Active deformation, fault segmentation, scarp morphology, seismic hazard assessment and geohazards along Muzaffarabad Fault, Hazara-Kashmir Syntaxis, Northwest Himalaya, Pakistan

Mirza Shahid Baig*1, Robert S.  Yeats 2, MonaLisa 3, Shahab Pervez 1, Basit Masood 4,
Mohammad Amer Sohail 4, Ishtiaq Ahmed Khan Jadoon 5, Allah Bakhsh Kausar 6

1Institute of Geology, University of Azad Jammu & Kashmir Muzaffarabad, Azad Kashmir, Pakistan
2Oregon State University, Corvallis, Oregon, USA.
3Department of Earth Sciences, Quaid-i-Azam University, Islamabad, Pakistan
4NESPK House, 1-C Block Model Town, Lahore, Pakistan
5Department of Environmental Sciences, COMSATS Abbottabad, Pakistan
6Geological Survey of Pakistan, Islamabad, Pakistan
* Email: drshahidbaig@yahoo.com or drshahidbaig@gmail.com

The NW-trending Hazara-Kashmir syntaxis (HKS) lies in the hinterland of the Himalayan collision zone. The NW-trending Indus-Kohistan Seismic Zone (IKSZ) is associated with an active blind wedge of thick-skin reverse faults which extends obliquely across the HKS [1, 2]. The IKSZ got reactivated during the October 08, 2005 Kashmir earthquake. It has been suggested that the IKSZ traces SE to the Riasi Thrust in India and the Soan thrust in the Kangra reentrant [3, 4]. The Main Boundary Thrust (MBT), Panjal Thrust and active Himalayan Frontal Thrust (Muzaffarabad Fault) are truncated by the active left-lateral reverse Jhelum Fault along the western limb of the HKS. The MBT has been displaced 38 km along the active left-lateral reverse ramp of the Jhelum fault from the northern apex of the HKS to Kohala (Berot Nalla) in the south [5]. The Jhelum Fault in higher structural levels shows evidence for back-steepening and dips 40-800 ESE in the southern segment and NE in the northern segment, respectively. The Jhelum Fault extends south from Kohala to Azad Pattan. In this segment of the fault, the Murree Formation lies in the hanging wall and Kamlial, Chingi and Nagri Formations in the footwall. The Nagri Formation in the footwall of the fault extends north of Kohala up to Zamanabad.

The splays of the fault south of Azad Pattan within Siwaliks extend to Nara, Salgran, Shah Khaki, Thamnoa and Taraki Gujar Khan. These thin-skin faults occur in the cover sequence of the Indian plate. The active Muzaffarabad Fault, also called the Himalayan Frontal Thrust-HFT [4, 5, 6, 7, 8] or Balakot-Bagh Fault [9], extends from Allai-Kohistan to Poonch in Jammu and Kashmir. The fault was segmented as result of left or right stepping during October 08, 2005 Muzaffarabad earthquake.

The fault segments trend NW-SE and dip 35-550 NE. The rupture zone from Allai-Kohistan to Hangari is not continuous, however, the tectonic deformation expressed as linear landslides is present north of Siran valley. In this area, the fault acts as a blind reverse fault of the IKSZ. The fault segment from Balakot to Mara Tanolian Muzaffarabad is along the overturned limb of the Balakot-Muzaffarabad anticline, and juxtaposes the Cambrian carbonates of the Muzaffarabad Formation and the Miocene lower Murree Formation. The fault in this segment behaves as a thrust except in the Shawai Nalla Muzaffarabad, where, due to back-steepening, it is a SW-dipping pseudo-normal fault. The lower Murree Formation along the Tanda to Sudangali fault segment was thrust over the upper Murree and Kamlial Formations. However, in the southeast from Sudangali to Chrikot and Poonch, the fault emplaces the Murree Formation on the Chinji and Nagri Formations.

