Keywords

1.1 Introduction

Neotectonics and active tectonics are the key geological agents, which are responsible for the modelling of present-day geomorphology on the earth. The tectonic processes are responsible for the many geological hazards to society. Among all the geological hazards, earthquakes have the most disturbing effect on society. In the field of earth science, the tectonic geomorphology is a rising domain due to its addition of distinctive tools like geodetics, geomorphology, geochronology. Additionally, these tools help in assessment of the deformation rate, incision upliftment, erosion and fault slip rates (Kumar et al. 2020b).

For a long time, the peninsular shield of India has been considered stable seismically and the region has the potential of generating only low-level seismicity at few places (De Montessus de Ballore 1911; Tandon and Chatterjee 1968; Krishnan 1968). However, this belief has been shattered after the occurrence of the 1967, Koyna Earthquake of M 6.2. The M 6.2 magnitude Koyna earthquake forced researchers to reconsider and reassess the seismic status of Peninsular India. The detailed studies conducted by (Chandra 1977; Auden 1949; Watts and Cox 1989; Bansal and Gupta 1998; Dole et al. 2000; Rajendran 1997; Sheth, 1998; Raj et al. 2003; Mohan et al. 2007; Kaplay et al. 2013, 2016; Naik and Awasthi 2003; Kale et al. 2016; Jade et al. 2017, Kumar et al. 2022, 2020a, b) show that, PFS zone, the Konkan coastal belt, Koyna are affected by tectonically generated deformation activities in the Deccan Volcanic region.

The profound accessibility of Geographic Information System and their role in the uninterpretation of digital elevation models has helped to the purposes of RIAT evaluation by means of geomorphic indices. The research on this subject are growing and have seen significant growth in last decades (Kumar et al. 2022). The GIS-based software enables to extract and analyse of landscapes with detailed information. The Assessment of RIAT from indices of geomorphic shows the rates of upliftment and deformation in the landscapes for the long time (Bull 1977; Kumar et al. 2022).

The current area under study is situated in western portion of DVP in the Western India (Fig. 1.1). Since late Triassic/early Jurassic to late Cretaceous periods, West Coast of India has evident persistent rifting events. The current study area is situated in the Panvel flexure seismic zone (Fig. 1.1), which is undergoing through earthquake events later 1618 (Rao 2005; Rao and Rao 1984; Kumar et al. 2020a) (https://isr.gujarat.gov.in/). Key purpose of this research is to evaluate the seismic hazard in area as there are no considerable studies associated to active tectonic and seismic hazard due active fault (s) in the area under study.

Fig. 1.1
A map of Western India marks the areas of Bhuj, Ahmedabad, Bharuch, Dahanu, Mumbai, Western Ghats Escarpment, Nashik, and Pune, among others in different color gradients. It also marks various fault lines in colors.

The Tectonic map of Western India (after Biswas 1982; Sheth 1998), KMF-Kachchh Mainland Fault, KHF-Katrol Hill Fault, ECF-East Cambay Fault, WCF-West Cambay Fault, NKF-North Kathiawar Fault, SNF-Son-Narmada Fault, NTF-North Tapti Fault

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1.2 Geological Setting and Study Area

The area under study is situated in western parts of India (Fig. 1.1). In the west, it’s confined by WCF (west coast fault), whereas in east is bounded by Western Ghats escarpment and the central portion is occupied by the Panvel flexure (Fig. 1.1). The Deccan basalt, trachyte and rhyolite complex dominates the study area with basic rock dykes. The central parts of the study area are occupied by alluvium these sediments are distributed in intermittent spots of major rivers (Kumar et al. 2022 and 2020a, b). During Pliocene, the Western Coast of is formed due to the faulting (Krishnan 1953). The WCF is the main tectonic structure in this part of India. Due to its NNW-SSE trend, the straight orientation in the west coast and up to the Gulf of Cambay in the north and continue to the south of Mumbai is considered to be due to this fault (Bombay) (Krishnan 1982).

