著者

神田 克久 武村 雅之

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.58, no.3, pp.177-198, 2005-12-25 (Released:2010-03-11)

参考文献数

41

被引用文献数

2

---

The inversion analysis using seismic intensity data is carried out to investigate the source characteristics of major damage earthquakes that have occurred in the sea off Miyagi Prefecture since 1861. It can evaluate short-period seismic wave radiation zones (SPRZ) on an earthquake fault plane. Comparison of the result of the inversion analysis for the May 26, 2003 Off-Miyagi earthquake with the source region deduced from reported aftershock activity verified sufficient accuracy of the present method. We compared the location of SPRZ among analyzed earthquakes and identified the kinds of earthquakes. The SPRZ of the June 12, 1978 earthquake was located near the seacoast of Miyagi Prefecture, adjacent to the large slip area obtained from the waveform inversion by Yamanaka and Kikuchi (2004). The SPRZ of the November 3, 1936 and July 27, 1937 earthquakes were located on the south of that of the 1978 earthquake, which may indicate that the rupture area of the 1936 and 1937 earthquakes were different from that of the 1978 earthquake. On the contrary, location of the SPRZ of the February 20, 1897 earthquake was similar to that of the 1978 earthquake. The SPRZ of the August 5, 1897 earthquake was located near the Japan Trench, while that of the April 23, 1898 earthquake was just beneath the coast of northern Miyagi Prefecture, suggesting that it was an intermediate-depth intraslab earthquake the same as the May 26, 2003 event. The location of the SPRZ of the 21 October, 1861 earthquake was mostly consistent with that of the 1978 earthquake. We consider that the 1861 earthquake should have occurred on the upper boundary of the subducting Pacific plate, not in the inland of northern Miyagi Prefecture.

著者

武村 雅之 小山 順二

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.36, no.3, pp.323-336, 1983-09-25 (Released:2010-03-11)

参考文献数

37

被引用文献数

1

---

Classification of low-frequency earthquakes has been made quantitatively by using a diagram of seismic-moment factor Me versus characteristic period Tc. Tc and Me correspond to period of corner frequency and seismic-moment density at Tc of each earthquake. About 3, 000 earthquakes from 1926 to 1978 along the Kurile, Japan, and Ryukyu trenches have been analyzed and MS's of those earthquakes cover the range from 3 to 8.3. Most of earthquakes beneath the inner trench slopes have been classified into low-frequency events which show large Tc for the same Me, while ordinary earthquakes have been commonly found in the frontal arc regions. Relations among magnitudes and seismic moment: MS-MJMA, MS-mb, and MS-Mo, for low-frequency events are also different from those for ordinary events. The relations, therefore, cannot be explained by a scaling model for ordinary earthquakes. A scaling model has been derived for low-frequency earthquakes in a statistical manner, taking into account constraints based on the relations among magnitudes and seismic-moment mentioned above. Corner frequencies of source spectra of the low-frequency model are always one half of those of the ordinary model with the same seismic-moment. This model has been also justified by the data of so called tsunami earthquakes in other subduction regions in the world, suggesting a similarity relation among destructive tsunami earthquakes and low-frequency earthquakes with small magnitude.

著者

武村 雅之 池浦 友則 野澤 貴

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.52, no.4, pp.425-444, 2000-03-25 (Released:2010-03-11)

