Mir and Parvez (2020) simulated bedrock level peak ground motion at 2346 sites on a regular grid of 0.2\(^{\circ }\)x0.2\(^{\circ}\) in NW Himalaya from 543 simulated sources, using the stochastic finite-fault, dynamic corner frequency method, with particular emphasis on Kashmir Himalaya. Acceleration time series generated are then integrated to obtain velocity and displacement time series, which are all used to construct a suite of hazard maps of the region. The expected PGA values for the Kashmir Himalaya and Muzaffarabad are found to be  \(\sim\) 0.3–0.5g and for the epicentral region of the 1905 Kangra event, to be 0.35g. The PGA values estimated in this study are in general found to be higher than those implied by the official seismic zoning map of India produced by the Bureau of Indian Standards.  Major events in Kashmir Himalayas, such as those of 1555, 1885 and 2005, are simulated individually to allow comparison with available results. This study provides a first-order ground motion database for safe design of buildings and other infrastructure in the NW Himalayan region.
    Kumar and Sharma (2019) performed a detailed study on the temporal evolution of seismicity in and around the seismic gaps in the Himalayan region. They segmented the region into four meridional regions (A) 80\(^{\circ}\)E to 83.5\(^{\circ}\)E, (B) 83.5\(^{\circ}\)E to 87.5\(^{\circ}\)E, (C) 87.5\(^{\circ}\)E to 90\(^{\circ}\)E, and (D) 90\(^{\circ}\)E to 98\(^{\circ}\)E along with a fixed latitude belt. A homogeneous catalogue with 3 \(\le\)M\(\le\) 6.5 was used for the spatial and temporal analysis of seismicity in terms of b-value. It is found that pockets of lower b-values coincide over and around stress accumulated regions. The observed low b-value before the occurrence of the Nepal earthquake of 2015-04-25 supports the argument of impending occurrence of moderate to large magnitude earthquake in Sikkim and north-east Himalayan region in future.
    Singh S.K., et al., (2020) documented the site amplification at 28 sites in the Indo-Gangetic Plains (IGP) using the RSS (ratio of the source spectrum) technique. The fundamental frequency (fo) of the sites increases from 0.12 Hz near the foothills of Himalaya to 2.0 Hz at the southern edge of the basin and the amplification reaches about 10. At several sites, is difficult to select and an amplification of \(\sim\) 5 in broadband is in the range 0.12–0.7 Hz. Application of standard spectral ration (SSR) technique to teleseismic S-wave data recorded in the IGP reveals that this approach may be useful in the estimation of amplification at low frequencies (< 0.5 Hz).
    Sharma N., et al. (2021) targeted two largest magnitude earthquakes (2017-02-06 M 5.1 and 2017-12-06 M 5.6) recorded by the CSIR-NGRI network in Uttarakhand. They estimated the peak ground acceleration (PGA), peak ground velocity (PGV) and peak ground displacement (PGD) from recorded waveforms and acceleration, velocity and displacement response spectra at different structural periods for districts like Haridwar, Rudraprayag, Almora, Tarikhet and Thakurdwar. Their simulation reveals that the displacement spectra for many Himalayan earthquakes obey a circular crack model with  fall. The high stress drop (70 and 100 bars) estimations suggest high release of energy in the seismic zone V region which hosted the 1991 Uttarkashi (M 6.8) and 1999 Chamoli (M 6.3) earthquakes. The PGA values estimated at Rudraprayag district (168 cm/s2) bring the district under moderate to severe intensity zone even for moderate size earthquakes.
    Strong motion data from the network operated by WIHG in the Himalaya have been used to quantify seismic hazard assessment in the form of attenuation, site effects, and simulation of strong ground motion for different sectors of the Himalaya, e.g. Garhwal and Kumaun (Uttarakhand), Kinnaur (Himachal Pradesh), Nubra-Shyok (Ladakh), and NE India (Sandeep et al., 2019a, 2019b, 2019c, 2020, 2022; Kumar P., et al., 2019). This work provides great insight into exploring recent trends in seismology and earthquake engineering for seismic hazard evaluation.
