Keywords

1 Introduction

Millions of people are affected by natural hazards every year, and their impact can be calamitous. From the destructions of building to the spread of disease, natural hazards can devastate entire countries overnight. Tsunamis, earthquakes, and typhoons do not just wreak havoc on the land; they also disrupt people’s lives in both densely populated cities as well as remote villages and rural areas. Increasing effects of natural hazards viz. landslides, tsunami, floods around the world have a direct social, economic, and environmental impact. In the domain of the Indian sub-continent, the ongoing collision between the Indian plate and the Eurasian plate has resulted in the formation of the Himalayas in the north and a subduction zone in the southwest along the Sumatra-Andaman trench. Indian sub-continent experienced several devastating catastrophic events, viz. the 1993 Latur earthquake (Mw 6.2) and 2001 Bhuj earthquake (Mw 7.6) in Stable Continental Region (SCR) of Western India; 2005 Muzaffarabad earthquake (Mw 7.6) in Kashmir; 2015 Gorkha earthquake (Mw 7.8) along the active plate boundary in Himalaya; and 2004 Sumatra-Andaman earthquake (Mw 9.3) associated with a transoceanic tsunami in the Indian Ocean [1].

In peninsula India, the state of Maharashtra, along with Gujarat and Madhya Pradesh, has suffered from frequent deadly and damaging earthquakes. The western coastal areas of Maharashtra along with the state capital Mumbai and Pune city have both been strongly shaken in the past in 1618 and 1764, respectively, [2]. Seismologists identified several faults in this region out of which many show evidence of movement during the Holocene epoch. The major Chiplin fault lies along the Sahyadri range running from the mouth of Bombay Harbor to the Sangammeshwar area in Ratnagiri district. While the west coast fault runs along the eastern shore of the Thane Creek in the Mumbai area, along the flanks of the Parsik Hills. The Vidharbha region is intersected by Kadam fault which runs from Bhusawal city into northern Andhra Pradesh. The 1967 Koyna earthquake (Mw 6.6) is the largest known case of Reservoir Induced Seismicity (RIS) in the world. The devastating Latur earthquake (Mw 6.2) of 1993 in southern part of the state has claimed an estimated 11,000 human lives [3].

To study these seismic events, various methods and techniques are used. Deterministic- and probabilistic-based seismic hazard analysis of the earthquake prone source-site area carried out to know about the peak ground acceleration and return interval of the earthquakes in a particular area or site. Geotechnical and geophysical data-based site characterization and ground response analysis help to divide the area into smaller zones giving a new direction toward microzonation. Liquefaction occurs generally due to rapid loading during seismic events, where there is not sufficient time for dissipation of excess pore-water pressures through natural drainage. Liquefaction Potential Index (LPI) is evaluated at each borehole location from the obtained Factors of Safety (FS) to predict the potential of liquefaction to cause damage at the surface level at the site of interest. This paper deals with the compilation of various studies carried out at different region of the Maharashtra based on several approaches. A critical review is presented on each of these methods, highlighting their limitations and suitability of application.

2 Earthquakes in Maharashtra

After 2001 Bhuj earthquake in Gujarat, seismic hazard map of India was updated by the Bureau of Indian Standards (BIS). The latest seismic zonation map of India includes the Beed, Osmanabad, and Latur, along with eastern sections of Ahmednagar, Pune, Satara, and Sangli districts in Zone III. Districts viz. Raigad, Ratnagiri, and Satara lie in Zone IV, where maximum expected intensity of seismic motion is VIII. The remaining western and northwestern districts of the state fall in Zone III [4, 5]. India Meteorological Department (IMD) under Government of India responsible for monitoring seismic activity in and around the country helping in collection the real-time earthquake data. Following Table 1 gives the detailed description about the major earthquakes event occurred in the Maharashtra region after 1950.

Table 1 Major earthquakes in Maharashtra after 1950
Full size table

3 Seismic Hazard Assessment

The seismic hazard could be defined as the likelihood of experiencing a specified intensity of any damaging phenomenon at a site or all over a region during a considered period of time. The development of the methodology for analyzing the probability of seismic hazards has originated from the engineering needs for better designs [6]. Seismic hazard assessment is analogous to long-term earthquake prediction [7, 8]. Two basic methodologies used for Seismic Hazard Assessment (SHA) are the Deterministic Seismic Hazard Assessment (DSHA) and the Probabilistic Seismic Hazard Assessment (PSHA) approaches. In the Deterministic Seismic Hazard Assessment (DSHA), the strong ground motion parameters are estimated for the maximum credible earthquake, assumed to occur at the closest possible distance from the site of interest, without considering the likelihood of its occurrence during a specified exposure period. On the other hand, in PSHA, rather than searching for elusive worst-case ground motion, all possible earthquake events and resulting ground motions along with their associated probabilities of occurrence are considered to estimate the level of ground motion intensity exceeded with some tolerably low rate.

