A Real-Time Processing System of Seismic Wave Using

Transcription

1 J. Phys. Earth, 40, , 1992 A Real-Time Processing System of Seismic Wave Using Personal Computers Shigeki Horiuchi,* Toru Matsuzawa, and Akira Hasegawa Observation Center for Prediction of Earthquakes and Volcanic Eruptions, Faculty of Science, Tohoku University, Aoba-ku, Sendai 980, Japan A multi-channel digital event recording system with automatic event detection and location has been developed by using personal computers with clock frequency of 8 MHz. The system is designed to record seismic signals of more than 100 input channels with a sampling frequency of 150 Hz. The sampled data are written on a hard disk with a cassette streamer, which can copy all the data in the hard disk to a cartridge tape. The capacity of the hard disk is 20 Mbytes, which corresponds to waveform data for 40 events. Although a personal computer is used, it takes only 2 min to pick P and S wave arrivals of 27 stations and to calculate a hypocenter location. The present personal computer system was used in the seismic observation of the 1986 Joint Seismological Research in the Western Nagano Prefecture. Waveform data of about 2,800 events were recorded during the observation period of 52 days, and 1,264 events that occurred in the aftershock area were precisely located in the seismic observation. Hypocenters of about 2,000 events, including about 1,600 local events, were determined automatically by using the data played back from the streamer tapes. 1. Introduction It has become clear that the crustal structure beneath Japan Islands is very complicated (e.g., Asano et al., 1985; Horiuchi et al., 1988; Ikami et al., 1986; Inamori et al., 1992; Mizoue et al., 1982), and a large number of seismic stations are necessary in order to determine precise hypocenter locations of shallow earthquakes. Members from almost all universities doing research work on earthquake prediction in Japan held a meeting and discussed about setting up a dense joint temporary seismic network to determine hypocenter locations, velocity structure, etc., as precise as possible. The conclusion of the meeting was to do the 1986 Joint Seismological Research in the Western Nagano Prefecture (JSR'86) by setting a dense seismic network at the aftershock area of the 1984 Western Nagano Prefecture Earthquake, where aftershocks were still frequently occurring. It was planned to set 26 telemetering stations with three or four components and to record collected waveform data at a center of the network (Aoki, Received September 30, 1990; Accepted June 30, 1991 * To whom correspondence should be addressed. 395

2 396 S. Horiuchi et al. 1988). Therefore a seismic recorder with more than 100 channels was required. Since the seismic observation was carried out in the area having high seismic activity, a large number of events were expected to be recorded by a large number of seismic stations. On an ordinary occasion of temporary seismic observations, seismograms are recorded on magnetic tapes. For determining hypocenter locations, we must reproduce seismograms on chart paper and pick arrival times of P and S waves manually. These processes require much work and it is rather difficult to determine hypocenter locations for a large number of events having large number of stations, such as in the present case. Actually, about 1,500 events were observed by the JSR'86 and only about one-third of them were located manually (Horiuchi et al., 1992). In such a case, if an automatic processing system which detects, locates and records seismic events is introduced, the work required for studying velocity structure, seismic activity, etc., will decrease drastically. Techniques to pick arrival times of the P and the S waves have been developed by many researchers (e.g., Morita and Hamaguchi, 1984; Shirai and Tokuhiro, 1979; Stewart, 1977; Yokota et al., 1981). These were designed to be operated by the use of mini-computers with high computation speed and with large memory size. Since temporary seismic observations are made usually at some small village, it is difficult to find some large building available to set mini-computers which require large capacity of commercial current and air conditioning. Horiuchi et al. (1985) developed an automatic processing system of event detection and location using a personal computer, which is portable and much cheaper than mini-computer. This system is applicable only for a seismic observation with channels less than 16. Recent development of electronic technique provides us with higher quality personal computers than before. In the present paper, we developed a 128-channel personal computer system with the functions of automatic detection, location and recording of seismic events. The present system was used in the JSR' Hardware The hardware configuration for the personal computer system used in the real-time observation of the JSR'86 is summarized in Fig. 1. Specification of the present system is listed in Table 1. We use two sets of PC-98XA made by NEC. One is for the multi-channel event detection and recording system which is already described in the previous paper (Horiuchi et al., 1987). The other is for automatic location of seismic events by picking P and S arrival times. Even if the next event may occur while the latter computer is locating hypocenter for the last event, the former computer will record the waveform data of the next event. The PC-98XA is a 16 bits personal computer with 768 kbytes RAM available for users and with an arithmetic floating point processor. Clock frequency is 8 MHz. Since PC-98XA has a high resolution display with 1,120 times 750 dots, which is almost four times larger than that for other normal mode displays, seismograms for about 30 channels can be clearly plotted on the screen simultaneously. Hard copies of seismograms displayed on the screen were taken for monitoring. Commercial A/D boards with accuracy of 12 bits are used. They have 16 channel J. Phrs. Earth

