Abstract:

On February 6, 2023, two destructive earthquakes struck southern and central Turkey and northern and western Syria. The epicenter of the first event(M_W7.8)was 37km west-northwest of Gaziantep. The earthquake had a maximum Mercalli intensity of Ⅻ around the epicenter and in Antakya. It was followed by a M_W7.7 earthquake nine hours later. This earthquake was centered 95km north-northeast from the first one. There was widespread damage and tens of thousands of fatalities. In response to these catastrophic events, in March 2023, a seismic scientific expedition led by China Earthquake Administration(CEA)was promptly organized to investigate the surface ruptures caused by these earthquakes. Here, we focus on the surface ruptures of the second earthquake, known as the Elbistan earthquake. The post-earthquake field survey revealed that the Elbistan earthquake occurred on the East Anatolian fault zone's northern branch(the Cardak Fault). This event resulted in forming a main surface rupture zone approximately 140km long and a secondary fault rupture zone approximately 20km long, which is nearly perpendicular to the main rupture.

We combined the interpretation of high-resolution satellite imagery and geomorphic investigations along the fault to determine the fault geometry and kinematics of the second earthquake event. The Elbistan earthquake formed a main surface rupture zone approximately 140km long, which strikes in an east-west direction along the Cardak Fault. The main rupture zone starts from Göksun in the west and extends predominantly eastward until the western end of the Sürgü Fault. It then propagates northeast along the southern segment of the Malatya fault zone. The entire Cardak Fault and the Malatya fault zone's southern segment are considered seismic structures for this earthquake. The overall surface rupture zone exhibits a linear and continuous distribution. Secondary ruptures show a combination of left-lateral strike-slip or left-lateral oblique-thrust deformation. Along the rupture zone, a series of en echelon fractures, moletracks, horizontal fault striations, and numerous displaced piercing markers, such as mountain ridges, wheat fields, terraces, fences, roads, and wheel ruts, indicate the predominance of pure left-lateral strike-slip motion for most sections. The maximum measured horizontal displacement is(7.6±0.3)m. According to the empirical relationship between the seismic moment magnitude of strike-slip faulting earthquakes and the length of surface rupture(SRL), a main rupture zone of 140km in length corresponds to a moment magnitude of approximately 7.6. Based on the relationship between the seismic moment magnitude and the maximum coseismic displacement, a maximum coseismic displacement of(7.6±0.3)m corresponds to a moment magnitude of about 7.5. The magnitudes derived from the two empirical relationships are essentially consistent, and they also agree with the moment magnitude provided by the USGS. Besides the main surface rupture zone, a secondary fault rupture zone extends nearly north-south direction for approximately 20km long. Unfortunately, due to the limited time and traffic problem, we did not visit this north-south-trending secondary fault rupture zone.

According to the summary of the history of earthquakes, it is evident that the main surface rupture zone has only recorded one earthquake in history, the 1544 M_S6.8 earthquake, which indicates significantly less seismic activity compared to the main East Anatolian Fault. Moreover, the “earthquake doublet” will inevitably significantly impact the stress state and seismic hazard of other faults in the region. Seismic activity in this area remain at a relatively high level for years or even decades to come. The east-west striking fault, which has not been identified on the published active fault maps at the western end of the surface rupture zone, and the north-east striking Savrun Fault, which did not rupture this time, will experience destructive earthquakes in the future. It remains unknown why the east-west striking rupture did not propagate to the Sürgü Fault this time. More detailed paleoearthquake studies are needed to identify whether it is due to insufficient energy accumulation or because this section acts as a barrier. If the Sürgü Fault, about 40km long, was to rupture entirely in the future, the magnitude could reach 7 based on the empirical relationship.

Considering the distribution of historical earthquakes along the East Anatolian fault zone, as well as the geometric distribution of the surface ruptures from the recent “earthquake doublet” and the surrounding active faults, it is believed that the future earthquake hazards in the northeastern segment of the East Anatolian fault zone, the northern segment of the Dead Sea Fault, and the Malatya Fault deserve special attention.

Key words: 2023 M_W7.5 Turkey Elbistan earthquake, Surface rupture, Maximum seismic slip, Cardak Fault, East Anatolian Fault

摘要：

2023年2月6日土耳其东南部发生2次破坏性地震, 中国地震局土耳其地震科学考察队对2次地震的地表破裂开展了详细调查。 震后现场调查表明, 其中第2次地震事件(Elbistan地震)发生在东安纳托利亚断裂带的北分支断裂(Cardak断裂)上, 形成了一条长约140km的主地表破裂带, 同时形成一条与主破裂带近垂直展布、 长约20km的分支破裂。 主破裂带西起Göksun, 沿近EW向展布至Sürgü断裂西端, 然后向NE传递至Malatya断裂带南段, Cardak断裂全段和Malatya断裂带南段为此次地震的发震构造。 地表破裂带总体呈线性连续展布, 各次级破裂呈左旋左阶拉张或左旋右阶挤压的雁列式组合, 沿线一系列冲沟、 山脊、 麦田、 田埂、 栅栏、 道路和车辙等位错标志指示断层以纯左旋走滑运动为主, 野外实测最大水平位错为(7.6±0.3)m。 结合东安纳托利亚断裂带上的历史地震分布和此次“双震”地表破裂与周边活动断裂的几何展布, 认为东安纳托利亚断裂东北段、 Sürgü断裂和Malatya断裂未来的地震危险性应该引起重点关注。

关键词: 2023年土耳其 M_W7.5 Elbistan地震, 地表破裂, 最大同震位移, Cardak断裂, 东安纳托利亚断裂