Preface This publication contains a list of origin times, hypocenters, and magnitudes of 70,000 earthquakes occurring in the Kanto area and its vicinity during the period from June 1971 through September 1980. Maps showing the distribution of these earthquakes are also presented. The focal parameters and their standard errors have been computed using data recoded by the network of highly sensitive seismograph stations of the Earthquake Research Institute with additional data supplied from several institutions on an exchange basis. Many members of the ERI have been involved in the observation and data processing of these earthquakes. Among them Dr. K. Tsumura, now at the Japan Meteorological Agency, took a leading role. Dr. R. S. Matsu'ura and Mr. I. Karakama have reexamined the records and redetermined or newly determined the hypocenters employing an algorithm developed by Dr. M. Matsu'ura of the Geophysical Institute, University of Tokyo. The output of the focal parameter computations has been printed here in two parts. The 400 km x 500 km area covered by this catalog is a most complex and interesting zone in view of the seismotectonics of Japan. It is our hope that this catalog will be used as a primary data source for various kinds of earthquake studies. Tokuji Utsu Director Earthquake Research Institute Additional Information In any oral or written presentation including these data, please reference: R. S. Matsu'ura, I. Karakama, and K. Tsumura, List of Earthquakes in the Kanto area and its vicinity for June 1971-September 1980, Part 1, Earthquakes Research Institute, University of Tokyo, 1988. R. S. Matsu'ura, I. Karakama, and K. Tsumura, List of Earthquakes in the Kanto area and its vicinity for June 1971-September 1980, Part 2, Earthquakes Research Institute, University of Tokyo, 1988. DATA FORMAT for files of List 1, 3, and 4: (5I2,F4.2,I3,2F3.1,F6.4,F3.1,F7.4,F3.1,F5.2, F3.1,3I2) year-1900, month, day, hour, min., sec., IT, Mf-p, Mamp, latitude in degrees, DY in km, longitude in degrees, DX in km, depth in km, DZ in km, number of stations, iteration number, number of region in List 5. The smaller the number of iterations (IT), the better the obtained hypocenter is and the less the result depends on the initial guess. If you find any error in the digital data, please write to R. S. Matsu'ura: 3-10-9-401 Umegaoka Setagaya Tokyo 154 JAPAN Index to the List of Earthquakes Part 1 Introduction 1 Notations 5 List 1. Well-converged hypocenters - part 1 7 (June 1971 - December 1976) Figure 1. Maps of epicenters - part 1 502 (June 1971 - December 1976) List 2. Stations 570 Figure 2. Maps of stations 573 Part 2 Notations 1 List 3. Well-converged hypocenters - part 2 4 (January 1977 - September 1980) Figure 3. Maps of epicenters - part 2 445 (January 1977 - September 1980) List 4. Hypocenters not well-converged 491 Figure 4. Yearly maps of epicenters 506 List 5. Regions 516 Figure 5. Map of regions 517 List 6. Interruption of observation at the 518 five stations of Dodaira microearthquake observatory List 7. The program for the hypocenter determination 522 Notations for Lists 1, 3, and 4 D H M S Origin time of an day hour minute second earthquake (Japan Standard Time). MAG Magnitudes determined from the F-P AMP duration time (F-P), and the maximum amplitudes (AMP). LAT North latitude of an epicenter in D M S degree, minute, and second. DY Epicentral error in the north-south direction in KM. LONG East longitude of an epicenter in D M S degree, minute, and second. DX Epicentral error in the east-west direction in KM. DEPTH Focal depth in KM. NS Number of stations where seismic waves are detected. NR Region number listed in List 5 and shown in Fig. 5. * An artificial earthquake or an explosion. Notations for List 2 CODE Station code. ORG Organization to which a station belongs. ABU: Abuyama Seismological Observatory, Kyoto University CRI: Central Research Institute of Electric Power Industory DPR: Disaster Prevention Research Institute, Kyoto University ERI: Earthquake Research Institute, University of Tokyo HKD: Hokkaido University IIS: International Institute of Seismology and Earthquake Engineering JMA: Japan Meteorological Agency NGY: Nagoya University NRC: National Research Center for Disaster Prevention THK: Tohoku University TKY: Faculty of Science, University of Tokyo UTS: Utsunomiya University NAME Station Name. LAT North latitude of a station in degree. LONG East longitude of a station in degree. H Station height in kilometers. COR Station correction (in seconds) used for the hypocenter determination. TERM The term during which data of a station is used for this work (not always equal to the operating term of the station). SEN The total gain of observation on the monitor (kine/mm). DATE The date when the amplifier gain was changed. Notations for List 6 M D H M Starting and ending time month day hour minute of a problem. ST. D: DDR D K O S T K: KYS O: OYM S: SRY T: TSK A problem occurred at stations marked by * or #. *: Power failure, radio problem, or other problems of electric signals. No substitute was available. #: Improper pen pressure, ink shortage, occasional grazing pen, or other problems in recording. Other monitoring instruments