Seismicity with reference to Pakistan

1. Introduction
A seismic hazard analysis is based on both the geological and seismological history of the region, including recent and historical seismicity (earthquake catalogue), supplemented with available paleoseismological information.
Pakistan is an earthquake prone country. A number of earthquakes have hit Pakistan resulting in losses of life and property. Pakistan Meteorological Department (PMD) presently has a network of eleven seismic stations. By using recorded data the seismicity and zoning maps of Pakistan have been developed by PMD.
From our experiences derived from the recent October 8, 2005, Kashmir earthquake it emphasized the importance of redefining the seismic zonation for whole Pakistan especially seismically active areas to minimize further losses of human lives and failure of infrastructure due to such earthquakes. It requires both research and scientific investigation and to introduce new disciplinary approaches for the mitigation of earthquake risks and post earthquake management of the disaster.
The Pakistan Meteorological Department (PMD) has its seismological network and Government of Pakistan has charged the PMD with the mandate to complete the micro seismic zonation of the whole country along with development of new seismic risk map needed for updation of building codes and accomplish the relevant research by further up-grading and strengthening of its seismic network.
2. Geographical and Tectonic location of Pakistan
Pakistan is situated in South Asian region between longitudes 61º & 76º E and latitudes 24º & 37º N covering a total land area of 796,095 sq. km. The country shares its borders with Iran to the West, India in the Southeast, Afghanistan in the Northwest, and China in the North. Figure 1 shows, the Geographical location of Pakistan in South Asia.
Tectonically, the Pakistan region comprises diverse nature of geological/tectonic features as it lies at the junction of Indian, Eurasian and Arabian plate boundaries. Himalayas are in the North and Hindukush in the Northwest. In the South, along the coast of Arabian Sea, there is a narrow marine zone belt. In rest of the country there is a belt of sub-mountain Potohar plateau and Salt range in Punjab, western bordering highlands the Balochistan plateau and southern Indus plain.

Figure 1 Pakistan’s geographical location in South Asia
3. Seismotectonics of Pakistan
Pakistan is located on the north-west region of the Indian plate that pushes into the Eurasian plate. The direction of horizontal compression has been mapped from the focal mechanism and is shown in Section 5 (Figure 2).
The Hindu Kush region generates regularly very large earthquakes, occurring down to
300 km depth, which are also felt in Pakistan. The compressive stress direction of this region is NNW-SSE, albeit with a somewhat bimodal nature as seen in Figure 2. The direction of crustal stress in Kashmir is NE-SW, perpendicular to the line of plate collision and MBT. In the Hindu Kush region the earthquake mechanism is generally thrust faulting occasionally normal faulting whereas in Kashmir, the earthquakes mainly show thrust fault mechanism with a clear NE-SW compression. Both the Karakoram and
Hindu Kush ranges are caused by the collision of the Indian plate into the Eurasian plate the Indian plate collides and under plates the Eurasian plate. The Hindu Kush and the
Pamir constitutes one of the most seismically active earthquake zones in the world.
In the Kashmir region we find the important Hazara-Kashmir Syntax (HKS), which was formed due to the change in the Himalayan thrust interface direction from NE in Kashmir to NW along the Indus. The Punjal thrust and the Main Boundary Thrust (MBT) are folded around this syntax and are subject to a 90o “rotation” from one side to the other.
The Panjal thrust, MBT and Muzaffarabad thrust are truncated by the active Jhelum fault (Baig and Lawrence, 1987). Beside other faults in this region, the Jhelum fault acts as an active left lateral oblique reverse fault. The general seismicity pattern of the Jhelum- Ambore zone is low activity of regular earthquakes with magnitudes ≤ 4.0. The historical and instrumental seismic data from this region show no earthquake with a size exceeding magnitude 6.8.
In the western Himalayas (Gilgit Agency), the seismic activity is associated with earthquakes of magnitude 5 and larger, and largely coincides with the surface trace of the Himalayan Main Central Thrust (MCT) rather than with the Himalayan Main Boundary Thrust (MBT) which represents the structural boundary. Many of the earthquakes that occur on the MBT take place at depths of 20 to 40 km and are associated with a shallow northward dipping subsurface extension of the MBT underlying the MCT (the under plating of the Eurasian plate). One section of the eastern Himalaya front thrust was relatively quite during the last decades. This is the source zone of the Kangra earthquake (Ms 8.0) which occurred in 1905 and which extended from Kangra to Dehra Dun, i.e. 76o E to 78o E (Middlemiss, 1910).
