Pn wave velocity and Moho geometry in north eastern India

Transcription

1 Pn wave velocity and Moho geometry in north eastern India S S RAI ], K S PRAKASAM 1 and N AGRAWAL 2 1 National Geophysical Research Institute, Uppal Road, Hyderabad ; e mail:postmast@csngri.ren.nic.in 2 Pan India Consultants Pvt. Ltd., Gurgaon, Haryana Pn velocity has been computed across the NE India and Moho geometry constrained, using regional earthquake travel times recorded by a network of 30 seismological stations operated during February- May Using an appropriate velocity model and the arrival times at the network stations, preliminary hypocentres of 16 regional earthquakes provided by NEIC were also improved. The average Pn wave velocity in NE India has been found to be km/s. It varies from 8.3 to 8.5 km/s beneath the Shillong Plateau, Mikhir hills and Assam valley, which is significantly higher than those in other parts of India. The crustal thickness in NE India is also high, varying from km under the Shillong plateau and the adjoining region to km in the convergence zone. The presence of a thick crust and high Pn velocity suggests that the lithosphere in NE India is colder, as also indicated by the observed deeper level (45-51 km) seismicity of the region. 1. Introduction North eastern India is composed of diverse geological terranes ranging in age from Archean to young orogenies. These are (figure 1): the Shillong massif, the eastern Himalaya, the Assam syntaxis, the Arakan Yoma ranges, the Brahmaputra valley and the Bengal basin, all fashioned in this assembly by the complex geological processes that followed the penetration of the Indian continent in to Eurasia and later in to the wedge that evolved with the wrapping of the Himalayan collision front around its northeastern edge to form the Arakan Yoma ranges. The narrow region intervening between these two ranges is the Brahmaputra valley, confined at its southern edge by the Shillong massif, an elevated Archean block that rises abruptly above the Bangladesh plains. The detailed tectonic set up of the region is described by Evans (1964), Nandy (1980). During the last one hundred years, this region has witnessed over a dozen devastating earthquakes, including the great earthquakes of 1897 and 1950, each of magnitude greater than 8.7. A special feature of the region's seismicity is the occurrence of deep crustal (45-51 km) earthquakes beneath the Shillong plateau (Chen and Molnar 1990) pointing to the existence of a relatively colder lithosphere in this region. Investigation of the crustal structure of NE India has been a subject of numerous studies over the last four decades (Tandon 1954; Kayal and De 1987; Sitaram and Saikia 1992; Mukhopadhyay et al 1995). Estimates of the P-wave velocity for the upper crust range from 5.6 to 5.99 km/s and 6.5 km/s for the lower crust. The Pn velocity is inferred to vary between 7.94 and 8.7 km/s, and estimates for the Vp/Vs ratio for the upper and lower crust are 1.74 and 1.76 respectively. Most part of NE India shows lower heat flow with an average of,,~ 45 mwm -2 (Ravi Shankar 1988) similar to that observed over the Archean terranes of south India. The Bouguer gravity anomaly (figure 2) in the region varies from ~ -20m gal over Shillong plateau to -250m gal in the Brahmaputra valley. Verma and Mukhopadhyay (1977) suggest that the crust underlying the Shillong massif is probably denser as well as thicker than the normal, while the Assam valley is underlain by a thicker crust. Estimates of the crustal thickness of the region derived from modelling of Bouguer gravity anomalies (Verma and Mukhopadhyay 1977; Gaur and Bhattacharji 1983) also show a variation from 45 to 63 km. Keywords. NE India; seismicity; crust; travel time; time term. Proc. Indian Acad. Sci. (Earth Planet. Sci.), 108, No. 4, December 1999, pp Printed in India 297

