Aseismic Layer Detected in the Mid-Crust of Nubra-Siachen Region , India Using Local Seismic Data

The local seismicity observed by seismic network in siachen-nubra region during January 2010December 2012 shows that the middle part of the crust (17 40 km) is aseismic. This aseismic layer (17 40 km) is sandwiched between two seismically active layers and depicts a good spatial correlation with the observations of low resistivity reported from magnetotelluric studies for the same region. The local seismicity shows a trend along the Karakoram fault and clustering of events in Shyok Suture zone and Karakoram shear zone. The moment magnitude of these events lies between 1.3 and 4.3. Most of these events have been originated in upper crust.


Introduction
The Himalayas, spanning length of 2500 km is a consequence of continent collision [1]- [3].The Himalayan region is known for its high seismicity for small to large magnitude earthquakes.Five large magnitude earthquakes have visited Himalayan region (mag.> 8.4) and nine earthquakes of magnitude > 7.0 since 1897.Beside these large magnitude earthquakes, numerous small magnitude (Magnitude < 3.0) earthquakes have visited the Himalaya.
Nubra valley locates in the eastern Karakorum region of northern Ladakh and extends from Siachen glacier system in North-West to Shyok-Nubra confluence in South East.It covers 4381.23 sq km area and amassed of various litho units such as Karakorum plutonic complex, Ladakh Plutonic Complex, etc. Siachen Glacier is more than a valley glacier; it is a system of glaciers, consisting of Northern, Central and Southern glaciers [4].Siachen glacier along with its tributaries covers more than 1500 sq kms.This is the largest glacier in the Himalaya and second largest among the known glacier outside the polar and sub-polar regions.Siachen lies between the Saltoro Range immediately to the west and the main Karakoram Range to the east.The melting waters of the glacier are the main source of the Nubra River, which meets the Shyok River near Khalasr forming Nubra-Shyok confluence (Figure 1).The Shyok in its onward journey joins Indus River; thus the glacier is a major source of water for the Indus.
A network of digital broadband seismograph equipped with 3-component digital seismometer (Trillium 120P) is operational in Nubra-Valley, since 2008.The seismic data recorded (Jan'10-Dec'12) at these stations have been used for this study.The hypocentral depth of the local events recorded at seismic stations is in the upper crust depth range 0 -17 km and 40 -75 km but few events are observed at the depth greater than 100 km in the mantle along the Karakorum fault.2) used in this study is shown on the Geological map [16] of the area.

Tectonic Set up of the Area
The collision between the Eurasian plate and the Indian plate started about 55 Ma ago [5], and is still continuing at the present time [6].This successive collision forms the Himalaya and the Earth's largest and highest plateau with anomalously thick crust.Different models have been proposed by various research workers to explain the origin of the Himalaya [2] [3] [7].The collision has also resulted into large scale thrusting and the same is progressed southward forming the Main Central Thrust (MCT) and Main Boundary Thrust (MBT).The Indus-Tsangpo Suture (ITSZ) located about 250 km.North of MCT marks the pre-collision boundary along which the Indian plate subducted below the Eurasian plate and the subduction ended in Eocene times [7].Along the MCT, the Higher Himalaya overthrusted the Lesser Himalaya along MBT, the Lesser Himalaya thrusted over the Siwaliks.
The crustal thickness using, 15 -60 s Rayleigh wave group velocities with receiver functions from teleseismic arrivals was mapped up to 75 km at Taksha station in Nubra region [8].Nubra region is tectonically highly disturbed [9] and this region is bisected by Karakoram fault [10]- [13].Approximately 800 km, dextral strike slip Karakoram fault is the prominent tectonic feature in the region [14] [15], this fault runs almost parallel to the main body of Siachen glacier.The other major tectonic features of this region are Karakoram shear zone, Shyok Suture zone, Khalsar thrust, etc. Karakoram Shear Zone (KSZ) is intensely mylonitized granite gneiss, volcanic, conglomerate, slate-phyllite-limestone intercalations, amphibolites and serpentinite intervene the Shyok Suture Zone and the frontal Asian Plate margin along the Nubra-Shyok Valleys for nearly 200 km to form the 1 -5 km wide KSZ [16].
This region falls in the zone IV of the seismic zoning map of India [17], and seismotectonic atlas of India and its enviors [18].Geographical position of study region is shown in Figure 2, and this region is tectonically active due to the presence of Karakoram fault and various structural features like Shyok Suture Zone (SSZ), Karakorum Shear Zone (KSZ).This region is almost virgin as far as local seismological observations are concerned, prior to the installation of this network.This region is remote and accessible through world highest motor able pass Khardung La (18,380 feet) for a very limited period in the year.Owing to remoteness and harsh climatic condition throughout the year, maintenance and operation of seismic network is a big challenge in nubra-siachen region.However four, broadband seismometers has been installed during 2008 in this region.This region did not experience any major earthquake till date (as per the literature/record available).However micro, small & large earthquakes frequently experienced by local residing in this region [19] [20].The local seismicity observed by this seismic network is mainly mapped along the Karakoram Fault, Karakoram Shear zone & Shyok Suture zone.