参考文献数

55

被引用文献数

1

---

Magnitudes for the 1923 Kanto earthquake and its major aftershocks were determined in JMA (Japan Meteorological Agency) scale. The original definition of the JMA magnitude is a magnitude that is calculated by the Tsuboi's formula from the maximum amplitudes in horizontal components of seismograms obtained by the regional observation network of JMA. The used seismograms were recorded by standard seismographs, which were the displacement type with the natural period of about 5s and damping ratio of about 8. However, the seismometers had not been yet standardized before 1925 and various types had been used whose instrumental responses were quite different from those of the standard seismographs. The purpose of the present study was that the JMA magnitudes of the 1923 Kanto earthquake and its major 3 aftershocks were determined in consideration of the difference of the instrumental responses. Fortunately, unsaturated seismograms by the Imamura's type strong motion seismographs (displacement type) have been preserved at 7 stations of JMA. The natural period and damping ratio of each seismograph have been evaluated from the free oscillation records preserved at each station. The records for the main shock and aftershocks were digitized and corrected in the instrumental responses to calculate the seismograms with the instrumental response of the standard seismograph of JMA. After that, the maximum amplitudes were measured on the corrected records and the magnitude was determined for each earthquake following the definition of the JMA magnitude. The determined JMA magnitude was 8.1±0.2 for the main shock. All the results were consistent within the difference of 0.2 with the customary results, which were determined from the uncorrected amplitude and seismic intensity data. The standard deviations were smaller than 0.2 for all the events, which shows higher reliability of the present results, comparing with the past ones.

著者

神田 克久 武村 雅之 宇佐 美龍夫

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.57, no.2, pp.153-170, 2004-12-27 (Released:2010-03-11)

参考文献数

36

被引用文献数

4

---

An inversion analysis has been developed to evaluate short-period seismic wave radiation zones on an earthquake fault plane using seismic intensity data. It is a useful method for historical earthquakes, for which neither strong motion nor tsunami data have been observed by instruments. Since the accuracy of the present method is verified by using ground motion waveforms synthesized by the stochastic Green's function method, it is concluded that the effects of directivity of the fault rupture process and the radiation pattern on seismic intensity distribution can be neglected in the frequency range that is effective for seismic intensity. The present method is applied to great earthquakes that have occurred since 300 years ago in the Nankai Trough seismogenic zone in southwestern Japan, where the Philippine Sea plate is subducting beneath the Eurasian plate. The reliability of their solutions is discussed based on a sensitivity analysis. It is noted that short-period seismic wave radiation zones are often adjacent to large slip areas, termed asperities, but don't always overlap them. Furthermore, they are sometimes located where fault rupture has stopped. Short-period seismic waves are not always radiated from the same zone. For example, fault zones in the Enshu-Nada Sea, the Kumano-Nada Sea, the Kii Channel and the Kochi seacoast have radiated short-period seismic waves at every event, but whether or not short-period seismic waves were radiated in the interior of Suruga Bay, off the Shiono Cape and off the Muroto Cape varied with the events. It is also concluded that crustal structures such as subducted seamounts and subducted ridges take an important role of radiation of short-period seismic waves.

著者

羽鳥 徳太郎

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.48, no.2, pp.229-233, 1995-08-25 (Released:2010-03-11)

参考文献数

22

被引用文献数

1

---

Ten years after the 1983 Nihonkai-Chubu tsunami, the 1993 Hokkaido Nansei-Oki tsunami (magnitude, m=3) hit Okushiri Is., Oshima Peninsula and other regions, Killing 231 people by tsunami and landslide. The distribution pattern of tsunami hfeights is different from that of the 1741 Oshima-Oki tsunami (m=3.5). This paper presents the distribution of cumulative energy, ∑H2, for each 150km segment alomg the Japan Sea coast for the time intervals of the recent 100-year (1894-1994) and of 1600 to 1893 (historical tsunamis). The amount of cumulative energy for the recent and historical tsunamis is largest at the Akita and Oshima-Tsugaru regions, respectively. The cumulative energy at the Akita, Shakotan and other regions for the recent 100-year tsunamis exceeds that of the historical tsunamis. On the contrary, the cumulative energy in the Oshima-Tsugaru and Niigata-Ishikawa regions for the recent tsunamis are small compared with those of the historical tsunamis. Precaution must be paid to these regions for seismic gap may exist there. The total tsunamigenic energy in the Japan Sea comprises 24% of that due to tsunamis in the whole Japan.

