How big earthquakes rattle spring dynamics

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The massive 7.6-magnitude earthquake that hit Nepal on 25 April has left unprecedented damage in life and property. The epicenter of the earthquake was in Gorkha district, 150 km west of Kathmandu. However, the ongoing series of aftershocks have occurred between Gorkha to the west and Dolakha to the east. This blog aims to highlight how earthquakes impact springs in the mountains. 

A major impact has been the movement of ground, and the land surface is reported to have shifted both horizontally and vertically by +- 1.5 meters. An interesting phenomenon related to this is the underground land deformation that has affected the groundwater flow movement. News reports have indicated three different types of impact on water flow movement: i) Water flowed from the dry (non-functional) stone spouts, ii) New springs were observed and old ones yielded more water, and iii) Rivulets dried out. 

Science indicates that these are all caused by changes in the water flow channel in the underground surface. But, let’s first understand the groundwater processes. 

Rainwater generally infiltrates into soil surface, and some of the water saturates the soil up to few meters depending upon the topography and soil types. Once the soil is saturated, the water further moves down which is known as percolation. The percolated water is then stored in underground geological structures such as fractures and cracks. The storage capacity and outflow from these structures depend upon the type of rocks and geological formations. In general, the more the cracks among rocks, more water is stored. When these cracks are filled with water, water starts to flow to the lower elevation areas under the influence of gravity. During this process, the water develops its interconnected pathways as shown in Figure 1 and the flow continues for many years. When this flow emerges on the surface, we call it a spring. Spring water is widely used for domestic purposes in rural areas, and springs are often the primary source of water in many mountain villages.

During large earthquakes, these interconnected pathways might change course due to the movement of the tectonic plates deep underneath the ground. So during this process, some portions of rocks and ground get squeezed while others expand, both leading to change in the underground water table. Due to this, on the one hand, the interconnected pathways might gain more water through widening of the cracks deep underground or by combinations of several water pathways. On the other hand, the water may disappear as the path gets closed. Therefore, after a big earthquake, there are always reports of changes in spring water flow.  

Figure 1: Groundwater storage in the pores of geological formations in the interconnected pathways which flows under the influence of gravity. Left: Fractures in granite. Right: Caverns in limestone (Source: USGS http://pubs.usgs.gov/circ/2003/circ1262/)

In the wake of the recent earthquake, I observed a few springs in two mountain districts. 

In Sipapokhare VDC of Sindhupalchok district (Photo 1), there were two springs on the slope (Figure 2). In between these two springs, a new crack developed after the earthquake from which a big flow of water emerged, around 30 liter per minutes (Spring #2). One can only guess that the new crack might have met the interconnected underground water flows which feed the new springs. Similarly, the flow of Spring #1 and 3 have also yielded more water after the quake. 

According to a woman who experienced the earthquake just in front of the springs, the new spring flowed immediately after the first tremor. Similarly, there are news reports that new springs have been observed in many mountain slopes (Table 1). Besides springs, there are many stone spouts in Kathmandu valley whose flow also increased. In urban areas, buildings can affect the underground water flow channel and after the earthquake, the channels might get reoriented and some stone spouts might get more water while others may see a decrease in water flow or may completely go dry. 

Photo 1: The three spring sources in Sipapokhare VDC of Sindhupalchowk district. The rectangle on upper left side is a new spring which feeds Spring 2. The flow of Spring 3 also increased after the earthquake

In Kavre district, where Nepal Water Conservation Fund (NWCF) and ICIMOD have collaborated on spring research project, local communities shared that many springs have yielded more discharge after the earthquake. At Darauni Pokhari village of Panchkhal municipality (the then Darauni Pokhari VDC) many springs have yielded more water in the northern slope. The discharge of the Thulo Dhara springs (Photo 2) which is widely used for drinking and bathing by the local community increased drastically after the earthquake. The Spring 1 discharge increased from 14 to 36 liter per minute, while the discharge of Spring 2 went up from 22 to 53 liter per minute1. However, in the southern slope, two springs have completely dried up (Photo 3). 

In general, there are reports of existing springs yielding more water and new springs emerging as compared to the springs that have dried up. However, it is uncertain if these springs will maintain the increased water flow permanently. It is likely that the increase might be temporary and may dwindle after a few months. Since the monsoon season begins in mid-June, these new developments could prevail until the rains stop. However, it will take some time to precisely understand whether the phenomenon is temporary or permanent. Since springs are the primary source of water in rural areas, it is important to understand how they function and what causes variation in their flows. Therefore, better understanding of geological aspect of spring hydrology is critical and further research needs to focus in these aspects.

Photo 2: A spring that went dry in the southern slope of Darauni Pokhari village  

Photo 3: The Thulo dhara springs in the northern slope of Darauni Pokhari village yielded more discharge after the earthquake

Table 1: News reported in local media

Untitled Document

Water supply types

Area

Sources

Stone spouts that yielded more discharge

Matatirtha, Muruhiti, Sheshnarayan, Pashupati area
 

http://www.onlinekhabar.com/2015/05/275174/

Two small rivers dried in Gorkha

Gorkha district, Barpak area

http://annapurnapost.com/News.aspx/story/11274

Springs that yielded more water

Kathmandu and periphery

http://annapurnapost.com/News.aspx/story/11829

The spring discharge is measured by Nepal Water Conservation Fund (NWCF) regularly as a part of the collaborative pilot study of NWCF and ICIMOD. The discharge is measured every mid-month. The last measurement after the earthquake was from 16 through 21 May 2015.