    In the Jammu & Kashmir sector of the Himalaya, the thrust faults are blind and large-scale folding is the only expression of active deformation at the surface, making it difficult to assess seismic hazard in this region. O’Kane et al. (2022) used field, satellite and seismological observations to determine the fault geometry for this region and modeled the potential hazard scenarios. Their results suggest that earthquakes that rupture the buried, shallow part of the locked Main Himalayan Thrust could generate PGVs that are >3 times larger than earthquakes of the same magnitude on its deeper portions.
    Gogoi et al. (2023) estimated ground motion parameters (GMPs) by processing 125 accelerogram records of 26 earthquake events, with 375 components, that originated in NE India and its vicinity with special emphasis on the 2016-01-M6.7 Tamenglong earthquake. Moreover, Ground Motion Prediction Equations were developed through multiple regression analysis on observed data of 8 GMPs namely, PGA, PGV, Arias Intensity, Characteristics Intensity, Housner Intensity, Cumulative Absolute Velocity, Effective Design Acceleration and Acceleration Spectrum Intensity. The developed equations are representative of statistics on changes in amplitudes of parameters with varying distances and magnitude in connection to NE India. Besides, newly developed GMPEs can be applied to 4–6.8 magnitude earthquakes and valid up to 525 km of distance.
    Boruah et al. (2022) attempted estimation of site amplification factors of different geomorphological units in Shillong city using the distribution of PGA due to maximum credible earthquakes that originated in nearby major faults and average shear wave velocity (Vs30) values for various geomorphological units. The amplification for the highly dissected land in the city is found to be maximum within a range of 2.77–2.92, while the high plateau segment is characterized by least values (2.01–2.16). Simultaneously, the effective ground motion mapped on the surface indicates a maximum value of 0.6–0.94g for a probable earthquake of Mw 8.1 on the Dauki fault. Similarly, Dhubri and Kopili faults might produce a ground motion of 0.05–0.08 and 0.22–0.33g for a maximum credible earthquake of Mw  \(\sim\) 7.0, respectively. 
    The distribution and dissipation of post-seismic stress have been investigated for the Garhwal-Kumaun region based on spectral analysis of P-waveforms of local earthquakes (Hajra et al., 2022a). The study detects significantly lower stress drop compared to the overall values for the Central Seismic Gap region suggesting incomplete dissipation of the accumulated stress. The high-stress drop events are predominant along the MHT at mid-crustal depths, with the upper \(\sim\) 10 km of the brittle crust rarely hosting any strong earthquake. The b-value is estimated as 0.64\(\pm\)0.08 for the Kumaun Himalaya, which is low compared to the Garhwal and the rest of the NW Himalaya.
    The Ladakh-Karakoram zone (LKZ) is a unique testing ground for understanding the geodynamic evolution of the Himalaya-Karakoram orogeny. Despite the accumulation of a large amount of strain energy that originated due to the India-Asia collision along the Karakoram fault, earthquakes of M\(\ge\) 7.0 are considerably less in the Karakoram Fault Zone compared to the Himalayan seismic belt in the northwest Himalaya. Earthquake source parameter study in the LKZ through spectral analysis of P-waves of local earthquakes reveals low-stress drop earthquakes ( \(\sim\) 0.06-64.36 bar) caused by the possible presence of aseismic creeping patches in the Karakoram Fault. A partial stress drop mechanism is proposed for low-stress drop in the forearc region (Paul and Hazarika, 2022). 
    Although the 1950 Assam earthquake (Mw 8.7) endures as the largest continental earthquake ever recorded, its exact source and mechanism remain contentious. Coudurier-Curveur et al. (2020) analyzed the spatial distributions of reappraised aftershocks and landslides to estimate the hitherto unknown surface rupture extent along the Mishmi and Abor Hills. Their results from two key sites (Wakro and Pasighat) suggest over twice as large co-seismic surface throw (7.6\(\pm\)0.2 m vs. > 2.6\(\pm\)0.1 m) and average thrust dips (25–28\(^{\circ}\) vs. 13–15\(^{\circ}\)) on the Mishmi Thrust (MT) and Main Himalayan Frontal Thrust (MFT).
