Indian Strong Motion Instrumentation Network

Transcription

1 Indian Strong Motion Instrumentation Network Ashok Kumar, Himanshu Mittal, Rajiv Sachdeva, and Arjun Kumar Ashok Kumar, Himanshu Mittal, Rajiv Sachdeva, and Arjun Kumar Indian Institute of Technology, Roorkee INTRODUCTION The Indian subcontinent is prone to earthquakes in both interplate and intraplate regions. Areas that have been identified as severe seismic regions include the Himalayan belt in the north from Kashmir to Manipur; Gujarat in the west; and the Andaman and Nicobar Islands in the southeast. The faults in these areas are capable of generating large-magnitude earthquakes that would subject neighboring areas to significant ground shaking. Thus, seismic hazard assessment is of prime importance in India. The relative velocity of the Indian plate with respect to the Eurasian plate near Delhi is about 5 cm/yr in the direction of N13 E (NUVEL-1A model of De Mets et al. 1994). The collision of these continental plates results in crustal shortening along the northern edge of the Indian plate. This process has given rise to three major thrust planes: the main central thrust (MCT), the main boundary thrust (MBT), and the main frontal thrust (MFT) (see, for example, Gansser 1964; Molnar and Chen 1982). The region has experienced several great earthquakes in the past hundred years or so (1897 Assam, 1905 Kangra, 1934 Bihar-Nepal, 1950 Assam). The Himalayan geodynamics and the occurrence of great earthquakes are well summarized by Seeber and Armbruster (1981), Khattri (1999), and Bilham and Gaur (2000). During the last episode of strain release, a 750-km-long segment, which lies between the eastern edge of the 1905 rupture zone and the western edge of the 1934 earthquake, remained unbroken (Figure 1). This segment, called the central seismic gap, continues to be under high strain (e.g., Singh et al. 2002). Large earthquakes occurred in this seismic gap in 1803 and 1833, but the magnitudes of these earthquakes were less than 8, and, hence, they were not gap-filling events (Khattri 1999; Bilham 1995). Based on these considerations and on a shortening rate of 20 mm/yr across the Himalayas (Lyon-Caen and Molnar 1985; Avouac and Tapponier 1993; Gahalaut and Chander 1997; Bilham et al. 1998), Khattri (1999) has estimated the probability of occurrence of a great 8.5 earthquake in the gap in the next 100 years to be The northeastern region of India is also regarded as one of the most seismically active regions worldwide. Seismicity data since 1897 show that the northeastern region has experienced two great earthquakes with magnitudes above 8.0 and about 20 large earthquakes with magnitudes varying between 8.0 and 7.0 (Kayal et al. 2006). The great 1897 earthquake (Mw 8.1; Bilham and England 2001), which took place in the Shillong plateau, caused tremendous damage in the northeastern region. The biggest earthquake of the 20th century, the great 1950 Assam earthquake with its epicenter at the Assam syntaxes (Tandon 1954) produced considerably more destruction in a widespread area. Apart from these prominent earthquakes, several other smaller earthquakes have also caused significant loss of life and property in the region. These devastating earthquakes occurred in northeastern India when the population was 10 times less than at present; if such earthquakes were to occur in the near future, they would be much more devastating, thus emphasizing the need for seismic hazard estimation in northeastern India. Studies related to hazard estimation depend on the availability of strong ground motion records from past earthquakes. Strong motion seismographs are instruments designed to record on-scale time histories of ground motions where the traditional high-gain seismographs used to routinely locate earthquakes go off scale. Strong motion seismographs are often called accelerographs because they measure acceleration of the ground. The most critical role of strong motion networks is to provide onscale recordings of potentially damaging motions over a broad frequency band (0 50 Hz or higher). Because continuous analog recording is extremely expensive and strong shaking is infrequent, strong motion seismographs are designed to record only during strong ground shaking (i.e., triggered recording). The main goal of strong motion seismology is to improve the scientific understanding of the physical processes that control strong shaking and to develop reliable estimates of seismic hazards for the reduction of loss of life and property during future earthquakes through improved earthquake-resistant design and/or retrofitting. The strong motion data are fundamental for earthquake engineering studies such as site effects, advanced structural analyses, seismic hazard evaluation, and calibration of ground motion prediction relationships. More than half of the area of India is susceptible to strong ground motions from earthquakes; therefore it is essential to know about the probable characteristics of strong ground motion of future earthquakes in this region. For earthquake engineering purposes, a number of different parameters are typically used to characterize strong motion records. These parameters include peak acceleration, peak velocity, peak displacement, duration of strong shaking, and response spectra. A rational assessment of the expected seismic hazard in different regions of the country will lead to substantial monetary savings in the design of structures and reduce potential losses from earthquakes. The Department of Earthquake Engineering, Indian Institute of Technology, Roorkee (formerly University of Roorkee), operated a network of about 200 analog strong doi: /gssrl Seismological Research Letters Volume 83, Number 1 January/February

