2020 Digana Tremor Report: The findings and recommendations

Colombo (News 1st);  On the 29th August and 02nd September 2020, minor tremors were experienced in the Digana-Haragama area, Kandy raising serious questions on what lies below the Sri Lankan surface.

The Geological Survey and Mines Bureau appointed a committee to study these events and its report outlined the initial observations of the events based on both seismographic information and ground observations. Earth tremors are a seldom occurrence within the landmass of Sri Lanka and most of the reported ones are mainly concentrated in SSE part of the island, as of the historical records. Sri Lanka basement being a part of a stable Precambrian shield is located very far from any active faults or plate margins and having a substantially thick old-cold crust does not provide a favourable site for occurrence of major/frequent earthquakes. However, the country has felt a few tremors in the past, of which the amplitude and the intensity are very low. Majority of these tremors were concentrated to SSE sector of the country and it is presumed that the crust is relatively thinner in this region and thermal activity is higher than the rest of the country. However, of the recent tremors occurred in Digana-Haragama area, the main event took place on 29.08.2020 (20.32 Local Time) and this site is located far from the SSE sector of the country. The area in which the tremors were felt by the dwellers (affected area) has notable geological and geomorphological characters. The area is mainly composed of dolomitic marble and charnockite rocks. Valleys are mainly formed along the marble bands while the high relief areas are underlain by charnockitic rocks which are resistant to weathering. The affected area is surrounded by water-bodies or located just adjacent to such water bodies, most of them being inundated areas of the Victoria Reservoir. There exist several dolomite mines in this area and quarrying with rock-blasting is a common practice in the environment. The mining is concentrated at the left-bank of the river and extends close to the reservoir. The inundated area is surrounded by marble rocks. The water levels of the reservoir has remained constant with very less fluctuations, during the last few months. The main event occurred at 20.32 (Local Time) on 29.08.2020 and this event has been felt by people living in Haragama, Milapitiya, Anuragama and Kiual-Linda areas of the right bank of the Mahaweli River as well as and dwellers at Ambakote, Aluthwatte and Kengalla areas of the left bank of the river. The energy dispersion seems to be anisotropic, which is strictly controlled by geology and geomorphological aspects of the area. Besides of the main event, two events were recorded on the same day prior to the main event and these two events have taken place at 14.18 hrs and 18.50 hrs (Local Time) respectively. Based on the observed seismic data, the epicentre of the main event should be within the proximity of 4km radius of the location of PALK seismic station with magnitude around 2.0 in Richter scale. People living at the edge of the reservoir and along the valley felt higher intensity; though the ground vibration seems to be directed along the valleys and houses built on the dolomitic marble felt relatively higher intensity than the rest of the area, which is composed of charnockite rocks. [embed]https://www.youtube.com/watch?v=yH36_rCrIQw[/embed] The waveform analysis of seismic stations and information received from the general public in the area of influence indicate that this train of events, including the main event, is caused by natural phenomenon; release of stress in the a form of seismic energy. A few critical observations were reported in reservoir close to Aluthwatte, as release of air-bubbles were witnessed coinciding with the 2nd pre-event. Conclusion by the Committee that probed the events are as follows;  1. The events that were taken place since 29.08.2020 (14.18 hrs- local time) is a natural phenomenon related to stress release. These events are presumably very shallow and the main event -29.08.2020 (20.32 hrs- local time) is having a magnitude of approximately 2.0 units in the Richter scale and recorded in the SE direction to the PALK station. The main event should be considered as a tremor, a classification for a very small earthquake. 2. Seismic data analysis confirms that this as a very shallow and a minor event that has taken place about 4km from the PALK station. The ground observations reveal that the location is about 5.2km from the PALK station. The mismatch between calculated and (intensity based) observed event-locution is due to anisotropic nature of the near surface layers, usually caused by local geology. Such ambiguities would have been negated having access to many stations. However, events in this caliber are hardly recorded in distant stations due less energy output and high rate attenuation. 3. Two pre-events that were recorded at the two stations are potentially foreshocks of the main event and the post event that was taken place on 02.09.2020 (07.06hrs, local time) could not be categorize as an aftershock of the main event. Considered the time-lag between the main event and the post-event, it is not possible to conclude the post event as an aftershock of the main event. 4. It could be concluded that all four events took place at approximately the same locality and energy may have been transferred linearly along the valleys. The weathered formations and marble rocks have enhanced the amplitude as many of the dwellers in such areas felt the event with relative high intensity than the near-by areas which are composed of charnockite rocks.   [embed]https://www.youtube.com/watch?v=a5PA6tXNg2w[/embed]

Recommendations:

1. The felt area shall be exposed to a continuous monitoring process by GSMB in a long term basis and based on the acquired data sets, descriptive models should be developed to understand the regional geological, and seismic-tectonic behaviour in order to make decisions on policy making on land use practices, educating the general public in the area about the possible vulnerabilities , and mitigating earth related risks. 2. It is recommended that GSMB shall run a temporary high-sensitive network covering interested areas enabling to study and understand such events. Possibilities for such initiatives shall be explored under the Memorandum of Understanding (MoU) with the GFZ , other foreign scientific-technical collaborations and through the funded projects conducted by Disaster Management Centre. 3. GSMB , as the focal institute for the relevant affairs, is in a need of having strengthened on its capacity with the technical, scientific and infrastructural support of all the other stakeholder institutes. Therefore, it is recommended to make all possible and effective ties that fixing maximum collaborations in this regard. 4. Quarry blasting of the area shall be conducted according to a strict time-windows, agreed pre-schedule, enabling to discard such artificial blasting in a possible future natural event(s). 5. Considered the widespread mining and blasting activities in the region, particularly at the left bank of the river, conducting a crack survey would be very appropriate to determine the background status of the area pertain to construction practices. It is apparent that the main event influenced on the pre-existing cracks as crack widening has been observed at several locations in the affected region. This proposed crack survey shall initiate immediately. 6. It is recommended to make all efforts to keep four seismic stations functional and and upgraded with buffers enabling to acquire data when required. Increasing the island wide monitoring capacity through establishing the proposed new station in North Western – Western zone is also highlighted. [embed]https://www.youtube.com/watch?v=IVDli2xYEJw&feature=youtu.be[/embed]