The second section of presently low seismic activity is near the eastern flanks of the Kashmir syntaxial bend. Note that the north western end of the zone of low activity in Kashmir stops against a zone of high activity which is the area of the destructive Pattan earthquake of December 28, 1974 (M=6.0) and the 8th October, 2005 Muzaffarabad earthquake (Mw =7.6).
The seismicity in the Kirther range is relatively diffuse compared to that in the Suleiman range. In the latter, the seismicity falls on or near a well defined fault scarp which offsets the range against the eastward extending Indus basin. The northern two thirds of the faults have ruptured in the large Much earthquake of August 27, 1931 (West, 1934).
The most remarkable clustering of seismicity occurs in the Quetta transverse range as far as 100 km south of its higher peaks and is located within or beneath the thick young tertiary sediments of the Indus basin. Figure 3 shows the Tectonic sketch map and the faults of Pakistan.
For the study of earthquake hazard in Pakistan, the country was divided into nineteen zones (Figure 5) including some portions of the neighbouring countries of Afghanistan, Tajikistan, Iran and India. The division of the region into these source zones is based on the seismicity, the fault systems and the stress direction analysis.
Figure 2 aggregated directions of maximum horizontal compression in the crust
4. Seismic Zones of Pakistan
Seismic zoning map of Pakistan have been developed by Pakistan Meteorological Department in 1998 by earthquake data collected from United States of Geological Survey (USGS), International Seismological Centre (ISC) and PMD for the period from 1905 to 1998. For this purpose the magnitude range for Seismicity map was taken as greater than 4.5 on Richter scale. Figure 4 shows, the Seismic Zoning Map of Pakistan.
Figure 3 Tectonic Sketch Map and the faults of Pakistan
The region of Pakistan is divided into four main Seismic zones in term of major, moderate, minor and negligible zones with respect to ground acceleration values as shown in figure above. Where g is the ground acceleration and its unit is in Gal.
4.1. Seismic provinces and area sources
For the study of earthquake hazard in Pakistan, the country was divided into 19 zones, including some portions of the neighboring countries of Afghanistan, Tajikistan, Iran and
India. The division of the region into these source zones is based on the seismicity, the fault systems and the stress direction analysis. The division was also based on the data processing of the whole catalogue regarding the seismicity, depth and the study of research papers. One of the basic principles for the zonation of a region is that the seismicity within a single zone remains uniform and homogeneous, even though this principle clearly is not always fulfilled as judged from the individual catalogues used in the study. The nineteen seismic zones are all having geometric shapes (polygons) and the coordinates of their corners are described below along with a Figure 5. There are altogether 19 area zones defined for this study, as delineated in the following.

Figure 4 Seismic Zoning Map of Pakistan
Zone 1; Kohistan-Kashmir
This zone includes mostly the Kashmir areas, some parts of the Gilgit region and the NWFP (North-Western Frontier Province) and some of upper Punjab. The zone has always been very active as it covers the plate boundary of the Eurasian and the Indian plate. The northern region of Punjab is included in this zone as it was always affected by the earthquakes originated from the faults present in the Kashmir region. Some areas of
Himachal Pradesh, India are also included in this zone. Lahore, a densely populated city of Pakistan and the capital of Punjab, was destructed in 1905 by the devastating earthquake in Kangra, India, which is also included in this zone. Chilas, Besham and Gilgit city are other important cities of this zone. The coordinates of this zone are:
31.2o N 75.5 o E
32.1o N 78.0 o E
36.1o N 74.4 o E
35.0o N 72.3 o E
Figure 5 Seismic Zones of Pakistan
Zone 2; Northern Baluchistan
Although the name given to this region is the Northern Baluchistan Seismic Zone, it also includes some areas of Punjab. The zone is the region of a large number of earthquakes with magnitudes more than 4.5 in the PDE catalogue. The historical data from PMD does not show any significant activity in this zone, however, one must then remember that the epicenters of the historical catalogue is highly uncertain, and to a large part also reflects on population density. The coordinates of the corners are:
30.0o N 68.7 o E
28.2o N 69.5 o E
31.1o N 71.4 o E
31.1o N 69.9 o E
Zone 3; Quetta-Sibi
The geological and the seismological features of this zone are almost similar to those of Zone 2. This zone also contains Quetta which in 1935 was affected by an earthquake of magnitude 7.6. Almost 30,000 people died during this seismic event and the whole city perished. Jacobabad is also a city of Baluchistan in this zone.
30.1o N 65.0 o E
27.6o N 68.4 o E
28.2o N 69.5 o E
30.0o N 68.7 o E
31.3o N 67.8 o E
Zone 4; Southern Baluchistan
Only four earthquakes are found in this zone in the ISC catalogue having M ≥ 6.0.
