**1. Introduction**

At ~860 Ma, Indian plate was broken from Antarctica and then it moved along with African plate until 184 Ma [1, 2]. Subsequently, it moved toward North until it got separated from Madagascar at 88 Ma [3]. At 65 Ma, it moved over the Reunion hotspot, which led to outpouring of Deccan volcanism resulting in the Deccan volcanic province occupying an area of 0.5 million km2 with 2000 m thick basaltic lava in the western and central parts of the Indian subcontinent within a short duration of time [4].

Occurrence of the initial outpouring of Deccan volcanism in Pakistan (at ~72–73 Ma) has been evidenced by the available 40Ar-39Ar age data of the Reunion Island like alkali lavas [3]. The presence of relatively younger basaltic intrusions (~68.5 Ma) in the northern India [5] has supported the fact that India moved

toward south. Also, younger basalts have been found further south [6]. Finally, at 65 Ma, the 95% basaltic outpouring of the Deccan volcanism took place in the western and central India [7]. This Deccan plume model has got further support from the presence of a low-velocity anomaly in the upper mantle extending up to a depth of 600 km below the north-western India as modeled by the regional mantle P-wave seismic tomography [8]. The presence of high <sup>3</sup> He/4 He ratio in Rajasthan has provided further support for the Deccan plume model [5]. Adding credence to this plume theory, local earthquake tomography, surface wave dispersion and receiver function modeling studies have imaged crustal mafic underplating, Moho upwarping and asthenospheric thinning underlying the 2001 Bhuj earthquake region [9, 10]. Recently, the shear-wave splitting study suggested that the upper mantle anisotropy in the KRZ is contributed both by lithospheric frozen anisotropy and asthenospheric flow induced anisotropy, which could be inherited from the plumelithosphere interaction during the Deccan/Reunion plume episode (~65 Ma) [11]. These observations suggest that imprints of the Deccan/Reunion mantle plume are still present in the crust and upper mantle below the north-western region.

The Kachchh region has been experiencing earthquakes since historical times [12]. The region has already experienced seven M6 earthquakes including two Mw 7.7 earthquakes in 1989 and 2001. The latter event has claimed a death toll of 20,000 people. The aftershock activity of this 2001 earthquake is continuing until today, with regular occurrences of Mw 3 events and occasional occurrences of Mw 4 events. The aftershock activity of the 2001 Bhuj earthquake is still continuing that includes 15 Mw ≥ 5, about 300 Mw ≥ 4 and about 6000 Mw ≥ 3 events. We feel that the enigmatic seismicity associated with the Kachchh rift zone is linked with its abovediscussed unique Geodynamic history. Aiming at understanding the influence of crustal-mantle structure in the genesis of uninterrupted occurrences of earthquakes since the occurrence of the 26 January Mw 7.7 Bhuj earthquake, in this chapter, the crust corrected P-wave residuals are estimated at 14 broadband stations and a 3-dimensional P-wave teleseismic tomography is performed using the estimated crust corrected residuals. Finally, modeling results are interpreted concerning the geodynamical processes responsible for generating intraplate earthquakes occurring in the Kachchh region.

### **2. Seismic network and data**

For the present study, broadband digital waveforms of 241 teleseismic events from 14 three-component broadband stations in Kachchh, Gujarat are used (**Figure 1a** and **b**). The sampling rate of recording is 50 sps. Station spacing in the above seismic network is 30–100 km, however, for our study we use data from 20 broadband stations consisting of 14 NGRI and 6 ISR stations as shown by a square in **Figure 2**. The station spacing for the network consisting of these 20 stations is 30–60 km. We selected 241 teleseismic events with magnitude ranging from 5.9 to 8.2, with epicentral distances of 29–90° (**Figure 1b**) with reference to the center of the network.

First, each trace of an event is used to pick the arrival time of the first P-wave maximum amplitude (either peak or trough). Then, these picks are correlated within the network. Following this procedure, the first P-wave onset times are picked from selected highest-quality traces recorded at different stations for one event. Here, the uncertainty of the picking is used to decide a quality factor for each measurement. Note that most of the measurements are found to be having an uncertainty of ±0.05 s. Finally, these quality factors are used to estimate the average data error, which is found to be ±0.06 s.

**33**

**Figure 1.**

*P-Wave Teleseismic Tomography: Evidence of Imprints of Deccan Mantle Plume…*

*(a) Elevation (in m) map showing station distribution in Kachchh, Gujarat. Filled red triangles mark the broadband seismograph stations (SIV—Sivlok; VJP—Vajepar; KNM—Kanmer; BCH—Bhachau; BEL—Bela; GDD—Gadhada; JUM—Jumkunaria; MTP—Motapaya; NGR—Nagor; BHU—Bhuj; NPR—Narayanpar; TPM—Tapar Mundra; MND—Mandvi; VGH—Vaghura; TPR—Tapar Anjar; and NDD—New Dudhai). The inset shows the key map for the area, where the study area is shown by a red open square. K and KL mark Kachchh, Gujarat and Killari, Maharashtra, respectively. An arrow shows the location of Cambay. The black filled portion marks the areal extent of Deccan volcanic province (DVP) in India. (b) Epicentral plot of 241 teleseismic events of Mw 6.0–8.4, whose broadband data are used for our P-wave teleseismic tomography study. A red triangle and green diamond symbols mark the center of our network (lat. 70°, long. 23°) and epicenters of* 

*selected teleseismic events. The size of the diamond symbols vary depending on their sizes.*

*DOI: http://dx.doi.org/10.5772/intechopen.83738*

*P-Wave Teleseismic Tomography: Evidence of Imprints of Deccan Mantle Plume… DOI: http://dx.doi.org/10.5772/intechopen.83738*

#### **Figure 1.**

*Forecasting Volcanic Eruptions*

ring in the Kachchh region.

