Introduction
On February 6, 2023, two massive earthquakes with magnitudes of 7.8 and 7.5 struck southern Turkiye and northwestern Syria. These earthquakes were felt in many neighboring countries and caused widespread casualties and destruction in the two countries of Turkiye and Syria. Following this events a team from International Institute of Earthquake Engineering & Seismology (IIEES) and RCECWA visited the earthquake affected areas in Turkiye and assessed different aspects of the earthquakes in order to outline the lessons from that events. A comprehensive report (in Persian) can be found at IIEES website (www.iiees.ac.ir). Here, a brief summary is presented in English.
Seismological Aspect
The first earthquake (Mw =7.8, Depth ≈18Km, Local Time = 04:17), centered about 35 Km northwest of Ghaziantep city was the strongest and most destructive earthquake in Turkiye since 27 December, 1939 Erzincan (North-East of Turkiye) earthquake, which had a similar magnitude, with the loss of around 33,000 lives. The resulting surface rupture can be followed for 200km (Fig. 3). This earthquake was followed by a shallow depth aftershock with 6.7 magnitude, 11 minutes later. 9 hours later (at 13:24 local time) the second earthquake (Mw =7.5, Depth ≈10Km) was occurred at a distance of 100Km north-east of the first one, near the Elbistan city on another faults.
The 6th February 2023 earthquakes originated on East Anatolian fault system, a large fault system with 700 Km long northeast-southwest left lateral transform fault (Fig. 1), which forms the boundary between the Anatolian and Arabian plates and accommodates the westward extrusion of Turkiye into the Aegean Sea, with a relative movement of about 10 mm per year (Aktug et al., 2016).
Geotechnical impacts and Geo-hazards
Local geological and geotechnical conditions can lead to variation in earthquake ground motion characteristics (amplitude, frequency content, duration). This phenomenon that is known as the site effect seems to have an important role in destructive power of Turkiye earthquakes (Fig. 2). On the other hand, these earthquakes triggered many other geological hazards, such as the slopes instabilities (landslide), soil liquefaction and lateral spreading, which directly and indirectly increased the destructive effects of these earthquake (Fig. 3). The most important landslides triggered by these earthquakes are the Tepehan and Islahiye landslides (Fig 4.). Soil Liquefaction was the most important geohazard triggered by these earthquake and resulted in sever damages to Hatay airport, roads, urban facilities and buildings in many affected cities like, Iskenderun, Dorthiol, Samandagh and especially Golbashi (Fig. 5). Also, in this earthquake, some geotechnical structures such as earth dams, retaining walls and road embankments were seriously damaged (Fig. 6)
Structural damages
The earthquakes of February 6, 2023 in Turkiye led to the destruction of a large number of buildings, even newly constructed. Investigation of the causes can be effective in preventing damages in future earthquakes. Some of the most important reasons from the structural engineering point are shown in figure 7 and can be listed as fallow:
- Material defects:
- Concrete poor quality (low resistance) (Fig7)
- Application of simple rebar
- The use of non-standard rebar (non-ductile)
- Implementation defects
- Improper concrete density
- Insufficient thickness concrete cover
- Obvious defects of reinforcement
- Using 90 degree hooks in stirrup
- Irregular and inappropriate spacing of stirrup in the critical area
- Failure to bend the ends of longitudinal rebar
- Absence of stirrup in the beam-column connection points
- Design defects
- Using rectangular columns without necessary preparations
- Use of heavy ceilings
- Excessive changes in hardness and resistance at altitude
- Non-embedding of interruption seam
- Strong beam – weak column behavior
- Shear failure in column and wall
- Poor performance of stairs
- Not complying with the rules of implementing wall posts and separating them
Fig. 1. Tectonic setting of the East Anatoly Fault Systems (EAFS (from Güvercin et al (2022). Thick and thin red lines show the East and North Anatolian Faults, respectively. Secondary faults are shown by thin black lines. Black arrows on the fault traces indicate the relative motion of the faults. White arrows indicate the GPS velocities with Arabia fixed reference frame
Fig. 2. Resonance frequencies estimated for some strong motion stations in earthquake affected region, show the potential of ground motion amplification (site effect)
Fig. 3. Surface Rupture and ground instabilities (landslides, liquefactions, lateral spreading and embankment dams failure) detected in February 6, 2023 Turkiye Earthquakes
Fig. 4. Example of slope instabilities during Feb. 6, 2023 Turkiye earthquakes: A) Tepehan Landslides, B) Islahiye Landslides.
Fig. 5. Example of damages caused by soil liquefaction, A) Anslope instabilities during Feb. 6, 2023 Turkiye earthquakes: A & B) Antakya- Reyhanli road; C, D, E & F) tilting and sinking of buildings in city of Golbashi.
Fig. 6. Example of failures in retaining wall and embankment dam during Feb. 6, 2023 Turkiye earthquakes: A) Marash- Malatya road, B) Sultansuyu Dam.
Fig. 7. Example of improper construction of reinforced concrete building. A & B) Concrete poor quality; C) simple rebar; D) Brittle rebar; E&D) Insufficient thickness concrete cover