    The Shillong Plateau is a peculiar geodynamic terrane hosting significant seismic activity outboard the Himalayan belt. This activity is often used as an argument to explain an apparent reduced seismicity in the Bhutan Himalayas. Although current geophysical and geodetic data indicate that the Bhutan Himalaya accommodates more deformation than the Shillong Plateau, it is aimed to quantify the extent to which the two geodynamic regimes are connected and potentially interact through stress transfer. Grujic et al. (2018) compiled a map of major faults and earthquakes in the two regions and computed co-seismic stress transfer amplitudes. Results indicate that the Bhutan Himalaya and the Shillong Plateau are less connected than previously suggested. The 1897 Assam earthquake (Mw 8.25) that affected the Shillong Plateau did not cause a stress shadow on the Main Himalayan Thrust in Bhutan as previously suggested. Similarly, the 1714 Bhutan earthquake (Mw 8\(\pm\)0.5) had negligible impact on stress accumulation on thrust faults bounding the Shillong Plateau.
    An earthquake of magnitude Mw 5.7 shook the northeastern region of India on 2017-01-03 at 14:39:0.5 local time. The duration of the tremor lasted for about 5–6 s and had its epicenter in Dhalai District, Tripura, India. Even though the earthquake was of moderate magnitude, it caused damage to several masonry dwellings in Tripura and triggered soil liquefaction, lateral spreading, and landslides near the epicentral area. Anbazhagan et al. (2019) reported field reconnaissance observations of geotechnical effects and damage to buildings following this earthquake. In addition, the distribution of surface PGAs caused by the earthquake was estimated from the empirical equations based on the available data.

6.1.2 Intraplate regions

Surve et al. (2021) performed seismic hazard studies for the Mumbai city (financial capital of India), having a population of over 18 million. Two seismicity models, linear and areal, were used to compute the seismic hazard of Mumbai with an updated earthquake catalogue and latest knowledge on seismotectonics of the region. The hazard values for Mumbai corresponding to 475-year and 2475-year return periods are computed. Hazard maps at bedrock level for 2% and 10% probability of exceedance in 50 years were prepared. The hazard levels obtained in the present study are lower than those reported for the same area by previous researchers. The lower seismic hazard can be attributed to the fact that in this study, the Koyna–Warna region is used as one of the five independent source regions and reservoir triggered seismicity at times might reduce the overall seismic hazard in nearby regions.
    Mandal and Asano (2019) modeled the low-frequency (0.1–1 Hz) ground motions excited by the 2006-04-06 event, using the finite difference method assuming a point source, to assess the robustness of the constructed velocity structure model. At most of the stations, the observed and simulated velocity waveforms are found to be in good agreement in terms of both amplitude and ground motion duration.  They also computed synthetic ground velocities at numerous locations within the basin, for both the 2001 Bhuj mainshock (finite-fault source) and the 2006 aftershock (point source) cases, using a 3-D velocity model. Their work revealed that the presence of low velocity sediments within the Kachchh rift basin plays a key role in modifying/amplifying the ground motions in 0.1–1.0 Hz range.
    The study of two earthquakes (2006-03-07 and 2006-04-06, Mw 5.5) in the Kachchh seismic zone by Mandal (2020) revealed that the estimated normalized response spectra at strong motion accelerograph sites in the Tertiary formations or near a zone of geological contact between the Jurassic/Tertiary formations, exceeded the design response spectra in the period range of 0.07–0.2 s, correlating with the complete collapse of low-rise buildings, water tanks and dams during the 2001 Bhuj earthquake. On the other hand, the normalized acceleration spectrum of corresponding to hard sediments (rock site) is found to not exceed the design spectrum, correlating with the lack of damage in the Mesozoic hill zone. It is also noticed that the spectral acceleration values at a few sites lying on the Quaternary formations have exceeded the design spectra at 3–4 s, suggesting these sites to be hazardous for engineered reinforced structures like bridges.