3.1 Deterministic Seismic Hazard Analysis (DSHA)

This method of seismic analysis is based on Maximum Credible Earthquake (MCE) and estimates the maximum size of an earthquake located at defined distance from the site. The basic steps involved in the process of deterministic seismic hazard assessment are as follows, as shown in Fig. 1.

Fig. 1
figure 1

Steps of deterministic seismic hazard analysis (DSHA) [9]

Full size image
  1. a.

    Identification of earthquake sources: These sources could be either clearly understood and defined faults or less well understood and less well defined geologic structures or seismotectonic provinces of many thousands of square kilometers. Earthquake sources could be either clearly understood and defined faults or less well understood and less well defined any geologic structures or seismotectonic region of many thousands of square kilometers. The individual sources could be configured as points, lines, areas, or volumes [6]. The geometry and potential of each earthquake source need to be defined well before jumping to next step.

  2. b.

    Evaluation of source-to-site distance: For each source zone, the shortest distance between the source zone and site of interest is selected.

  3. c.

    Determination of motion parameters: The earthquake potential of each source specified in the first step should be defined in terms of a maximum earthquake known as either "maximum credible earthquake" which is the maximum earthquake capable of occurring in an area or on a given fault during the current tectonic regime. They are also defined as "maximum expectable earthquake," or "design earthquake." For regional seismicity, two levels of maximum earthquakes could be considered as in the case of the U.S. Nuclear Regulatory Commission: "Safe shutdown earthquake" and "operating basis earthquake" selected for hazard assessment.

  4. d.

    Formal definition of seismic hazard: This can be defined as the hazard at the site of interest represented by specific peak ground motion such as response spectrum ordinate values, describing the earthquake effects [10] (Fig. 2).

    Fig. 2
    figure 2

    Steps of probabilistic seismic hazard analysis (PSHA) [11]

    Full size image

3.2 Probabilistic Seismic Hazard Analysis (PSHA)

Probabilistic seismic hazard analysis is a rational solution to the different types of dilemmas posed by uncertainties. The first methodology applied to most of the probabilistic seismic hazard analyses was defined by [6] in 1968. The four steps involved in this method are explained below:

  1. a.

    Identification of earthquake sources: The types of the sources may range from small planar faults to large seismotectonic provinces. The source-site distances for each source are calculated considering the spatial uncertainty. The distances from all possible locations to the site within the earthquake source should be certainly considered since these earthquakes are assumed to happen anywhere within that source.

  2. b.

    Creation of recurrence relationships: In contrast to the deterministic analyses which try to use one controlling earthquake or one maximum earthquake for each source separately, each source is characterized by an earthquake probability distribution or simply by a recurrence relationship in probabilistic analyses.

  3. c.

    Determination of motion parameters: A set of earthquake attenuation or ground motion curves relating a ground motion parameter to distance for an earthquake of a given size is required.

  4. d.

    Formal definition of seismic hazard: The hazard curve which integrates the effects of all the earthquakes of different sizes occurring at different locations within different earthquake sources at different probabilities of occurrence is formed to show the probability of exceeding different levels of ground motion levels at the site during a specified period [10] (Table 2).

    Table 2 Advantages and disadvantages of PSHA and DSHA [9]
    Full size table

The seismic hazard of Greater Mumbai region is evaluated using DSHA methodology [12]. The spatial variation of 50 and 84th percentile of Spectral Acceleration (Sa) values shown in Fig. 3 for 0.2 and 1 s period. Seismic hazard for Peak Horizontal Acceleration (PHA) increases steadily from relatively low to high seismicity from the center to southern and northern parts of the Greater Mumbai region. The same hazard pattern is observed for Sa values at short period (0.2 s); however, for long period (1 s), the hazard is observed to be concentrated on southern part of the region with a steady decrease toward north side. The PHA values for Greater Mumbai region ranges between 0.076 and 0.328 g.

Fig. 3
figure 3

Spatial variation of 50th percentile and 84th percentile values for a 0.2 s, b 1 s at bedrock [12]

Full size image

4 Liquefaction Hazard Assessment

Liquefaction Potential Index (LPI) is a parameter used to represent regional liquefaction potential [13, 14]. LPI at a specific site is computed by integrating the Factors of Safety (FS) along the soil column. A weighting function is added to give more weight to the layers closer to the ground surface. The Liquefaction Potential Index (LPI), proposed by [15, 16] is expressed as follows:

$${\text{LPI}} = \mathop \smallint \limits_{0}^{20} F.w \left( z \right){\text{d}}z$$
(1)

where z is depth of the midpoint of the soil layer), and dz is differential increment of depth. The weighting factor, w(z), and the severity factor, F(z), are calculated as per the following expressions:

$$\begin{aligned} F\left( z \right) \, & = 1 { } - FS\quad {\text{for}}\quad FS < 1.0 \\ F\left( z \right) & = 0\quad {\text{for}}\quad FS \ge 1.0 \\ w\left( z \right) & = 10 - 0. 5z\quad {\text{for}}\quad z < 20\;{\text{m}} \\ w\left( z \right) & = 0\quad {\text{for}}\quad z > 20\;{\text{m}} \\ \end{aligned}$$

