M 7.6 - 21 km NNE of Muzaffar?b?d, Pakistan

Tectonic Summary

The October 8, 2005, M 7.6 Pakistan earthquake occurred as the result of shallow reverse faulting near the convergent boundary between the India and Eurasia plates. Focal mechanism solutions indicate that rupture occurred on either a steep southeast-striking reverse fault or a moderately dipping northwest-striking reverse fault. Of these two possible fault orientations, finite-fault modeling of globally recorded seismic data is more consistent with slip on the northwest-striking fault. At the location of the earthquake, the India subcontinent moves northward at a velocity of about 40 mm/yr, colliding with Eurasia. Earthquakes and active faults in northern Pakistan and adjacent parts of India and Afghanistan are the direct result of this collision. The boundary also produces the highest mountain peaks in the world, including the Himalayan, the Karakoram, the Pamir, and the Hindu Kush Ranges. As the India plate moves northward, it is being subducted or pushed beneath the Eurasia plate.

Much of the compressional motion between these two colliding plates has been and continues to be accommodated by slip on a suite of major thrust faults that are at the Earth’s surface in the foothills of the mountains and dip north-northeastward beneath the ranges. These include the Main Frontal Thrust, the Main Central Thrust, the Main Boundary Thrust, and the Main Mantle Thrust. These faults have a sinuous trace as they arc across the foothills in northern India and into northern Pakistan. In detail, the modern active faults are actually a system of faults composed of a number of individual fault traces. In the rugged mountainous terrain, it is difficult to identify and map all of the individual thrust faults, but the overall tectonic style of the modern deformation is clear in the area of the earthquake; north- and northeast-directed compression is producing thrust faulting. Near the city of Muzaffarabad, about 10 km southwest of the earthquake epicenter, active thrust faults that strike northwest-southeast have deformed and warped alluvial-fan surfaces of Pleistocene age into anticlinal ridges. The strike and dip direction of these thrust faults is compatible with the style of faulting indicated by the focal mechanism solutions from the nearby October 8, 2005, M 7.6 earthquake.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse-faulting events of the size of the October 8, 2005, earthquake are typically about 90x40 km (length x width); modeling of this earthquake implies dimensions of about 70x35 km, predominantly up-dip of the hypocenter.

Few large earthquakes have occurred in the immediate vicinity of the October 8th event in recent history, though several have occurred just over 200 km to the northwest in northeastern Afghanistan, four with M 7 and greater over the preceding 40 years. The most recent event, one of the largest, was a M 7.4 earthquake in March 2002 that resulted in at least 150 fatalities, several injuries, and 400 houses being damaged or destroyed by a landslide that dammed and flooded the Surkundara Valley.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)