339 CHAPTER 4: STRONG EARTHQUAKE MOTION OBSERVATIONS GENERAL ASPECTS OF STRONG-MOTION OBSERVATIONS DURING THE TANGSHAN EARTHQUAKE Peng Kezhong, Yu Shuangjiu and Guo Yuxue* The main shock accelerograms were recorded at most of the stations of the strong-motion accelerograph network in North China during the Tangshan earthquake of magnitude 7.8. Strong-motion records were also recorded at some of the stations during the Luanxian earth- quake of magnitude 7.1 and the Ninghe earthquake of magnitude 6.9. After the main shock of the Tangshan earthquake occurred some mobile observation stations were quickly deployed in Tangshan City and its nearby regions, and a number of near-source records from the aftershocks were obtained thereafter. I. Strong-Motion Accelerograph Network in North China After the 1966 Xingtai earthquake in Hebei Province a strong-motion accelerograph net- work of China was first established and developed slowly in North China. Prior to the July 28, 1976 Tangshan earthquake a total of 23 accelerographs had been installed. Among them, 18 accelerographs (Model RDZ1-12-66) were operated by the Institute of Engineering Mechanics, Academia Sinica (IEM), 3 accelerographs (Model RDZ1-12-66) were operated by the Institute of Earthquake Engineering, Chinese Academy of Building Research (IEE), one accelerograph (Model SG-4-12) by the Institute of Scientific Research, Ministry of Water Conservancy and Electric Power (ISR), and one accelerograph (Model SMAC-B2) by the Institute of Geophysics, State Seismological Bureau (IGP). After the Tangshan earth- quake of magnitude 7.8 another accelerograph was supplemented at Baliqiao in Tongxian County by IEE. The total of 24 accelerographs was deployed. The distribution of the strong- motion accelerograph stations in North China is shown in Figure 1. The basic information on stations of the network is given in Table 1. II. Instrumentation There were 3 types of strong-motion accelerographs used in the permanent network and the mobile stations: Models RDZ1-12-66, SG-4-12 and SMAC-B2. * Institute of Engineering Mechanics, State Seismological Bureau 340 (1) RDZ1-12-66 strong-motion accelerograph The RDZ1-12-66 is a multi-channel instrument with galvanometer registrations. It con- sists of one electromagnetic optical oscillograph of Model RDZ1, eight horizontal trans- ducers of Model RPS1, four vertical transducers of Model RZS1, two mechanical triggers of Model RCF1 and one protective support for the oscillograph (see Photos 1–4). They were designed by IEM and produced by the Beijing Geological Instrument Factory (Huang Zhen- ping et al., 1977). The RDZ1-12-66 is used with external batteries. Both the horizontal and vertical transducers were velocity sensing pendulum. While the coils of the transducer move in the magnetic field during an event, the current generated in the coils is directly proportional to the acceleration of the measured point and the current would drive the high-frequency galvanometer coupled with the transducer to record the acceleration. To connect the transducer with the galvanometer for a recording channel, a dual T-shaped network of resistance coupling shown in Fig. 2 is adopted. In this figure, Rs was the interior resistance of a sensor coil, R1 the damping resistance of the sensor, R2 an adjustable resistor for changing the sensitivity of the recording channel, rg is the interior resistance of a galvanometer coil, rd and R3 represent a matched resistance for adjusting the sensitivity of the recording channel and the damping resistor of the galvanometer respec- tively. The primary technical specifications of the transducer, galvanometer and trigger are given in Tables 2 to 4. The technical specifications of the accelerograph are as follows: 1. Flat frequency response: 0.5 to 35 Hz (Fig. 3). 2. Recording sensitivity: 0.5 to 10 gal/mm (with Model FC6-120 galvanometer), adjust- able. 5 to 100 gal/mm (with Model FC6-400 galvanometer), adjustable. 3. Triggering sensitivity: minimal triggering level of 1 gal, adjustable. 4. Start-up time: full operation within 0.2 to 0.25 Sec. 5. Optical arm length of oscillograph: 308.38 mm. 6. Recording medium: photographic paper, width of 200 mm, length of 20 m. 7. Paper speed: 2.1, 4.8, and 11.3 cm/s, selectable, paper speed variations less than 3% within 1 m. 8. Hold-on time: 2 to 3 Sec. Continuous running is possible when acceleration is larger than triggering level. 9. Timing mark: 20 ±0.1 Hz. 10. Power: DC 25 volts (nominal), rechargeable batteries. 11. Size of oscillograph: 490 ×358 ×353 mm. 341 12. Weight of oscillograph: 35 kilograms. 