Seismologically speaking the week of 19-25 September was dominated by the magnitude 7.7 (M7.7) earthquake which struck southern Pakistan. One of the largest to have occurred this year, it may turn out to be one of the most deadly as reports of death and injuries continue to emerge.
That apart, the week was relatively quiet. The United States Geological Survey’s real time earthquake map showed 1477 earthquakes (all magnitudes in the US and its territories and at least M4.0 elsewhere) of which 30 were ≥M5.0. Of these, five were associated with the Pakistan tremor.
The majority of the rest occurred around the complex boundary between the Pacific and Australian tectonic plates in the western Pacific, with four tremors of at least M5.0 occurring in the Papua New Guinea-Solomon Islands area.
The Week’s Largest Earthquake: M7.7, Balchistan Province, Pakistan
The largest earthquake of the week was an M7.7 which occurred in Baluchistan province, in the south of Pakistan, on 24 September. Initial reports did not indicate any deaths or injuries but as news began to filter through from this (relatively remote) region, it has become clear that there’s significant damage and loss of life.
News reports suggest that, 24 hours after the earthquake, the death toll is at least 320 with hundreds more injured. The number of fatalities is expected to rise: ABC news quoted a local official as saying that “many more deaths were feared.” Damage is also thought to be extensive, with several villages, constructed from mud bricks, completely obliterated by the tremor; while it’s also reported that the earthquake caused a new island to emerge off the Pakistan coast.
Pakistan is tectonically unstable, located as it is across the series of long north-south trending strike-slip faults which mark the western boundary of the collision between India and Eurasia and with the Makran subduction zone, where the Arabian plate is forced underneath the Eurasian continent, to the south. Both of these boundaries are seismically active. Early USGS reports suggest that the cause of the tremor was rupturing within the Eurasian plate as a result of subduction.
US Earthquakes: Wyoming
In Alaska the sequence of aftershocks from the M7.0 of 30 August at last seems to have died down after a fortnight of on-going but decreasing tremors. This largest tremor in the lower 48 states was an M4.9 which occurred in Wyoming, in the Wind River Range. Large, but not exceptional in magnitude, this earthquake occurred at the extreme eastern edge of the Basin and Range province, which his governed by extensional tectonics and where normal faulting (downward slippage) is the usual fault mechanism.
The Deadly Earth
Although the death toll from this week’s earthquake in Pakistan is small compared to the loss of life following other major tremors in the country (in 2005 an M7.6 in the north of the country killed an estimated 80,000) it illustrates the vulnerability of certain areas to earthquake activity, as traditional building prove unable to withstand shaking. Had the area not been so sparsely settled, it’s likely that many more deaths would have occurred. People in seismically-active areas with traditional building methods and high populations will continue to be at risk of considerable loss of life from earthquakes of this magnitude.
ABC news. Pakistan earthquake kills more than 320 people with death toll expected to rise. (2013). Accessed September 25, 2013
BBC news online. Pakistan earthquake: Hundreds dead in Balochistan. (2013). Accessed September 25, 2013
USGS. M7.7 – 66km NNE of Awaran, Pakistan. (2013). Accessed September 25, 2013
USGS. Real time earthquake map. Accessed September 25, 2013
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.