104-01-Week Two

  Central Nepal Earthquake - April 25th, 2015

A large earthquake of magnitude Mw7.8 occurred in central Nepal on April 25th, 2015. This earthquake is responsible for more than 5000 deaths and massive destructions in the city of Kathmandu and the surrounding areas.

       The April 25 earthquake has been followed by many aftershocks, as for example a Mw6.7 event on the day that followed the main shock. Most of the aftershocks are close to edges of the ruptured zone as it may be approximately delineated from seismological and geodetic observations. They are more particularly concentrated near its eastern edge. On May 12, a new earthquake of magnitude Mw7.3 occurred in this eastern zone with the same mechanism as the main shock (shallow dipping thrust). For its characteristics and location, that earthquake appears to belong to the same sequence than the main shock and should be considered as strong aftershock.

Based on geological cross-sections, crustal structure model, and the preliminary seismological data available few days after the main shock, it appears that the earthquake has ruptured a piece of the Main Himalayan Thrust (MHT). The rupture started at the epicenter, about 80km to the northwest of Kathmandu and propagated eastward for about 130 km, rupturing the area directly located under the capital city of Nepal. Preliminary models of co-seismic slip actually suggest that the largest amount of slip (2 m to 5 m) on the fault would be located just below the city. The focal mechanism derived from the analyses of seismological data shows a fault plane striking N143°E, with a very shallow dip of 7° towards the North.

Numerous aftershocks continue to happen since the occurrence of the main shock. Some of these aftershocks have a magnitude way above 6 with two events with magnitude respectively of 6.6 and 6.7. The first one was located in the epicentral area and the later one occurred to the North East of Kathmandu. Many more aftershocks with smaller magnitude continue to occur every day. The aftershocks are distributed in an area roughly 150 km long and 50 km wide, with the majority of the aftershocks located in the eastern part of the ruptured area. At first order, the area defined by the bulk of aftershocks corresponds to the ruptured area on the fault plane. The location of the ruptured area, together with the shallow dip of the focal mechanism, suggest that it is most probably the sub-horizontal part of the MHT that has ruptured, mostly the deeper part of it. The possible spatial extent of the ruptured area is shown in red on the cross-section of the Himalaya front at the crustal scale. The quasi absence of aftershocks to the south of the valley of Kathmandu suggests that the rupture has not propagated southward to reach the surface along the Main Frontal Thrust. This, however, will have to be further assessed in the weeks to come.

The Himalayan front has been the location of many large earthquakes in the past. Although they are not all well documented, recent paleoseismological investigations provided a better idea of the past seismicity of the area, and more precisely of the lateral extent of the Mw8.2 event that devastated Kathmandu in 1934. In the same study, the penultimate event has also been identified, in 1255AD, and the series of previous events, yielding a return time close to 700 – 800 yrs (Sapkota et al., 2013, Bollinger et al., 2014). The most recent work by the international team formed of French researchers (L. Bollinger and Y. Klinger for CEA and IPGP respectively), Singaporian researchers (P. Tapponnier and his team, EOS) and Nepalese researchers (S. Sapkota and his team, DMG) has shown, based on the balance between the energy dissipated by past earthquakes and the long term tectonic loading, that the area located between the earthquake of 1934, to the east, and the event of 1505, to the west, that did rupture last in 1344AD, was very close to rupture, as proved by the event of April 25th, 2015.

Structure of the Lead:

Who-not given

Where- Nepal

When- April 25th, 2015

What- A large earthquake of magnitude Mw7.8

Why-not given

How-not given

Keywords:

1.magnitude:大小

2.ruptured:破裂
3.seismological:地震學上的
4.preliminary:初步
5.epicenter:震中
6.propagate:傳播
7.seismicity:地震
8.penultimate:倒數第二
9.tectonic:構造