Question: In September 2011 what magnitude earthquake struck the city where Northern Railway is headquartered?

full_context: Document 1: New Delhi: New Delhi has one of India's largest bus transport systems. Buses are operated by the state-owned Delhi Transport Corporation (DTC), which owns largest fleet of compressed natural gas (CNG)-fueled buses in the world. Personal vehicles especially cars also form a major chunk of vehicles plying on New Delhi roads. New Delhi has the highest number of registered cars compared to any other metropolitan city in India. Taxis and Auto Rickshaws also ply on New Delhi roads in large numbers. New Delhi has one of the highest road density in India.
Document 2: Southern California: Many faults are able to produce a magnitude 6.7+ earthquake, such as the San Andreas Fault, which can produce a magnitude 8.0 event. Other faults include the San Jacinto Fault, the Puente Hills Fault, and the Elsinore Fault Zone. The USGS has released a California Earthquake forecast which models Earthquake occurrence in California.
Document 3: 2008 Sichuan earthquake: According to a study by the China Earthquake Administration (CEA), the earthquake occurred along the Longmenshan fault, a thrust structure along the border of the Indo-Australian Plate and Eurasian Plate. Seismic activities concentrated on its mid-fracture (known as Yingxiu-Beichuan fracture). The rupture lasted close to 120 sec, with the majority of energy released in the first 80 sec. Starting from Wenchuan, the rupture propagated at an average speed of 3.1 kilometers per second 49° toward north east, rupturing a total of about 300 km. Maximum displacement amounted to 9 meters. The focus was deeper than 10 km.
Document 4: Houston: The Houston area has over 150 active faults (estimated to be 300 active faults) with an aggregate length of up to 310 miles (500 km), including the Long Point–Eureka Heights fault system which runs through the center of the city. There have been no significant historically recorded earthquakes in Houston, but researchers do not discount the possibility of such quakes having occurred in the deeper past, nor occurring in the future. Land in some areas southeast of Houston is sinking because water has been pumped out of the ground for many years. It may be associated with slip along the faults; however, the slippage is slow and not considered an earthquake, where stationary faults must slip suddenly enough to create seismic waves. These faults also tend to move at a smooth rate in what is termed "fault creep", which further reduces the risk of an earthquake.
Document 5: New Delhi: New Delhi lies on several fault lines and thus experiences frequent earthquakes, most of them of mild intensity. There has, however, been a spike in the number of earthquakes in the last six years, most notable being a 5.4 magnitude earthquake in 2015 with its epicentre in Nepal, a 4.7-magnitude earthquake on 25 November 2007, a 4.2-magnitude earthquake on 7 September 2011, a 5.2-magnitude earthquake on 5 March 2012, and a swarm of twelve earthquakes, including four of magnitudes 2.5, 2.8, 3.1, and 3.3, on 12 November 2013.
Document 6: New Delhi: With a total area of 42.7 km2 (16.5 sq mi), New Delhi forms a small part of the Delhi metropolitan area. Because the city is located on the Indo-Gangetic Plain, there is little difference in elevation across the city. New Delhi and surrounding areas were once a part of the Aravalli Range; all that is left of those mountains is the Delhi Ridge, which is also called the Lungs of Delhi. While New Delhi lies on the floodplains of the Yamuna River, it is essentially a landlocked city. East of the river is the urban area of Shahdara. New Delhi falls under the seismic zone-IV, making it vulnerable to earthquakes.
Document 7: Seattle: Due to its location in the Pacific Ring of Fire, Seattle is in a major earthquake zone. On February 28, 2001, the magnitude 6.8 Nisqually earthquake did significant architectural damage, especially in the Pioneer Square area (built on reclaimed land, as are the Industrial District and part of the city center), but caused only one fatality. Other strong quakes occurred on January 26, 1700 (estimated at 9 magnitude), December 14, 1872 (7.3 or 7.4), April 13, 1949 (7.1), and April 29, 1965 (6.5). The 1965 quake caused three deaths in Seattle directly, and one more by heart failure. Although the Seattle Fault passes just south of the city center, neither it nor the Cascadia subduction zone has caused an earthquake since the city's founding. The Cascadia subduction zone poses the threat of an earthquake of magnitude 9.0 or greater, capable of seriously damaging the city and collapsing many buildings, especially in zones built on fill.
Document 8: New Delhi metro station: New Delhi is a station on the Yellow Line of the Delhi Metro. It is within walking distance from the Indian Railways New Delhi station. It is on the Ajmeri Gate (Platform Number 16) side of the New Delhi Railway Station.