**2. Seismic network and data**

data error, which is found to be ±0.06 s.

toward south. Also, younger basalts have been found further south [6]. Finally, at 65 Ma, the 95% basaltic outpouring of the Deccan volcanism took place in the western and central India [7]. This Deccan plume model has got further support from the presence of a low-velocity anomaly in the upper mantle extending up to a depth of 600 km below the north-western India as modeled by the regional mantle

provided further support for the Deccan plume model [5]. Adding credence to this plume theory, local earthquake tomography, surface wave dispersion and receiver function modeling studies have imaged crustal mafic underplating, Moho upwarping and asthenospheric thinning underlying the 2001 Bhuj earthquake region [9, 10]. Recently, the shear-wave splitting study suggested that the upper mantle anisotropy in the KRZ is contributed both by lithospheric frozen anisotropy and asthenospheric flow induced anisotropy, which could be inherited from the plumelithosphere interaction during the Deccan/Reunion plume episode (~65 Ma) [11]. These observations suggest that imprints of the Deccan/Reunion mantle plume are

still present in the crust and upper mantle below the north-western region.

The Kachchh region has been experiencing earthquakes since historical times [12]. The region has already experienced seven M6 earthquakes including two Mw 7.7 earthquakes in 1989 and 2001. The latter event has claimed a death toll of 20,000 people. The aftershock activity of this 2001 earthquake is continuing until today, with regular occurrences of Mw 3 events and occasional occurrences of Mw 4 events. The aftershock activity of the 2001 Bhuj earthquake is still continuing that includes 15 Mw ≥ 5, about 300 Mw ≥ 4 and about 6000 Mw ≥ 3 events. We feel that the enigmatic seismicity associated with the Kachchh rift zone is linked with its abovediscussed unique Geodynamic history. Aiming at understanding the influence of crustal-mantle structure in the genesis of uninterrupted occurrences of earthquakes since the occurrence of the 26 January Mw 7.7 Bhuj earthquake, in this chapter, the crust corrected P-wave residuals are estimated at 14 broadband stations and a 3-dimensional P-wave teleseismic tomography is performed using the estimated crust corrected residuals. Finally, modeling results are interpreted concerning the geodynamical processes responsible for generating intraplate earthquakes occur-

For the present study, broadband digital waveforms of 241 teleseismic events

First, each trace of an event is used to pick the arrival time of the first P-wave maximum amplitude (either peak or trough). Then, these picks are correlated within the network. Following this procedure, the first P-wave onset times are picked from selected highest-quality traces recorded at different stations for one event. Here, the uncertainty of the picking is used to decide a quality factor for each measurement. Note that most of the measurements are found to be having an uncertainty of ±0.05 s. Finally, these quality factors are used to estimate the average

from 14 three-component broadband stations in Kachchh, Gujarat are used (**Figure 1a** and **b**). The sampling rate of recording is 50 sps. Station spacing in the above seismic network is 30–100 km, however, for our study we use data from 20 broadband stations consisting of 14 NGRI and 6 ISR stations as shown by a square in **Figure 2**. The station spacing for the network consisting of these 20 stations is 30–60 km. We selected 241 teleseismic events with magnitude ranging from 5.9 to 8.2, with epicentral distances of 29–90° (**Figure 1b**) with reference to the center of

He/4

He ratio in Rajasthan has

P-wave seismic tomography [8]. The presence of high <sup>3</sup>

**32**

the network.

*(a) Elevation (in m) map showing station distribution in Kachchh, Gujarat. Filled red triangles mark the broadband seismograph stations (SIV—Sivlok; VJP—Vajepar; KNM—Kanmer; BCH—Bhachau; BEL—Bela; GDD—Gadhada; JUM—Jumkunaria; MTP—Motapaya; NGR—Nagor; BHU—Bhuj; NPR—Narayanpar; TPM—Tapar Mundra; MND—Mandvi; VGH—Vaghura; TPR—Tapar Anjar; and NDD—New Dudhai). The inset shows the key map for the area, where the study area is shown by a red open square. K and KL mark Kachchh, Gujarat and Killari, Maharashtra, respectively. An arrow shows the location of Cambay. The black filled portion marks the areal extent of Deccan volcanic province (DVP) in India. (b) Epicentral plot of 241 teleseismic events of Mw 6.0–8.4, whose broadband data are used for our P-wave teleseismic tomography study. A red triangle and green diamond symbols mark the center of our network (lat. 70°, long. 23°) and epicenters of selected teleseismic events. The size of the diamond symbols vary depending on their sizes.*

#### **Figure 2**

*(a) Crust corrected relative P-residuals in Gujarat. (b) Directional mean of P-residuals by subtracting an average residual from the eight stations in Kachchh. Red open circles mark negative residuals while blue open circles show positive residuals. CRZ marks the Cambay rift zone, which is shown by dotted lines. Black dotted elliptical area mark the central Kachchh rift zone characterized by negative residuals. And, black square area shows our study area considered for our P-wave teleseismic tomography.*
