A cartographic cum site-effectuated review of greater Shillong region of northeast India

The cartographic cum site-effectuated view of Shillong region of northeast India is presented here. Starting from the existing tectonics, the prevalent geological settings of the study area is comprehensively delineated. The seismic prone area is further overviewed in the context of site effects with accompaniment of available borehole information. The resonance frequency estimates form ambient noise survey along with receiver functions are outlined which implicates a heterogeneous subsurface. This further helps in segregating the region into two compelling profiles, thereby enabling us to get a deeper insight in the probable subsurface as well as heterogeneity. Eventually, the influence of topography over strata was also highlighted and interpreted as well.

from the above studies, Shillong Plateau of northeast India also drew attention of several researchers because of its unique geological structure and intense seismo-tectonic activity. The area of the present study is actually embedded in the Shillong Plateau.
Keeping all these in view, we present here a cartographic cum site-effectuated review of this seismically active area. Accordingly, the tectonic settings of the Shillong Plateau are initially overviewed in general. It is then followed by seismicity and geological settings of Shillong City. Apart from this, we also highlight site effects from ambient noise and their implications in particular in this study.

Tectonic Settings of Shillong Plateau
The Shillong Plateau together with the Brahmaputra valley in the northeastern region (NER) of India lie between 25 0 N to 26.5 0 N in latitude and 89.5 0 E to 93.5 0 E in longitude. It is seismically one of the most active zones in the World. The tectonic settings of the Shillong Plateau and its adjoining area are portrayed in Figure 2. The Shillong Plateau is a part of Indian Shield that was uplifted during the Tertiary period [15] which is associated with several E-W, N-S, NE-SW and NW-SE oriented faults and lineaments [16]. The Shillong Plateau consists of crystalline rocks partly covered by gently dipping Tertiary and younger sediments [9]. Geomorphologically, the Shillong Plateau has been quite intriguing. It represents a flat high land of about 1.0-1.5 km average regional elevation limited in an area of about 100 x 300 sq. km. The Shillong Plateau has an average elevation of about 1500 m and high-grade metamorphic basement rocks of Achaean age, which are overlain by the mildly deformed Protozoic shelf sediments [17]. In fact, the Shillong Plateau lies at the junction of the Himalayan orogeny on the three sides viz. north, east and NE. Basement  , however, interprets these as strike-slip earthquakes and associates them with the Sylhet lineament. Also, [6] opined that the earthquake of   [13], which was felt rigorously. According to [13], since 1970, there has been gradual decrease in P-wave velocity yielding a speculation that the region is experiencing a dilatancy stress, precursory to a large earthquake. According to [17], the Shillong Plateau shows a pertinent seismic activity with an average of more than ten small magnitude earthquakes per day which is again enriched by instrumental records of more than 30 large earthquakes. With the advent of digital seismic networks set up by NEIST (Formerly known as RRL) Network, IMD, there has been a tremendous improvement in the recording of microtremors.
Accordingly, the seismicity in the Shillong area can be presumed to be moderate. On a closer look, Borapani Shear Zone may be perceived to be the limiting region of this moderate seismicity. Very recently, the no. of felt tremors with their epicenters located in proximal areas of Shillong City has been rising considerably. All these complex tectonic settings contribute towards making Shillong region a seismically vulnerable zone.  [38] . These kinds of rocks are highly weathered and the degree of weathering is found to be more in topographic depressions than in other areas. The metabasic rocks are more prone to weathering than quartzite rocks. In low-lying areas like Laban, Richa Colony and Rylbong etc., valley fill sediments are also prominent.