Document 9: Hyderabad: The most commonly used forms of medium distance transport in Hyderabad include government owned services such as light railways and buses, as well as privately operated taxis and auto rickshaws. Bus services operate from the Mahatma Gandhi Bus Station in the city centre and carry over 130 million passengers daily across the entire network.:76 Hyderabad's light rail transportation system, the Multi-Modal Transport System (MMTS), is a three line suburban rail service used by over 160,000 passengers daily. Complementing these government services are minibus routes operated by Setwin (Society for Employment Promotion & Training in Twin Cities). Intercity rail services also operate from Hyderabad; the main, and largest, station is Secunderabad Railway Station, which serves as Indian Railways' South Central Railway zone headquarters and a hub for both buses and MMTS light rail services connecting Secunderabad and Hyderabad. Other major railway stations in Hyderabad are Hyderabad Deccan Station, Kachiguda Railway Station, Begumpet Railway Station, Malkajgiri Railway Station and Lingampally Railway Station. The Hyderabad Metro, a new rapid transit system, is to be added to the existing public transport infrastructure and is scheduled to operate three lines by 2015.
Document 10: New Delhi: New Delhi is a major junction in the Indian railway network and is the headquarters of the Northern Railway. The five main railway stations are New Delhi railway station, Old Delhi, Nizamuddin Railway Station, Anand Vihar Railway Terminal and Sarai Rohilla. The Delhi Metro, a mass rapid transit system built and operated by Delhi Metro Rail Corporation (DMRC), serves many parts of Delhi and the neighbouring cities Faridabad, Gurgaon, Noida and Ghaziabad. As of August 2011, the metro consists of six operational lines with a total length of 189 km (117 mi) and 146 stations, and several other lines are under construction. It carries millions of passengers every day. In addition to the Delhi Metro, a suburban railway, the Delhi Suburban Railway exists.
Document 11: Istanbul: Istanbul is near the North Anatolian Fault, close to the boundary between the African and Eurasian Plates. This fault zone, which runs from northern Anatolia to the Sea of Marmara, has been responsible for several deadly earthquakes throughout the city's history. Among the most devastating of these seismic events was the 1509 earthquake, which caused a tsunami that broke over the walls of the city and killed more than 10,000 people. More recently, in 1999, an earthquake with its epicenter in nearby İzmit left 18,000 people dead, including 1,000 people in Istanbul's suburbs. The people of Istanbul remain concerned that an even more catastrophic seismic event may be in the city's near future, as thousands of structures recently built to accommodate Istanbul's rapidly increasing population may not have been constructed properly. Seismologists say the risk of a 7.6-magnitude or greater earthquake striking Istanbul by 2030 is more than 60 percent.
Document 12: Hazrat Nizamuddin railway station: Nizamuddin हज़रत निज़ामुद्दीन Rapid transit, Light rail, Commuter rail & Regional rail station Location New Delhi, Delhi India 28 ° 35 ′ 21 ''N 77 ° 15 ′ 15'' E ﻿ / ﻿ 28.58917 ° N 77.25417 ° E ﻿ / 28.58917; 77.25417 Elevation 206.7 metres (678 ft) Platforms 7 Construction Structure type Standard (on ground station) Parking Yes Other information Status Functioning Station code NZM Zone (s) Northern Railway zone Division (s) Delhi Electrified Yes Traffic Passengers (Daily) 360,000 + Location Nizamuddin Location within Delhi
Document 13: Urban rail transit in India: There are currently 11 operational rapid transit (also called 'metro') systems in ten cities in India. As of November 2017, India has 425 kilometres (264 miles) of operational metro lines and 347 stations. A further 500 + km of lines are under construction. Metro rail lines in India are composed of mainly standard gauge. Projects like the Kolkata Metro and Delhi Metro used broad gauge for their earliest lines but all new projects in India are on standard gauge as rolling stock imported is of standard gauge.
Document 14: New Delhi: The Delhi Metro is a rapid transit system serving New Delhi, Delhi, Gurgaon, Faridabad, Noida, and Ghaziabad in the National Capital Region of India. Delhi Metro is the world's 12th largest metro system in terms of length. Delhi Metro was India's first modern public transportation system, which had revolutionised travel by providing a fast, reliable, safe, and comfortable means of transport. The network consists of six lines with a total length of 189.63 kilometres (117.83 miles) with 142 stations, of which 35 are underground, five are at-grade, and the remainder are elevated. All stations have escalators, elevators, and tactile tiles to guide the visually impaired from station entrances to trains. It has a combination of elevated, at-grade, and underground lines, and uses both broad gauge and standard gauge rolling stock. Four types of rolling stock are used: Mitsubishi-ROTEM Broad gauge, Bombardier MOVIA, Mitsubishi-ROTEM Standard gauge, and CAF Beasain Standard gauge.
Document 15: April 2015 Nepal earthquake: Thousands of houses were destroyed across many districts of the country, with entire villages flattened, especially those near the epicenter.
ground_truth: 4.2-magnitude
sub_queries: What Indian city is the headquarters of the Northern Railway? Answer: New Delhi
What was the magnitude of the earthquake that affected New Delhi on September 2011? Answer: 4.2-magnitude