5.1 Timing, Sources and Magnitude of Paleoearthquakes
The stratigraphic criteria, geometry and cross-cutting relations of deformational features provide evidence for repeated liquefaction events to seismic activity. The deformational features found in the present study, such as sandblows, sand dikes and flame structures, are correlated to earthquake induced mechanism because their morphology indicates sudden application of a large upward-directed hydraulic force of short duration (Obermeier 1996) and presence of clasts. Similar features were observed in different geological and tectonic regimes during strong historical earthquakes (Obermeier 1996; Tuttle and Atkinson 2010; Mugnier et al. 2011; Cox et al. 2012, 2014) and have been reported earlier from different depositional environments in India including the Brahmaputra plains of NE India (Sukhija et al. 1999; Sukhija et al. 2002; Rajendran et al. 2004; Thomas et al. 2007; Jayangondaperumal and Thakur 2008; Reddy et al. 2009; Jayangondaperumal et al. 2011; Mugneir et. al. 2011; Kumar et al., 2016; Lakshmi and Gawali 2022).
In the present study along Kolong River basin, from the structural, stratigraphic, and age relations at sites suggest that the paleoliquefaction features probably formed as the result of five earthquake events (Table 1) in a closely timed earthquake sequence including the 1869 Cachar earthquake. Previous studies demonstrates that using OSL, ‘direct dating’ of prehistoric earthquakes may be possible, if sand blows from liquefied dykes are preserved (Thomas et al. 2007; Porat et al. 2007). The ages obtained from present study is grouped in to five seismic events (Fig. 6). Evidence for Event I is recorded in Trenches 1 and 2 (Figs. 3 and 4). According to the OSL ages measured at this site (Table 1), the Event I is constrained by both maximum and contemporaneous ages (Fig. 6). The maximum age (AD 1848–1885) of Event I is provided by youngest liquefaction features in Trench 1 (Fig. 3) at Namgaon site. This event is also inferred from contemporaneous age (AD 1889–1915) in trench 2 and is recorded in youngest sandblow. Evidence for Event II liquefaction which took place sometime during the period AD 1782–1826 has been recorded in Trench 1 (Fig. 6). Event III has been inferred from the age constraints of sand blow in Trench 1 (Figs. 3 & 6) at Namgaon site and Sand blow 2 in Trench 2 (Figs. 4 & 6) at Nampani site. Two contemporaneous OSL ages AD 1692–1770 and AD 1640–1716 points to an earthquake in 17th and 18th century. Present study also provide Event IV in Trench 2 and the contemporaneous OSL age 1540–1626 points to an earthquake in the 16th and 17th century (Fig. 6). Event V occurring at AD 1057–1211 is also identified from Trench 3 at Satargaon site (Fig. 6).
Instrumental records over the last five decade suggest that Cachar earthquake of 1869 (Mw-7.4) and Hajoi earthquake of 1943 (Mw 7.2) occurred in the study area which was associated with the NW-SE trending KF (Figs. 1 & 2) (Kayal et al. 2012). These data allow us to correlate the Events I and II to one of the two large earthquake occurred in this time interval in this area: 1869 or 1943 events. As far as the age of the events is concerned, given the limited results of the dating available (Table 1) we can conclude that Events I and II may be associated to the 1869 Cachar, which is the nearest and strongest event for which historical sources report liquefaction in the area. The 1869 event occurred in the southeastern end of the KF and has been well described by Oldham (1883). It is equally possible that the AD 1943 Hajoi earthquake, on the basis of historical accounts, may have produced liquefaction if it was similar to the mechanism of 1869 earthquake. However, a little information is available for the 1943 event except newspaper report (Dasgupta 2011) which occurred farther north of 1869 event. However, there is no large event in the KF zone since the 1943 M 7.3 Hajoi earthquake. Although it is tempting to compare Event III (AD 1692–1770 and AD 1640–1716) with the earthquake of AD 1697 Sadiya, this event is unlikely to have generated liquefaction in the region, where our study sites are located due to the distance to the assumed source, However, this data support to tally the liquefaction episodes to 250 ± 25 year BP in the same geological conditions along KF Zone (Kumar et al., 2016). Although AD 1548 earthquake (Iyengar and Sharma 1998) is the closest historical event for Event IV (AD 1540–1626) from the present study, it is hazy to correlate the two, based on one exposure of a liquefaction layer. Kumar et al. (2016) dated the liquefaction feature that occurred about 900 year BP at the Kakotigaon along Kolong River which is near by present study sites. This data suggest that the liquefaction Event V (AD 1057–1211) at Satargaon site in the present study may correspond to 900 year BP, but only one age precludes this conclusion (Fig. 6). Dating of more paleoliquefaction features at additional sites in the KF would greatly improve our determination of the timing and the likely seismic source.
The contiguous spreading of liquefaction fields (~ 41 km) along the Kolong River indicate local earthquakes of approximately M > = 6.0 using empirical relation of Ambraseys (1988) for moment magnitude to distance of farthest liquefaction to epicentre. If all the liquefaction events resulted from the KF, then according to Ambraseys (1988) the earthquakes were approximately M − 6.5. The present study liquefaction fields recorded at least five local earthquakes in 1000 years, with a conjectural average recurrence interval of approximately 200 years for M 5.5 to 6.5 events.
In the past, the KF has experienced several earthquakes of magnitude 4.5 to 6.2, three of magnitude 6 to 7 and two earthquakes of magnitude greater than 7 viz., 1869 Cachhar and 1943 Hajoi earthaquakes. This region has been identified as a probable region for future generating large magnitude earthquakes (Kayal et al. 2006, 2010, 2012). Recently, it is argued that KF cuts across the Himalayas and caused displacement and curvilinear structure at the Main Boundary Fault and Main Central Thrust zones (Kayal et al. 2010, 2012). Paleoseismic investigation in the KF zone by Kumar et al. (2016) revealed seismogenic liquefaction features near Kopili and Kolong Rivers which correspond to the occurrence of three (250 ± 25 year BP, between 400 and 700 year BP and 900 ± 50 year BP) causative seismic events. However, both the studies, Kumar et al. (2016) and present study, indicates that an earthquake of comparable magnitude occurred in the Assam, which has generated liquefaction features of similar dimensions in the source zone nearby KF zone. But there is also a possibility that even a distant great earthquake could generate liquefaction in the KF zone. The limited dating constrains based on only limited liquefaction features are presented here and are less than sufficient to develop a timeline of earthquakes that impacted the KF.