Geology, Intensity and Aftershocks of Gorkha Earthquake

Geology

M6+ Himalayan region earthquakes, 1900–2014
Main articles: Geology of Nepal and Thrust tectonics
Nepal lies towards the southern limit of the diffuse collisional boundary where the Indian Plate underthrusts the Eurasian Plate, occupying the central sector of the Himalayan arc, nearly one-third of the 2,400 km (1,500 mi) long Himalayas. Geologically, the Nepal Himalayas are sub-divided into five tectonic zones from north to south and, east to west and almost parallel to sub-parallel. These five distinct morpho-geotectonic zones are: (1) Terai Plain, (2) Sub Himalaya (Sivalik Range), (3) Lesser Himalaya (Mahabharat Range and mid valleys), (4) Higher Himalaya, and (5) Inner Himalaya (Tibetan Tethys). Each of these zones is clearly identified by their morphological, geological, and tectonic features.

The convergence rate between the plates in central Nepal is about 45 mm (1.8 in) per year. The location, magnitude, and focal mechanism of the earthquake suggest that it was caused by a slip along the Main Frontal Thrust.

The earthquake's effects were amplified in Kathmandu as it sits on the Kathmandu Basin, which contains up to 600 m (2,000 ft) of sedimentary rocks, representing the infilling of a lake.

Based on a study published in 2014, of the Main Frontal Thrust, on average a great earthquake occurs every 750 ± 140 and 870 ± 350 years in the east Nepal region. A study from 2015 found a 700-year delay between earthquakes in the region. The study also suggests that because of tectonic stress buildup, the earthquake from 1934 in Nepal and the 2015 quake are connected, following a historic earthquake pattern. A 2016 study on historical great (M ≥ 8) earthquake pairs and cycles found that associated great earthquakes are likely to occur in West China region through the 2020s.

Intensity

Isoseismal map for the Gorkha earthquake annotated with values on the Mercalli intensity scale
According to "Did You Feel It?" (DYFI?) responses on the USGS website, the intensity in small parts of Kathmandu was IX (Violent).[1] In most of Kathmandu the intensity was VI, as evidenced by the numerous undamaged water towers installed on top of undamaged multi story buildings. Tremors were felt in the neighboring Indian states of Bihar, Uttar Pradesh, Assam, West Bengal, Sikkim, Jharkhand, Uttarakhand, Gujarat, [better source needed] in the National capital region around New Delhi and as far south as Karnataka.[better source needed] Many buildings were brought down in Bihar. Minor cracks in the walls of houses were reported in Odisha. Minor quakes were registered as far as Kochi in the southern state of Kerala. The intensity in Patna was V (Moderate).[not in citation given] The intensity was IV (Light) in Dhaka, Bangladesh.[1] The earthquake was also experienced across southwestern China, ranging from the Tibet Autonomous Region to Chengdu, which is 1,900 km (1,200 mi) away from the epicenter. Tremors were felt in Pakistan[46] and Bhutan.

Aftershocks

Main article: List of aftershocks after the 2015 Nepal earthquake
A series of aftershocks began immediately after the mainshock, at intervals of 15–30 minutes, with one aftershock reaching 6.6Mw within 34 minutes of the initial quake. A major aftershock of magnitude 6.9 Mw occurred on 26 April 2015 in the same region at 12:54 NST (07:08 UTC), with an epicenter located about 17 km (11 mi) south of Kodari, Nepal. The aftershock caused fresh avalanches on Mount Everest and was felt in many places in northern India including Kolkata, Siliguri, Jalpaiguri, and Assam. The aftershock caused a landslide on the Koshi Highway which blocked the section of the road between Bhedetar and Mulghat.

A model of GeoGateway, based on a United States Geological Survey mechanism of a near-horizontal fault as well as location of aftershocks showed that the fault had an 11° dip towards the north, striking at 295°, 50 km (31 mi) wide, 150 km (93 mi) long, and had a dip slip of 3 m (9.8 ft). The USGS says the aftershock registered at a shallow depth of 10 km (6.2 mi).

Assuming that 25 April earthquake was the largest event in this seismic episode, Nepal could expect more than 30 aftershocks greater than magnitude 5 over the following month. As of 24 May 2016, 459 aftershocks had occurred with different epicenters and magnitudes equal to or above 4 Mw (out of which 51 aftershocks are equal to or above 5 Mw and 5 aftershocks above 6 Mw) and more than 20,000 aftershocks less than 4 Mw.

Source : wikipedia.org