著者

羽鳥 徳太郎

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.52, no.1, pp.43-50, 1999-06-30 (Released:2010-03-11)

参考文献数

19

---

The Noto Peninsula coast has been suffered from the tsunamis originated along the eastern margin of the Japan Sea. According to the old documents, the 1833 Yamagata-Oki tsunami reached 5.3m at Wajima (tip of Noto Peninsula), and killed 47 persons. Inundation heights of the 1964 Niigata, 1983 Nihonkai-Chubu and 1993 SW. Hokkaido tsunamis were 2-4m at Wajima and its neighboring area. These tsunami heights are more than 2-3.5 times larger than the ones expected from the average tsunami magnitude. Based on the refraction diagrams and the shoaling-refraction factors around the peninsula, amplification factors estimated by the Green's formula are 3.0-4.0 at the north coast, 1.5 at the west coast and 1.0 at the east coast. The distribution patterns of the calculated factors nearly agree with those of the inundation heights for each tsunami. In case of the 1964 Niigata tsunami having the period of about 20min, the seishe of Nanao Bay (east side of peninsula) may be excited, because the maximum wave occurred 2.7 hour late from the initial wave. Tsunamis generated off Yamagata and SW. Niigata are strongly affected by the shoaling-refraction.

著者

村松 郁栄

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.15, no.4, pp.341-342, 1962-12-25 (Released:2010-03-11)

参考文献数

6

被引用文献数

1

著者

鈴木 康弘 池田 安隆 渡辺 満久 須貝 俊彦 米倉 伸之

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.42, no.2, pp.151-159, 1989-06-24 (Released:2010-03-11)

参考文献数

17

被引用文献数

5 2

---

Many active faults trending N-S along basin-mountain boundaries are recognized in Northeast Japan, but only a few of them have experienced surface faulting in historical time; most of them seem to have been quiescent in the past several hundred years or more. Thus earthquakes are anticipated to occur from these active faults in the near future. To detect the recurrence intervals of faulting, which can be obtained by the excavation study, is indispensable for the long term prediction of earthquakes.We excavated a trench at Kitasakai, Sakata City, across the Kannonji fault, one of the eastern boundary faults of the Shonai plain, Northeast Japan, in order to reveal its late Holocene activity including a possible faulting event associated with the Shonai earthquake (M=7.0) of 1894 A. D., which caused severe damage along this fault.Our excavation has revealed that (1) the last surface faulting event on the Kannonji fault occurred in a period from 2, 500 years B. P. to 1894 A. D., and that (2) no surface faulting occurred (at least at the trenching site) in association with the Shonai earthquake of 1894. Careful examination of historical records, however, strongly suggests that the earthquake of 1894 was also generated from this fault; it is likely that thick, unconsolidated sediments prevented the rupture from propagating up-dip to the surface. These results indicate that the interval between the last two earthquakes originating from the Kannonji fault is less than 2, 500 years. It could be 1, 000 years, because the event revealed by excavation is possibly correlated to the historically-documented earthquake of 850 A. D..

著者

多田 堯

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.38, no.1, pp.1-12, 1985-03-25 (Released:2010-03-11)

参考文献数

15

被引用文献数

2 18

---

The N-S extensional strain field in the Kyushu is understood as the result of the crustal deformation caused by the rifting and the spreading of the Okinawa Trough which crosses the Central Kyushu in the E-W direction. The spreading velocity and the spreading direction of the Okinawa Trough in the Kyushu have been estimated as 1.4cm/a and N-S direction, respectively, by the analysis of the distance survey. The horizontal and the vertical crustal movements, the normal active faults, the gravity anomaly, the volcanism, and the fault plane solutions of the earthquakes in the Shimabara Peninsula suggest that the Unzen Volcanic Graben is an active rift of the Okinawa Trough. The beginning of the spreading of the Okinawa Trough in the Kyushu has been estimated to be during the period from 0.8Ma to 0.5Ma B. P. by the spreading velocity and the width of the Unzen rift. The distributions of the active faults, the low gravity anomaly belts, the hypocenters of the shallow earthquakes and their fault plane solutions suggest that Okinawa Trough in the Kyushu consists of the three rifts whose strikes are E-W direction and the three transform faults like shear fault zones whose strikes are the NE-SW direction.

