewer_legend

Tectonic overview

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.

Basic Datasets

Earthquake Catalogue

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).

nwa_catalogue

Figure 1 - The Mw-homogenized earthquake catalogue prepared by GEM for NWA.

Hazard Model

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.

area_sources

Figure 2 - The proposed source zonation for NWA. Different colors are used to represent the seismicity level of the region.

Seismicity analysis

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
2 GEOAS059 active_shallow 4 1.05 7.1
4 GEOAS066 active_shallow 2.8 1 7
5 GEOAS072 active_shallow 3.7 0.87 7.3
6 GEOAS073 active_shallow 4.4 1.06 7.3
7 GEOAS901 active_shallow 4.51 1.08 6.1
8 GEOAS076 active_shallow 3.7 1 7
9 AZEAS077 active_shallow 4.3 1 7.3
10 AZEAS079 active_shallow 4.3 1 6.8
11 GEOAS080 active_shallow 2.95 0.75 7.3
12 GEOAS081 active_shallow 3.3 0.95 7.1
13 AZEAS082 active_shallow 3.5 1 7.3
14 AZEAS114 active_shallow 3.5 1 7.7
15 AZEAS115 active_shallow 4.32 1.02 7.3
16 ARMAS168 active_shallow 4.57 1.09 7.8
18 AZEAS171 active_shallow 4 1 7.3
19 AZEAS172 active_shallow 4 0.9 7.3
20 GEOAS173 active_shallow 4.18 0.98 7.3
21 TURAS178 active_shallow 4.3 1 7.1
26 AZEAS972 active_shallow 4.63 1 7.3
43 RUAS13 crotonic 1.31 0.99 6.3
44 RUAS12 crotonic 1.18 0.98 6.3
46 UKRAS11 active_shallow 2.37 0.98 6.5
47 UKRAS12 active_shallow 2.38 1 6.5
48 UKRAS13 active_shallow 2.5 1 6.5
49 RUAS02 active_shallow 2.78 0.98 7
50 RUAS03 active_shallow 2.65 0.99 6.5
51 RUAS04 active_shallow 2.86 0.99 7
52 RUAS07 stable_continental 1.35 1 6.1
53 UKRAS14 active_shallow 1.87 1 6.3
54 BSAS00 active_shallow 2.5 1 7.5
55 ARMAS060 active_shallow 3.75 0.9 7.3
56 ARMAS061 active_shallow 4.35 1.05 6.8
57 RUAS05 active_shallow 1.9 1 6.1
58 AZEAS116 active_shallow 3.05 0.99 6.3
59 AZEAS115 active_shallow 4.72 1.02 7.3
61 EMCA51 active_shallow 1.64 1 6.2
63 RUAS06 stable_continental 1.15 1 6.1
64 RUAS01 active_shallow 1.64 0.99 6.3
72 UAAS230 stable_continental 1.94 0.95 6.3
73 BYAS036 stable_continental 2.55 1 6.3
74 LVAS035 crotonic 2.35 1 6.3
75 LVAS024 crotonic 2.5 0.98 6.3
76 FIAS032 crotonic 2.1 1 6.3
77 FIAS026 crotonic 2.65 1 6.3
78 RUAS025 stable_continental 1.8 1 6.3
82 EMCA02 active_shallow 2.3 1 6
83 EMCA05 active_shallow 2.9 1 6
84 EMCA60 active_shallow 2.9 1 6
85 EMCA03 active_shallow 2.3 0.72 6.5
86 EMCA04 active_shallow 2.81 0.74 6.5
89 EMCA38 active_shallow 1.16 0.72 6.5
90 EMCA39 active_shallow 2 0.72 6.5
91 EMCA40 active_shallow 2.46 0.72 6.5
92 EMCA41 active_shallow 2.11 0.72 6.5
93 EMCA42 active_shallow 4.13 1.03 6.5
94 EMCA52 active_shallow 3 1 6
95 EMCA40 active_shallow 2.46 0.72 6.5
96 EMCA41 active_shallow 2.11 0.72 6.5
97 EMCA42 active_shallow 4.13 1.03 6.5
98 EMCA52 active_shallow 3 1 6

Table 1 - Seismicity parameters used in the NWA model.

Smoothed Seismicity

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.

smoothed_seismicity

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.

Cratonic Crust Weight
SomervilleEtAl2009YilgarnCraton 1.0
Stable Continental Crust Weight
AtkinsonBoore2006Modified2011 0.5
PezeshkEtAl2011NEHRPBC 0.5
Active Shallow Crust Weight
ChiouYoungs2014 0.5
AkkarEtAlRjb2014 0.5

Table 2 - GMPEs used in the NWA model.

Results

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.

References

Frankel, A. (1995). Mapping seismic hazard in the Central and Eastern United States. Seismological Research Letters 66:4, 8-21.