Panjgur, and Sonmiani bay of Baluchistan and Larkana of Sindh are some of the important places in this zone. The Central Brahui range, Siahan range, Kirthur range and the Central Makran mountain ranges run through this zone.
30.1o N 65.0 o E
27.2o N 63.0 o E
25.0o N 66.7 o E
27.6o N 68.4 o E
Zone 5; Northern Afghanistan-Tajikistan
This zone covers mostly the border area of Pakistan, Afghanistan, Tajikistan and China.
The city of Sost, a very important town on the
Karakoram Highway
 and the Kunjarab pass are both located in this zone, as well as a small area of the province of China, Xinjiang. Tashkurgan is a Tajik town in western Xinjiang, China. It is almost on the borders of both Afghanistan and Tajikistan, and close to the borders of Kyrgyzstan and Kazakhstan. Badakhshan in Afghanistan is also in this zone.
36.1o N 71.0 o E
36.1o N 76.0 o E
38.0o N 76.0 o E
38.0o N 71.0 o E
Zone 6; Hindu Kush
This zone entirely covers the Afghanistan region. Several earthquakes with their
epicentres in or around the Hindu Kush ranges have affected the Northern areas of
Pakistan. The earthquakes in 1983, 1985 and 1991 in the Hindu Kush had magnitudes 7.4, 7.4 and 6.7, respectively, and it has been reported that more than 300 people died as a result of these events, in the regions of Peshawar, Chitral, Swat and Malakund (the PMD database). A few records of large historic earthquakes have also been found in the PMD data base. Any earthquake in Hindu Kush with magnitude greater or equal to 6 is reported to be felt (although weakly) also in the southern Punjab.
36.5o N 66.4 o E
35.0o N 66.0 o E
36.1o N 71.0 o E
37.0o N 71.0 o E
Zone 7; North Western Afghanistan-Tajikistan Border Region
This zone covers the area just to the south of Dushanbe, the capital of Tajikistan. In addition to Tajikistan and Afghanistan it also covers some areas of Uzbekistan and
Turkmenistan. Being situated near the Hindu Kush region and a few hundred kilometers away from Pakistani region, this is the region of large earthquakes according to the ISC and PDE catalogues. A few records of large earthquakes are found in the PMD historic database, in this zone.
38.0o N 65.9 o E
36.5o N 66.4 o E
37.0o N 71.0 o E
38.0o N 71.0 o E
Zone 8; Eastern Afghanistan
This zone covers mainly the Afghan territories up to Baghlan in the west, and the border region with Pakistan, in the south east. Kabul, Kandhar, Nuristan and Mazar-Sharif are the major cities of Afghanistan covered in this zone. Many active faults are located in this zone, which have generated significant earthquakes in the past as well as during the recent few decades. The seismicity increases as we go from south to north in this zone and the PMD historic database gives information about a few significant earthquakes in this zone, however according to the PDE catalogue, most of the southern parts of this zone are inactive.
30.1o N 65.0 o E
31.3o N 67.8 o E
34.75o N 71.2 o E
36.1o N 71.0 o E
35.6o N 68.5 o E
33.1o N 66.1 o E
Zone 9; Makran Coast
Tsunamis have also affected the coast of Pakistan. An active subduction zone exists off the Makran coast and the 1945 earthquake originated in this region. The magnitude of this earthquake was 8.2 Ms according to the PMD catalogue and a tsunami was generated by this huge earthquake which struck the Makran coast. As discussed earlier in this report a wave height of 12 m was reported during this tsunami.
From the PMD historical database, only two considerable events in 1851 and 1864 are reported in the Gwadar area. The intensities of these earthquakes were VII.
27.2o N 63.0 o E
26.0o N 62.0 o E
23.0o N 62.0 o E
23.0o N 67.9 o E
25.0o N 66.7 o E
Zone 10; Runn of Kuchch
The area under consideration in this zone belongs mostly to India (Runn of Kuch). The zone contains a reasonable number of earthquakes having M ≥ 4.5 as reported in the PDE catalogue. In the historical data from PMD, there are two major earthquakes in this zone.
The seismic activity in this area has always affected also Pakistan.
23.0o N 67.9 o E
23.0o N 72.0 o E
24.5o N 70.1 o E
23.7o N 67.5 o E
Zone 11; Sindh-Punjab
This is the largest zone with respect to area, but it has the least seismic activity. It covers most parts of Punjab and Sindh including the western areas of India. In the south it covers the coastal areas of Sindh where a few but significant historical earthquakes are found, however there are generally few earthquakes in this zone.
