The 2014 Earthquake Model of the Middle East: seismogenic sources

Abstract

The Earthquake Model of Middle East (EMME) project was carried out between 2010 and 2014 to provide a harmonized seismic hazard assessment without country border limitations. The result covers eleven countries: Afghanistan, Armenia, Azerbaijan, Cyprus, Georgia, Iran, Jordan, Lebanon, Pakistan, Syria and Turkey, which span one of the seismically most active regions on Earth in response to complex interactions between four major tectonic plates i.e. Africa, Arabia, India and Eurasia. Destructive earthquakes with great loss of life and property are frequent within this region, as exemplified by the recent events of Izmit (Turkey, 1999), Bam (Iran, 2003), Kashmir (Pakistan, 2005), Van (Turkey, 2011), and Hindu Kush (Afghanistan, 2015). We summarize multidisciplinary data (seismicity, geology, and tectonics) compiled and used to characterize the spatial and temporal distribution of earthquakes over the investigated region. We describe the development process of the model including the delineation of seismogenic sources and the description of methods and parameters of earthquake recurrence models, all representing the current state of knowledge and practice in seismic hazard assessment. The resulting seismogenic source model includes seismic sources defined by geological evidence and active tectonic findings correlated with measured seismicity patterns. A total of 234 area sources fully cross-border-harmonized are combined with 778 seismically active faults along with background-smoothed seismicity. Recorded seismicity (both historical and instrumental) provides the input to estimate rates of earthquakes for area sources and background seismicity while geologic slip-rates are used to characterize fault-specific earthquake recurrences. Ultimately, alternative models of intrinsic uncertainties of data, procedures and models are considered when used for calculation of the seismic hazard. At variance to previous models of the EMME region, we provide a homogeneous seismic source model representing a consistent basis for the next generation of seismic hazard models within the region.

Appendix: OpenQuake: tailored to fit

OpenQuake was used for calculating the seismic hazard over the entire region (Şeşetyan et al. 2017, this issue). OpenQuake is built on open-source and open-standards and freely available (www.globalearthquakemodel.org). The key features of OpenQuake (here we refer to the hazard library as OQ-hazard engine) are the state-of-the art seismic source representation, advance treatment of uncertainties and various options for hazard calculators (Pagani et al. 2014).

We used the default seismic source definitions of OQ-hazard engine (version 1.5) as the blueprints to design our source models; Hence, individual sources were parameterized according to the User’s Manual (Crowley et al. 2015). According to the software manual, geometry parameters and seismicity occurrence models represent each seismic source. The geometry implies definition of source location, style-of-faulting, and depth. In particular, for the area and point sources, the style of faulting is important.

The software allows defining extensive ruptures linked to the magnitude distribution; hence, a seismic source is not anymore a point source. Generation of extensive ruptures, as tuned by style-of-faulting parameters and magnitude, allows correctly computing the distance definitions used by new ground motion models (Bommer and Akkar 2012). The impact of using extensive ruptures on the hazard estimates regarding the point-rupture approximation, leads to a significant increase in the probabilities of exceedence for specific level of motion (Monelli et al. 2014). In our model, the area sources and gridded smoothed seismicity models share the same attributes for style-of-faulting and depth distribution. Specifically, there are three depth values and three style-of-faulting (i.e. normal, thrust, strike-slip) assigned to each individual seismic source.

Style-of-faulting of future earthquake ruptures is assessed source by source based on various data sets, including earthquake focal mechanisms, stress indicators, stress orientation and geological structure. Results of the assessment are relative frequency of strike-slip versus normal and reverse faulting averaged across each seismic source. Style-of-faulting frequency values are treated as aleatory variability and converted to probabilistic weights for seismic hazard integration (see Fig. S4 in the Electronic Supplement of this manuscript).

Additional parameters are the lower and upper seismogenic depth describing the region where source specific extensive ruptures are allowed to propagate. These parameters were obtained mainly from seismicity focal depths, location of top and bottom edges of the faults and the crustal model CRUST 2.0 (Bassin et al. 2000). Crustal faults are modelled as simple faults, and the subduction interface zones are represented as complex faults. A simple fault describes a fault surface projected along strike and dip. A complex fault does not require a dip angle because the geometry can be described by combinations of fault edges to describe top, mid or bottom of a fault surface. Common to all sources is the magnitude scaling relationship (Wells and Coppersmith 1994); the scaling relationship controls the size of floating ruptures as a function of magnitude.

A truncated GR (Gutenberg and Richter 1944) magnitude frequency distribution, defined by the activity parameters (a- and b-value), lower and upper magnitude is used to characterize all seismic sources. Minimum magnitude used in the probabilistic hazard calculation is 4.5 M_w. whereas the upper bounds vary accordingly to the M_max logic tree (see Fig. S4 in the Electronic Supplement of this manuscript). M_max is treated as aleatory to overcome the computational difficulties arising from multiple factors including complex seismogenic source model, extensive ruptures generation and software optimization.