SUPPLEMENTARY MATERIAL FILE 

A.	The Zagros Fold-Thrust Belt - Tectonics, Stratigraphy, Physiography, and Surficial Gravity Collapse Structures 

Tectonics (folding, faulting, active deformation) and seismicity 

The Seymareh rock avalanche initiated by a massive failure of a segment of the northeast flank of the Kabir Kuh anticline (Harrison and Falcon, 1937;1938). Kabir Kuh is located in the actively-deforming Zagros fold-thrust belt (Figure 2 in main text), also known as the Simple-Folded Zone (e.g., Falcon, 1974)), which constitutes the foreland belt of the Zagros Orogen. Topography of the fold-thrust belt is dominated by the surface expression of deformation in the 8-12 km thick sedimentary cover of the Arabian Plate driven by the continental collision of Arabia with Eurasia (e.g., Falcon, 1974; Blanc et al., 2003; McQuarrie, 2004; Alavi, 2007; Mouthereau et al., 2012). 

Ductile folding of the cover rocks has led to the formation of spectacular fold trains characterized by resistant limestone “whale-back” anticlines that dominate the geomorphology of the belt (Figure 3 in text; Oberlander, 1965; Sattarzadeh et al. 2002; Burberry et al. 2008; Tavani et al., 2011)). The structural grain of the Zagros, developed parallel to the plate margin, is broadly NW-SE (Figure 2 in main text). The topography of the Zagros fold-thrust belt has been dissected by contemporaneous fluvial erosion since the Tertiary (Oberlander, 1965, 1985; Burberry et al., 2008). 

The Zagros folds have average amplitudes of 1-10 km and wavelengths of 5-20 km (Falcon, 1974). Anticlinal hinges are generally parallel to the structural trend of the belt, but are frequently offset in an en-echelon manner, suggestive of segmentation by transverse NE-SW trending belt-perpendicular faults. Folds are typically asymmetric, with axial planes that generally dip at more than 60° NE (Falcon, 1974); thus, with few exceptions, the steepest limbs of the Zagros anticlines are SW sides of the folds. 

Faulting within the sedimentary cover is dominated by northeast-dipping belt-parallel reverse faults (Figure 2 in main text). Some faults that reach the surface run nearly the entire length of the belt or form fault systems that do (Figure 2 in main text).  N-S belt oblique transverse faulting is concentrated in the central and northwest of the Zagros fold-thrust belt (Tatar et al. 2002) due to the accommodation of oblique plate convergence by right-lateral shear (Stephenson et al., 2007; Tatar et al., 2004).  

Evidence for active surface deformation of the Zagros fold-thrust belt is found in the deformation of ancient archeological features (Lees and Falcon, 1952), deformation of Holocene terrace gravels (Oberlander, 1965), active faulting (Talebian and Jackson, 2002; Bachmanov et al., 2004), GPS-measured surface movement (Tatar et al., 2002; Hessami et al. 2006) and shallow earthquake activity in the sedimentary cover (Talebian and Jackson, 2004) related to present- day thrust faulting. In the Lorestan fold zone, in the vicinity of Kabir Kuh, GPS data suggests that convergence is 18 mm/yr to the north, oblique to structural trend of the belt (Navabpour et al. 2008).

The Zagros fold-thrust belt is one of the most seismically active regions of the world (Ambraseys and Melville, 1982; Talebian and Jackson, 2004; Engdahl et al., 2006) with frequent earthquakes above M6.0 (Figures 2B). Earthquakes generated by Arabia-Eurasia collision occur within the sedimentary cover and crystalline basement (Enghdal et al. 2006). Analysis of recent earthquake data (1957-2000) from the Zagros fold-thrust belt by Talebian and Jackson (2004) indicates that present day reverse faulting is relatively restricted to lower topography (500-1000 m a.s.l.) on the southwest edge of the belt with strikes parallel to the structural trend and fold axes. This area of active faulting, located towards the foreland margin, also corresponds to the area containing the hypocenters of the larger recent earthquakes (M > 5.3) occurring in the belt (Figure 2B).

Regional stratigraphy of the sedimentary cover 

The sedimentary cover in the Lorestan region is a multi-layer Cretaceous and Tertiary succession of interbedded resistant carbonate units (characterized by the Asmari Limestone, a major ridge-forming and petroleum reservoir unit) and weaker marls and shales (Figure 5 in main text; James and Wynd, 1965; Alavi, 2004). The geotechnical kinematic contrast between these two rock mass groups has important implications for the geomechanical behavior of both near surface rock masses (in response to tectonic forcing – Colman-Sadd, 1978; Sattarzadeh et al., 2002; Sepehr et al., 2006; Alavi, 2007; Farzipour-Saein et al., 2009) and surface rock slopes (in response to erosion – Harrison and Falcon, 1934, 1936; McQuillan, 1973, 1974; Burberry et al. 2008).