Data Used and Methodology
The local seismological network comprising of four broad band recording station has been installed in Nubra Valley region, Jammu & Kashmir.The layout of the network is shown in Figure 3 and the details of recording stations are given in Table 1.The four recording stations used the same type of instrumentation i.e. the Trillium 120P broadband seismometer and Taurus 24 bit data acquisition system, manufactured by M/s Nanometrics, these instruments are operational in continuous mode.The seismic data recorded at Nubra valley seismological network and used for present analysis is given in Table 2.The 3-C seismogram of one of the recorded event are shown in Figure 4. Data were recorded with a sampling rate of 100samples per second.For computation of hypocentral parameters of Local earthquakes three types of data is required such as phase data of local earthquake, accurate geographical co-ordinates along with elevation of recording stations and the local velocity model of the region covered by the recording stations.Additionally a computinally efficient and stable computer program is a major requirement to allow conversion of travel time data to estimate hypocentral parameters [21] [22].The seismic velocity model determined by [23] at Taksha seismic station (TKS) has been used for estimation of hypocentral parameters of local earthquakes.The "HYPCENTER 3.2" given by [24] and incorporated in SEISAN environment has been used for the estimation of hypocentral parameters of local earthquakes.The moment magnitude (MW) has been estimated using [25] given as below.

Result and Discussions
The hypocentral parameters of local earthquake are estimated with the help of "HYPOCENTER 3.2" [24] from the phase data recorded at seismic stations.The standard error in the estimation of hypocentral parameters for these events are <0.5 seconds in origin time, <5.0 Km in epicenter (ERH) and <6.0 in focal depth (ERZ).The local seismicity is mainly oriented with the Karakoram Fault and clustered in Shyok Suture Zone and Karakoram shear zone.The depth trend of earthquake events (Figure 5) shows that the seismicity in the Siachen region is mainly occurring at upper crustal depth.However few events are occurred at a depth of 40 km and more but no local event is reported in a depth range from 17 -40 km below the surface of the earth.This seismically inert layer (17 -40 km) shows that no stress get accumulated in this layer.This layer shows good spatial correlation with the low resistivity layer in the mid crust of NW Himalaya determined from the magnetotelluric studies of NW Himalaya [26] & Partially melted upper middle crust revealed by Rayleigh wave dispersion [23].Due to the partially melted nature of mid-crust (depth range 17 -40 km) no stress is accumulated in the midllecrust, so no earthquake is reported in mid-crustal depth range in Siachen region.This infers the presence of aseismic layer in the mid-crust part.This aseismic layer is sandwiched between seismically active layers in Siachen region, India and consistent with the geological section of the area prepared by [24] (Figure 6(a)).
Considerable amount of broken ice chunks were observed near the snout of siachen glacier during field visit in Aug'2011 (Figure 7).The field personnel deployed at field stations reported that the cracking mainly observed contemporaneous to the shacking.The close association of log book maintained at field sites for keeping   record of shaking and seismic data recorded at field seismic stations indicates that these cracking mainly associated with tremors occurring near the snout of the siachen glacier.Moreover few earthquake events are located in the snout region of glacier it is evident that these earthquake events are responsible for cracking in the snout of glacier.As the Karakoram fault passes through the main body of siachen glacier as well as through snout region of siachen glacier (Figure 1) such tremors will keep occurring due to the presence of this active fault and theses tremors may have very serious implication on the snout of the siachen glacier.

Conclusion
Aseismic layer (17 -40 Km) is detected in the mid-crust of Nubra-Siachen region of NW, Himalaya with the help of locally observed seismic data.This layer is sandwiched between two seismically active layers.This aseismic layer shows the good spatial correlation with the low resistivity layer [26].Results of this study support the proposed "partially melted crust for this region" by [24].The observed local seismicity for the period Jan' 2010-Dec' 2012 in Siachen-Nubra region of Jammu & Kashmir is mainly oriented along the Karakoram Fault and clustered in Shyok suture zone & Karakoram Shear zone.The moment magnitudes of these events lie between 1.3 and 4.3.The depth distribution of earthquake events shows that most of the local earthquakes in this region have been occurred at upper part of the crust.Such shallow seismic activity in the glacier region may affect the glaciers.These aspects need to be studied in more detail.

Figure 1 .
Figure 1.The earthquake events (Table2) used in this study is shown on the Geological map[16] of the area.

Figure 2 .
Figure 2. Geographical position of study area.

Figure 3 .
Figure 3. Layout plan of local seismic stations of Nubra-Siachen region shown on Google earth image.

Figure 4 .
Figure 4. 3-C Seismogram of Local earthquake recorded at seismic network in Nubra-Siachen region (Z, N-S, E-W from top to bottom).

Figure 5 .
Figure 5. Plot of number of earthquakes versus focal depth.

Figure 6 .
Figure 6.(a) Geological cross-section through the Northern parts of NW Himalaya and Karakorum, after [16]; (b) Aseismic layer in the mid-crust.

Figure 7 .
Figure 7. Field Photographs showing cracking in the snout region.

Table 1 .
Details of seismic station installed in area of study.

Table 2 .
List of Local events recorded at Nubra-Siachen seismic Network.