3 A Real-Time System Using Personal Computers 397 Fig. 1. Block diagram of the 128-channel digital event recording system with functions of automatic event detection and location. NDP and PR indicate arithmetic floating point processor and printer, respectively. Table I. Specifications of the digital event recording system. A/D converters, and 4 bits digital input and output. By using 16 pieces of 8 channel C-MOS multiplexers, the number of input channels is increased to 128 (Horiuchi et al., 1987). The multiplexers are controlled by 3 bits of the 4 bits digital output of the A/D board connected to the former computer. The latter computer gets switching information of the address of the multiplexers through the digital input to know correct sampling moments corresponding to each channel. Trigger pulse is sent from the former to the latter computer by the remaining one bit of the digital output. Time code signals generated by a X'tal clock were put into an analog channel of the multiplexers. The clock has a function of automatic correction by receiving the broadcasted time signal of the JJY. Accurate trigger times are determined from the data of the time code. The former computer detects seismic events and stores sampled data on a hard disk with streamer, which can copy all the data in the disk to a cartridge Vol. 40, No. 2, 1992

4 398 S. Horiuchi et al. tape at a time. The capacity of the hard disk is 20 Mbytes. It takes about 5 min to copy 20 Mbytes data. In the JSR'86, the number of input channels, sampling frequency and data acquisition time for each event are 104, 150 Hz and 16 s, respectively. The hard disk can record seismograms for 40 events. In general, a shortcoming of the digital recording system is the lack of recording media that can store a large amount of data. The capacity of the present hard disk is not enough. We had to take a copy of the hard disk to cartridge tape twice a day. Sometimes even if hard disk data were copied at night, the disk became full at midnight of that day and the observations were stopped till next morning. Now, an optical disk with a capacity of 400 Mbytes per side has become available. Use of an optical disk or other media having large capacities removes the shortcoming of the present system. 3. Software Software for the personal computer system is separated into three parts: (1) event detection and data recording, (2) automatic picking of P and S waves, (3) hypocenter location. The main part of the software system is developed by using FORTRAN. Assembler is also used to control A/D converter, and to write a large number of waveform data on hard disk. All computer programs were made under MS-DOS. 3.1 Event detection The method of event detection has been shown by Horiuchi et al. (1987), and only its outline will be described below briefly. Considering a large variation in the amplitude of ground noise generated by falling rain, blowing wind, automobiles, etc., a recursive filter is used to eliminate high frequency noise. Then the ratio of the short-term to long-term averages of the absolute values of the filtered data (STA/LTA), is calculated. Ten stations whose ground noise levels were considerably low were selected for the event detection in the actual observation. However, because of the limitation of the computation speed of the personal computer, calculation of STA/LTA values for only 5 stations are feasible in the case of setting sampling frequency to be 150 Hz. Therefore, STA/LTA values are computed with sampling frequency of 75 Hz by calculating values for 5 stations and those for the other 5 stations alternately. Values of cut-off periods for the short- and long-term averages are 0.2 and 30 s, respectively. The criterion of the triggering is as follows: STA/LTA values at more than 3 stations among the 10 stations should become larger than trigger levels assigned to each station ranging from 2.5 to 3.0. We record not only waveform data but also an event file mentioning date of events stored on the 20 Mbyte disk, correct trigger times, name of waveform files for each event, sampling frequency, and number of recording channels. Information on waveform data for some event can be easily found by displaying data of this event file on the screen by using the type command of MS-DOS. In our recording system, observation is stopped when one computer, which is used for event detection and recording, writes waveform data on the hard disk or when data stored on the disk are copied to. the streamer tape. It took about 1 min to write 640 kbytes waveform data on the hard disk. Software to write data on a hard disk was J. Phys. Earth

