Seismic Hazard Assessment of Tripura and Mizoram States along with Microzonation of Agartala and Aizawl Cities

Abstract

Tee present research focuses on seismic hazard studies for the states of Tripura and Mizoram in the North-East India with taking into account the complex sesismotectonic characteristics of the region. This area is more prone to earthquake hazard due to complex subsurface geology, peculiar topographical distribution, continuous crustal deformation due to the under thrusting of Indian and the Eurasian plates, a possible seismic gap, and many active intraplate sources identified within this region. The study area encompasses major seismic source zones such as Indo Burmese Range (IBR), Shillong Plateau (SP), Eastern Himalayan arc (EH), Bengal Basin (BB), Mishmi Thrust (MT) and Naga Thrust (NT). Five historical earthquakes of magnitude Mw>8 have been listed in the study area and 15 events of magnitude Mw>7 have occurred in last 100 years. Indian seismic code BIS-1893-2002 places the study area with a high level of seismic hazard in the country (i.e. seismic zone V). More than 60% of the area is hilly steep-terrain in nature and the altitude varies from 0 to 3000 meters. Recent works have located a seismic gap, known as the Assam gap since 1950 between the EH, SP, and IBR with the Eurasian plate. Various researchers have estimated the return period, and a large size earthquake is expected in this region any time in future. The area is also highly prone to liquefaction, since rivers in Tripura (Gomati, Howrah, Dhalai, Manu, Bijay, Jeri, Feni) and the rivers in Mizoram (Chhimtuipui, Tlawng, Tut, Tuirial and Tuivawl etc.) are scattered throughout the study area where soil deposits are of sedimentary type. In 2011, both the states together have experienced 37 earthquakes (including foreshocks and aftershocks) with magnitudes ranging from 2.9 to 6.9. Of these events, there were 23 earthquakes (M>4) of magnitudes M6.4 (Feb 4th 2011), M6.7 (March 24th 2011), M6.9 (Sept.18th 2011), M6.4 (October 30th 2011), M6.9 (Dec 13th 2011), M5.8 (Nov 21st 2011), M5 (Aug 18th 2011), M4.9 (July 28th 2011), M4.6 (Dec 15th 2011), M4.6 (Jan 21st 2011), M4.5 (Dec 9th 2011), M4.5 (Oct 21th 2011), M4.5 (Oct 17th 2011), M4.5 (Sept 18th 2011), M4.3 (Oct 10th 2011), M4.3 (Sept 22nd 2011), M4.3 (April 4th 2011), M4.2 (Sept 9th 2011), M4.2 (Sept 18th 2011), M4.1 (April 29th 2011), M4.1 (Feb 22nd 2011), M4 (June 9th 2011), and M4 (Dec 2nd 2011) which occurred within this region [source: IMD (Indian Metrological Department), India]. The earthquake (M6.9) that occurred on Sept. 18th 2011 is known as the Sikkim earthquake, and it caused immense destruction including building collapse, landslides, causalities, disrupted connectivity by road damages and other infrastructural damages in Sikkim state as well as the entire North-East India. In the cities of Agartala and Aizawl of Tripura and Mizoram, construction of high rise building is highly restricted by the Government. Being the capital city, many modern infrastructures are still pending for growth of the city planning. Although many researchers have studied and reported about the status of seismicity in North-East Region of India, very few detailed studies have been carried out in this region except Guwahati, Sikkim and Manipur where almost the whole of the study area is highly vulnerable to severe shaking, amplification, liquefaction, and landslide. From the available literature, no specific study exists for Tripura and Mizoram till date. In the present research, seismic hazard assessment has been performed based on spatial-temporal distribution of seismicity and fault rupture characteristics of the region. The seismic events were collected from regions covering about 500 km from the political boundary of the study area. The earthquake data were collected from various national and international seismological agencies such as the IMD, Geological Survey of India (GSI), United State Geological Survey (USGS), and International Seismological Centre (ISC) etc. As the collected events were in different magnitude scales, all the events were homogenized to a unified moment magnitude scale using recent magnitude conversion relations (region specific) developed by the authors for North-East Region of India. The dependent events (foreshocks and aftershocks) were removed using declustering algorithm and in total 3251 declustered events (main shocks) were identified in the study area since 1731 to 2011. The data set contains 825 events of MW < 4, 1279 events of MW from 4 to 4.9, 996 events MW from 5 to 5.9, 131 events MW from 6 to 6.9, 15 events MW from 7 to 7.9 and 5 events MW ≥8. The statistical analysis was carried out for data completeness (Stepp, 1972). The whole region was divided into six seismic source zones based on the updated seismicity characteristics, fault rupture mechanism, size of earthquake magnitude and the epicentral depth. Separate catalogs were used for each zone, and seismicity parameters a and b were estimated for each source zone and other necessary parameters such as mean magnitude (Mmean), Mmax, Mmin, Mc and recurrence periods were also estimated. Toposheets/vector maps of the study area were collected and seismic sources were identified and characterized as line, point, and areal sources. Linear seismic sources were identified from the Seismotectonic atlas (SEISAT, 2000) published by the GSI, in addition to the source details collected from available literature and remote sensing images. The SEISAT map contains 43 maps presented in 42 sheets covering entire India and adjacent countries with 1:1million scale. Sheets representing the features of the study area were scanned, digitized and georeferenced