The Sarvak Formation ( ~ 725 m), known in earlier work as the Bangestan Limestone (James and Wynd, 1965), is a thin-bedded fine-grained argillaceous limestone with marl partings, that grades upward into more thickly bedded limestone (James and Wynd, 1965; Figure 5).  Above, the Surgah Formation (~ 100 m) is composed of shale with subordinate fine-grained, thin-bedded limestone and has disconformable contacts with the underlying and overlying formations.  The Ilam Formation (~ 150 m), which is a regular bedded, fine-grained argillaceous limestone with thin fissile shale partings and is conformably overlain by the Gurpi Formation (~ 475 m). The Gurpi  is a  tan- to blueish grey-weathering marl and shale with subordinate marly limestone; its only feature-forming unit is the Emam Hasan Member is (~ 110 m), a white-weathering marly limestone with marl interbeds (Figure 5 in main text).  

The Pabdeh Formation (~ 525 m) mostly consists of gray calcareous shales and marls with subordinate limestone interbeds. Its lower contact with the Gurpi is disconformable and marked by the base of the informal purple shale member, an important marker in field mapping.  It grades upward into the resistant well jointed cream-weathering limestone of the Asmari Formation (~ 225 m) (Figure 5 in main text).  

The Miocene Gachsaran Formation (~ 2000 m) conformably overlies the Asmari Formation (Figure 5 in main text). In the study area, nearly all the Gachsaran has been removed by erosion, exposing the Asmari Formation on the surface of the anticlines. but is preserved in synclines.  The lowermost member (~ 40 m), composed of alternating thick-bedded anhydrite and thin-bedded limestone, is still in contact with the Asmari along the toes of the anticline flanks.

Physiography

Tectonic structural elements and stratigraphy together determine physiography in the Zagros fold-thrust belt, since destruction by surficial faulting and erosion is relatively limited.  Folds are commonly dissected by transverse drainage incised perpendicular to their fold axes since the Tertiary (Oberlander, 1965, 1985; Burberry et al., 2008).  The resistant Asmari Limestone carapaces of most anticlines are largely preserved, imparting a ‘whale-back’ form to each (Figure 3 in main text).  Anticlines with breached carapaces expose cores of underlying units, as in the case of Kabir Kuh. Synclines preserve Tertiary units overlying the Asmari (Figure 3 and 5 in main text) as well as Quaternary sediments. The gypsiferous Gachsaran Formation (Figure 5) predominates in many synclinal valley bottoms where it is commonly rilled and karstified to form badlands exhibiting up to 100 m of relief.

Superficial gravity collapse structures 

Gravity collapse structures, formed in response to the Quaternary erosion of the Zagros folds, are described by Harrison and Falcon (1934, 1936) and are important to consider in examining the initial failure mechanism of the Seymareh rock avalanche. The structures develop on the limbs of anticlines where resistant limestones, most commonly the Asmari Formation, forming the surface slope of the anticline, cap less-resistant units dominated by shales and marls. The structures form by the ductile deformation of the subjacent less-resistant unit and the subsequent deformation and downslope transport of the overlying limestone cap. A prerequisite for the formation of these features appears to be the breaching of the axial zones of anticlines by erosion (Harrison and Falcon, 1934). 

A variety of structures have been described by Harrison and Falcon (1934, 1936). Of relevance here are the structures that develop when the resistant Asmari overlies the less resistant marl and shale units of the Pabdeh and Gurpi Formations (Harrison and Falcon (1936)) leading to the development of a “knee fold”. In this process, downslope movement of the Asmari cap on the subjacent ductile Pabdeh and Gurpi, forms a knee fold. The fold may then break through the Asmari carapace along a low-angle thrust fault surface. This process forms a sheet of Asmari limestone, which Harrison and Falcon (1936) term a “slip sheet”, which is then free to move downslope over that part of the same Asmari limestone unit which has remained in place. As a result of this process the subjacent ductile units are disaggregated, the Asmari exhibits steep irregular dips in the vicinity of the knee fold, and if the slip sheet continued its movement downslope, it would overly the younger Gachsaran Formation in the valley bottom (see Figure 2 in Harrison and Falcon, 1936). No highly ductile evaporite units are involved in this particular process since no evaporites are present in the Pabdeh and Gurpi Formations. The geomechanics and the rates of formation of the Zagros gravity collapse structures have not been investigated in detail, particularly with respect to the mechanism of strength loss in the ductile units (cf. Morley and Guerin, 1999). 

B.	Re-analysis of the volume of the Baga Bogd rockslide, Mongolia, using SRTM and LANDSAT data

The volume of the gigantic Baga Bogd rockslide, Mongolia was reported by Philip and Ritz (1999) as 50 Gm3. We suggest that the volume was overestimated by Philip and Ritz (1999) because of overestimation of the debris area.  The area they report (300 km2) is more extensive than even that defined by the maximum dimensions of the debris (ca. 14 km wide by 15 km long; ~210 km2) (Figure B1).