1.3 Seismotectonics of the Study Area

The area is experiencing earthquakes since 1856. Two historical events (MS5 in 1935 and MS5.7 in 1856) were recorded in the study area especially concentrated in the southern part (Kumar et al. 2020b; Bansal and Gupta 1998; Chandra 1977). At present, the seismicity in the region endorses the active nature of the present tectonic features; epicentres of the earthquakes are focussed beside these tectonic units (Kumar et al. 2020a). A substantial number of earthquakes between M1 to 5.7 (Chandra 1977; Bansal and Gupta 1998 and Kumar et al. 2020b) are documented in the area under study. The disruption due to the tectonism is even now marked by several earthquakes in western India (Fig. 1.2).

Fig. 1.2
A set of 2 maps. The left map marks the areas of Junagadh, Bhavnagar, Bharuch, Surat, and Silvassa, among others in different color gradients along with the earthquake magnitude and epicenters in circles of various sizes. The right map marks the earthquake magnitude and epicenters in circles for the study area in Silvassa.

a The Seismotectonic map of western India b The Seismotectonic map of the study area (After Kumar et al. 2020b)

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1.4 Methodology

The targeted research work is distributed into three parts (i) evaluation of RIAT, (ii) soft-sediment deformation study and (iii) estimation of seismic hazard due to an active segment of the fault.

1.4.1 Evaluation of RIAT

For evaluation of the RIAT, the remote sensing (RS) and geographic information system (GIS) techniques are used. The network of streams and the demarcation of watershed boundaries are done utilizing Survey of India toposheet at 1:50,000 and SRT Digital Elevation Models (30 m) in the GIS system. The recognition of linear feature like faults, lineaments and dykes, processing of image, production of the FCC, and preparation of shaded relief maps are prepared. The indices, i.e., Bs, HI, SL, Vf, are assessed and after calculation of all, sub-watersheds are classified in three category on the basis of the value of index. Finally, these values are added and each every sub-watershed has been grouped according to the value of the RIAT (Relative Index of Active Tectonics).

1.4.2 Soft Sediments Deformation (SSD) Structures

The study related to seismite (SSDS) is completed in the steps as follows:—seismites are identified, mapped in alluvial sediments pile up along Damanganga river banks in the study area. These seismites were measured and their association with the surrounding layers of sediments was done. Then the literature related to the seismites has been reviewed and the reasons (whether primary or secondary) behind the formation of these seismites are studied. In addition, the mechanism of trigger, the earthquake distribution and the manifestation of active faults in the region have been investigated.

1.4.3 Seismic Hazard Assessment Due to Active Fault Segment

To determine the seismic hazard of any area, the future earthquake potential valuation is mandatory. Precisely, it is essential to estimate the size of the earthquakes that might be produced by any specific fault. The magnitude of earthquake may be related to rupture parameters like length and displacement (Iida 1959; Tocher 1958; Chinnery 1969). To estimate these parameters, prior paleo-seismic and geologic studies of active faults are required. The parameters/data from the geological and geomorphic studies can be used to evaluate the time of historical earthquakes, the extent of displacement of each event, and the segmentation of the fault zone (Schwartz and Coppersmith 1986; Schwartz 1988; Coppersmith 1991) in the study area. To interpret these source features into estimates of earthquake size, the empirical relationship between rupture parameters and the measure of earthquake size, typically magnitude, is required (Wells and Coppersmith 1994).

Numerous published realistic relationships are available to relate magnitude to various fault rupture parameters, like fault rupture displacement versus rupture length and magnitude versus rupture area (subsurface and surface both), magnitude contrasted with total fault length (Tocher 1958; Iida 1959; Albee and Smith 1966; Chinnery 1969; Ohnaka 1978; Slemmons 1977, 1982; Acharya 1979; Bonilla and Buchanon 1970). There are research works also available that relate the seismic moment and magnitude to the rupture length, width, and an area of the rupture (as assessed from the amount of deformations at surface, the aftershock zone extent, or functions of earthquake source time) (Utsu 1970; Kanamori and Anderson 1975; Wyss 1979; Singh et al. 1980; Purcaru and Berckhemer 1982; Darragh and Bolt 1987). The empirical relationships proposed by Wells and Coppersmith (1994) were well-tested and used in a number of significant studies in the seismic Hazard Assessment (Mohan et al. 2017, 2018, 2021). Therefore, the same relationship has been used in the present study to estimate the earthquake magnitude from the observed displacement, estimation of rupture area, rupture length and rupture width. The details are as follows.