2 Figure 1. Tectonic map of the Himalayas. Hatched areas denote intensity greater than or equal to VIII. The segment between the rupture areas of the 1905 and 1934 earthquakes is known as the central seismic gap. MCT, main central thrust; MBT, main boundary thrust (after Singh et al. 2002). motion accelerographs in the northern and northeastern parts of the country starting in This network produced a wealth of strong motion data, which are being used at the national and international level by engineers and scientists. The network covered parts of Himachal Pradesh, Punjab, Haryana, Uttaranchal, Uttar Pradesh, Bihar, West Bengal, Sikkim, and northeastern India. Dense networks of strong motion instruments at about 40 km spacing were located only in the Himachal Pradesh, Uttaranchal, and Shillong regions, whereas in other regions, only sparse instrumentation was installed. Not many accelerograph stations were operated in seismic zone III of India, although this region has high seismic vulnerability due to its large population density. These strong motion instruments performed well during their active period and provided good quality recordings. However, due to the unavailability of components/spare parts and to obsolete technology, most of the strong motion accelerographs installed in the early 1980s are no longer functional. To overcome this problem, the Department of Science and Technology (DST), a division of the government of India, sanctioned a project titled National Strong Motion Instrumentation Network to the Indian Institute of Technology, Roorkee (IITR), in February 2004 under its mission mode program on seismology. Under this program about 300 state-of-the-art digital strong motion accelerographs were installed in northern and northeastern India to monitor earthquake activity in seismic zones V and IV, and in some heavily populated cities in seismic zone III. This network was further strengthened in 2007 when another project, Strong Motion Instrumentation Network in Delhi, was sanctioned by DST to IITR. Under this project 20 digital strong motion accelerographs were installed in the Delhi region. In the following sections, we discuss details about this instrumentation and its objectives and achievements. STRONG MOTION INSTRUMENTATION NETWORK The strong motion instrumentation network of IITR covers the Indian Himalayan range from Jammu and Kashmir to Meghalaya. The accelerographs were procured in September 2005, and their installation started in November In total, 293 strong motion stations have been installed in the states of Himachal Pradesh, Punjab, Haryana, Rajasthan, Uttarakhand, Uttar Pradesh, Bihar, Sikkim, West Bengal, Andaman and Nicobar Islands, Meghalaya, Arunachal Pradesh, Mizoram, and Assam. The map in Figure 2 shows the location of these instruments. The first phase of the project concerned the choice of the new sites for installations. Site selection has been generally a compromise between network geometry, logistics, and safe installation. Based on these criteria, sites falling in seismic zones IV and V of the country and in some densely populated cities of zone III were selected and instrumented. Average station-to-station distance is kept at between 40 to 50 km, which ensures triggering of at least two or more accelerographs (set at trigger level of 5 gals) if a magnitude 5 or larger earthquake occurs anywhere in northern or northeastern India. Typically the instruments are installed in a room on the ground floor of (preferably) a government-owned one- or two-story building where proper logistics are available, which means that the 60 Seismological Research Letters Volume 83, Number 1 January/February 2012

3 Figure 2. Maps showing the location of instruments along the Himalayan belt. Insets: Map showing the location of instruments in the Andaman and Nicobar Islands and Delhi. instrument is secure from tampering and 220 v AC power supply is available. In Delhi, however, free field instruments were installed inside a specially fabricated housing. A schematic diagram of a typical installation is shown in Figure 3. Networking Modern telecommunications are utilized to allow networking of these accelerographs. An understanding with the National Informatics Centre (NIC) permits the use of the NICNET facility at district headquarters. Telecommunication links to the entire instrumentation are planned so that each instrument can be accessed from headquarters or from other remote locations. Currently, all the instruments installed in the field at district level are connected to the VSAT/leased line of NIC. Data flows through VSAT/leased line from these field stations to NIC headquarters in Delhi. From Delhi to Roorkee data flows on a 2 MBPS leased line of Bharat Sanchar Nigam Limited (BSNL) (Figure 4). The instruments situated in sub-divisions/ towns are connected through the State Wide Area Network (SWAN). All 20 Delhi installations are connected to Roorkee through the MahaNagar Telephone Nagar Limited (MTNL) network. With this connectivity, about 220 accelerograph stations can be accessed from headquarters at Roorkee. These instruments are now routinely checked remotely for their health and for data downloads when earthquakes occur. OBJECTIVES OF THE PROJECT Strong ground motion records in the form of time histories or response spectra provide the basic information for earthquake engineering. A major earthquake is an experiment, which provides observations on the performance of existing structures subjected to strong ground motion. Such records also constitute the basic data for evaluation of seismic hazard of an area and for improving design practices from the understanding of structural response and eventually the basics of earthquake engineering research. The main goals of the project are both to collect data with a wide range of magnitude, thus allowing us to increase knowledge of the Himalayan area; and to assure the recovery of high quality datasets in cases of strong events. Further objectives include retrieving, processing, interpreting, and disseminating the strong motion data; archiving the data in Seismological Research Letters Volume 83, Number 1 January/February