5 A Real-Time System Using Personal Computers 399 improved after the observation and it became only 10 to 15 s. 3.2 Automatic picking of P and S waves Methods to measure arrival times of P and S waves automatically have been developed by Shirai and Tokuhiro (1979), Yokota et al. (1981), Morita and Hamaguchi (1984), Kitagawa and Takanami (1985) and other researchers. Some of these are successfully applied to the actual seismic networks in Japan (e.g., Hasegawa et al., 1986; Hori and Matsumura, 1987), where several computers with high computation speed and large memory size are available. However, it is difficult to use such computers in temporary seismic observations, which are usually made at some isolated villages in mountain ranges. Moreover, most of the temporary seismic networks are set up at areas of high seismicity. This circumstance necessitates development of an automatic processing system that can determine hypocenters in a very short time; otherwise, subsequent events may occur during the computation time of the former event. Considering these limitations mentioned above, much attention was paid to shorten computation time in the present system. An autoregressive function, (1) is introduced to decrease amplitude of noise, where lin is n-th observed data and Ck is a constant determined by the method of least squares which minimizes, where M is the number of data in a time window before P or S wave arrivals. We use time windows for 0.5 s of noise data in the case of the P wave picking and for 0.5 s of the P wave data in the case of the S wave picking. The method to determine approximate arrival times of P and S waves with short computation time is described in Horiuchi et al. (1985). A characteristic of this method is that the Akaike's Information Criteria (AIC) is used not only to determine final solution of picking times but also to determine approximate arrival times. Arrival times of P and S waves are determined as follows. At first, a time series of waveform data is divided into several tens of blocks by introducing A(n), defined as: (2) (3) where L is a constant defining number of data in each block. Approximate arrival time of the P wave is determined by finding minimum of, where N is the number of blocks. The final solution is obtained by applying Eq. (4) (4) Vol. 40, No. 2, 1992

6 400 S. Horiuchi et al. also to data around the approximate solution with L in Eq. (3) to be 1. An approximate time of P arrival is determined by using the data in the time range from the beginning of the triggered data to the moment when amplitude of the absolute value of sampled data becomes maximum. The arrival time of S wave is measured by using the data in a horizontal component in the time range 0.1 s after the arrival of P wave. 3.3 Hypocenter determination It is difficult to make an automatic processing system that measures correct P or S wave arrivals without mistakes. Our automatic system mentioned above sometimes picked an arrival time of S wave as that of P wave. It also picked an arrival time of P wave of the next event as that of S wave. Since most of these errors are fetal to the correct location of hypocenter, it is very important to find out mispickings and determine hypocenters by eliminating them (Hasegawa et al., 1986). We determine hypocenters as follows. At first, a mean value of P wave arrival times is calculated. Then, we eliminate P arrival time data whose differences from the mean value are larger than some critical value expected from the largest distance of separation among stations and the P wave velocity. S wave arrival time data having large picking errors are eliminated by comparing an origin time estimated from P and S wave arrival times at each station with the mean value of origin times estimated by all the stations having data of both P and S arrivals. Next, hypocenter location is calculated by putting its initial value to a point just beneath the station having the earliest P arrival. If a convergent solution is not obtained by the ordinary least squares method, the damped least squares method is used. Only the origin time is calculated in the case where no convergent solution is obtained by both the trials. Then, the P or the S reading which has the largest error is eliminated and hypocenter is calculated again. This procedure is repeated till the root mean squares of the residuals becomes a value less than some critical value. In the case where a convergent solution is obtained, we pick again P and/or S wave arrivals for stations which are not read in the first trial or for those eliminated in the above procedure of the hypocenter location. Since narrow time windows suitable for re-picking of P or S waves can be taken by the use of a convergent solution, there are stations having only S wave arrivals. This re-picking is very important, and about half of arrival time data are measured in this procedure. The final hypocenter location is calculated by adding the data of re-picking. 4. Real-Time Operation As mentioned by Horiuchi et al. (1992), 26 stations equipped with three or four channel telemetries were set up in and around the aftershock area of the 1984 Western Nagano Prefecture Earthquake in the temporary seismic observation of the JSR'86. Using data obtained by this dense joint network, Horiuchi et al. (1992), Yamazaki et al. (1992), Hirahara et al. (1992) and Ikami et al. (1992) determined detailed hypocenter locations, spatial distribution of focal mechanisms, and three-dimensional velocity structure, respectively. Data from all the telemetering stations were collected and recorded at the J. Phys. Earth