1.4.3.1 Maximum Earthquake Magnitude

The length of surface rupture and the maximum displacement on continental fault traces are the most commonly used parameters to conclude magnitudes for paleo-earthquakes (Wells and Coppersmith 1994). Here, we have used the maximum displacement method (Wells and Coppersmith 1994) to calculate the maximum magnitude of an earthquake along the identified faults present in the study area.

1.4.3.1.1 Maximum Displacement Method

The maximum displacement method involves determining the maximum displacement (MD) estimated from the paleoseismological investigations associated with a paleoearthquake, and comparing that value to the maximum displacement measured or computed for an instrumentally recorded earthquake (Wells and Coppersmith 1994).

The empirical relationship between Moment magnitude (M) and MD will have the form of:

$${\text{M}} = {\text{ a }} + {\text{ b}}*{\text{ log }}\left( {{\text{MD}}} \right)$$

Regressions coefficient derived by Wells and Coppersmith (1994) for Moment magnitudes (M) and maximum displacement (MD) are:

$${\text{a}} = {\text{ 6}}.{\text{69 and b }} = {\text{ }}0.{\text{74}}$$

Along the normal active faults mapped in the study area, the maximum surface displacement of ~0.25 m is measured. Thus in the above equation with MD = 0.25, the possible Moment magnitude of Mw 6.2 is estimated.

1.4.3.2 Estimation of Seismic Hazard

The seismic hazard can be estimated using two different methodologies (i) Deterministic Seismic Hazard Assessment and (ii) Probabilistic Seismic Hazard Assessment. In the case of seismic designing and retrofitting of structures, the DSHA has an advantage (McGuire 2001). The DSHA is also useful to check the worst-case scenarios (the largest magnitude at the closest distance) and in the training and plans for emergency response and post-earthquake recovery (McGuire 2001).

In the present study, the deterministic seismic hazard assessment has been conducted to estimate the seismic hazard due to the active segment of the Kilvani Fault (Fig. 1.3), where a displacement of 0.25 m was observed. The Strong motion simulation involves the rigorous mathematical exercise covering the earthquake source/rupture (geometry, nucleation, and propagation) and seismic wave propagation (between the source to the site) through different rock boundaries in the earth’s crust. While passing through different subsurface layers, the seismic waves change (amplifies/deamplifies) and reach the site. Cancani (1904) initiated the simulation of strong motion (SM) by generating the SM parameters from the seismic intensity. Later on, Housner (1947) proposed the concept of black-box simulation for simulating SM by using white Gaussian noise. Presently, mainly five types of SM simulation techniques are available. These are (1) composite source modelling (Saikia and Herrmann 1985; Saikia 1993; Zeng et al. 1994; Yu 1994; Yu et al. 1995), (2) stochastic simulation (Boore 1983; Lai 1982; Boore and Atkinson 1987), (3) empirical Green function technique (EGF) (Hartzell 1978, 1982; Hadley and Helmberger 1980; Kanamori 1979; Mikumo et al. 1981; Irikura and Muramatu 1982; Irikura 1983, 1986; Muguia and Brune 1984; Hutchings 1985; Kamae and Irikura 1998; Irikura and Miyake 2011), (4) semi-empirical approach (Midorikawa 1993; Joshi and Midorikawa 2004; Joshi et al. 2001; Mohan 2014), and (5) Stochastic Finite Fault Source Modeling Technique (SFFMT) (Motazedian and Atkinson 2005). Every simulation technique follows certain conditions for the assumptions of source, path, and site effects and rarely estimates all three in one step. Due to advancements in the research methodologies, the SM simulation can be effectively done by dividing it into three major parts (i) source characterization and rupture propagation, (ii) wave propagation from source to base rock/Engineering bedrock (EBR), and (iii) wave propagation from EBR to surface considering near-surface effects gathered in the form of site amplification from geotechnical or/and geophysical parameters like Vs. Generally, one can choose any technique based on available input parameters (source, path and site conditions). The SFFMT is a well-tested SM technique of simulation and well tested in Gujarat by Chopra et al. (2010, 2013), Mohan et al. (2017, 2018, 2021) for seismic hazard assessment. In view of this, the technique has been selected to estimate the strong motion at a grid interval of 10 km × 10 km. A significant portion of the study area is covered with sediments. The United State Geological Survey (USGS) provided the worldwide Vs30 values based on the topographic slope (Allen and Wald 2009). The Vs30 values in the study region vary from 250 m/sec to 900 m/sec. Therefore, the strong motion has been simulated at B/C Boundary at Vs30 of 760 m/sec and crustal amplifications suggested by Boore and Joyner (1997) for the Vs30 of 760 m/sec. The near-surface wave attenuation/Fall-off of the high frequency (>1 Hz) Fourier amplitude spectrum (Anderson and Hough 1984)/Kappa values (ĸ) is taken as 0.03 as used by Chopra et al. (2010) for the estimation of seismic hazard in the adjacent Mainland Gujarat. The Quality factor and stress drop were also considered as suggested by Chopra et al. (2010) for the adjacent Mainland Gujarat area. The input parameters considered for the simulation of ground motion are given in Table 1.1.