13. Operating temperature: 0°C to 40°C for oscillograph, −20°C to +40°C for transducer. (2) Model SG-4-12 strong-motion accelerograph** The SG-4-12 is a multi-channel galvanometer recording system designed to record strong earthquake motion. It consists of an electromagnetic optical oscillograph of Model SG-4-12, developed by the Institute of Scientific Research, Ministry of Water Conservancy and Elec- tric Power, and DJ651-B seismic detectors manufactured by the Xian Chemical Instrument Factory. The natural frequency of the sensor is about 26 Hz and the damping constant is 0.35 to 0.5. The sensor is considered as an acceleration transducer for recording strong ground motion, because the output current of the sensor is directly proportional to derivative of acceleration at a measured point. Integral galvanometers or low frequency galvanometers with the natural frequency, f2 , of 10 Hz and the damping constant of 0.15 to 0.2 were used to respond to the sensor's current and then to record the acceleration at the measured point. Two types of triggers were used in this accelerograph. One was the electromagnetic type and the other was the mechanical trigger. The former consisted of a DJ651-B sensor, an elec- tronic amplifier and a control circuit and the latter consisted of a horizontal pendulum, a ver- tical pendulum, a set of contact switches and a control circuit. The appearance of the SG-4-12 is shown in Photo 5. A resistance coupling network shown in Fig. 4 was used to couple a sensor with a galvanometer. In this figure, Rs is the interior resistance of the sensor coil, R1 is the damping resistance of the sensor, R2 is a vari- able resistor for changing the sensitivity, and rg is the interior resistance of a galvanometer. Rd is a matched resistance to adjust the damping of the galvanometer. The primary technical specifications of the sensor and the galvanometer for the SG-4-12 strong-motion accel- erograph are given in Table 5 and Table 6. The technical specifications of the SG-4-12 accelerograph are as follows: 1. Flat frequency response: 0.5 to 20 Hz. 2. Dynamic range: 1 to 200 gal. 3. Triggering sensitivity: from 1 gal, adjustable, for the mechanical trigger. Four levels (1, 2.5, 5, 10) are available for the electromagnetic trigger. 4. Hold-on time: 3 to 60 Sec., adjustable. 5. Optical arm length: 0.32 m. 6. Photographic paper: width of 120 mm, sensitivity of 140 DIN. ** The specifications of SG-4-12 is provided by Mr. Su Kezhong of ISR. 342 7. Paper speed: 5 cm/sec. 8. Number of channels: a total of 12, 10 channels for recording, 2 channels for timing marks. 9. Timing mark: 10 Hz. 10. Power : AC 220 volts; DC 12 or 24 volts. (3) Model SMAC-B2 strong-motion accelerograph The SMAC-B2 strong-motion accelerograph is a direct mechanical scratch recording sys- tem, which is manufactured by AKASHI SEISAKUSHO, LTD., Japan (Photo 6). Three torsional type accelerometers with the natural frequency of 7.14 Hz in an orthogo- nal position were fixed in the accelerograph. The air piston damper is used for critical damping. The recording system was operated by a mechanical spiral spring. Both the hori- zontal and vertical triggers were available: the former is a horizontal one controlled by a mechanical fall-ball system and the latter is a vertical electric pendulum. Its technical specifications are as follows: 1. Flat frequency response: 0 to 7 Hz. 2. Sensitivity: 12.5 gal/mm. 3. Dynamic range: 6 to 500 gal. 4. Timing mark: 1 Hz. 5. Recording speed: 10 mm/sec. 6. Recording medium: wax paper, width of 280 mm and length of 10 m. 7. Number of channels: a total of 4, 3 channels for recording, l channel for timing mark. 8. Triggering sensitivity: 100 gal. for the fall-ball type trigger; 10 gal. for the vertical pendulum starter, adjustable. 9. Single recording duration: 3 minutes, up to 5 times of recording. 10. Power: DC 6 volts. 11. Size: 540 ×540×370 mm. 12. Weight: 100 kilograms. 343 III. Strong-Motion Data Acquisition of the Tangshan Earthquake (1) Records from the main shock of magnitude 7.8 During the Tangshan earthquake of magnitude 7.8 a total of 58 well recorded accelera- tion traces were recorded at 7 stations (8 accelerographs) of the strong-motion network in North China: the Beijing Hotel Station (2 accelerographs), the Hujialou Building Station, the Miyun Reservoir Dam Station, the Sanlihe Building Station, the Guanting Reservoir Dam Station, the Fengcun Bridge Station and the Hongshan Ground Station (see Table 7). In addition, 6 accelerographs installed at the Friendship Hotel Station, the Fensiting Station, the Babaoshan Subway Station, the Baijiatan Station, the Tianjin Hospital Station and the Secondary Dam Station of Huangbizhuang were triggered, but records were not transferred due to some faults. The other 9 accelerographs were not triggered because of environmental reasons or the distance from the epicenter. (2) Records from the Luanxian earthquake of magnitude 7.1 and the Ninghe earthquake of magnitude 6.9 During the Luanxian earthquake of magnitude 7.1, which occurred at 18 h 45 m, July 28, 1976, 45 acceleration traces were recorded at 5 stations: the Beijing Diplomatic Apartment Station, the Hujialou Station, the Fengcun Bridge Station of Hebei Province, the Huangbiz- huang Reservoir Emergency Spillway Station and the Normal Spillway Station (Table 8).