December 17, 2005

Between July and October, several jolts were felt in coastal parts of Karachi, particularly in some areas of the Defence Housing Authority, causing considerable anxiety among residents. Interestingly, these tremors were only recorded at the Karachi Seismic Station of the Pakistan Meteorological Department (PMD).

The other seismic stations of PMD — in Quetta, Peshawar, Islamabad and Lahore — did not record them, most probably due to their low magnitudes and shallow depths. Between August and September of 1998 too minor jolts were felt in Karachi, causing anxiety among residents of Malir, Quaidabad, Landhi and Korangi.

In both the cases, the location of epicentres and focal depths could not be calculated, mainly because data from at least three stations are required to compute them accurately. A number of views from different geologists and engineers appeared in the newspapers about the possible causes of these low-intensity quakes. These opinions were based on assumptions, in the absence of scientific data.

It’s true that three-dimensional patterns of earthquake hypocentres may help define patterns of dominant earthquake fault zones. But in the case of Karachi, the earthquake hypocentres cannot be determined accurately because of the localized nature of the low-intensity seismic events, at apparently shallow depths.

Geologically, Karachi is located on the southern margin of the geological trough which lies in the southern extension of the Kirthar range. The trough is delineated by severely deformed mountain ranges — namely the Mor Range, Pab Range and Bela ophiolite/melange zone to the west, Kirthar Range to the north and the east, and by the Indus delta and the Arabian Sea creeks to the south. The trough may be subdivided into three principal regions.

Northern relatively uplifted region: This includes a number of anticlinal folds, namely Sari Singh, Hundi, Rahuja, Benir and Kand structures in the Thano Bola Khan and Karchat areas. To the east of this region, recent and continuous uplift of the Lakhra anticline has been reported on the basis of strong geomorphic evidence. This uplift is estimated to be to the tune of 30 to 45 metres during the past 5,000 years.

Southern submerged region: This part of the trough is occasionally overlain by Pliocene Manchhar Formation and Quaternary sediments in the south. A number of linearly stretched folds of small amplitude are developed in this part of the trough.

Gaj Formation consists of dominantly limestone-forming prominent ridges. Otherwise, the younger and other formations contain mostly sandstones and shale-forming flat areas as part of the valleys.

Western monocline: It consists of Manghopir and Hub River (Cape Monze) anticlinal nose-like structures. This part gives a drag effect on aerial photos by the changing trend of the plunge of fold axes.

The northwest-southeast trending faults associated with the Manghopir anticlinal nosing structure have been over-interpolated, without the support of scientific data, as an extension of the Rann of Kutch inferred fault passing across the southern submerged region. This interpolation, covering about an area of 500kms between Karachi and Allah Band fault site in the Indian Rann of Kutch, has created great geological confusion.

Based on the magnitude-wise distribution of the earthquakes recorded from 1970 to 2005, four seismic zones have been identified in and around Karachi. One seismic zone lies to the west of Karachi, passing across the Sonmiani area. It extends southwestwards into the Arabian Sea in alignment with the submarine Murray Ridge and seems to extend towards Uthal-Bela areas in the north.

The second seismic zone seems to follow the southeastern margin of Kirthar Range from the north to the south, swinging ultimately towards the southwest. This zone includes Karchat, Thano Bola Khan, Lakhra, Jhimpir, Jungshahi, Thatta and areas further south. The third zone passes across the eastern vicinity of Badin in the northeast-southwest direction along the eastern margin of the Indus Delta. The fourth seismic zone lies across the Pakistan-India border.

Earthquakes of low to moderate magnitudes — 3.1 to 4.0M and 4.1 to 5.0M — dominate in these zones. Quakes of higher magnitudes (above 5.0M) take place only in the Rann of Kutch, which is a known high-risk area.

Records show that earthquakes of low to moderate magnitudes took place in the other zones, that is Murray Ridge-Sonmiani-Uthal, southeastern Kirthar, and NE-SW Badin areas. Moderate magnitude earthquakes in Jangshahi, Thatta, Jhimpir and Thano Bola Khan areas were often felt with low to moderate intensity in Karachi. For instance, the 1985 quakes were felt in Karachi.

Interestingly, some earthquakes of more than 4M that took place to the west of Karachi in the Arabian Sea at about the same distance as that of the 1985 earthquakes were not felt in Karachi. Similarly, the great Bhuj earthquake of 2001 — which measured 8.0 on the Richter scale and which caused enormous damage in Ahmedabad and its surrounding areas — did not cause much damage in Karachi or Hyderabad even though these two Pakistani cities are situated as far away from the epicentre as Ahmedabad.

Much like the tremors resulting from the micro-quakes occurring from July to October of 2005, jolts were felt as well between August and September of 1998. During the two-month period, some 11 micro-earthquakes, with magnitudes ranging from 2 to 3.9M, took place in addition to numerous smaller ones. These events were only recorded at PMD’s Karachi Seismic Station.