Fig. 1.3
A contour map marks the areas of Bhilad, Vapi, Dadra, Silvassa, Rakholi, West Coast Fault, Kilvani Fault, and Panvel Flexure Fault in different color gradients and layers. It has a color scale of P G A in centimeters per square second that ranges from 40 to 360.

The PGA (in cm/sec2) distribution map at a Vs of 760 m/sec due to an earthquake of Mw6.2 along the Kilvani Fault

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Table 1.1 The selected model parameters for the simulation of ground motion
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Site amplification plays a significant role in the estimation of seismic hazards in any area. In the study area, the Vs30 values proposed by USGS have been used to estimate the site amplification factors at a grid interval of 10 km × 10 km by using the velocity–amplification relationship proposed by Matsuoka and Midorikawa (1994). The PGA distribution map thus prepared at Vs30 of 760 m/sec, the site amplification map (between the Vs of 760 m/sec and the surface Vs) and the PGA distribution map at the surface level have been shown in Figs. 1.3, 1.4, and 1.5, respectively.

Fig. 1.4
A contour map marks the areas of Bhilad, Vapi, Dadra, Rakholi, Silvassa, Rakholi, West Coast Fault, Kilvani Fault, and Panvel Flexure Fault in different color gradients and various values. It has a color scale of site amplification that ranges from 0.8 to 2.

The site amplification map of the study area

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Fig. 1.5
A contour map marks the areas of Bhilad, Vapi, Dadra, Silvassa, Rakholi, West Coast Fault, Kilvani Fault, and Panvel Flexure Fault in different color gradients and layers. It has a color scale of P G A in centimeters per square second that ranges from 40 to 440.

The PGA (in cm/sec2) distribution map of the surface level due to earthquake of Mw6.2 along the Kilvani Fault

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1.5 Result and Discussions

1.5.1 Faults and Lineament Mapping

During the field geological mapping, a normal fault has been mapped near Kilvani village trending N170°–N350°, with a sharp dip of 72° in the SW direction (Fig. 1.6a). It is evident by the impressive growth of slickensides, the slickensides were occupied by fine-grained white zeolites and calcite. The slickensides zone is very well visible in a depth of 2–4 m in road cuttings (Fig. 1.6a). The slickenlines are suddenly tending towards the south-SW on the surface of fault. The smoothness in touch in the downward direction on slickeside surface and upward direction roughness is observed (Fig. 1.6b), which suggests that the missing western block moved down relative to the block east of the fault (Doblas 1998; Argles 2010). The exposed bedrock along the rock cutting is mainly Basalt, which is found sheared and very closely spaced fractures are formed due to the faulting. The presence of normal fault with a trend N170°–N350° dipping 72° SW suggests the NE-SW extension in the study area. The slickensides on striated fault planes were recorded in the expose rock section at Kilvani and Meghwal, (Fig. 1.6). Generally, they present on fresh outcrops showing, thin (~1–5 mm), mineralized (secondary zeolite and quartz, and calcite.) the planes of fault that display primarily a normal slip. Mineralized layers are likely to erode (Doblas 1998; Whiteside 1986; Kranis 2007). The Kilvani fault is the younger fault in the study area as along this fault the displacement in the sediments has been mapped. Though other faults (like the WCF and PF) are also present in the region but along these faults, the signature of displacement or movement has not been found in the study area. The Kilvani Fault also follows the trend of the major faults and the epicentres are occurring along the trend of these faults. Therefore, to estimate the hazard related to seismic event in the area and to estimate the maximum seismic potential, the Kilvani Fault has been considered.

