The purpose of this article is to examine the Chaman transform fault by extensive literature review and utilizing the open-source data for making maps for historical earthquake events, digitizing faults, and understanding the stresses, seismology, segmentation, and recurrence period.

Since 1980, more than 10000 people died due to earthquakes in Afghanistan, and the country estimates yearly losses of $80 million (Disaster Risk Profile, Afghanistan 2017). In Pakistan, the Mw 7.6 earthquake in 2005 killed 86000 people and left 4 million without homes (USGS). The location of the western boundary between the Indian and the Eurasian plates is in Afghanistan and Pakistan territories, where the tectonic movement is accommodated with left-lateral strike-slip fault systems. This system consists of the Ornach-Nal fault, the Ghazaband, fault, and the Chaman fault (Riaz et al., 2019). The Chaman fault segment is responsible for accommodating as much as 33% of the total movement between the two plates (Barnhart, 2017). For that reason, It is among one of the deadliest strike faults in the world, with registered events since the XVI century. The current deformation of the Chaman transform fault is dispersed among the Chaman, Ghazaband, Hoshab, and Ornach-Nal faults rather than being constrained along a single fault.
The Chaman Fault System is a left-lateral transform boundary between the Indian and Eurasian plates notable for its minimal seismicity, which has been attributed to either a lengthy recurring period of major earthquakes or aseismic creep (Ambraseys & Bilham, 2003). The Chaman fault system is around 1200km long, and the main fault is approximately 650km long. It originates in the Makran subduction zone (Ahmed et al., 2021) where the Arabian plate subducts under the Eurasian plate, in southwest Pakistan near 28.3ºN, and extends to the north through Afghanistan to approx. 35ºN near the country’s capital, Kabul.

Geological basics, seismology, and segmentation

In northern Pakistan, transpression occurs as a result of direct collision along the Chaman fault (Farah et al., 1984). The lobate Sulaiman fold belt southeast of the Chaman fault was formed as a result of this oblique convergence (Sarwar & DeJong, 1979). Micro fragments that were separated and drifted from Gondwana and accreted to the Eurasian landmass from the late Paleozoic to Cenozoic eras are thought to make up the Afghan block, located northwest of the Chaman fault (Boulin, 1981). The western boundary of the Indian subcontinent is marked by the left-lateral strike-slip Chaman fault. The transform zone’s width varies significantly. The Ornach Nal fault is around 100 kilometers wide in the south and west. It narrows to 25-40 kilometers from Khuzdar to just north of Quetta. It widens dramatically to about 200 kilometers in the Zhob belt, then progressively narrows to roughly 175 kilometers around Kabul (Lawrence et al., 1981). For the slope map of Baluchistan, The Google Earth Engine (GEE) is used to acquire the SRTM DEM (30m resolution). The selection of GEE over USGS Earth explorer is that several DEM scenes were required for covering 347,190 km² of Baluchistan area and the post computation of DEM in ArcMap was also time-consuming with lower computation power and no hard disk space.

Study area map illustrates the Chaman transform fault system along with historical earthquakes
The tectonic framework of the Chaman transform fault system (modified after (Mohadjer et al., 2010)) is illustrated over SRTM DEM.

(Ul-Hadi et al., 2013) detected a geomorphic slip rate of 33.3 mm/yr along a strand of the Chaman fault between 30-31°N, which is about double the geodetically calculated rate of 18 mm/yr (Mohadjer et al., 2010). The difference between geomorphic and geodetic slip rates in this location might be due to the area’s relatively low viscosity, as evidenced by the existence of a 20–25 km thick crystalline crust underneath the Chaman fault (Jadoon & Khurshid., 1996). The majority of the activity is expected to occur in the central section of the Chaman fault system, with a slip rate of 49.1 mm/yr inside a 60-kilometer zone, whereas the slip rate in the north and south of this zone is estimated to be 3.1 mm/yr and 0.5 mm/yr, respectively. (Ambraseys and Bilham., 2003) estimated a slip deficit in a 60 km broad western plate boundary zone based on the century’s seismic record, which might produce one or more Mw >7 events.