2 298 S S Rai, K S Prakasam and N Agrawal 28 ~,o N 91 o f.m.j L. C~ SPA 9 & / M A//V BO\ 94ot -J / / \ op -..J ~. BRAHk~AP~ X -,......,.,,. / A NGP \ S HILLONG HMNA/ ~ A PLATE AU ANGN SHL / 9 ".._ J ~,... / z" 9 &UMS k..~,._-... f\j JWI?.5 ~ DAUKI FAULT "~. DWK /.-) B A N G L A D ES H /,I /../,-.J 4o...,[7"',,,,, i,, i /1". PNB. '.. r/, //SNAI MIKHIR l j '~ r =v,,./ & TML MLRjI/,/ / ~u KPGJ I ") : UKLI ~ 9 ~. AtMP /g ABS P /'2 /'~ Figure 1. Regional geology and location of temporary seismic stations in northeastern India. These observations suggest distinct different materim properties of the underlying crust and upper mantle in this region. A comprehensive program to model the velocity structure of NE India was accordingly formulated during 1993 and implemented by the National Geophysical Research Institute. A hybrid network of 60 short period vertical seismometers with analog smoke drum recorders were operated for a period of three months (15th February-15th May, 1993). Of these, only 30 seismic stations with reliable time synchronisation were included for data analysis. We present here the Pn velocity and crustal thickness variation in the region using the time term approach. 2. Data We used P-arrival times over the seismic stations in NE India (figure 1), each equipped with 1 Hz vertical seismometer and a smoke drum recorder, for 16 regional earthquakes of mb > 4.5 listed in the NEIC bulletin. Due to the structural complexity and absence of horizontal component seismograms we refrained from using the S-phase data. Using the initial hypocentre of these earthquakes taken from the NEIC and the velocity model presented in table 1 (Gupta et al 1984) we recomputed their hypocentre (table 2) through a grid search over a depth range varying from 4 to 50 km discretized at intervals of 1 km. With this exhaustive search, we obtained the average travel time residual of 0.4s. The RMS error in epicentre was < 3 km and the depth error < 5 km. The travel timedistance graph for these earthquakes and the recording station is presented in figure 3(a). The Pn crossover distance was found to be around 180km as reflected in the reduced travel time-distance plot shown in figure 3(b). 3. Pn velocity computation We follow the time-term method (Sheidegger and Willmore 1957; Kind 1972; Hearn 1984) to compute

3 Moho structure in NE India 299 ' ;o" tl I~ SMOOTHED BOUGUER GRAVffY ANOMALIES IN NORTHEASTERN INDIA ~jl, Contour interval : I0 0 i,. ~ ~ SO (I 0 z4...t "t---,,,, N B All values are negative 88 ~ ~ ~'~. 94 ~ 96 i I I. I... I, I Figure 2. Bouguer gravity field over NE India. Shillong plateau has characteristic relative higher gravity anomaly (0 to -50regal) compared to significant negative over the Indo-Burmese (up to -150mgal) and Indo-Asian (up to -300mgal) convergence. Table 1. Velocity km/s Velocity model used for locating earthquakes. P-wave velocity model Depth km the Pn velocity using the relation: tij = ai -[- bj -{- xij/v where tij is the Pn travel time at a station i for an earthquake j; ai and bj are time terms for the recording and source site; xij the epicentral distance and v is the velocity of the underlying refractor. We rewrite the above equation as Rij = tij -- Xij/V ---- ai + bj where R/j is the travel time residual, the difference between observed and predicted time. The station residual ai is the same for all the events while bj varies with station and event, such that Ej bij ~-O, therefore, ai = ~ Rij. To compute the travel time residual the travel time data for each of the event was fitted to a straight line in the least squares sense. The slope of this line is the reciprocal of the apparent velocity across the array. To avoid over-emphasis by events from the same region, earthquakes were grouped into four quadrants and an average was computed for each quadrant. These averages were further averaged to obtain the station anomaly. These values of ai were subtracted from the computed travel times and the procedure to compute the least square velocity and ai was repeated to obtain new estimates. After 10 iterations the velocity and station anomaly became stable showing only marginal variation with further iterations. The station anomaly and Pn velocity

4 300 S S Rai, K S Prakasam and N Agrawal Table 2. Hypocentre parameter of earthquakes. Date Origin time Latitude Longitude Depth yy mm dd hr. min. sec. (~ (~ km ~', 7O 5 m "-" 60 r $ $ $ N 50 ~ I I I I I I I I O00 Distance (kin) 1o- v O O 8- O I 4 *** 9 ** ****~ J~ ~$*~ ~* * 2 * **' **, $ **, **~** * 0 I l 1 I I I Distance (kin) Figure 3(a). P wave travel time shown as a function of source to receiver distance for earthquakes listed in table 2. (b). Reduced travel time plot including Pg and Pn data.