Fig. 1.6
2 photographs labeled A and B. A is a photo of a hill-type side near a road with 4 arrows and a bounding box pointing at the fault lines. B is a close-up photo of a surface with slickenlines moving downwards.

a Normal fault near Kilvani village (20°18′1.70"N, 73° 5′53.55"E) road exposures with strike N170°–N350° and dip amount 70° in SW direction, b Slickensided fault plane showing the direction of movement by black arrows (After Kumar et al. 2020b)

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The lineament map has also been generated in the study area, and the results of the analysis depict that these lineaments display maximum resemblance with the trend of the tectonic features present in the area. The lineament density analysis was performed in GIS platform by dividing the study area into four sectors, the results of the lineament density analysis show that the highest density of the lineaments is in the central portion of the study area along the axis of the Kilvani Fault and other tectonic features (Fig. 7b), while the lowest lineament density in alluvial portion. The high lineament density (Fig. 7b) observed in the central portion (in a black circle) was linked with the regional tectonic features present in the study area. Furthermore, the interpretation of the rose diagram and overlay investigation shows that maximum lineaments/linear geological structures are aligned to sub-parallel (N–S direction) to the Kilvani Fault and other tectonic structures (Fig. 7b inset).

Fig. 1.7
2 images labeled A and B. A is a map marking the area around Silvassa with an oval circle in different color gradients. It plots the lineament density that ranges from 0 to 255 in different colors. The circle points to a rose diagram on the inset labeled B.

Structural lineament map of the area: a lineament density map in which the flat area shows low concentration as compared to flanks, b rose diagram of lineaments with a major trend in N-S direction (inset) (After Kumar et al. 2020b)

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1.5.2 Relative Index of Tectonic Activities