著者

宮川 康平 中西 一郎 三浦 勝美 田中 聡

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.51, no.1, pp.113-121, 1998-07-03 (Released:2010-03-11)

参考文献数

18

---

The Geiyo earthquake occurred on June 2 in 1905 in the western Seto Inland Sea between the Honshu and Shikoku Islands, Japan. The seismograms of the earthquake obtained at the stations of Central Meteorological Observatory were newly found at Earthquake Research Institute of the University of Tokyo. They are recorded by the Omori seismometers and tromometers, which are superior to former seismometers with respect to continuous recording. For the estimation of the magnitude and source mechanism from the seismograms, we digitize 3 records, which are Hongo EW component, Hitotsubashi EW component and Tokyo tromometer. We have to know the response of the Omori seismometer to estimate the ground motion during the earthquake. In order to know the frequency characteristics of the seismometers, we calculate their Fourier amplitude spectra. The spectra of the Hongo EW component and Hitotsubashi EW component show clear peaks which may be considered as the natural periods of the seismometers. The natural periods of Hongo EW component and Hitotsubashi EW component are about 60s and 25s, respectively. The damping constant estimated from the free oscillation record of Omori seismometer at Ishinomaki observatory is less than 0.01, and the friction is 1.7mm.

著者

竹内 均 金森 博雄

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.21, no.4, pp.317-317, 1969-02-25 (Released:2010-03-11)

被引用文献数

1

著者

瀬野 徹三

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.29, no.2, pp.197-200, 1976-05-15 (Released:2010-03-11)

参考文献数

9

著者

瀬野 徹三

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.30, no.3, pp.253-264, 1977-12-05 (Released:2010-03-11)

参考文献数

18

被引用文献数

1

---

In the previous paper (SENO, 1977), the method of calculation of the recurrence time interval of great earthquakes in the Nankai and the Sagami trough regions was improved and refined. The recurrence time interval T and the value d (so-called recovery ratio) were estimated at the several sites on the seismic crustal movement areas along these troughs.In the present paper, the vertical crustal movement observed by precise levellings and the defomation of the Holocene terraces in the south Kanto district are reexamined because the deformation of the coastal terraces have some complicated relations with the seismic crustal deformations in this region.In the Shonan district, the deformation of the Holocene terraces is attributed to the crustal deformation associated with the 1923 Taisho type earthquakes and the recurrence time T of 180-400 years and the value d of 0.25-0.50 are obtained. In the southern Boso peninsula, the deformation of the Holocene terraces is divided into two: the crustal deformation associated with 1923 type earthquakes and that caused by the fault off the Boso peninsula which moved at the time of the 1703 Genroku earthquake. The recurrence time T and the value d for this fault off the Boso peninsula are estimated to be 950-2500 years and 0.20-0.54, respectively.

著者

松浦 充宏

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.44, no.Supplement, pp.53-62, 1991-07-24 (Released:2010-03-11)

参考文献数

36

被引用文献数

4

---

Geophysical models are generally estimated from observed data by using some criterion. In most cases observed data are inaccurate, insufficient and inconsistent. To deal with such data we must introduce a new criterion in place of the classical least-squares criterion. The history of the development of inversion theory in geophysics is the history of quest for the new criterion. From such a standpoint we review the development of inversion theory in the last score.In a series of papers published in 1967, 1968 and 1970, G. Backus and F. Gilbert introduced the concept of model resolution and formulated the geophysical inverse problem as the problem of compromising reciprocal requirements for model resolution and estimation errors in some natural way. This was the starting point of the fruitful development of inversion theory in geophysics. Various criteria to compromise model resolution and estimation errors have been proposed in the early part of the 1970s, but the objectivity of these criteria were not clear. At the end of the 1970s D. D. Jackson introduced the concept of prior information about unknown model parameters and combine two reciprocal criteria for model resolution and estimation erros in a natural way. Jackson's approach for linear inversion, which is based on the minimum variance criterion, has been soon exteded to nonlinear cases by A. Tarantola and B. Valette and also by D. D. Jackson and M. Matsu'ura on the basis of probability theory. The geophysical data inversion is now understood as the process of extracting new information from observed data, combining it with prior information about model parameters, and constructing the more clear image of a physical model.