Considering a more accurate debris area (180 km2) measured from an orthorectified Landsat ETM+ image (Figure B1) and the maximum deposit thickness reported by Philip and Ritz (1999) (200 m), the volume of the rockslide cannot exceed 37 Gm3.  Consideration of the mean deposit thickness (~ 167 m; Philip and Ritz, 1999) yields a more conservative volume estimate of 30 Gm3.

Thus, based on the post-failure volume determined in this work (44 Gm3), the Seymareh rock avalanche is larger than the Baga Bogd rockslide, making it the largest rock slope failure and the largest sub-aerial non-volcanic landslide on the present topographic surface of the Earth.

C.	Methods
Geometrics of the Seymareh rock avalanche (Table 1 in main text) were determined in GIS from orthorectified Landsat imagery (horizontal distances), SRTM-3 DEM (heights and thicknesses), and GPS-controlled field traverses using simple geometric calculations (area, volume, mobility) outlined by Roberts (2008).  Remotely-sensed datasets included Landsat-7 ETM+ imagery (30 m multispectral, 15 m panchromatic), SPOT-5 imagery (10 m multispectral), small-scale (1: 60, 000) aerial photographs and a digital elevation model (DEM) derived from Shuttle Radar Topographic Mission (SRTM-3) (3 arc second) data (see Farr et al. (2007) for mission details).  
Accuracy is greatest for measured horizontal distances.  Estimation of thickness of masses with only one visible surface introduced the greatest errors. In general, accuracy is highest for dimensions measured directly from remotely sensed datasets or GPS surveys.  Errors increase with the incorporation of additional calculation and estimation steps due to the cumulative effects of possible errors. We estimate errors to be as follows; elevations (± 10 m), heights (± 20 m), horizontal distances (± 20 m), distances along dipping surfaces (± 30 m), areas (± 5 km2), volumes (± 1 Gm3). The absolute vertical errors for SRTM-3 data in the Zagros region is generally less than 10 m (see Figure 14 in Farr et al. 2007).

D.	Modification of the Geological Strength Index (GSI)  for the geologic sequence of Kabir Kuh

The Geological Strength Index (GSI) provides quantitative estimations of rock-mass strength based on observed lithology, structure and discontinuity surface condition (Marinos et al., 2005).  Hoek et al.’s (2005) extension of the GSI for tectonically undisturbed rock-masses (Figure 10 in Hoek et al., 2005) was adapted for the moderately disturbed limestone-mudrock sequence present in Kabir-Kuh.  Because it was developed for stratigraphically variable classic sedimentary lithologies (mainly conglomerates, sandstone, siltstone and shale), Hoek et al.’s (2005) extension of the GSI considers rock composition and structure similar to those present in the limestone-mud rock sequences of Kabir Kuh (Asmari to Sarvak Formations in (Figures 4 and 10 in main text).  The resulting modified GSI chart (Figure D1) was used to approximate relative strength values for units of the failed sequence of the Seymareh rock avalanche, which are represented in Figure 10 in the text.  Because of anisotropy of the rock-mass, the GSI was not used for stability assessment (cf. Marinos et al., 2005).

The overall form of Hoek et al.’s (2005) GSI was maintained.  Competent (conglomerate / sandstone) and incompetent (siltstone / shale) units described by Hoek et al. (2005) were substituted with competent (limestone) and incompetent (shale) units, respectively, present in Kabir Kuh. Rows describe three types of units: non-interbedded competent units (strongest), competent-incompetent interbedded units (intermediate strength) and non-interbedded incompetent units (weakest).  Hoek et al. (2005) described only a single class for non-interbedded competent units.  Due to the variation of strength of competent (predominantly limestone) units in the sequence of Kabir Kuh, however, the modified GSI (Figure D1) differentiates between three classes of non-interbedded competent unit (K1 – K3) on the basis of bedding thickness.  Classes defined for competent-incompetent interbedded units (K4 – K6) and non-interbedded incompetent units (K7) closely follow Hoek et al. (2005).

Columns describe surface conditions of discontinuities, which are usually bedding surfaces.  Surface conditions of discontinuities are largely affected by composition and structure and, consequently, generally decrease (from very good to very poor) with weakening rock unit (from class K1 to K7).  Zones in the ‘composition/structure-surface condition’ space represent the strength of each class and broadly follow positions of Hoek et al. (2005).  Midpoints of each zone were used to quantitatively represent the relative rock-mass strength differences between units (Figure 10 in main text).

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FIGURE CAPTIONS FOR SUPPLEMENTARY DATA

Figure B1. LANDSAT ETM+ scene of the Baga Bodg landslide, Mongolia, showing general morphology (outlined) and approximate maximum dimensions of the debris.  Image is a false color composite (band 4, red; band 3, green; band 2, blue).

Figure D1. Modified GSI for the limestone-mudrock sequence of Kabir Kuh, modified from Hoek et al. (2005).