For the soil profiles with the depth less than 20 m, LPI is calculated using the following expression [4],

$${\text{LPI}} = \mathop \sum \limits_{i = 1}^{n} w_{i} F_{i} H_{i}$$
(2)

where Hi is thickness of the discretized soil layers; n is number of layers; Fi is liquefaction severity for ith layer; FSi is the factor of safety for ith layer; wi is the weighting factor (=10–0.5zi); and zi is the depth of ith layer (m).

To predict the potential of liquefaction causing damage at the surface level of the site of interest evaluated the Liquefaction Potential Index (LPI) at each borehole location from the obtained Factors of Safety (FS) for Greater Mumbai [17]. LPI contour maps prepared for Mumbai region showed that a high degree of liquefaction damages is likely to occur at many sites in the city during severe seismic event. These LPI contour maps help to check the vulnerability of the area against liquefaction. These contour maps can also be used effectively for seismic safety plans and in the seismic hazard mitigation programs followed by seismic microzonation.

5 Seismic Response and Geophysical Investigation

Seismic hazard analysis gives the ground shaking at the bedrock. However, the ground shaking and the associated damage to engineered structures can be strongly influenced by the geology and topography in their vicinities. The influence of local site conditions on the nature of the ground motions and the damage they may cause have been observed and studied [13]. The early work of seismologists studied site amplification shows that they used to assume linear soil behavior, and they rarely considered the soil non-linearity in their assessments of site conditions [7, 18]. On the other hand, the non-linear approach is complicated and cannot model the actual hysteretic stress–strain behavior of cyclically loaded soil. Therefore, the equivalent linear method that modifies the linear approach may be an acceptable estimation of ground response for practical problems of interest [16]. The equivalent linear model provides few parameters that represent typical soil behavior when subjected to cyclic loading. In the equivalent linear method, an iteration procedure is used to make sure the parameters used in the analysis which are compatible with the computed strain level in all the layers (Fig. 4).

Fig. 4
figure 4

Spectral shapes for different site conditions [7]

Full size image

Shear wave velocity (Vs) is an essential parameter for evaluating the dynamic properties of soil in the shallow subsurface [8, 19]. Several geophysical methods have been proposed for near-surface characterization and measurement of shear wave velocity by using a great variety of testing configurations, processing techniques, and inversion algorithms. The most widely used technique is Multichannel Analysis of Surface Waves (MASW). The MASW method was first introduced in geophysics by [20, 21]. Following Fig. 5 represents the steps considered while doing geophysical survey using MASW.

Fig. 5
figure 5

Procedure followed during MASW survey [22]

Full size image

Characteristics of strong ground motion and ground response analysis for different sites in Nanded city of the Maharashtra state studied using MASW [20, 23]. Shear wave velocity averaged over top 30 m of the soil is obtained from MASW survey at 60 sites at depths of 20–50 m below the ground surface. The high VS30 values (900–1000 m/s) are found toward the northeast and southern part of the city. Around the Chikhli Khurd and Shrinagar areas observed shear wave velocity is 500 m/s. The eastern phase areas are characterized by relatively low VS30 values (<200 m/s). They are associated with the Godavari River carrying alluvial deposits composed of brown clay with intercalated bands of sand and gravel.

6 Summary and Conclusion

Maharashtra state suffered many earthquakes in the past few decades including major events like 1967 Koyna earthquakes and 1993 Latur earthquakes. These earthquakes caused property damages and killed thousands of people. To study theses earthquakes and their post effects, various techniques and methods are used. There are basically two methods used for seismic hazard assessment viz. Deterministic Seismic Hazard Assessment (DSHA) and Probabilistic Seismic Hazard Assessment (PSHA). DSHA is an easy and quick method and provides a simple framework for the evaluation of the worst-case ground motions associated with the catastrophic failure damages of the large engineering structures such as nuclear power plants and large dams based on maximum credible earthquake. The deterministic seismic hazard analysis is always associated with the subjective predictive decisions, regarding for instance the earthquake potential, of the combined opinions and judgments of seismologists, engineers, risk analysts, economists, social scientists, and finally government officials. PSHA is an improvement over DSHA by considering all possible uncertainties into the hazard estimation procedure, arising in magnitude scales; types of earthquake sources and its nature of physical processes; using various kinds of earthquake recurrence models and its inter-model or intra-model assumptions and estimation of strong ground motion using appropriate ground motion prediction equations. MASW is one of the easiest seismic methods that provide highly favorable and competent results. Shear wave velocity obtained from MASW survey gives a new direction toward liquefaction hazard assessment and seismic microzonation.