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Seismic Hazard in Karachi, Pakistan: Uncertain Past, Uncertain Future

Cooperative Institute for Research in Environmental Sciences and the Department of Geological Sciences
University of Colorado at Boulder
Boulder, Colorado 80309-0399 USA
roger.bilham{at}colorado.edu
(R.B.)

View larger version (53K): [in this window] [in a new window]   Figure 1. Tectonic settings of Karachi and Los Angeles to same scale with the growth in their populations shown at right. A 250-km-radius circle around each city in the figure is shown with simplified tectonics.

View larger version (81K): [in this window] [in a new window]   Figure 2. Locations of active faults and dated historical earthquakes with inferred ruptures outlined. Locations of moderate events shown only by date; smaller shocks (3.8 < M < 5.5) as squares proportional to magnitude. The Sonne fault offsets ridges in the accretionary wedge at 2-5 mm/yr (Kukowski et al. 2000). No large earthquake is known historically on the Ornach Nal system. The dashed oval is the inferred 1765 event depicted by Byrne et al. (1992); we show its date and size and location to be conjectural. Although the 1819 earthquake was apparently similar or larger in magnitude than the 2001 Bhuj event, little damage occurred in Thatta and Hyderabad in 1819 compared to 2001 even though the former event was closer.

View larger version (135K): [in this window] [in a new window]   Figure 3. Henry Cousens' 1897 photo (Cousens 1929) of the ruined tower at Brahmanabad (Bhamanabad), whose excavation had been first reported by Bellasis (1857a, b; Cousens 1905). Crushed skeletons and scattered coins indicate its destruction by an earthquake within the decade or so following A.D. 975, the date of the youngest coins buried at the site.

View larger version (32K): [in this window] [in a new window]   Figure 4. Mughal revenues from towns in Thatta province in 1596 in millions of dams, the 16th-century coinage. Numbers for the town of Thatta are missing from the A'in-i Akbari listings. Samawani is clearly in the second rank of the 51 shown.

View larger version (97K): [in this window] [in a new window]   Figure 5. Locations of historically damaging earthquakes at Brahmanabad and Samawani and elsewhere in Sindh province. We show Habib's 1982 location for Samawani. The five jurisdictions of the Mughal administrative division of Thatta (Habib 1982) are shaded within a dashed envelope: S = Sewistan or Sehwan, N = Nasarpur or Nassirpoor, H = Chakrahala, G = Chachgan, and T = Thatta. The destruction of Brahmanabad and Samawani by earthquakes in central Sindh, followed by avulsions of nearby rivers, suggests a common tectonic cause. Hyderabad was founded on the ruins of Nerunkot in 1768.

View larger version (44K): [in this window] [in a new window]   Figure 6. Before and after the 1819 earthquake (see figure 2 for location of section). Upper profile uses Baker's 1844 leveling data north of points E and D. Although the pre-earthquake surface morphology between E and C is disputed, the depth of the river was reported 2 fathoms (4 m) by Grindlay (1808). Lower profile is the section leveled by Baker (1846) across and through the Allah Bund (Oldham 1898) corrected for river sinuosity. Baker's 56-km-long river profile has an estimated cumulative error of < 20 cm and shows that the base of the Puram (Narra) was typically 4 m below the bank (consistent with Grindlay). Fort Sindri (on the banks of the Puram) sank < 1 m in 1819 but the Sunda region shallowed in 1819 (Burnes 1833; Grant 1837).

View larger version (49K): [in this window] [in a new window]   Figure 7. (A) GSHAP hazard map of Pakistan (Giardini et al. 1999; color scale indicates peak ground acceleration (m/s/s) with 10% probability of exceedance in 50 years) compared to (B) a recently revised hazard map following the 2005 earthquake (working group on Pakistan Hazard 2006; zonation 4 is most hazardous, 1 is least hazardous).

View larger version (35K): [in this window] [in a new window]   Figure 8. Enlarged view of contours of peak ground acceleration (g) for 10% probability of exceedance in 50 years from the GSHAP map and from the new 2007 map for Pakistan. Neither is reliable.