著者

三雲 健 大塚 道男 尾池 和夫

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.23, no.3, pp.213-225, 1970-09-28 (Released:2010-03-11)

参考文献数

17

被引用文献数

1

---

The focal mechanism of 20 microearthquakes in the Wakayama region have been determined from the seismograms recorded at ten temporary stations and eight routine network stations covering the region.The hypocenters of the earthquakes have been determined for five different crustal models (with a continuously varied velocity profile), to estimate the accuracy of their location and the emergent angle of seismic rays at the focus. The radiation pattern of P wave first motions for the 15 shocks, which was corrected for an appropriate crustal structure, can be interpreted by the double-couple type mechanism with dipping nodal planes.The average of the maximum pressure axes is oriented nearly horizontally along the N70°W-S70°E direction, while the axes of the maximum tension show a steep dip in many cases. If a slip dislocation is assumed to be a likely model of the earthquakes, the source would be thrust faulting with a predominant component of dip slip. The other five earthquakes shows, however, the radiation pattern inconsistent with the double-couple type mechanism.

著者

石川 有三 尾池 和夫

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.35, no.2, pp.171-181, 1982-06-25 (Released:2010-03-11)

参考文献数

31

被引用文献数

3

---

Ten reservoir induced earthquakes were investigated. The relation between the dam height and the maximum magnitude of the induced earthquake was found. We show that there are the magnitude limit of earthquake related with the dam height. The Shenwo earthquake, this was only one exception of this relation, wasn't far from the 1975 Haicheng earthquake. It occurred only 43 days before the Haicheng earthquake. We think the Shenwo earthquake occurred in the precursory period of the Haicheng earthquake. So we concluded that it occurred under the special tectonic condition just before the large earthquake.Fault plane solutions for reservoir induced earthquakes were also compared with those for natural ones. We got the result that the directions of P-axis and T-axis for induced earthquakes were not always same to those for natural ones. So, we concluded that induced earthquakes occurred in the faults which were weakened by the permeated water rather than in the faults which were easily dislocated by the tectonic force. Large aftershocks of the Xinfengjiang earthquake were investigated and we showed the examples of the shock induced by loading and unloading.The b-values for the aftershocks of induced earthquakes were calculated, but we got only one b-value for natural one which could be compared to induced one and there was no significant difference between them.

著者

山田 功夫 深尾 良夫 石原 靖 青木 治三

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.42, no.1, pp.21-31, 1989-03-25 (Released:2010-03-11)

参考文献数

9

被引用文献数

3

---

Since late 1986, we have been operating a three-component set of STS seismometers (STS-1) at Inuyama Observatory, Nagoya University, Japan. This is a preliminary report of the two years observation. The STS-1 seismometer equips with three component BRB (BRoadband Band) outputs and three component LP (Long Period) outputs. The lowpass filtered LP outputs are digitized and recorded continuously at a sampling interval of 10s in a dynamic range of about 120dB. These continuous data have been used so far to determine seismic moment or centroid moment tensor of large Japanese earthquakes. The BRB output is recorded on an event recordering system, where the BRB and LP outputs after an eight-pole analog anti-aliased filtering are digitized and recorded for 50 minutes at sampling intervals of 0.1 and 0.4s, respectively. The dynamic range of this system is limited at present by the A/D converter to about 90dB. The triggered BRB data have been used so far to determine the source time functions of large Japanese earthquakes, which suggest the occurrence of backswing of fault motion. To maintain the horizontal component instruments in a stable condition, a care must be taken for variation of atmospheric pressure. The results of the analyses indicate a unique importance of nearby observation of large earthquakes by broadband, wide dynamic range and high precision seismometers such as the STS instruments.