Lithologs in & around Shillong City
In conformity with local geology of Shillong, it was possible to acquire an appreciable amount of borehole information of fourteen particular sites in & around Shillong City. These data sets were procured from government departments like Central Ground Water Board and Public Health Engineering Department exclusively located in Shillong City. So far, the litholog information is concerned, only fourteen sites were available. The stratigraphic information covers up to a depth of 100 meters in most cases. Barring some of the sites, deep borehole data beyond 250 meters of depth were also prominent. The litholog information of ten borehole sites are incorporated as they mainly fall in the urban part of Shillong City and in the immediate neighborhood of ambient noise location points as illustrated in Figure 4.
Additionally, the site effectuated view of greater Shillong region is tried to understand from the resonance frequency estimations through H/V ratio from ambient noise survey [For details, see [43]]. Figure 5 illustrates the estimates of resonance frequency as well as mapping of it spanning the entire Shillong City. Figure 5a and 5b exemplify the estimation of resonance frequency corresponding to two sites whereas 5c depicts the contour plot of resonance frequency. We see a considerable variation of resonance frequency throughout the area under study. In order to emphasize the heterogeneous distribution of resonance frequencies, we plot two profiles AB and CD in close proximity of the borehole locations.
Further, we include more results oriented towards site effects. A temporary network of stations was installed in three sites of Shillong region, which were in the immediate neighborhood of boreholes. As for illustration, Figure 6 highlights those results. Figure 6a shows the locations of the station and Figures 6 (b)-(d) represent the findings as computed from receiver function technique corresponding to three stations IIG, SETUK and NEHU [for more details, see [44]]. The results pertaining to site response at the three investigation sites through this receiver function technique emerge out to be similar in the range of 4-8 Hz. They tally with the estimates of fundamental frequencies estimated through ambient noise recordings at these same sites. These findings implicate changes in geology and soil conditions prevailing in the region.
Pertaining to ten sites where borehole-drilling data are available, the underneath subsurface structure is constructed. However, it is clarified that the borehole drilling were executed with the sole aim of exploring/tapping ground water. The highest depth up to which geological information is available is found to be more than 250 meters.
Further, as stated earlier, two profiles AB and CD are defined inclusive of the borehole sites. Corresponding to these two sections, the topographical profile is plotted in Figure 7 that shows an undulating topographical variation. So far the distribution of resonance frequencies across these two profiles are concerned, it appears to be totally heterogeneous; accompanied by a declining elevation pattern. In case of AB cross section, the variation of resonance frequencies follows some pattern. As revealed in Figure 5c along with Figure 7; the higher resonance frequencies are somewhat confined to sites with higher elevation; while; the less elevated regions seem to yield lower frequency estimates. However, in case of CD cross section, the pattern appears to be random; ruling out any correlation between resonance frequencies with elevation. As such; without a greater coverage of area and adequate survey points, it would be too ambitious to draw out a fine correlation between resonance frequency of estimates and altitudes.

Discussions and inference
Shillong and its neighboring region have always grabbed the attention of geologists, seismologists for its interesting geological footprint. So much diversity lies in this small area that it is adequate to result in unique seismic signature, which is only characteristic to itself. Some researchers extensively studied about the path and site characteristics prevailing in this region. As for example, the attenuation characteristics existing in Shillong and neighboring region were comprehensively delineated by [35][36]. They made use of micro earthquake spectra where they found disparities in signal content in very nearby stations corresponding to identical events, which eventually led to the strong establishment of site effects; thus implying varying local lithology existing in and around Shillong. Meanwhile, Biswas et al [37] utilized modified form of Nakamura method to formulate suitable relationship of resonant frequency with depth as well as overburden thickness. As per [37][38], the subsurface suffers from considerable heterogeneity. The study was further accentuated by Biswas and Boruah, 2015 [39] who executed a nonlinear earthquake site response analysis. From the recorded accelerograms, they estimated fundamental frequencies. The frequency estimates seemed to comply with the passive seismic noise recordings; thus giving more foresight in the heterogeneous stratification as observed in the Shillong region. Biswas and Boruah [44] combined noise data with earthquake recordings where they found identical variation of resonant frequencies. As observed, the resonant frequency estimates from ambient noise are quite a match with receiver function corresponding to earthquake recordings in the range of 3 to 7. In another study by [45], H/V ellipticity intermingled with SPAC analysis resulted in the subsurface profiles with varying shear wave velocities. In the same note, there was compelling evidence of varying shear wave velocity profiles prevalent in Shillong region, which also advocated the existence of low velocity zones at some pocket areas [46]. In another recent study [47], there is indication of the existence of stiff soil and rocks at different depths whereas resonant frequency; thus revealing a vulnerability rate of 60%. Accordingly, the topographical profile as shown in Figure 7 bears much significance. As seen in the two demarcated profiles, the topographical undulations bear quite resemblance with the heterogeneous stratifications as observed in the previous literatures.

Conclusion:
Conclusively, the cartographic and site effectuated analysis have been presented.
There emerges to be active seismotectonics in the study region. Starting from the existing tectonics, the prevalent geological settings of the study area is comprehensively delineated. Along with this, we analyze the site effects pertaining to resonance frequency estimates. We further complement it with receiver function results. The seismic prone area is further overviewed in the context of available borehole information, which helps in segregating the region under study into two compelling profiles, thereby helping to get a deeper insight in the probable subsurface as well as heterogeneity in conformity with the published literatures. Eventually, the influence of topography over strata was also highlighted and interpreted, as well. There is ample evidence of topography over strata present in this seismically active area. The results are an indication of highly vulnerable index for the study region.   . Towards left, the observed profiles AB and CD in conformity with the boreholes are shown. Corresponding Elevation profiles for these two sections AB and CD are given towards right (Modified after [43][44][45]).

Longitude
Latitude