4 Figure 3. A figure showing the installation of instruments in the field. Figure 4. A diagram showing the connectivity of instruments from the field to Roorkee. 62 Seismological Research Letters Volume 83, Number 1 January/February 2012

5 Figure 5. A sample strong motion (kinemetrics) record of an Mw 4.1 earthquake from the Delhi-Haryana border region recorded at the IMD station (epicentral distance 28 km) on 25 November a strong motion data bank; and further research such as strong motion attenuation characteristics of the region, strong motion characterization of a region with respect to geological setting and source to site distance, determination of local site effects and seismic loading, and site-dependent seismic risk evaluation. Instruments All 300 strong motion accelerographs consist of internal AC-63 GeoSIG triaxial force-balanced accelerometers and GSR-18 GeoSIG 18-bit digitizers with external GPS. The 12 strong motion accelerographs installed in Delhi are K-2 (Kinemetrics K-2s) with internal accelerometer (model Episensor) and 18-bit digitizer. The recording for all instruments is in trigger mode Figure 6. A sample strong motion (Geosig) record of an Mw 6.5 earthquake from the Hindukush region recorded at the Keylong station (epicentral distance 717 km) on 17 September at a sampling frequency of 200 sps. The triggering threshold was initially set at g for all the instruments. The recording is done on a 256-MB GeoSIG or 1-GB Kinemetrics compact flash card. Performance This instrumentation network has recorded around 130 earthquakes in a span of four years. Several prominent earthquakes in the northern Himalayas as well as in the northeast Himalayas were recorded. Figure 5 shows an example of a strong motion accelerograph record from the Mw 4.1 Delhi-Haryana border region earthquake that occurred on 25 November 2007, and Figure 6 shows a strong motion accelerograph record from the Mw 6.5 Hindukush region earthquake that occurred on 17 Seismological Research Letters Volume 83, Number 1 January/February

6 Figure 7. Plot for <5 magnitude multistation recorded earthquakes in the northern Himalayas. September Records were obtained for near-source earthquakes at distances of 10 km and for a far-source earthquake at a distance of 1,000 km. Based on experience with the performance of the instruments, the trigger thresholds of all the instruments placed in Himachal Pradesh, Uttarakhand, and some of the northeast instruments was lowered to.002 g from g. By doing this we can now record the lower magnitude earthquakes. After due monitoring, we are planning to lower the triggering threshold of all other instruments connected through NICNET. Figures 7 and 8 show plots of magnitude (M) vs. hypocentral distance (R) for <5 and for 5 magnitude earthquakes in the northern Himalayas, while Figure 9 shows a plot of M vs. R for multistation recorded earthquakes in the northeastern Himalayas. Processing of Records All downloaded accelerograms are processed before dissemination. Computer programs developed by the first author of this paper are used to process the records. As the headers of ASCII files are entirely different for records obtained from GeoSIG and Kinemetrics accelerographs, for ease of execution it was essential to develop two different computer programs. In general, both computer programs read headers of ASCII files of records, generate the output header as shown in Figure 10, baseline correct the record, and rotate the horizontal components to get N-S and E-W components. Instrument correction of records is not done since the natural frequency of the sensors is quite high (about 200 Hz), and thus it will not make any difference. Also no frequency filters are applied, and it is left to users to apply filters according to their need. However, raw accelerograms are available to users on request. A typical data file for an earthquake record Figure 8. Plot for 5 magnitude multistation recorded earthquake in the northern Himalayas. Figure 9. Plot for multistation recorded earthquakes in the northeastern Himalayas. is given in Figure 10. The Mw 5.1 Chamoli earthquake of 14 December 2005 is the first earthquake recorded by the digital instrumentation. In total, eight instruments of the network were triggered by this earthquake and provided records. Table 1 gives the computed peak displacement for three components of various stations during this earthquake. 64 Seismological Research Letters Volume 83, Number 1 January/February 2012

