2013 Balochistan earthquakes

The 2013 Balochistan earthquakes took place in late September in southwestern Pakistan. The mainshock had a moment magnitude of 7.7 and a maximum Mercalli intensity of IX (Violent). At least 825 people were killed and hundreds more were injured. On 28 September, a M6.8 aftershock occurred to the north at a depth of 14.8 km, killing at least 22 people.

Tectonic setting
On a broad scale, the tectonics of southern and central Pakistan reflect a complex plate boundary where the India plate slides northward relative to the Eurasia plate in the east, and the Arabia plate subducts northward beneath the Eurasia plate in the Makran (western Pakistan). These motions typically result in north–south to northeast–southwest strike-slip motion at the latitude of the 24 September earthquake that is primarily accommodated on the Chaman Fault, with the earthquake potentially occurring on one of the southernmost strands of this fault system. Further, more in-depth studies will be required to identify the precise fault associated with this event. Although seismically active, this portion of the Eurasia plate boundary region has not experienced large damaging earthquakes in recent history. In the past 40 years, only one significant event has occurred within 200 km of this event, which was a Mw 6.1 earthquake in July 1990 that killed six people.

Earthquake


The United States Geological Survey reported that the earthquake took place on Tuesday 24 September 2013 at 11:29:48 UTC. The depth was reported to be 15 km. The earthquake reportedly lasted about a minute, causing panic in cities of southern Pakistan such as Karachi and Hyderabad.

The earthquake occurred as a result of the oblique-strike-slip type motion at shallow crustal depths. The location and mechanism of the earthquake are consistent with rupture within the Eurasia plate above the Makran Trench subduction zone. The event occurred within the transition zone between northward subduction of the Arabian plate beneath the Eurasia plate and northward collision of the India plate with the Eurasia plate. A 200 km surface rupture occurred as a result. Rupture initiated along a near-vertical fault and propagated mainly southwest along a curved fault that dipped 50° to the north. The fault segment is part of the Chaman Fault; during which its southern segment ruptured. Despite a mostly southwest rupture direction (with some rupture to the north), most of the aftershocks occurred north of the epicenter. Backprojection studies by Wang et al. suggest possible supershear rupture propagation velocity along some segments.

The source fault, identified as the Hoshab Fault, was likely an old thrust fault due to its shallow dip angle. It formed within an accretionary wedge related to the Makran Trench. During the earthquake, displacements along the fault was exclusively strike-slip. A possible explanation for the faulting behavior is due to the accretionary wedge being pushed eastwards towards the Indian Plate. Much of the inferred displacements occurred no less than 10 km beneath the surface. Peak slip was estimated at 10 m while the average slip across the rupture was 6 m.

Impact
The earthquake killed at least 825 people and injured hundreds of others. The earthquake struck a sparsely populated region of Pakistan. Most homes and buildings in the region are constructed of mud bricks and collapsed during the earthquake and aftershocks. An official in the Balochistan province claimed that 80 percent of the homes in the Awaran District had collapsed or were damaged. In the regional capital Quetta, some areas appeared to be badly damaged. Officials from the Balochistan government estimated that at least 21,000 houses had been completely destroyed by the tremor, while some areas remained beyond the reach of rescue services 48 hours after the initial quake.

PAGER impact estimates from the United States Geological Survey included a red alert level for initial shaking-related fatalities (35% chance of 1,000–10,000 fatalities, 27% chance of 10,000–100,000 fatalities) and an orange alert level for economic impact (35% chance of US$100 million–$1 billion, 26% chance of US$1–10 billion).

The earthquake was felt in major cities across Pakistan, including Karachi, Hyderabad, Rawalpindi/Islamabad, Larkana, and Lahore. The quake was also felt in Delhi, India, where some buildings shook, and Muscat, Oman—800 km from the epicenter—where mild tremors shook tables and cabinets. The earthquake shook in the parts of UAE, Iraq, Afghanistan, Iran and Qatar. The earthquake also shook Saravan, Iran without causing any damage or casualties. There were also minor tremors and aftershocks in the United Arab Emirates.

Ground effects


The earthquake was apparently powerful enough to raise a small island, later dubbed Zalzala Jazeera, meaning "quake island", in the Arabian Sea, variously reported as being 350 m to 1 km off the shore of Gwadar. The island is partially composed of rock, but mostly consists of mud and sand. The oval-shaped island was 60 – high and about 200 – long according to local people that visited the island the day after it appeared. They noticed a smell of gas and managed to ignite it, later extinguishing it with difficulty.

The island is believed to be the result of a mud volcano. In the region, deposits of frozen gas hydrates—which have a large methane content—exist beneath 300 to(-) of compressed sediment. After such a large earthquake, the gas hydrates converted from frozen to gaseous form through the heat of friction and either raised the overlaying sediment enough to create fissures through which it escaped or rose through fissures resulting from the earthquake itself. Another possible contributing factor in the island's creation was the liquefaction of the seabed, which allowed finer, loosely packed sediments to become liquid-like and squeeze up through fissures in overlying compressed sediment. Locals who ventured to the island shortly after it formed heard a hissing noise at one end and started a fire which was difficult to extinguish. There are several mud volcanoes inland near Zalzala Jazeera and they are common in the vicinity of subducting plate boundaries; in fact, similar islands have appeared in the same region following earthquakes in 1945, 1999, 2001, and 2010. Because of its composition of softer sediments, the sea was predicted to erode the island completely within a few months. By the end of 2016, the island had completely disappeared.

Tsunami
Despite its inland epicentre, the earthquake generated a tsunami with a maximum height of 0.57 m in eastern Oman.

Aftershocks

 * 5.8 earthquake at 27.209°N, 65.567°W occurred at 11:36:27 UTC (16:36:27 PKT) on 24 September at a depth of 10.0 km.
 * 5.6 earthquake at 27.313°N, 65.62°W occurred at 13:01:39 UTC (18:01:39 PKT) on 24 September at a depth of 14.5 km.
 * 5.5 earthquake at 27.131°N, 65.471°W occurred at 17:20:13 UTC (22:20:13 PKT) on 24 September at a depth of 10.5 km.
 * 6.8 earthquake at 27.183°N, 65.505°W occurred at 07:34:06 UTC (12:34:06 PKT) on 28 September at a depth of 12.0 km.

Aftermath
On 26 September, two days after the disaster, two rockets were fired at a helicopter carrying Maj. Gen. Muhammad Saeed Aleem, the National Disaster Management Authority chairman, as well as other officials and members of the media. Government sources blamed Balochi separatists, who are very active in the Awaran area.