Seismic tomography of the area of the 2010 Beni-Ilmane earthquake sequence, north-central Algeria

Issam Abacha 0 Ivan Koulakov 1 2 Fethi Semmane 0 Abd Karim Yelles-Chaouche 0 0 Centre de Recherche en Astronomie, Astrophysique et Geophysique , BP. 63, Bouzareah, Alger , Algeria 1 Novosibirsk State University , Novosibirsk , Russia 2 Trofimuk Institute of Petroleum Geology and Geophysics, SB RAS , Prospekt Koptyuga, 3, Novosibirsk 630090 , Russia The region of Beni-Ilmane (District of M'sila, north-central Algeria) was the site of an earthquake sequence that started on 14 May 2010. This sequence, which lasted several months, was triggered by conjugate E-W reverse and N-S dextral faulting. To image the crustal structure of these active faults, we used a set of 1406 well located aftershocks events and applied the local tomography software (LOTOS) algorithm, which includes absolute source location, optimization of the initial 1D velocity model, and iterative tomographic inversion for 3D seismic P- and S-wave velocities (and the Vp/Vs ratio), and source parameters. The patterns of P-wave low-velocity anomalies correspond to the alignments of faults determined from geological evidence, and the P-wave high-velocity anomalies may represent rigid blocks of the upper crust that are not deformed by regional stresses. The S-wave low-velocity anomalies coincide with the aftershock area, where relatively high values of Vp/Vs ratio (1.78) are observed compared with values in the surrounding areas (1.62-1.66). These high values may indicate high fluid contents in the aftershock area. These fluids could have been released from deeper levels by fault movements during earthquakes and migrated rapidly upwards. This hypothesis is supported by vertical sections across the study area show that the major Vp/Vs anomalies are located above the seismicity clusters. - Introduction Northern Algeria has been the location of many destructive earthquakes, including the recent 21 May 2003 strong (Mw6.9) event at Boumerdes (e.g., Yelles et al. 2004). Several recent moderate seismic events have also occurred in this region, such as the 2006 Lalam earthquake (Mw5.2) (Beldjoudi et al. 2009) in the Kherrata fault system north of Setif and the 2006 Tadjena earthquake (Mw5.0) (Beldjoudi et al. 2011). In addition, in 2010, an earthquake sequence occurred in the region of Beni-Ilmane (Yelles et al. 2013), a small village located in the southern part of the Tellian Atlas and about 200 km southeast of Algiers (Figure 1a). This earthquake sequence caused 3 fatalities and injured 170 people, and generated substantial damage to houses and local infrastructure in the epicentral area. An earthquake sequence such as the 2010 Beni-Ilmane sequence is uncommon in Algeria in several respects. First, the sequence consisted of three main shocks of very similar magnitudes (5.0 < Md <5.2) over a period of nine days in a localized area. Second, the sequence was characterized by strong aftershock activity (more than 24,000 events were recorded by a temporary seismic network during the first week of the sequence) that lasted for several months. Third, the earthquake sequence occurred in a region of lower seismic activity in northern Algeria, and more precisely between two geological transition zones; namely, the Bibans-Hodna Mountains and the Tellian Chain High Plateaus (Figure 1a). Prior to the Beni-Ilmane sequence, the study region was characterized by low seismic activity, although some historical events have occurred here. The most notable event occurred close to the epicenter of the 2010 Beni-Ilmane event on 21 February 1960, and was of magnitude M5.6. This event generated similar macroseismic Figure 1 Seismo-tectonic context of the 2010 Beni-Ilmane seismic sequence (from Yelles et al. 2013) with the seismic networks. The brown rectangles represent the drillings in the region and some among them attained around 4000 m of depth. The red dashed box shows the tomography area shown in Figures 3, 4 and 6. effects to those of the Beni-Ilmane event, and its maximum intensity was VIII (MSK) (Benouar 1994). To understand the crustal structure in the Beni-Ilman region, and to enhance the accuracy of source locations there, we performed a tomographic inversion that uses the simultaneous determination of P- and S-wave velocity distributions as well as the relocation of aftershocks. Local earthquake tomography (LET) is a useful tool for imaging lateral heterogeneities in the upper crust based on the relationship between the pattern of P- and S-wave velocity anomalies and the clustering of aftershocks along active fault zones. LET has been used for studying the distribution of seismic velocity in several major fault systems, including the San Andreas Fault in the western US (e.g., Chiarabba and Amato 1994; Dorbath et al. 1996; Thurber et al. 1997) and the North Anatolian Fault in Turkey (e.g., Salah et al. 2007, 2011; Koulakov et al. 2010; Yolsal-evikbilen et al. 2012). Tomography studies in Algeria have been implemented mainly for studying aftershocks at three sites where the greatest number of large earthquakes have been recorded, namely in the region of the 1980 El-Asnam earthquake (Chiarabba et al. 1997), the region of the 1985 Constantine earthquake (Bounif et al. 1998), and the epicentral area of the 2003 Boumerdes earthquake (Ayadi et al. 2008). The seismic tomography of the El-Asnam region (west-central Algeria) shows a complex preexisting structure along which the evolution of a rupture is controlled by the heterogeneity of the crust (Chiarabba et al. 1997). Seismic tomography of the Constantine earthquake source region (eastern Algeria) has yielded the 3D P-wave velocity to depths of 12 km. The resulting seismic model revealed the locations of possible fault planes and a segmentation of the aftershocks into three ruptured segments (Bounif et al. 1998). For the Boumerdes earthquake region (offshore Algeria), the high-resolution study of aftershocks and velocity structure helped to constrain the fault geometry and structure (Ayadi et al. 2008). Seismotectonic context In northern Algeria, seismic activity resulting from the convergence of the African and Eurasian plates is concentrated mainly in the Tellian chain, where moderate to large seismic events have occurred. The most seismically active region is the coastal part of the chain along the junction with the offshore part. Many large events have been recorded in this region, including the most recent earthquake in Boumerdes in 2003, and those documented in historical catalogues, such as the Oran event of 1790, the Algiers events of 1365 and 1716, and the Djidjelli event of 1856. In the western and central parts of the country, seismicity is related to NESW reverse faults or NWSE conjugate faults, which may be explained by regional compression due to the convergence of Africa with Eurasia. In the eastern part of the country, strike-slip faults are more common, as are found, for example, in the Mila and Guelma basins. The Beni-Ilmane sequence took place in a tr (...truncated)