using MapInfo 10.0 version. After this, tectonic features and seismicity events were superimposed on the map of the study area to prepare a Seismotectonic Map with a scale of 1:1million. In seismic hazard assessment, a state of art well known methodologies (deterministic and probabilistic) was used. In deterministic seismic hazard analysis (DSHA) procedure, hazard assessment is based on the minimum distance between sources to site considering the maximum magnitude occurred at each source. In hazard estimation procedure a lot of uncertainties are involved, which can be explained by probabilistic seismic hazard analysis (PSHA) procedure related to the source, magnitude, distance, and local site conditions. The attenuation relations proposed by Atkinson and Boore (2003), and Gupta (2010) are used in this analysis. Because in this region two type activities are mostly observed, regions such as SP, and EH are under plate boundary zone whereas IBR is under subduction process. These equations (GMPEs) were validated with the observed PGA (Peak ground acceleration) values before use in the hazard evaluation. The hazard curves for all six major sources were prepared and compiled to get the total hazard curve which represents the cumulative hazard of all sources. Evaluation of PGA, Sa (0.2s and 1.0s) parameters at bedrock level were estimated considering a grid size of 5 km x 5 km, and spectral acceleration values corresponding to a certain level of probability (2% and 10%) were done to develop uniform hazard spectrum (UHS) for both the cities (Agartala and Aizawl). To carry out the seismic microzonation of Agartala and Aizawl cities, a detailed study using geotechnical and geophysical data has been carried out for site characterization and evaluation of site response according to NEHRP (National Earthquake Hazard Response Program) soil classifications (A, B, C, D, and E-type). Seismic site characterization, which is the basic requirement for seismic microzonation and site response studies of an area. Site characterization helps to have the idea about the average dynamic behavior of soil deposits, and thus helps to evaluate the surface level response. A series of geophysical tests at selected locations have been conducted using multichannel analysis of surface waves (MASW) technique, which is an advanced method to obtain direct shear wave velocity profiles from in situ measurements for both the cities. Based on the present study a major part of Agartala city falls under site class D, very few portions come under site class E. On the other hand, Aizawl city comes under site class C. Next, a detailed site response analysis has been carried out for both the cities. This study addresses the influence of local geology and soil conditions on incoming ground motion. Subsurface geotechnical (SPT) and geophysical (MASW) data have been obtained and used to estimate surface level response. The vulnerable seismic source has been identified based on DSHA. Due to the lack of strong motion time history of the study area, synthetic ground motion time histories have been generated using point source seismological model (Boore 2003) at bedrock level based on fault rupture parameters such as stress drop, quality factor, frequency range, magnitude, hypocentral distance etc. Dynamic properties such as the shear modulus (G) and damping ratios (ζ) have been evaluated from the soil properties obtained from SPT bore log data collected from different agencies such as PWD (Public works Department), and Urban Development Dept. of the State Government, in situ shear wave velocity has been obtained from MASW survey in different locations, and following this, a site response analysis has been carried out using SHAKE-2000 to calculate the responses at the ground surface in combination of different magnitudes, distances and epicentral depth for a particular site class. An amplification factor was estimated as the ratio of the PGA at the ground surface to the PGA at bedrock level, a regression analysis was carried out to evaluate period dependant site coefficients, and hence, the period dependant hazard impact on the ground surface could be calculated to obtain the spatial variation of PGA over the study area. Further, liquefaction potential of the site (Agartala) was also evaluated using available SPT bore log data collected and using presently estimated surface level PGA. The results are presented in the form of liquefaction hazard map representing as a Factor of safety (FS) against liquefaction with various depths such as 1.5m, 10m, and 15m respectively. It has been seen that Agartala city shows highly prone to liquefaction even up to 15 m depth. Hence, site specific study is highly recommended for implementing any important project. The liquefaction hazard assessment could not be conducted for the Aizawl city because of non availability of the SPT-N data, however, the city stands on hills/mountains, and therefore, such a study is not applicable in this area. Further, seismic microzonation maps for both the cities have been prepared considering Analytical Hierarchy Process (AHP) which support to the Eigen value properties of the system. Two types of hazard maps have been developed, one using deterministic and another using the probabilistic seismic microzonation maps. These maps can be directly used as inputs for earthquake resistant design, and disaster mitigation planning of the study area. However, an investigation has also been made in forecasting a major earthquake (Mw>6) in North-East India using several probabilistic models such as Gamma, Weibull and lognormal models. IBR and EH show a high probability of occurrences in the next 5 years (i.e. 2013-2018) with >90% probability.