The indices like stream length index, valley to floor ratio, hypsometric integral, and basin shape index are calculated, and their collective results were combined to assess the relative index of tectonic activity (RIAT) in the study area. The stream length is an important tool to estimate the relative tectonic activities of any area. The aberration in the profile of river from the steady state may be due to the effect of the lithological, or climatic and tectonic reasons (Hack 1973). The SL index value has been estimated and the area is distributed into 54 sub-basins. Based on the results and the values classified into three classes; Class I (SL, ≥ 600), Class II (300, < SL < 600), and Class III (SL, ≤ 300). The 07 numbers of sub-basins come in class-I, a sum of 10 sub-basins comes in class-II and 10 sub-basins comes in Class-III. The results of the study disclose the presence of moderate and high activities in the eastern and northern portions, individually. The central and western portion is moderately least tectonically active along with fairly high stream length index value. The valley to floor ratio index is measured to differentiate among V and U shaped valleys. These are (V-shaped) developed in response to upliftment and flat-floored (U-shaped) wide valleys formed as a reaction to the stability of base level (Bull 1977). The incision by river results into uplift,emt, while low Vf is associated to progressive incision rate and uplift. The < 1 Vf value is related to the V-shaped valleys, linear streams shape with and revealed active upliftment and non-stop downgrade cutting. The > 1 Vf value is associated to flattened or valleys with U shaped, which displays attainment of erosion of base level mainly in response to relative tectonic inactivity (Keller 1986; Keller and Pinter 2022). In the region, the valley to floor width index is calculated in the main streams of sub-basins. Three numbers of classes were classified in this case also; Class I, (Vf ≤ 0.5), Class II, (0.5 < Vf < 1.0), and Class III, (Vf ≥ 1.0). The findings of the study reveal that the majority of the area comes in Class 1, which shows the V-shape and therefore discloses a remarkably higher degree of tectonic activity. The hypsometric integral index is unbiased of area of the basin and is usually consequent for a precise drainage basin. Usually, the HI outlines the elevational dispersal of an exact area of land, mainly a drainage basin (Strahler 1952). The high value of hypsometric index is possibly related to the current tectonic activity, whereas, the low values signify the mature landscapes, which have been further eroded and less affected by the recent tectonic activities (Strahler 1952). After the results of the analysis, in relations of concavity and convexity of hypsometric curve, the HI may be categorized into three classes, Class 1, (HI ≥ 0.5) shape of concave curve; Class 2, (0.4 < HI < 0.5) a shape of concave-convex curve, Class 3, (HI ≤ 0.4) the convex shape of curve. The quantity of the breadth of sub-basins varies as of one place to another hence the average value is taken to assess the shape of studied river basin. As per Elias et al., 2019, the index of basin shape (Bs) comprises three classes: (Class I) basin with Elongated shape (Bs ≥ 4); (Class II) basin with semi-elongated shape (3 ≤ Bs < 4), and (Class III) basin with Circular shape (Bs < 3) (Fig. 1.8). The results of the study reflect that high values of Bs are associated with the basins with elongated shapes, generally connected to relatively enhanced tectonic activities, and low values of Bs entitled to basins with a circular shape generally associated with low tectonic activities.

Fig. 1.8
A map marks the areas with a basic shape index of class 1, class 2, class 3, faults, Panvel Flexure, Western Ghats Escarpment, lineaments, and drainage networks in different color gradients. The areas are numbered from 1 to 54.

Basin shape index distribution in the sub-watersheds in the study are (after Kumar et al. 2022)

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The eruption of Deccan flood basalt took place at ~65 Ma and covered > 500, 000 km2 (Chandra 1977; Cox 1988; Acharya et al. 1998; Ramesh and Estabrook 1998). The earlier research in the Deccan province ascribed the viewed variations basically to change in climate, geomorphology, riverine systems, fluctuations in sea levels, and only devoted to the Deccan upland region connection with movements related to neotectonism (Dikshit 1970; Kale and Rajaguru 1987; Watts and Cox 1989; Widdowson and Cox 1996; Renne et al. 2015; Kale et al. 2016). In the present research, an effort is made to evaluate RIAT. The values of the indices computed are added to compute Relative index of Tectonic Activities and then appraised the spatial extent and dispersal of tectonic activities in the study area. The value of RIAT attained by addition of all the indices is grouped in three categories to describe the grade of RIAT in the region, which are given as: 1.3 ≤ RIAT, < 1.5 in Class II with high activities; 1.57–1.86, class III with moderate activities; and 2.0–2.33 Class IV, with low comparative tectonic activities separately. The distribution of these categories is shown in (Fig. 1.9). The river basins 44,42, 21, 2 fit in to class II (with high activities); the basins 52,43,8,4,3,1 fit into class III (with moderate activities); left all sub-basins fit into class IV (with low activities). The relative index of tectonic activities is high alongside the UGF (Upper Godavari fault), the WGE (Western Ghats escarpment), new lineaments and faults, present in the study area.

Fig. 1.9
A map marks the areas of Silvassa, Khanvel, Dudhani, Aslona, Peint, and Hattipada with relative index of tectonic activity that includes high class 2, moderate class 3, low class 4, faults, Panvel Flexure, lineaments, Western Ghats Escarpment, localities, and drainage networks in different color gradients.

Distribution of relative index of active tectonics (RIAT) in the Darda and Nagar Haveli and surroundings (after Kumar et al. 2022)

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In the study area, various types of seismites also mapped from various location in the river sediments during the field investigation. The seismites are primarily found in sandy silt, silty clay and sandy gravels. Major seismites in the area include dykes of intrusive nature and sills of sediments, sediments with slumping structures, clast chunks with suspended nature and bedding with convolute shape.