Zonation of 1998 quakes

The intensity of an earthquake is judged by the effect it produces in terms of deformations in the Earth’s crust. After an earthquake, it is possible to classify the effects on the Modified Mercalli (MM) Scale, delineating seismicity zones of different intensities based on the nature of shaking and the resulting damages in the region.

In the Modified Mercalli Scale, twelve levels of intensity have been arranged according to the effects of the motion on people and their surroundings, structures of all kinds, and the nature of land. Based on some field observations, the highest intensity level of IV on the MM scale was assigned to the Quaidabad-Landhi-Korangi areas. The trend in the isoseismals shows a general elongation of seismicity zone in the northeast-southwest direction. Anomalous trend becomes wider in Quaidabad-Landhi-Korangi area, south of the Malir River, where it attains its course in the east-west direction.

In this area, residents did not report any displacement of household furniture or other items. In the central part of the study area, some people woke up at 00.30am, particularly due to noticeable shaking during the Sept 19 quake which had a maximum magnitude of 3.9 on the Richter scale. In general, suspended objects swung and the sensation of a rumbling sound was clearly felt.

However, to the north such effects were observed in a relatively narrow zone between Thaddo Nadi and Malir River. These effects, having an intensity of II on the MM scale, were also felt over large areas in the southwest direction, up to the Clifton beach. All these areas are situated in the lower part of the Malir River basin, where relatively thick unconsolidated sediments have deposited, whereas the residents and industrial developments in eastern part are on bedrock where loose materials are relatively thin.

A possible reason behind the amplification of shaking in the Quaidabad-Landi-Korangi area was its landfills and relatively thick unconsolidated soil deposits, on which houses are built without proper masonry reinforcement. The quake intensity, and consequent hazard, is greater on loose ground than on solid bedrock. Moreover, in this area relatively higher rates of water content is observed, possibly due to enormous leakage from water and sewerage lines and the presence of inherent saline water. Such a situation can contribute favourably to liquefaction and subsequent amplification in the seismic intensity levels.

Possible causes

The causes of an earthquake can be divided into active (tectonic) or passive (non-tectonic) ones. Active reasons are influenced by tectonic processes prevailing over the globe, like convergence of plates, divergence of plates and/or transformation of plates. Tectonic earthquakes are generated by abrupt rock slippage through the mechanism of elastic rebound.

Strain energy is built up slowly in the Earth’s crust due to regional tectonism. Relief comes when a rupture takes place (or an existing rupture gets re-activated) along the plane of weakness — the fault. When a fault occurs or an existing fault re-activates, there is an elastic snapping back of the strained rock. This produces a vibration that travels through the ground. This is what we call an earthquake. The faults are ubiquitous. But not all faults produce earthquakes. Each fault can be studied to determine its potential to generate quakes.

Passive sources are influenced by anthropogenic activities of a diversified nature and also by some natural hazards, such as meteorites crashing to Earth and major landslides. The non-tectonic earthquakes are those that are induced by the water load in reservoirs, by enormous extraction of fluids from the ground, by injection of highly compressed fluids into the ground, by overloading of water-saturated unstable areas, by quarrying and/or mining, or by other similar activities.

Such earthquakes are generally small and have very shallow focal depths. Their characteristics are not related to fault zones and they occur confined to or closely adjacent to areas of loading or unloading.

As mentioned earlier, in 1998 as well as 2005 the epicentral locations and focal depths of micro-earthquakes in and around Karachi could not be calculated mainly because not enough information was available. In such a situation, a research-cum-applications study, aimed at collecting the baseline data to determine the causative factors, needs immediate attention.

Generally, it is observed that small earthquakes can have a widespread distribution unrelated to major structural features. They can occur wherever there is some distortion in an otherwise uniform stress field. The several shallow and low-magnitude activities in the coastal areas of Karachi in 1998 and 2005 were most probably non-tectonic quakes resulting from developmental or other activities in the city.

Because of the geological conditions and the developmental activities there, it is expected that DHA will be affected more by the low-magnitude earthquakes as compared to other areas of Karachi, because this area has entirely been reclaimed from the sea.

Moreover, the subsurface material are loose, which are generally saturated by saline water on which overloading can easily become one of the causative factors for generating seismic activities. Under such conditions, the area becomes more prone to seismic deformation due to the possibility of liquefaction.

Recommendations

It is recommended that the following studies are undertaken without delay in order to better understand the possible reasons behind the tremors rocking Karachi. Failure to undertake such studies may lead to disaster:

Comprehensive seismicity studies; detailed geological mapping; hydro-geological studies showing groundwater flow dynamics; studies describing the chemical, physical, and geo-technical characteristics of subsurface soils, rocks and water; demographic studies; studies on the changes in coastal configuration; studies on the ecological changes in rivers; and, geophysical studies.

All these studies should be integrated to develop a GIS model for the entire zone. Guidelines for remedial and restoration measures should be also formulated. In addition, an efficient seismic recording network needs to be set up on a priority basis.

Dr N.A. Zaigham zaigham@gerrys.net works as a professor for Karachi University’s geology department while Z.A. Nayyer life9to5@yahoo.com is a postgraduate student of applied physics at the same university

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