Palaeoseismic trenching to discover ancient earthquakes (by Silvia Crosetto)

Myanmar is one of the most earthquake-prone countries in the world. Since opening to foreign researchers five years ago, it has become clear that several important but poorly-known tectonic faults in the country have the potential to cause devastating earthquakes.

The Royal Holloway Travel Award gave me the opportunity to integrate my funding and make a complete analysis of one of these faults, the Kyaukkyan Fault, which last generated an earthquake of magnitude 7.7 in 1912, the largest ever recorded in Myanmar. My project is motivated by the urgent need to discover the history of past earthquakes caused by the Kyaukkyan Fault, which are the key to create a probabilistic model for future seismic events. The Kyaukkyan Fault affects a region where population has little seismic hazard awareness, despite two blooming towns, Taunggyi and Pyin Oo Lwin, lying on the fault.

During this field campaign I could literally dig into the fault to find out its geological features, using a field technique called ‘paleoseismic trenching’, developed on the intensely studied San Andreas Fault in the 1970s and widely used to study past earthquakes.

When the fault moves, and the earthquake occurs, rocks on the opposite side of the fault plane move from their original position and two parts of the same layer can be offset along the fault plane; the paleoseismic trenching technique consists in digging a trench across the fault-scarp in order to expose ancient soils that record evidence of historic seismic deformation with their offsets. Trenches are normally around 2 m deep, 2 m wide to allow people to work inside it, and several meters long, according to the features the site. Once the work is completed, the trenches are filled and closed, and their location recorded.

The trenching sites along the Kyaukkyan Fault were identified using remote sensing interpretation, which involves the analysis of aerial and satellite images, and through field-survey, which was undertaken in 2014 and 2015. Once the sites had been chosen, we had to arrange for permissions, given by the chairman of each village, and machinery. This operation created a funny net of word of mouth, attracting an interested public who attended our operations: local people were very interested not only about the practical part of the technique, but also in finding out their new living-on-a-faultline condition.

With the help of counterparts and researchers of Yangon, Mandalay and Taunggyi Universities, the side walls of each trench were manually cleaned and gridded as preparation for the study. We then measured every layer and drew a wall-log that is used for interpretation and location of the samples that are collected; hundreds of pictures were taken in order to build a detailed photomosaic of the trench.

Two out of four trenches showed several faults that were identified thanks to offset layers of travertine, a hard rock that represents a good marker for its white colour, standing over the

brown of the soils. The travertine, which forms as a horizontally continuous layer, in the trenches was laterally discontinuous, and in some cases sharply truncated, meaning that the fault has been active after the formation of this layer.

In order to chronologically locate the seismic event(s) that deformed the travertine, sample dating is necessary. Samples of charcoal, datable with radiocarbon dating technique, were collected below the travertine and above the uppermost deformed layer: the seismic event will be included within these two ages.

In the northern part of the fault recent layers are deformed, suggesting a relation with the 1912 earthquake, whose epicentre was located in the same area. In the southern part, close to the town of Taunggyi, the faults offset soil horizons that seem older and therefore not related to the same event, which would imply the existence of a pre-1912 seismic event not included in the historical records.

If dating results will confirm this hypothesis, it would be of genuine significance scientifically and socially, and most of all a crucial contribution for the seismic zonation of the area.