    A rare lower crustal earthquake occurred on 2021-07-25 near Hyderabad, India.  Mandal et al. (2022) used waveforms from 9 broadband stations and computed the average corner frequency, seismic moment, moment magnitude, stress drop, and source radius as 3.87 Hz, 7.14E+14 N-m, 3.75, 3.92 MPa, and 229 m, respectively. The mean crustal Q for the region was modeled to be 2182±1178, suggesting lower crustal attenuation below the Hyderabad region. However, the spatial distribution of the modeled crustal Q values revealed a high Q zone to the east of Hyderabad city, while a moderate Q zone was found west of the city. It is inferred that this lower crustal intraplate earthquake with a large stress drop of 3.9 MPa might have been generated due to the sudden movement on an almost vertical fault due to high pore-fluid pressure caused by the presence of CO2-rich mantle fluids.
    Srinagesh et al. (2021) analyzed a sequence of about 965 earthquakes in the magnitude range of 0.1–4.6.  The main shock of moderate-sized earthquake (2020-01-26, ML \(\sim\) 4.6) is located in the Palnadu sub-basin of the Cuddapah basin. It was felt both in the states of Telangana and Andhra Pradesh. The earthquakes prior and after the ML 4.6 quake are located close to the thrust and along the periphery of the backwaters of the Pulichintala reservoir. The epicentral parameters obtained from double difference technique, using a minimum 1-D velocity model, illuminated a steep seismogenic structure extending down to 8 km depth,. The b-value estimate is 0.82 for a completeness magnitude of Mc 1.8 and could be associated with an intraplate event having a longer recurrence time. The focal mechanism solution obtained from waveform inversion reveals a pure double-couple mechanism of a strike-slip motion with a reverse component on a N–S trending focal plane.
    Sharma A., et al. (2021) derived a new ML scale, using the Grey Wolf Optimization, a swarm intelligence-based global optimization technique, for the first time, that is ML=logA+1.2588logR+0.0002789R-2.2265, where A is the amplitude measured in millimeters, and R is the hypocentral distance in km. The new ML scale derived here is valid up to 400 km. The newly derived scale has a drop of 21.37% in the overall standard deviation of all magnitude residuals when station corrections are considered in comparison to the previously used scale.
    Sivaram (2021) simulated high-frequency ground motions at five stations in the National Capital Region (NCR) of India for a large hypothetical Mw 8.5 earthquake and an intermediate Mw 6.8 earthquake in the Himalayan central seismic gap, at fault-distances of about 200–300 km, and indicated that the far-reaching and adverse ground-motion intensities might affect intermediate-high rise structures (period 0.4–0.8 s) in the NCR due to the predominance of fluvial deposits. Nagamani et al. (2020) identified different zones of seismic amplification in the Surat district of Gujarat, India, which are the hub of many mining and industrial projects like oil and natural gas.
    Strong motion data from the network operated by ISR have been used in several engineering seismology applications, such as, estimation of source parameters, site characterization, development of ground motion prediction equation and ground motion modeling. The data recorded from the past decade have been used to characterize various sources in the Kachchh rift and other parts of Gujarat (Kamra et al., 2020). It is found from the study that stress drop of earthquakes (M 4.0-5.1) in the Kachchh rift are in the range 2.3-10.4 MPa with an average of 5.3 MPa. The estimated seismic moment and the source radius are in the range  dyne-cm and 0.43-1.32 km, respectively. The same exercise was carried out for the Saurashtra region (Kamra et al., 2021). For this region, the stress drop varies in the range 0.9-6.9 MPa with an average of 3.3 MPa. A regression relationship between observed accelerations and accelerations estimated from broadband data has been developed exclusively for the Gujarat region (Chaudhary et al., 2022).