Could A Large Earthquake Strike Karachi

Apparently yes. In fact, what seems to surprise geologists is why a large earthquake hasn't struck Karachi in the recent past.

Excerpts from Seismic Hazard in Karachi, Pakistan: Uncertain Past, Uncertain Future (1)

Our present, imperfect understanding of earthquake hazard in Karachi reveals an apparent paradox. On the one hand, Karachi sits close to an active plate boundary and is literally surrounded by active faults. We note a striking parallel between its setting and that of another well-known megacity: Los Angeles, California.


On the other hand, in contrast to Los Angeles, Karachi has experienced no damaging earthquakes in the past 150 years and few events large enough to be felt. The question currently faced by earthquake engineers is whether Karachi truly enjoys an aseismic setting or whether the absence of damaging earthquakes is only due to Karachi's short and incomplete history.

A review of the known historical data on earthquakes within 500 km of the city shows that the historical record prior to 1800 is limited and unreliable. Having summarized what is known about tectonics in and around Karachi, it is clear that key questions still remain.

At best the hazard in Karachi might be low. At worst, however, the hazard in Karachi could be roughly comparable to that in Los Angeles, or perhaps even worse in Karachi given its proximity to the subduction zone, for which Los Angeles has no analog.

Considering the number of known active faults that menace Karachi from almost every direction, however, it seems possible if not probable that hazard is higher than that assigned by recent national and global hazard maps.

A review of historical seismicity near Karachi reveals that although it is within striking distance of one or more Mw 8 subduction zone events to the west, reverse faulting earthquakes with 6 < Mw > 8 in Kacchh region to the east, Mw < 7.9 strike ruptures to the northwest, and Mw 6 earthquakes near and possibly beneath the city, little or no data are available to characterise return times and probabilities for any of these events.

A consideration of relative seismic risk in Karachi and Los Angeles leads to even more alarming conclusions. Seismic building provisions, including the Field Act to protect public school buildings, were first adopted in California following the 1933 Long Beach earthquake.

Although Karachi adopted the Uniform Building Code (UBC) provisions for California in 1979, the code was never effectively enforced, a circumstance common to most of Pakistan at that time. Following the Bhuj earthquake of 2001, Karachi was reassigned to be in zone 4, the zone of highest hazard.

It is not clear, however, how long the engineering community will take to adopt the new guidelines, or whether (and how) the government will enforce the new zoning for Karachi. In any case, new building codes will not remedy the known and worrisome vulnerability of existing structures in which 14 million people now work and live.

Moreover

Tsunami hazards exist in Karachi and its contiguous coastline that we have not examined in this article. The > 1-hour delay between the mainshock and the arrival of the damaging tsunami associated with the 1945 earthquake was very probably caused by submarine slumping offshore rather than direct uplift of the coast. If this were indeed the case, even a quite modest earthquake in the Kachchh region might be sufficient to trigger a submarine slide that would endanger the Karachi shoreline.

(1) Roger Bilham, Sarosh Lodi, Susan Hough, Saria Bukhari, Abid Murtaza Khan and S. F. A. Rafiqee, 'Seismic Hazard in Karachi, Pakistan: Uncertain Past, Uncertain Future,' Seismological Research Letters, November 2007; v. 78; no. 6; p. 601-613. (Copyright: Seismological Society of America)