7 DISCUSSION AND CONCLUSIONS This state-of-the-art instrumentation has partially filled the long-felt need for monitoring ground motion generated by earthquakes in the Himalayas using modern strong motion instruments. Since its installation, the instrumentation has been working in a satisfactory manner and has captured about 300 strong ground motion records from 130 earthquakes. The data recorded by this network is disseminated through the Web site About 100 research workers have registered at this Web site and are now using these records for their work. Acquisition software of present installations are proposed to be upgraded so as to achieve real-time streaming of data with the intent of developing of an earthquake early warning system. It is hoped that this instrumentation will serve as a launching platform for several useful studies and products for the Himalayan region. ACKNOWLEDGMENTS We are thankful to the Department of Science and Technology (DST), government of India, and the Ministry of Earth Sciences (MoES) for providing funds to execute this project. Successful implementation of a project of this size requires support from several individuals and agencies, which we thankfully acknowledge. We received tremendous support from the National Informatics Centre, New Delhi, during the course of the project. Support from the district administration of each instrument location was also very important for us. We received good support from the staff of SWAN of various states, for which we are thankful. We are thankful to the faculty of the Department of Earthquake Engineering and to the Dean of Sponsored Research and Industrial Consultancy, Indian Institute of Technology, Roorkee, for their constant support and encouragement. Comments from a reviewer were useful in improving the manuscript. We are thankful to Mr. Rakesh Gondhi, Mr. Veer Pal, Mr. Babu Ram, Mr. Amit Srivastava, Mr. Ghanshyam Tiwari, Mr. V. K. Sharma, and Mr. K. G. Mittal, who have made direct contributions in the installation and maintenance of this network. We are also thankful to the technical and administrative staff of the Department of Earthquake Engineering, who have made important contributions in this project. REFERENCES Figure 10. Example of header file of each component. Avouac, J., and P. Tapponnier (1993). Kinematic model of active deformation in central Asia. Geophysical Research Letters 20, Bilham, R. (1995). Location and magnitude of the Nepal earthquake and its relation to the rupture zones of the contiguous great Himalayan earthquakes. Current Science 69, TABLE 1 Computed Peak Ground Displacement of Longitudinal, Transverse, and Vertical Components of Different Stations (Modified after Mittal et al. 2006) Hypocentral Peak Displacement (Cm) values of LTV components Stations Distance Longitudinal Transverse Vertical Bageshwar Chamoli Champawat Pauri RudarPrayag Tehri Uttarkashi Seismological Research Letters Volume 83, Number 1 January/February

8 Bilham, R., F. Blume, R. Bendick, and V. K. Gaur (1998). The geodetic constraints on the translation and deformation of India: Implications for future great Himalayan earthquakes. Current Science 74, Bilham, R., and P. England ( 2001). Plateau pop-up in the great 1897 Assam earthquake. Nature 410, Bilham, R., and V. K. Gaur (2000). The geodetic contribution to Indian seismotectonics. Current Science 79 (9), 1,259 1,269. DeMets, C., R. G. Gordon, D. F. Argus, and S. Stein (1994). Effect of recent revisions to the geomagnetism reversal time scale on estimates of current plate motions. Geophysical Research Letters 21, 2,191 2,194. Gahalaut, V. K., and R. Chander (1997). On interseismic elevation changes and strain accumulation for great thrust earthquakes in the Nepal Himalaya. Geophysical Research Letters 24, 1,011 1,014. Gansser, A.(1964). Geology of the Himalayas. New York: Interscience, 289 pp. Kayal, J. R., S. S. Arefiev, S. Barua, D. Hazarika, N. Gogoi, A. Kumar, S. N. Chowdhury, and S. Kalita (2006). Shillong plateau earthquakes in northeast India region: Complex tectonic model. Current Science 19 (1), Khattri, K. N. (1999). An evaluation of earthquake hazard and risk in northern India. Himalayan Geology 20, Lyo-Caen, H., and P. Molnar (1985). Gravity anomalies, flexure of the Indian plate, and structure, support, and evolution of the Himalaya and Ganga basin. Tectonics 4, Mittal, H., S. Gupta, A. Srivastava, R. N. Dubey, and A. Kumar (2006). National strong motion instrumentation project: An overview. In 13th Symposium on Earthquake Engineering, Indian Institute of Technology, Roorkee, Dec 18 20, 2006, , New Delhi: Elite Publishing. Molnar, P., and W. P. Chen (1982). Seismicity and mountain building. In Mountain Building Processes, ed. K. Hsu, New York: Academic Press. Seeber, L., and J. G. Armbruster (1981). Great detachment earthquakes along the Himalayan arc and long-term forecasting. In Earthquake Prediction: An International Review, e d. M. Ewing, Series 4. Washington, D C : American Geophysical Union. Singh, S. K., W. K. Mohanty, B. K. Bansal, and G. S. Roonwal (2002). Ground motion in Delhi from future large/great earthquakes in the central seismic gap of the Himalayan arc. Bulletin of the Seismological Society of America 92, Tandon, A. N. (1954). Study of the great Assam earthquake of August 1950 and its aftershocks. Indian Journal of Meteorology and Geophysics 5, Department of Earthquake Engineering Indian Institute of Technology Roorkee, India himanshumitt10@gmail.com (H. M.) 66 Seismological Research Letters Volume 83, Number 1 January/February 2012