Probabilistic Seismic Hazard Analysis For Quetta City, Pakistan by Shafiq Ur Rehman, Najeeb Ahmed Amir,Conrad Lindholm and Zahid Rafi Pakistan Meteorological Department and , Norway EARTHQUAKE HAZARD, QUETTA, PAKISTAN Technical Report No. PMD-16/2012 January, 2012 Page 2 of 49 EARTHQUAKE HAZARD, QUETTA, PAKISTAN Table of Contents Preface ………………………………………………………………………………..5 Summary ....................................................................................................................... 6 1 Introduction ...................................................................................................... 7 2 Technical approach .......................................................................................... 9 2.1 Design codes and construction details ............................................................... 9 2.2 Methodology of probabilistic seismic hazard analysis .................................... 11 2.3 Probabilistic seismic hazard analysis ............................................................... 13 2.3.1 Theoretical framework ................................................................................... 13 2.3.2 The earthquake recurrence model ................................................................... 14 3 Geologic and seismotectonic setting ............................................................. 15 3.1 Mode of faulting around Quetta ....................................................................... 16 4 Earlier seismic hazard and zoning results ................................................... 17 5 Assessment of earthquake potentials ........................................................... 18 5.1 The largest earthquakes within the greater area ............................................... 18 5.2 Earthquake catalogues ..................................................................................... 19 5.2.1 Historical earthquakes .................................................................................... 19 5.3 Geographical distribution of earthquake databases ......................................... 21 6 Seismotectonic zonation and quantification ................................................ 22 6.1 Magnitude conversions .................................................................................... 22 6.1.1 Some manual magnitude assessments ............................................................ 24 6.2 Completeness, aftershocks and magnitude-frequency relation ........................ 24 6.2.1 Aftershocks ..................................................................................................... 24 6.2.2 Completeness .................................................................................................. 24 6.3 Seismic zonation .............................................................................................. 27 6.3.1 The local faults included in the hazard model ................................................ 28 6.4 Model parameters............................................................................................. 30 6.4.1 Zonation and distribution of activity .............................................................. 33 6.5 Fault modeling ................................................................................................. 33 7 Ground-motion models .................................................................................. 34 7.1 General review of models ................................................................................ 35 7.2 Considered models ........................................................................................... 35 7.2.1 Sigma (σ) values ............................................................................................. 37 8 The computational model .............................................................................. 37 9 Seismic hazard and loading results .............................................................. 39 10 References ....................................................................................................... 41 11 Glossary .......................................................................................................... 44 Page 3 of 49 EARTHQUAKE HAZARD, QUETTA, PAKISTAN Illustration on front cover: Quetta at night seen from south Page 4 of 49 EARTHQUAKE HAZARD, QUETTA, PAKISTAN Preface Page 5 of 49 EARTHQUAKE HAZARD, QUETTA, PAKISTAN Summary Quetta city is located in the most active seismic zones of Pakistan with shallow focus earthquakes, recorded very close to the city. Over the past century Quetta has been jolted by several large events. One earthquake in 1935 destroyed almost entire city and causing casualties of around 35,000 people. Over the historical time span at least nine earthquakes with intensities between VII and X have been reported around Quetta. This stipulates the need for seismic hazard analysis of Quetta city, based on probabilistic state-of-the-art methodology. The present research comprises of the study of active tectonics of the region, with focus on updating the existing fault systems of the study area for preparation of a seismotectonic map. The data was collected from various contributing agencies to form a composite earthquake data catalogue. The seismotectonic map and catalogue was used as the basis for the Probabilistic Seismic Hazard Analysis (PSHA) relying on area and line source modeling techniques, applying the CRISIS software (Ordaz et al., 2003). No well documented ground motion prediction equations exist for the Pakistan territory. Therefore we applied a relation developed from data of similar tectonic regimes (active compression). This technique was found suitable for shallow focus seismicity for different return periods and spectral ordinates. The results of the probabilistic seismic hazard analysis of Quetta for the return period of 500 years indicate a PGA ground motion of 4.8 m/s2. Spectral accelerations were calculated for four different sites (Quetta North, Quetta South, Quetta East and Quetta West) of Quetta city with period between 0.003s to 2.5s. The highest spectral acceleration of 11.6 m/s2 was observed at 0.20 s for Central Quetta. Page 6 of 49 EARTHQUAKE HAZARD, QUETTA, PAKISTAN 1 Introduction Quetta is the capital of Balochistan, and at the same time it is a trade center and commercial hub for a main trade route between Pakistan and Afghanistan (Fig.1.1). Quetta and its surroundings are among the most earthquake active areas in Pakistan. In 1935, the city was devastated by a major earthquake event documented as a magnitude M=8.1 event. Figure 1.1. The location of Quetta in western Pakistan bordering Afghanistan. In the early morning on 31 May 1935 a violent earthquake took place, which lasted for three minutes and followed by continuous aftershocks. Although there were not good enough instruments to precisely measure the magnitude of earthquake. However, estimates cite the magnitude as being a minimum of Mw 7.7 and possibly as high as Mw 8.1. The epicentre of the quake was established at 4-kilometres south-west of the town of Ali Jaan in Balochistan, that is 153-kilometres away from Quetta. The earthquake caused destruction in Quetta and almost all the towns around the city. Its tremors were felt as far as Agra in India. The largest aftershock of 5.8 Mw was measured three days after the main earthquake. It did not cause any damage in Quetta but the towns of Mastung, Maguchar and Kalat were seriously affected by this aftershock. According to some authors as many as 35,000 were killed by the Quetta, 1935 earthquake (Din Muhammad 2008). In 1931 this region already experienced two major earthquakes. The first of these near Sharigh, of Mw 6.8 on 24th August 1931 followed by the Mach earthquake of Mw 7.3 on 27th August 1931. Both these earthquakes caused huge damages to the property and loss of lives, though not comparable in devastation with the 1935 earthquake. Page 7 of 49 EARTHQUAKE HAZARD, QUETTA, PAKISTAN Figure 1.2. Location of Ali Jaan, assumed to be the epicenter position of the 1935 earthquake (red marker) as well as the city of Quetta (black marker) is shown. Figure 1.3. Location of Bolan Pass is assumed to be close to one of the 1931 epicenters (Szeliga et al, 2009). Page 8 of 49 EARTHQUAKE HAZARD, QUETTA, PAKISTAN 2 Technical approach 2.1 Design codes and construction details The U.S. Army Corps of Engineers have issued a manual for Engineering and Design (U.S. Army Corps of Engineers, 1999) in which several general guidelines are included. Te technical approach in that manual is generally deterministic but it contains key concepts that are applicable to the present study. The seismic assessment follows the key steps as below: Establishment of earthquake design criteria. In the present case this means that the definitions of Maximum Design Earthquake (MDE) and Operating Basis Earthquake (OBE) are commonly understood. Development of ground motion, corresponding to the MDE and OBE levels. Establishment of analysis procedures, i.e. procedures applied to reveal how the structure responds to the specified ground motions. Development of structural models. Prediction of earthquake response of the structure. Interpretation and evaluation of the results. In the present study we will exclusively focus on the second point as above, except that we refrain from using the terms MDE or OBE. Because these terms are relevant for sensitive structures in particular, definitions of MDE and OBE as produced below gives the