1.5.3 Deformation Mechanism

In previous studies in the central regions of Maharashtra the occurrence of SSDS, warping/flexures of sediments, remarkable displacement and deformation in alluvial deposits were documented (Dole et al. 2000, 2002; Rajendran 1997; Kaplay et al. 2013, 2016; Kale et al. 2016). There are various deformation mechanisms, which describe the formation of the seismites. Mills (1983), suggested that the seismites are produced by the disruption of non-lithified and sedimentary layers with water saturation. Researchers like Mills (1983), Lowe (1975), Owen (1987, 2003), Moretti and Sabato (2007) have recommended various deformation mechanisms behind the formation of seismites. The seismites may be formed by the failure in slope due to slumping, liquidization and shear stresses. It might happen if driving force results in reverse density (Allen 1982). The liquefaction or fluidization of the sediments is the most important reason in development of seismites in cohesion-less and water-rich sediment layers (Allen 1982). Normally, the process of the cause and the distortion can be instigated because of the results of exterior instruments like groundwater fluctuations, gravitational and storm currents, and an event of earthquake (Sims 1975; Lowe 1975; Owen 1987, 1996).

1.5.3.1 Trigger Mechanism

There are several probable trigger mechanisms described by various researchers most of them are summarized in this section. The commonly accepted trigger mechanisms are (a) loading of sediment (Moretti and Sabato 2007; Anketell et al. 1970), (b) storm and turbiditic currents (Molina et al. 1998; Dalrymple 1979; Alfaro et al. 2002), (c) sudden collapse in sediments (Waltham and Fookes 2003; Moretti et al. 2001; Moretti and Ronchi 2011), (d) liquefaction of soil through previous fissures (Holzer and Clark 1993; Guhman and Pederson 1992), (v) an earthquake event (Lowe 1975; Seilacher 1969; Sims 1975; Rossetti 1999; Calvo et al. 1998; Alfaro et al. 1999). In the study area, the sediment loading appears to be of least significance for features observed in the alluvial deposits within the study area. Seismites mapped in the study area are present in a large area, which recommends a further regional trigger mechanism in comparison to the limited acts of loading of sediment and storms current, collapse structures, turbiditic currents, and liquefaction via previously existing fissures. Seismic shaking due to the earthquake event could be the most plausible trigger mechanism and it might be the major reason for the development of the seismites within the study area, while present study area is bordered by faults which are active in nature (neotectonically), the Panvel Flexure Fault and its sympathetic faults. The deformed sediments found in the study area may probably be categorized as seismites, based on their extent, nature (river deposits), shapes and dimensions (Owen 1996; Sims 1975; Rossetti 1999; Calvo et al. 1998). The seismites are formed due to earthquake shock after its occurrence and for the development of these features; an area must have undergone to tectonic event and earthquake activities (Moretti and Sabato 2007; Jones and Preston 1987). The ground Shaking done by an earthquake is the widely accepted and famous phenomena behind sediment fluidization. All through the incidence of an earthquake, the pressures in pores are increased for the short time, which results into the loss of contact with grain–grain and short-term loss of strength as of limited pore water expulsion (Allen 1977). In study area, these seismites are qualified for earthquake origin on the basis of the explanations as follows: (a) undeformed beds of soil are present below and above the deformed beds; (b) the size of soil grains of deformed sediments falls in the range of soil liquefaction because of shaking due a seismic ecevnt (Balkema 1997); (c) seismites and their extent, shape, magnitudes, sedimentological properties and facies, are common to the studies on seismites by Rossetti (1999), Sims (1975), Vanneste et al. (1999) and Jones et al. (2000); (d) the presence of active faults in the present study area (Kumar et al. 2020a,b, 2022) and has been experiencing earthquakes with magnitude M ≥ 5, thus the seismites in the alluvial soil from the area meet with key conditions to be characterized as seismites. To trigger liquefaction in the soil, an earthquake of magnitude 2–3 is enough (Seed and Idriss 1971). For causing liquefaction in the soil, an earthquake magnitude must be >4.5 (Marco and Agnon 1995). The presence of active faults within 15 km to 50 km distance of the study area also affirms the seismites of seismic origin (Fig. 1.10). In view of all the above evidence, it has been postulated that the seismites present in the study area are developed due to the earthquake event of magnitude M ≥ 5. It has also been proposed that the earthquake, which might have generated the seismites, possibly will be between magnitude 5 and 7 in the surrounding region.