were occasionally used as a substitute. Introduction In order to reveal the mechanism of earthquake occurrence and to achieve the scientific earthquake prediction, the study of microearthquakes is very important. The construction of microearthquake observation networks and the improvement of the data acquisition and processing systems for those networks have been from the beginning a major part of Japan's national project for earthquake prediction. With data obtained by those networks, research has been done on many notable seismic activities and published in the Report of the Coordinating Committee for Earthquake Prediction or other publications. However, data from microearthquake observation networks have been rarely utilized for studying seismicity features for a longer term. The earthquake catalog of the Japan Meteorological Agency (JMA) is mainly used for this kind of study even now. Valuable data from microearthquake observation networks must be used not only for the analysis of individual activities, but also for extracting universal features of earthquake occurrences. For the latter kind of research, data from a network operated continuously over a long interval of time are desirable. Few such data have been published from microearthquake networks. This work contains lists of earthquakes which occurred in the Kanto area and its vicinity during the 1970's, when the Earthquake Research Institute (ERI) was most exclusively engaged in the detection of microearthquakes in the area. It is based on the microearthquake data set prepared by correction, amendment, modification, supplementation, and revision of about 400,000 cards of the routine data by referring original seismograms in storehouses. Hypocenters in the lists are newly determined from these new data. i) Data For the hypocenter determination, data from stations in List 2 and Figure 2 are used. The basic stations among them are the following: 1) Stations of Dodaira Microearthquake Observatory (DDR, KYS, OYM, SRY, TSK) Signals were radio telemetered and recorded at ERI on long-term drum recorders (paper speed 4mm/sec, tick marks every second). 2) Stations operated by the fund for the South Kanto Project (HAT, HNG, INB, KBH, OKN, TKE) Signals were recorded at individual sites on drum recorders (paper speed 4mm/sec, tick marks every second). 3) Stations of the Seismological Mobile Survey of ERI in the Izu Peninsula (DSZ, HGW, HOK, ICY, KWZ, NGH, NRM) Recording system was the same as 2). 4) Stations of Fujigawa Crustal Movement Observatory of ERI (FUJ, UMJ) Signals were recorded and data were read in the observatory. Maintainance of these observations was done by the following ERI staff members: Kenji Kanjo, Ikuo Karakama, Hideteru Matsumoto, Isao Nakamura, Izumi Ogino, Masayoshi Takahashi, Masaru Tsujiura, Kenshiro Tsumura, and Shigeru Watanabe. The data of arrival times and other parameters were read routinely by Kenshiro Tsumura for the period June 1971-August 1972, and by part-timers for the rest of the time. Data supplied from stations of the National Research Center for Disaster Prevention (NRCDP) on a mutual exchange of basis were included for the period July 1971-February 1979. In addition to these data, data from many temporary stations and temporarily exchanged data with other universities or organizations were mixed with the routine data. We first mixed data from JMA stations in List 2 taken and these data were examined. For data of 1980, part of NRCDP data was also mixed from the Seismological Bulletins of NRCDP. Since the data processing in those days was done on an off-line basis and much manual labour was involved, there were a lot of mistakes and problems in the data. We marked data of teleseismic signals and excluded phantom local earthquakes. We also marked apparent explosions in quarries or other artificial earthquakes. (Those explosions are marked by an asterisk in the lists and excluded from all the figures.) One-minute difference among data using different clocks often resulted in duplicate counts of an earthquake. In addition to these errors, various kinds of errors are corrected. Almost 40% of the cards were subjected to some change. Those changes were made by referring original records when they were available. Every page of stripchart records (paper speed 60mm/min) for the stations in 1) were also examined to recover omitted data and about 50,000 data records were added to correct deficits in the old data. In List 6, interruptions of observation at the stations in 1) (the most fundamental stations in the network) are listed for the sake of time-series analysis. ii) Hypocenter determination Mikumo's model E-3A3 (1966) is used as the velocity structure for the calculation. This model is often used for the Kanto area, and the old routine also used it. However, for the areas far from the Kanto area, this model does not represent the structure well. Therefore, earthquakes located outside of the area defined by [32.5 N, 37.5 N] and [137.8 E, 142.2 E] were excluded from List 1 and List 3. Station corrections (Ishida and Asano, 1979) for this model are used in the calculation. The old routine method of hypocenter determination had some problems. The origin time of an