著者

池田 隆司 笠原 敬司 伊藤 健治 多田 堯

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.37, no.4, pp.549-557, 1984-12-25 (Released:2010-03-11)

参考文献数

4

---

In order to obtain information on the underground structure around the Karasuyama-Sugaonuma fault, we carried out a temporary observation for artificial explosions which were blasted at four sites in Saitama and Ibaraki Prefectures in March 1982. Crossing the southern part of the fault, seven seismometers were deployed on an east-west striking observation line 30km long. For three of the explosions, good records were obtained at all the stations. Travel time analyses provided three layers; apparent p-wave velocities in the layers are 1.7km/sec, 2.8-3.1km/sec and 6.1km/sec from top to bottom. Travel times for the 6.1km/sec layer jump near Sashima-cho, Ibaraki Prefecture, where the western branch of the fault is supposed to lie. For the wave propagating through the topmost layer between the eastern and western sides of the north-south striking fault, a systematic difference in the wave form was observed. Those observed features strongly suggest that not only the basement layer (VP=6.1km/sec) but also upper layers are displaced or fractured by the Karasuyama-Sugaonuma fault.

著者

都司 嘉宣

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.34, no.3, pp.401-411, 1981-10-25 (Released:2010-03-11)

参考文献数

18

---

In the early morning of Dec. 31, 1703 (Genroku 16), an enormous earthquake with magnitude of 8.2 occurred in the south sea region of the Kanto District. It is well known that after the occurrence of the earthquake a huge tsunmi was generated, and that the coasts of the Boso Peninsula, the north coasts of Sagami Bay, and the east coasts of the Izu Peninsula were seriously damaged.Recently several old documents of the Genroku tsunami were also discovered on the coasts of the Tokai district, the Kii peninsula, and the Shikoku Island. In two towns, Nishina and Toi, on the west coast of the Izu Peninsula, inundation height was estimated 3 meters, and the residential areas of these towns were slightly submerged. At Miho village in Shimizu city on the west coast of Suruga Bay, residential areas were intermittently submerged for more than ten days, and the people took refuge in higher places. On the mouth district of the Lake Hamana, seaside banks were eroded, and the mouth of the lake became broader. Thirty-three large junks out of 36 anchored at the open sea of Arai town near the lake, were wrecked. In Ono town on the Chita Peninsula inside the Bay of Ise, a garden of Naiku shrine was washed and eroded, where inundation height was about 2 meters. Tsunami was also noticed at the port of Nagoya. Miwasaki and Taiji towns and Haida village on the south east coast of the Kii peninsula were seriously damaged in spite of the long distance from the tsunami origin. In these places 46 houses were washed away in total and the observed tsunami climbed up to 3 or 5 meters. Tidal irregulaity was also noticed in several ports of Kochi Prefecture on the Shikoku Island.

著者

檜垣 強

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.26, no.1, pp.93-100, 1973-06-30 (Released:2010-03-11)

参考文献数

4

---

A pulse-like minute oscillation happened to be recorded simultaneously at the four temporary seismic stations installed 2.5-9.0km apart each other, near Kumamoto city. The oscillation lasted only half a second which appears to be too short against the common understanding of earthquake phenomena. Amplitude is incredibly small as well.However, there are a few evidences to support that it is an “earthquake.” It is followed by an earthquake of moderate size, the observed time intervals between two events being about 18 seconds at every stations. The observed directions of initial P motion are reversed between these two events at every stations suggesting that they are physically connected. The hypocenter of this micro-earthquake can be accurately located by seismic technique.The provisional estimation of its magnitude has shown that it is less than -3.