2013 Balochistan earthquakes

26°57′04″N 65°30′04″E / 26.951°N 65.501°E / 26.951; 65.501TypeOblique-slipAreas affectedPakistanMax. intensityMMI IX (Violent)Aftershocks50 ≥M_w 4.0 (as of 25 September 2014)
Largest is M_w 6.8 Casualties870 killed, 965 injured

• 825 killed, 815 injured (mainshock)
• 45 killed, 150 injured (September 28 aftershock)

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). It had an epicenter located in Awaran District, 113 km (70 mi) northwest of Bela.^[1] At least 825 people were killed and hundreds more were injured, making it the deadliest earthquake to occur in 2013. On 28 September, a M_w  6.8 aftershock occurred at a depth of 14.8 kilometres (9.2 miles),^[2] killing an additional 45 people.^[3]

On a broad scale, the tectonics of southern and central Pakistan reflect a complex plate boundary where the Indian 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 kilometres (120 mi) of this event, which was a M_w 6.1 earthquake in July 1990 that killed six people.^[1]

The United States Geological Survey reported that the earthquake took place on 24 September 2013 at 11:29:48 UTC. The depth was reported to be 15 kilometres (9.3 mi). The earthquake reportedly lasted about a minute, causing panic in cities of southern Pakistan such as Karachi and Hyderabad.^[4]

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 Eurasian Plate and northward collision of the Indian Plate with the Eurasia Plate.^[1] A 200 km (120 mi) 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.^[5]

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 (6.2 mi) beneath the surface. Peak slip was estimated at 10 m (33 ft) while the average slip across the rupture was 6 m (20 ft).^[6]

The earthquake was felt in major cities across Pakistan, including Karachi, Hyderabad, Rawalpindi, Islamabad, Larkana, and Lahore.^[4] The quake was also felt in Delhi, India, where some buildings shook,^[7] and Muscat, Oman—800 kilometres (500 mi) from the epicenter—where mild tremors shook tables and cabinets.^[8] The earthquake shook in the parts of UAE, Iraq, Afghanistan, Iran and Qatar.^[9] The earthquake also shook Saravan, Iran without causing any damage or casualties.^[10] There were also minor tremors and aftershocks in the United Arab Emirates.^[11]

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 metres (1,150 ft) to 1 kilometre (0.62 mi) off the shore of Gwadar.^[12][13][14] The island is partially composed of rock, but mostly consists of mud and sand. The oval-shaped island was 60–70 feet (18–21 m) high and about 200–300 feet (61–91 m) 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.^[13]

The island is believed to be the result of a mud volcano.^[15] In the region, deposits of frozen gas hydrates—which have a large methane content—exist beneath 300 to 800 metres (980–2,620 ft) of compressed sediment.^[13] 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.^[13] By the end of 2016, the island had completely disappeared.^[16]

Despite an epicenter 173 km (107 mi) inland, the earthquake generated a tsunami with a maximum height of 57 cm (1.87 ft) at Qurayyat, Oman, 17 cm (0.56 ft) at Chabahar, Iran and 2 cm (0.066 ft) at Okha, India.^[17]

As of 25 September 2014, there were at least 50 aftershocks above magnitude 4.0 recorded by the USGS; most of them occurred within a 70 km (43 mi) radius of the epicenter.^[18] The largest aftershock, measuring M_ww  6.8, occurred at 07:34:06 UTC (12:34:06 PKT) on 28 September at a depth of 12.0 km (7.5 mi).^[2]

The earthquake killed at least 825 people and injured 815 others.^[19][3] Most homes and buildings in the region were constructed of mud bricks and collapsed during the earthquake and subsequent aftershocks. An official in the Balochistan province claimed that 80 percent of the homes in the Awaran District had collapsed or were damaged.^[20] There were 386 deaths in the district, where 32,638 houses were destroyed and 14,118 more were damaged.^[21] There were also 254 deaths in Kech, 60 in Khuzdar, 17 in Kalat, nine in Chagai and six in Gwadar.^[22] Severe damage was also reported in areas of Quetta.^[23][12] Some areas remained beyond the reach of rescue services two days after the initial quake.^[24] The September 28 aftershock killed an additional 45 people and injured 150 others.^[3]

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).^[25]

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.^[24]

• List of earthquakes in 2013
• List of earthquakes in Pakistan

Sources

• M7.7 - 61km NNE of Awaran, Pakistan – United States Geological Survey.
• Pakistan quake island off Gwadar 'emits flammable gas' – BBC News
• The International Seismological Centre has a bibliography and/or authoritative data for this event.
• ReliefWeb's main page for this event.