Fig. 1.10
A map marks the areas of Junagadh, Bhavnagar, Bharuch, Surat, Silvassa, and Mumbai, among others in different color gradients along with earthquake magnitude and epicenter circles in different sizes. It also marks S S D structures in Silvassa and high-density earthquake zones near Mumbai in ellipses.

The variation in epicentre distance of seismites (blue ellipse) with their association to 1618, 1856 earthquake (M6.9 and 5.7) affected the study area (after Kumar et al. 2020a)

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In an area, if you observe seismic activeness through RIAT, the presence of seismites etc., then it becomes essential to estimate the seismic hazard based on the seismic potential of identified active seismic source(s). In the present study, a displacement of the order of 25 cm (0.25 m) has been estimated along the Kelvani fault. Based on the displacement–magnitude empirical relationship, an earthquake potential of Mw 6.2 has been estimated along this fault. The PGA distribution map of the region based on a scenario earthquake of MW 6.2 along the Kevani fault at Vs of 760 m/sec2 and surface using site amplification factor estimated through Vs have been simulated using SFFMT. A PGA value of the order of 40 cm/s2 to 1.360 cm/s2 has been estimated at Vs 760 m/sec with the maximum value in the western part (towards the dipping direction) of the Kelvani Fault near Silvasa (Fig. 1.3). A site amplification of the order of 0.9–2.15 has been estimated in the study area with a maximum value in the N and NW part (near Vapi) (Fig. 1.4). The higher value of site amplification is estimated in the area covered with the sediments. A surface PGA of the order of 40 cm/sec2 to 440 cm/sec2 has been estimated in the study area with a maximum value in the western part of the Kelvani fault (near Silvasa and Rakholi, towards the dip direction) (Fig. 1.5).

1.6 Conclusion

The Dadra-Nagar Haveli and the surrounding region, in western India, have been experiencing moderate seismicity (more than 200 earthquakes (1.0 ≤ M ≤ 5.7) since 1856 including two historic events (Magnitude Ms 5 in 1935 and Magnitude Ms 5.7 in 1856). A study is conducted to map the tectonic structures in the region and their associated tectonic-geomorphic features to infer the tectonic behaviour and their impact on seismic hazard in the study area. RIAT of the watersheds of main rivers has been estimated through the geomorphic analysis SL gradient, HI, BS and VF and 03 groups (1.3 ≤ RIAT < 1.5 in class II with high activities, 1.5 ≤ RIAT < 1.8 in class III—with moderate activities, and 1.8 ≤ RIAT in class IV—with low activities, have been found in the study area indicating it a seismically active region. The study area falls within the Panvel seismic zone with the presence of faults with slickenside bearing planes, shear zones with brittle behaviour, extensional features and deformed dykes suggesting that the study area has faced neotectonic activities and is still active seismically. Through geological fieldwork, the evidence of past major seismic events (>5.5) is also found well preserved in the form of SSDS/seismites in quaternary sediments. The extent and dimension of these seismites indicate that the mechanism to trigger these and forces driven for the source of these structures were shock waves by an earthquake. The maximum moment magnitude of Mw 6.2 has been estimated based on the maximum displacement recorded along the normal active fault mapped in the study area (Kelvani Fault), which trends N170°–N350°, with a sharp dip of 72° in the SW direction. The seismic hazard assessment of the area considering scenario earthquake of Mw 6.2 along this fault located east of Silvasa city has been estimated using the Stochastic Finite Fault Modelling simulation technique. A maximum PGA of the order of 360 cm/sec2 has been estimated at the EBR with the Vs of 760 m/sec and 440 cm/sec2 has been estimated at the surface level with a maximum site amplification factor of 2.15 in the area.