23.7 o N 67.5 o E
24.5 o N 70.1 o E
31.2 o N 75.2 o E
31.1 o N 71.4 o E
28.2 o N 69.5 o E
27.6 o N 68.4 o E
25.0 o N 66.7 o E
Zone-12; Pamir-Kunlun
This is a rectangular-shaped zone covering only the Chinese territories near the Pakistani border with China. Kongur Tagh (also referred to as Kongur or Kongur Shan) is the highest peak of the Kunlun Mountains in China.
36.1o N 76.0 o E
36.1o N 78.0 o E
38.0o N 78.0 o E
38.0o N 76.0 o E
Zone 13; Indian Kashmir
The northern areas of Pakistan, Kashmir, Xinjiang of China and Himachal Pradesh of
India is the main areas covered by this zone. It is not a densely populated zone but some seismic activity is present.
32.1o N 78.0 o E
36.1o N 78.0 o E
36.1o N 74.4 o E
Zone 14; Upper Punjab-NWFP
According to the PDE catalogue this zone seems to be very active, especially parts of NWFP. One of the important aspects of this zone is that it includes two provincial capitals: Peshawar and Lahore, the country capital of Islamabad and one densely populated city of India, Amritsar. Malakund and Mardan are other important places in this zone. According to the historical PMD database, there is a belt of uniform seismic activity which exists in this zone. This belt starts near Rawalpindi and ends near Peshawar. In addition to the geological conditions of the area, it is thought that one of the reasons for many earthquake reports from this area is that it has remained densely populated since ancient times.
31.2o N 75.5 o E
35.0o N 72.3 o E
34.7o N 71.2 o E
31.1o N 71.6 o E
Zone 15; Chitral
This is a very small zone with respect to area. Chitral and Drosh of Pakistan and Asadabad of Afghanistan are the important cities in this zone.
34.75o N 71.2 o E
35.0o N 72.3 o E
36.1o N 74.4 o E
36.1o N 71.0 o E
Zone 16; Koh e Sulaiman
Some areas of Baluchistan, western Punjab, and most areas of the NWFP are included in this zone. The seismic activity in this zone is very low as compared to that in the neighboring zones i.e. Zones-2 and 3 in the south and 15 in the north.
30.0o N 68.7o E
31.1o N 69.9 o E
31.1o N 71.6 o E
34.7o N 71.2 o E
31.3o N 67.8 o E
Zone 17; South West Iran
The Harvard and ISC catalogues contain three earthquakes each having M ≥ 6.0 whereas the PMD historic database has no significant reports from this zone.
23.0o N 60.0 o E
23.0o N 62.0 o E
26.0o N 62.0 o E
27.2o N 63.0 o E
28.0o N 60.0 o E
Zone 18; Western Baluchistan
The old cities of Zahidan of Iran, Nimruz of Afghanistan and the Chaghi hills of Pakistan are all included in this zone. The western parts of Baluchistan are noted to be inactive according to the PMD historic database. The areas of Baluchistan and the border areas with Iran and Afghanistan contain few large earthquakes, according to the ISC catalogue.
However, the PDE catalogue provides reports on several events with M ≥ 4.5 in this region.
28.0o N 60.0 o E
27.2o N 63.0 o E
30.1o N 65.0 o E
32.0o N 60.0 o E
Zone 19; Central & Southern Afghanistan
According to the PDE catalogue only a few small earthquakes are found in this area with magnitudes near 4.5. No earthquake with M ≥ 6.0 is reported. Except a few areas of Iran, this zone mainly covers the Afghan territories e.g. Lashkargah, Chaghcharan and Negah are the major cities of Afghanistan. The coordinates of the corners of this zone are as below:
32.0o N 60.0 o E
30.1o N 65.0 o E
33.1o N 66.1 o E
35.6o N 68.5 o E
35.0o N 66.0 o E
4.2. Zones of high seismic activity in Pakistan
It was observed that most of the large earthquakes occurred in the following regions:
  1. Hindu Kush region
  2. Northern areas of Pakistan and Kashmir
  3. North-western part of Baluchistan and
  4. The coastal areas of Pakistan (near Makran region)
  5. The south-eastern corner of Pakistan (Runn of Kutch)
The combination of PMD and international data bases results in a catalogue with different kinds of magnitudes, essentially body wave magnitude (mb) for the smaller events and surface wave (Ms) and moment (Mw) magnitudes for the larger events.
Figure 5 Seismicity of the region (map with the 19 zones overlaid in Google Earth)

 

posted by Geology on 09:54

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