The HFT has been reactivated during the October 08, 2005 earthquake along a pre-existing active structural boundary. It shows right-stepping near Bandi Muzaffarabad, Sali-da-Katha and Lasdana areas in which it has left-lateral slip instead of its dominantly right-lateral slip. The right-lateral Muzaffarabad Thrust near Balakot splays into southern and northern segments that show 2.5 to 4 m vertical and 2 to 2.5 m right-lateral slips. The southern segment cuts the western margin of the HKS in the northwest and reactivates the MBT about 4 km in the Hangarai area. The MBT (previously called the Murree Fault) here was reactivated in the opposite sense:  northern-eastern side up whereas the MBT had that side down. The HKS was seismically not active, since the surface rupture during October 08, 2005 earthquake and zone of seismicity cut across the syntaxis. The Balakot city and Balakot tectonic ridge lie in the active hanging-wall anticline of the southern segment of Muzaffarabad Thrust. The active Balakot hanging wall anticline has been locally imbricated between the southern and the northern segments of the Muzaffarabad Thrust. The active Bandi anticline formed due to active folding of the hanging wall of the Muzaffarabad Fault. Holocene terraces are tilted, uplifted and folded in the Bandi anticline as well as in the Balakot hanging wall anticline.

The 0.5-0.75 km brittle shear zone, active linear old and new landslides, southwest tilted (30-450) trees, active alluvial fans, and cut, tilted (8-250), uplifted and folded terraces are present along the Muzaffarabad Fault. Active reverse and normal ruptures are present in the rupture zone ranging from 1 to 3 km in width. The active reverse ruptures are parallel to the trace of the fault. However, the normal ruptures are parallel to the crests of active hanging wall anticlines or the topographic front of the Muzaffarabad Fault and are the bending-moment faults. The reverse and normal ruptures trend NW-SE and dip 60-800 NE or SW. In the areas where reverse ruptures dip NE and SW, these faults form outcrop scale triangle zones. The local left stepping of right-lateral ruptures shows outcrop scale pressure ridges in Sudangali-Bagh area.

The reverse ruptures are the result of active NE-SW Himalayan compression. The normal ruptures represent the NE-SW late extension in the area. The fault scarp morphology includes vertical, folded and compression arch scarps. The folded scarps are associated with the Balakot and Bandi active hanging wall anticlines. The fault scarp morphology at places has been modified as a result of gravity collapse of the hanging wall due to strong ground shaking during the earthquake.

The horizontal shortening across the scarp measured through strain markers varies from 3 to 3.5 m. The vertical scarp height and lateral offset along the fault vary from 1.5 m to 15 m and 0.75 m to 3 m, respectively. The average vertical uplift along the Muzaffarabad Fault is 3 m [5], but it reached to a maximum of 7.5 m during the Muzaffarabad earthquake. The scarp height up to 15 m is the cumulative result of possibly multiple earthquakes in the Holocene. Historical and palaeoseimological earthquake data have not existed in Muzaffarabad area before the October 08, 2005 Kashmir earthquake for the previously known active Muzaffarabad Fault. The area is characterized by moderate to high seismicity [10]. Seismic Hazard Assessment (SHA) was carried out before the occurrence of the Kashmir earthquake using seismological data till 2004 [11]. This earlier work has been revised, incorporating all the available earthquake data till January 2007 within 50 km of Muzaffarabad. The critical tectonic features in the area have been reinvestigated to carry out SHA using deterministic approach. The fault length of 120 km from Balakot to Poonch has been considered on the basis of geological and structural data for the Muzaffarabad Fault. They assigned a maximum magnitude of 8.0 for a future earthquake at Balakot-Bagh Fault, with the peak ground acceleration (PGA) value of 0.5g as calculated with the help of the attenuation equation. MonaLisa et al [12] initially trenched and dated the Muzaffarabad Fault near Bandi (Nisar Camp) between 2000 to 2005 years and interpreted the two recent faulting events. The maximum earthquake recurrence interval for 8 magnitude earthquake along the Muzaffarabad fault based on paleoseismological data is about 1000 years. The geohazards in the area include landslides, active hanging wall anticlines, steep slopes, slope failure, active faults, back-steepen fault segments, active rupture zones, loose gouge and breccia, liquefaction, cliffs and banks of rivers. The linear active landslides along the Muzaffarabad Fault are the result of ground shaking and NE-SW late extension in the area.

Ground shaking, seismic amplification, liquefaction, slope failure, basement failure, structural collapse, structural design, old buildings, asymmetric structures and building material are the causes for the damage of civil structures and loss of more than 80,000 human lives. A detailed study of uplift rates, slip rates, strain buildup, scarp morphology, hazard zonation, Quaternary geochronology, recurrence intervals, seismicity and seismic hazard assessment is needed to avoid major human loss during a probable future earthquake along the Holocene Muzaffarabad Fault.

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