earthquake was fixed in the calculation. Errors of data were not appropriately treated in the calculation. Epicenters were often distributed on stripes with a space determined from the data accuracy. When a hypocenter came above the earth's surface during the iteration, the focal depth was fixed at 5 km. This resulted in diffusing hypocenters and in an obscure image of seismicity. In the present work, a new method (Matsu'ura, 1984) is used for the hypocenter determination. In this method: 1) prior information on the hypocenter is used, 2) the origin time is determined after the hypocenter is located, and 3) errors in each datum are carefully treated in the algorithm, and no jump of a hypocenter location due to a false datum occurs. It is robust and only 1.4% of all hypocenters are not well- converged due to lack of good arrival time data. These poorly converged hypocenters are listed separately in List 4 for future revision. The list of computer program for the hypocenter determination is shown in List 7. The program is available upon request. As prior information for the calculation (an initial hypocenter and assumed errors), we use the following: 1) For shallow earthquakes in the aftershock area of the 1974 Izu-hanto-oki earthquake: 34.6 N +/- 40 km, 138.8 E +/- 40 km, 5 km +/- 5 km. 2) For shallow earthquakes in the aftershock area of the 1978 Izu-Oshima-kinkai earthquake: 34.75 N +/- 40 km, 139.1 E +/- 40 km, 5 km +/- 5 km. 3) For other shallow earthquakes in the Izu area: 34.88 N +/- 40 km, 139.05 E +/- 40 km, 5 km +/- 5 km. 4) For shallow earthquakes near Kobugahara station: 36.65 N +/- 20 km, 139.4 E +/- 20 km, 5 km +/- 5 km. 5) For shallow earthquakes in the Matsushiro area: 36.5 N +/- 20 km, 138.2 E +/- 20 km, 5 km +/- 5 km. 6) For other shallow earthquakes, the initial epicenter is assumed to be 0.1 north and 0.1 west from the nearest station, the epicentral errors are assumed to be 50 km, and the initial depth and its error are assumed to be 20 km. 7) For earthquakes in the aftershock areas of the 1972 Hachijojima-kinkai earthquake and the 1972 Hachijojima-toho-oki earthquake: 33.5 N +/- 50 km, 141.0 E +/- 50 km, 50 km +/- 30 km. 8) For other earthquakes, the initial hypocenter is set as a hypocenter obtained by the calculation from the first three P-arrival times with the semi-infinite half space of Vp = 6 km/sec, and the epicentral errors and the error in depth are assumed to be 50 km. When the initial depth is 30 km or shallower, its error is assumed to be 30 km. All earthquakes were located for which the onset of P-wave was detected at three or more stations, except ones whose S-P time at the nearest station was larger than 40 seconds. About 70,000 hypo- centers are listed in List 1 and List 3. The number of hypocenters is increased by 30% from the old routine result. In Figures 1, 3, and 4, hypocenters in these lists within the range of magnitude 0 or above, depth 300 km or shallower are plotted monthly and yearly. In the first pages for Figures 1 and 3, hypocenters for the entire period of List 1 and 3 are also shown. From these maps, a clear seismic gap is seen near the Boso Peninsula. iii) Magnitude The magnitude of an earthquake is determined from the total duration times (Tsumura, 1967) according to Hori's formula (1973). M = 2.94 log (F-P ) - 2.56 F-P (sec) The magnitude is also determined from the maximum amplitudes. For velocity amplitudes, Watanabe's formula (1971) is used. For displacement amplitudes, Tsuboi's formula (1954) is used for earthquakes of depth 60 km or shallower, and Katsumata's formula (1964) is used for deeper earthquakes. We are grateful to Prof. Tokuji Utsu for helpful suggestions, continuous encouragement throughout this work, and critical reading of the manuscript. Computations and data processing were done with a HARRIS H-300 at the Seismological Mobile Survey of the Earthquake Research Institute, University of Tokyo. Authors strongly hope that following this publication, earthquake lists from many microearthquake networks become available to the seismological community. References Hori, M., Determination of earthquake magnitude of the local and near earthquake by the Dodaira microearthquake observatory, Res. Notes Earthq. Res. Inst., Univ. Tokyo, 10(4), 1-4, 1973 (in Japanese). Ishida, M., and S. Asano, A new seismicity map in the Kanto District, Japan, J. Phys. Earth, 27, 435-453, 1979. Katsumata, M., A method to determine the magnitude of deep-focus earthquakes in and near Japan, Zisin II, 17, 158-165, 1964 (in Japanese). Matsu'ura, M., Bayesian estimation of hypocenters with origin time eliminated, J. Phys. Earth, 32, 469-483, 1984. Mikumo, T., A study on crustal structure in Japan by use of seismic and gravity data, Bull. Earthq. Res. Inst., Univ. Tokyo, 44, 965-1007, 1966. Tsuboi, C., Determination of the Gutenberg- Richter's magnitude of earthquakes occurring in and near Japan, Zisin II, 7, 185-193, 1954 (in Japanese). Tsumura, K., Determination of earthquake magnitude from total duration of oscillation, Bull. Earthq. Res. Inst., Univ. Tokyo, 45, 7-18, 1967. Watanabe, H., Determination of earthquake magnitude at regional distance in and near Japan, Zisin II, 24, 189-200, 1971 (in Japanese).