Seismic Risk in Northern Pakistan and the KILLER Earthquake of 8th October 2005
Approximately 10,000 large and small earthquakes have been accurately located within a 400 by 500 Km (200,000 Sq Km) area in northern Pakistan.
FIG.1. MAP SHOWING REGION WITH HIGH SEISMIC ACTIVITY. AREAS OTHER THAN QUATERNARY BASINS ARE DOTTED.
LAMONT-DOHERTY GEOLOGICAL OBSERVATORY USA CONTRIBUTION NO. 2690 SEISMICITY OF THE HAZARA ARC IN NORTHERN PAKISTAN BY LEONARDO SEEBER AND JOHN ARMBRUSTER
These modern day data are obtained from two tele metered seismic networks installed and operated under the auspices of the Pakistan Government Agencies (WAPDA and PAEC) for assessing the seismic risk at engineering sites of National importance. The data supports high seismicity in Hazara arc in the Northern Pakistan. (Fig.1).
Recent earthquake 8-10-2005 Pattan earthquake 1974
FIG 2. SHOWS THE SEISMICITY PATTERN OF PAKISTAN AND NEIGHBORING COUNTRIES AS REPORTED BY THE USGS FOR SHALLOW EARTHQUAKES DEPTH < 70 KM FROM 1961-1974. THE PATTAN EARTHQUAKE OF 1974 AND THE RECENT EVENT OF 8-10-2005 WITH THE EPICENTER LOCATED 95 KM NNE OF ISLAMABAD ARE SHOWN. THESE EPICENTERS ARE LOCATED ALONG THE CONSPICUOUS ALIGNMENT OF THE EPICENTRAL TRACTS WITH THE REGIONAL FAULTS. (ADAPTED FROM ARMBRUSTER AND OTHERS, 1978)
The GSP has amassed an exhaustive contribution of reports, maps and papers since the last three to four decades among which the isoseismal map, seismic hazard zone map, seismo-tectonic map, seismic risk map, photo-grammetric compilation of active fault map of Pakistan and many more. These maps form the basic archive to define, refine and use them to draw concrete conclusions for identification and classification of regions into zones having a lower or higher vulnerability to earthquakes by the Town planners and structural engineers to plan and design cities and structures in consonance with the needed seismic parameters for minimizing the human sufferings and National losses in the event of an earthquake .
The sudden and unpredictable natural assassin had once again delivered the unwanted - the accumulated strain gestated for decades by the Mother Earth. The primary suspect manifesting on 8.10.2005 in the morning with a Richter Magnitude of 7.6 and epicenter located about 95 km NNE of Islamabad in the Hazara-Kashmir Syntaxes in close proximity to the MBT (focal depth = 10-20 Km a shallow focus quake ) wrecked havoc causing colossal damage to life and property . The death and destruction in Muzaffarabad, Hazara, Kohistan, NWFP, Islamabad, AJK and disputed Kashmir areas is a geological reckoner of the seismically active , tectonically unstable region of northern Pakistan where the Indo-Pak and Eurasian convergent Plates characterized by the continent-continent collision , obduction and thrusting is alive. This north collisional belt of intense folding, faulting and the presence of the Main Karakoram Thrust (MKT), the Main Mantle Thrust ( MMT) and the Main Boundary Thrust ( MBT ) backed with the historical and instrumental seismicity records from 25 AD to present show more than 150 events with Intensity ranging from VI-X and magnitude in the range from V-VIII and focal depth in general in the range of 50-80 Km with a few occasional departures of 100 – 200 Km depth indicate the northern areas to be under going active tectonic transformation rendering the area as a highly seismically hazardous zone where significant to severe damage by an earthquake could be expected.
The Muzaffarabad Earthquake of 8th October 2005
The recent event of Richter Magnitude 7.6 corresponds to an Intensity Range of IX-XI with inferred Peak Ground Acceleration (PGA) in the range of 0.3 to 0.6 g releasing about 2.59 * 10^ 22 ergs (energy). Ref: Seismic Danger Zone map of Pakistan by GSP Fig.3.
FIG.3 The map shows the extreme northern parts and areas surrounding Islamabad to lie in significant to serious seismic danger zones with peak ground acceleration from 0.15 to 0.6g with a corresponding Magnitude range of 6.5 – 8.5 and IX to XII equivalent Intensity range . Just as a reference masonry are damaged by events at Intensity VIII, Foundations damaged at Intensity IX, Buildings destroyed at Intensity X ~ 0.6g It can be very confidently inferred that the tectonic boundaries (thrust faults) and their off shoot faults with an area of influence from 10 to 200 km depending upon the focal depth and magnitude have brought many important cities of the northern part under lurking danger.
FIG.4. The active tectonic structures and shallow crustal seismicity has deduced from the seismic network monitoring data
Based on the seismo- tectonic studies of Hazara and Kohistan the MBT , west of the syntaxis shows downward extension towards north in Kohistan and is called the Indus –Kohistan seismic zone. On the basis of the composite fault plane solution the IKSZ is considered a thrust and is seismically active . The epicenter of the Pattan earthquake of 1974 was located in this zone. The magnitude was 6.5 with a focal depth of 10-20 Km . It was the Pattan- MMT fault contributing to that event . The IKSZ trends NNW-SSE for about 40 Km connecting the Hazara syntaxis with the MMT in the vicinity of Pattan. (Fig.4).
The recent earthquake which is a major one has made it mandatory to update the existing National Seismic Hazard Map. Any event in the future of these dimensions triggered on or along the regional faults or off shoots would therefore wreck havoc to cities in close proximity like Islamabad, Rawalpindi, Attock, Muzaffarabad, Pattan, Parachinar, Miran Shah, Mian wali, Peshawar, Saidu Sharif Gilgit, Chitral situated in the northern part bounded by Lat 32° to 37° and Long 69° to 76° .
The need of the hour is an initiation of an effective National Seismic Hazard Study program in which expansion of seismic monitoring network, research work in earthquake monitoring be done to demarcate and delineate high risk zones with more precision and to make better efforts for mitigation of this natural calamity.