5 Moho structure in NE India 301 Table 3. Sector wise residual pattern, station anomaly and Pn velocity at different stations. Station code Station anomaly sector wise AVR. BKD NA 1.09 NA 1.14 BKG NA BOJ NA NA 1.15 BSP NA NA CDL NA NA CHK NA NA DWK NA GAU NA GOP NA 0.80 NA GWH HMN IMP JWI NA 0.67 NA 0.41 KPG NA NA MKG NA 0.06 MLR NA NA NGN NA 0.69 NA NGP NA PBR NA 0.23 NA PNB 0.80 NA NA SHL NA 0.18 NA SJA NA 0.54 SNA NA 0.05 SPA NA NA TML NA TSG NA 0.35 TUR NA 0.46 NA TZR NA 0.08 UKL NA UMS NA WLN NA 0.54 NA 0.09 Average Pn velocity km/s (corrected for time term) for different stations is presented in table 3. An average Pn velocity of 8.5 km/s was obtained for the region. (b) Epicentre-Station Distribution 4. Crustal thickness estimate / 7 The relative travel time difference across the array can be utilised to map the depth of Moho below different points (Shearer and Openheimer 1982; Openheimer and Eaton 1984). We consider a two layer laterally homogeneous velocity model, the difference in travel r, nee Figure 4(b). Schematic depicting relative Pn ray path for layer over a half space model used in computing Moho depth variation. o i I ~ E41~hq~ke vma,tle ~ " time being assumed to be wholly due to the dip of the Figure4(a). Location ofearthquakeepicentre, seismic station Moho (figure 4(a)). The relative Pn travel time t/j and Pn ray paths used. between a reference station and the ith station within

6 302 Figure 5. (a) Crustal station anomaly in NE India. (b). Uppermost mantle Pn velocity in NE India. Contour indicates difference of 0.1 km/s in velocity structure imaged by Pn wave. (c).variation in Moho depth in NE India relative to Shillong where average depth is about 45 kin.

7 Moho structure in NE India 303 the array for the jth event, may be expressed as tij = (sin ~/vc)(dij - doj) + (1/vm)(xij + xoj) where a = cos-l(vc + vm). v~ represents the mean crust velocity and vm the mean mantle velocity, d is the perpendicular distance to the Moho beneath a station, x the epicentral distance and the symbol o refers to the reference station. Considering dij = dij -- doj and Xij : Xij -- Xoj, we can write dij = (tijvm - xij)/ tan a. Accordingly, we computed the Moho depth beneath all the 30 stations considering SHL as the reference point. Figure 4(b) shows the location of seismic stations, earthquake epicentres and the Pn ray path distribution in the region. We observe fairly good illumination and hence better solution reliability for velocity and Moho depths in the geographical region enclosed by 25-26~ and 90-94~ 5. Results and discussion Based on the methodology presented, the travel time residual, Pn velocity and depth of the Moho were computed at 30 stations. These are presented in figure 5(a-c). Station anomaly over the ShiUong plateau varies between 0.0 and 0.2s. A negative anomaly (-0.2 to -0.8 s) is observed between Shillong and the Mikhir hills and its northwest extension coinciding with the Kopili lineament. This could be representative of either a faster velocity crust or a thinner Moho. Pn velocity in the NE region is found to vary betw~n 8.3 and 8.7km/s. Over the Shillong plateau and Mikhir hills Pn velocity varies between 8.3 and 8.5km/s. Similar Pn velocity pattern is observed over the Indo- Burmese region. Higher Pn velocity (upto 8.7kin/s) is observed to the east of Mikhir hills. The Moho depth variation is presented in relation to Shillong. Significant crustal thickening (~ 8kin) is observed beneath the Mikhir hills and the adjoining Assam valley. Beneath the convergence zone, Moho is observed to be thickened by 8-16km. Similar observations were reported by Gaur and Bhattacharji (1983) through modelling of Bouguer gravity field observation (figure 6). Theoretically, we expect a relation between the crustal thickness(h) and the Pn velocity(v) as 5V 5V 5T 5V 5P 5--H = 6--T" 6H + 6P 6H where T and P are temperature and pressure respectively. Laboratory experiments (reviewed in Braile et al 1989) provide a working relationship between variations in the upper mantle velocity with pressure and temperature. Using a pressure derivative (5 V/SP) of 88 ~ 90 ~ 92 ~ 94 ~ 96" 9 I I I 30 i MOHO DEPTH (in Km.)IN NE INDIA ~6.~.. ) 98" I 3o- I i zs"l I r-~-'~-~-.~-- --''-" ~ I./ /! ",~ _~/~ /./.." I i 2z! ~;, l ~"-r I I 88 b 90 ~ 92 ~ 94 ~ 96 ~ Figure 6. Moho depth deduced from Bouguer gravity anomaly (redrawn from Gaur and Bhattachaxji 1983).

