EXPERT REACTION: Nepalese earthquake

Unfortunately, this was a tragedy waiting to happen – a 7.8 magnitude earthquake in a highly seismic zone striking a low socio-economic country with local construction that is highly vulnerable - a blend of abode construction, unreinforced masonry and poorly detailed reinforced concrete structures. Reviewing the damage, there were no surprises and very sadly the death toll will rise.

A strong earthquake with magnitude of 7.8 struck Nepal on 25 April 2015 (06:11:26 - UTC) and the epicentre of the earthquake was 34km away from city of Lamjung and 77km away from Kathmandu. Several aftershocks have occurred since, with the largest having magnitude of 6.7 occurred 25hrs after the main earthquake and 19km away from City of Kodari (67km east of Kathmandu). The population growth rate in Kathmandu has been more than 3 per cent per year in the last 30 years. Kathmandu is located in a synclinorium tectonic valley controlled by faults in the central zone. Typical stratigraphy of the area shows deep layers of alluvium consisting of sand and clay deposits extended more than 100m below the ground surface. Thus it is expected that high rise buildings with high natural vibration period would be more influenced in the region. 

More than 80 per cent of Nepal has hilly/mountainous terrain, leaving less than 20 per cent in the Terai marshy lands and forests. Since the region is one of the most activate tectonic areas with the India plate converging with Eurasia plate (at a rate of more than 40 mm/yr on average) and intense rainfalls, large landslides are expected. The intense rainfalls occur typically in monsoon season in Nepal between June and September each year. Areas in the proximity of Tamakoshi River in central Nepal may be susceptible to large landslides after this recent earthquake and aftershocks. Areas close to natural slopes with intense precipitation rate should receive particular attention now.

Nepal has more than eight major dams and hydropower stations; large embankment dams such as Kulekhani Dam (114m high) located 40km away from Kathmandu should be inspected immediately for possible cracks or flow slides. Large earthquakes can change the stress state in the embankment dams increasing tensile stresses and in some occasions decreasing the shear strength due to the increase in the pore water pressure.

The devastating Nepal earthquake illustrates the cause of sad frustration for seismologists. We can learn much from past earthquakes, but cannot predict future earthquakes. In 1991 I was working on an earthquake hazard study for Kathmandu. The previous large earthquake in the area was in 1934, and before that 1833. It was common local belief that a major earthquake would not occur for about 40 years.

Little seismological data was available, and most of that was only since improved seismograph coverage in 1960. Nepal has a written history dating back about 1,000 years, mainly relating to royalty, but including references to major earthquakes. Collating data from several sources, local seismologist M.R. Pandey and I found devastating earthquakes with magnitudes greater than about 7.5 had occurred on ten occasions, on average about every 100 years! However the interval between them varied from 23 years (1810 to 1833) to 273 years (1408 to 1681). This is a very short recurrence interval for a devastating earthquake. We can say that there will be another earthquake in Kathmandu, we cannot say when, but on a scale of tens to hundreds of years it will be sooner rather than later.

A large earthquake in the Himalaya region occurs at relatively shallow depth, so can cause more damage than the more common large offshore earthquakes on faults that dip deep underneath the adjacent continent, as in recent Japan, Chile or Indonesia earthquakes. In mountainous areas, widespread landslides, rockfalls and creation of weak temporary natural dams that then fail catastrophically can be a major problem. Unfavourable soil conditions in the Kathmandu region give increased likelihood of damage. The Nepal earthquake will be more similar to the 8 May 2008 magnitude 7.9 Wenchuan earthquake in Sichuan, China, which caused 69,000 fatalities and caused many landslides, or the October 2005 Pakistan earthquake.

The Nepal earthquake on 25 April was, unfortunately, a tragedy waiting to happen. The earthquake occurred in an area where the Indian subcontinent is pushing northward into Eurasia, causing uplift of the Himalayan mountain range. This uplift occurs via repeated large earthquakes on faults throughout the region, the latest being the M=7.8 event on Saturday.

Although the epicentre was approximately 77 km NW of Kathmandu, the earthquake rupture area extended more than 100 km to the SE with the largest amount of slip immediately north of Kathmandu. This, combined with earthquake prone construction (brick and unreinforced masonry) led to the collapse of many buildings in Kathmandu, and likely in surrounding rural areas. The earthquake also triggered landslides and avalanches in the steep mountain terrain leading to additional loss of life.

Large aftershocks are likely to continue over the next days to weeks and these may cause further building collapses, landslides/avalanches, and loss of life.

The tragedy is that homes and shops in Nepal will be rebuilt from the rubble in the same style as the former home. The next earthquake will knock it down again.

The Australian Government is sending Band-Aids - what the people need is education and now is when they need it, education and demonstrations on how to make their buildings more resilient before they rebuild. Some good Australian reinforcing steel would be very useful, that and the tools to use it to reinforce their masonry buildings.

The Australian Government should reinstate aid programs they have cut from South West Pacific countries where the earthquake hazard is double that in Nepal. They need a Building Code including a hazard map, unlike the draft Nepal standard from 1993 which rates the hazard about the same as Sydney and Melbourne and has never been adopted let alone been subject to enforced regulation.