7 A Real-Time System Using Personal Computers 401 Fig. 2. An example of vertical component seismograms for an aftershock. Automatically picked P onset times are shown by solid lines with U, D or P, and S onset times by solid lines with S. The symbols U, D, and P indicate polarity of initial motions of P wave, i.e. compressional, dilatational and unknown, respectively. observation center of the network. The real-time data processing was made in the JSR'86 by the use of the present system. One telemetering station, at the east end of the network, was added later, because it was found by the present system that many earthquakes were occurring in the area beneath this station. The present automatic system recorded waveform data from the 27 stations and clock signals. Total number of input channels was 104. The observation of the JSR'86 was made for 52 days from September 1 to October 22, The present system was operated from the beginning of the observation. An example of vertical component seismograms recorded by the present system can be seen in Fig. 2, showing that not only the P wave arrival times but also the S wave are picked with high accuracy. The automatic system reads polarities of the initial motions of P wave. The symbols near the P arrivals indicate the polarities of the initial motions of the P wave-u, D, and P being compressional, dilatational and unknown, respectively. Magnitude is also determined automatically by measuring maximum amplitude. There are stations at which only S wave arrivals are picked. Figure 3 is an example of seismograms for an event whose hypocenter is determined to be at a very shallow depth near the top of Mt. Ontake, an active volcano that erupted in It is clear that surface wave amplitude for this event is larger than that in Fig. 2. The predominant frequencies of the P and S waves are also lower than those in Fig. 2. Vol. 40, No. 2, 1992

8 402 S. Horiuchi et al. Fig. 3. An example of vertical component seismograms for an event occurring at a shallow depth near the top of Mt. Ontake, which is an active volcano that erupted in The hypocenter distribution determined by the present real-time processing system is shown in Fig. 4. Total number of the located events is 1,264. Comparison of hypocenter distribution determined by the manually picked data to that by the automatic system indicates that hypocenters determined by the latter are very accurate. It is clear that depths of the deepest events become deeper toward west. It was found after the observation of the JSR'86 that input data of the location for one station had been shifted by 2 km from the actual location. However, the present automatic system eliminated almost all arrival time data of the P and S waves for this station. 5. Discussion and Conclusion A multi-channel digital recording system with automatic event detection and location has been developed by the use of two personal computers, one for event recorder and the other for automatic location. This real-time processing system was used in the seismic observation of the JSR'86 and located 1,264 hypocenters of the aftershocks during the observation period of 52 days. Some events occurred while one computer for the automatic location was calculating hypocenters or was taking hard copies of seismograms displayed on the screen. In this case, hypocenters for these events were not located but their digital seismograms were J. Phys. Earth

9 A Real-Time System Using Personal Computers 403 Fig. 4. Hypocenter distribution of events(open circles) located by the present automatic processing system. Crosses show the locations of observation stations. Hypocenters of 1,264 aftershocks were located during the observation period of 52 days. Fig. 5. Hypocenter distribution of events located by applying the automatic processing system to the data played back from the cassette streamer. Vol. 40, No. 2, 1992

10 404 S. Horiuchi et al. Table 2. Number of events recorded or analyzed by the present automatic system. stored on the hard disk by the other computer. Therefore, we can determine hypocenters of these events after the present seismic observation by the use of data stored on the tapes. Hypocenter distribution determined by using the data played back are shown in Fig. 5. We made a program to locate hypocenter automatically with using data stored on a hard disk. The new program was made by changing a sub-program to operate A/D converter in-the present system with that to read waveform data on a disk. Table 2 summarizes the result of the observation by the use of the present automatic system. Total number of recorded events, including events triggered by the noise or triggered manually for the testing of instruments, is 2,881. Total number of events whose hypocenters are determined is 2,054, which is 71% of the total recorded number. In addition to the present automatic processing system, 8 sets of FM data recorders, multi-channel delay memories and hard trigger units were used for recording waveform data (The Group for the Seismological Research in Western Nagano Prefecture, 1988, 1989) in the seismic observation of the JSR'86. About 1,500 events were recorded by this system and one-third of them were located manually. Comparison of the number of recorded events and the number of located events by this system with those by the present automatic system, shows the effectiveness of using personal computers in temporary seismic observations. The authors would like to express their sincere gratitude to the members of the 1986 Joint Seismological Research in Western Nagano Prefecture, who operated the seismological network. REFERENCES Aoki, H., The 1986 Joint Seismological Research in the Western Nagano Prefecture-Short note for the seismological network-, Gekkan Chikyu (Earth Monthly), 10, , 1988 (in Japanese). Asano, S., K. Wada, T. Yoshii, M. Hayakawa, Y. Misawa, T. Moriya, T. Kanazawa, H. Murakami, F. Suzuki, R. Kubota, and K. Suyehiro, Crustal structure in the northern part of the Philippine Sea plate as derived from seismic observations of Hatoyama off-izu Peninsula explosions, J. Phys. Earth, 33, , Hasegawa, A., N. Umino, A. Yamamoto, and A. Takagi, Automatic event detection and location J. Phys. Earth

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