(Ahmed et al., 2021) calculates the b-value and seismic moment release of the Chaman transform fault and compared it with the slip rates. In the northern (i.e., >200 km) and southern (i.e., between 25 and 100 km) portions of the Ghazaband fault, the b-value is estimated to be about 1.2, which presumably indicates partly creep sections that are unlikely to create major earthquakes. Similarly, between 25 and 100 km, the southern Ornach-Nal fault line has a larger b-value > 1.5, which confirms the fault’s creeping nature. In the zone where prior large earthquakes occurred, b-values range from 0.65 to 1.0, but the Chaman fault’s partial creep section between 110 and 140 km has a b-value of 1.68, and the creeping stretch between 210 and 310 km has a b-value of 2.0. Furthermore, the center part of the Ornach-Nal and Ghazaband faults have b-values ranging from 0.7 to 1.0. The lower b value also indicates the higher stresses. The higher slip rate of approximately 16 mm/yr for Ghazaband fault as compared to approximately 8 mm/yr for the Chaman fault Fattahi et al, 2016 also indicates that the Ghazaband fault section of the Chaman transform fault system is one of the most hazardous faults of the plate boundary zone. These results can also be inferred from the figure, where a higher number of seismic events occurred along the fault segments suggested by all the authors above.

Earthquake historical events and recurrence

There are five earthquakes of magnitude 6.5 or bigger registered related to the Chaman fault since 1892, as (Riaz et al., 2019) researched in their paper, but there is also one other important event around 400 years before. The Ghazaband Fault threatens Quetta, the capital of Baluchistan province, which was devastated by an earthquake in 1935, killing 35,000 people (Ambraseys & Bilham, 2003). The northern Chaman Fault threatens Kabul on the Paghman fault, which was devastated by an earthquake of 7.3 Mw on July 5, 1505(Ambraseys & Bilham, 2003). The earthquakes of 1975 and 1978 are inferred to have ruptured the Chaman fault. On the 24th and 28th of September 2013, two massive earthquakes with magnitudes of 7.7 and 6.8 hit the Awaran area of Baluchistan, respectively. The 1505 earthquake produce 40 to 60 km of surface rupture(Ambraseys & Bilham, 2003; Quittmeyer & Jacob, 1979). The one in 1892 generate as much as 75 cm of left-lateral movement and drop the left side of the fault by around 30 cm (Quittmeyer & Jacob, 1979).

The most significant trapped Chaman Segment is 95 kilometers long and was the site of the 6.5 Mw earthquake in 1892 (Ambraseys & Bilham, 2003). A GPS transect covering the Chaman Segment revealed slip rates of 8.5 mm/yr and a locking depth of 3.4 km, according to (Szeliga et al., 2012). With these factors, they predicted a 100-year recurrence period for the 1892 earthquake. (Barnhart, 2017) results collaborate this conclusion: a 95 km 3.4 km locked fault with an 8.5 mm/yr slip deficit would produce one M6.6 earthquake every 100 years, therefore, being in the late part of its seismic cycle. As a result, a comprehensive probabilistic seismicity hazard assessment is needed for future research and safety measures.

The total of 1982 historical earthquakes for Baluchistan is compiled from the open-source United States Geological Survey (USGS), Pakistan Meteorological Department (PMD), and National Geophysical Datacenter (NGDC).

Conclusions and hazard assessment

The seismic hazard reported by (Waseem et al., 2020) is high for cities like Quetta and Chaman with PGA of 0.29 and 0.27 respectively corresponding to a recurrence period of 475 years. The building code of Pakistan 2007 PGA values depict the higher PGA values where the historical seismic activity is dominant (Figure 1). (Shnizai, 2020) suggests that an earthquake of magnitude 7.3, like the one in 1505, could have devastating human losses and building destruction, in a 4 million population city like Kabul. The World Bank assumes, in a report that analyzes the fiscal disaster risk assessment for Pakistan, that if an earthquake like the one in 2005 (Mw 7.6) happens nowadays, it would generate property losses of around US$2.8 billion. Even though, it is a matter of consideration that after the implementation of the Seismic Provisions from the Building Code of Pakistan (2007), the number of better infrastructures would lower the actual losses (Kim et al., 2020).

Such a number of casualties and losses demonstrates the weak and improvised mud-blocked housing that is frequent in rural areas. More precisely, (Ahmed et al., 2021) shows in 2012, that around 20% of buildings in Pakistan are made of Adobe masonry or are reinforced with wood or bamboo. Infrastructure destruction is not the only hazard in the region. The coseismic landslides are very likely in a moderate to high relief area near the Chaman fault. The place with more hazards due to possible earthquakes from the Chaman fault is Kabul city.