8 304 S S Rai, K S Prakasam and N Agrawal 2.67 x 10-4 km/s/pa, and an average crustal density of 2.9g/cm 3, the pressure effect becomes km/s/km as a function of depth. Thus due to pressure, a 10 km increase in the thickness of the crust would result in an expected increase in the upper mantle velocity of km/s. Assuming an ultramafic composition of the uppermost mantle, and temperature derivative (5 V/~ T) of km/s/~ and about 12~ temperature gradient for the upper and sub- Moho mantle respectively, the effect of the depth increase on velocity (~V/~H) was found to be km/s/km. Figure 5(b) and 5(c) indicate a broad correlation between the crustal thickness and the respective Pn velocity; the sub Moho temperature gradient beneath the Indo-Burmese ranges and the Himalayan convergence zone, being assumed to be > 12 ~ C/km. This study brings out two important results (i) the presence of a high Pn velocity ( km/s) in NE India possibly representing a colder lithosphere and (ii) significant variations in the crustal thickness (45-61km) from the Archean blocks to the younger convergence zones. The thick crust and colder lithosphere is possibly responsible for deeper level seismogenesis in the sub-moho olivine dominated layers beneath the Shillong plateau and Assam valley. Acknowledgements We are grateful to all those who contributed to the operation of field stations in a rather difficult terrane. Critical review by Prof. Vinod Gaur helped to improve the manuscript. References Braile L W, Hinze W J, Van Frese and Keller G R 1989 Seismic properties of crust and uppermost mantle of the conterminous United States and adjoining Canada, in Geophysical Framework of United States (ed.) L C Parkiser and W D Mooney Mem. Geological Soc. Am Chen W P and Molnar P 1990 Source parameters of earthquakes and intraplate deformation beneath the Shillong plateau and the northern Indo Burmese ranges; J. Geophys. Res Evans P 1964 The tectonic framework of Assam; J. Geol. Soc. India Gaur V K and Bhattacharji J C 1983 Gravimetric determination of the shape of Moho in peninsular and NE India, IUGG Assembly, Hamburg, Germany, August, Gupta H K, Singh S C, Dutta T K and Saikia M M 1984 Recent investigation in NE Indian Seismicity, in Proc. Intl. Symp. on continental seismicity and earthquake prediction, (Beijing: Seismological Press) Hearn T M 1984 Pn travel times in south California; J. Geophys. Res Kayal J R and De R 1987 Pn velocity study using a temporary seismographic network in the Shillong plateau, NE India; Bull. Seism. Soc. Am Kind R 1972 Residual and velocity of Pn waves recorded by the San Andreas seismograph network; Bull. Seism. Soc. Am Mukhopadhyay S, Khattri K N and Chander R 1995 Seismic velocity and related elastic parameters of the crust in the Shillong massif; J. Himalayan Geology Nandy D R 1980 Tectonic patterns in northeastern India; Indian J. Earth Sci Openheimer D H and Eaton J P 1984 Moho orientation beneath central California from region earthquake travel times; J. Geophys. Res Ravi Shankar 1988 Heat flow map of India and discussion on its geological and economic significance; Indian Minerals Scheidegger A E and Willmore P L 1957 The use of least squares method for the interpretation of data from seismic surveys; Geophysics Shearer P M and Openheimer D H 1982 A dipping Moho and crustal low velocity zone from Pn arrivals at Geysers, Clear lake, California; Bull. Seism. Soc. Am Sitaram M V D and Saikia M M 1992 Results of a seismic network in NE India; Curr. Sci Tandon A N 1954 A study of Assam earthquake of August 1950 and its aftershocks; Indian J. Meteorol. Geophys Verma R K and Mukhopadhyay M 1977 An analysis of the gravity field in northeastern India; Tectonophysics MS received 2 November 1999; revised 18 November 1999