γ線スペクトロメトリにより地下水に含まれるラドン濃度を求めるための計数効率について検討した。前回の報告ではU8容器に密封された標準γ線体積線源と，細口テフロン容器に詰めた密度1.3 g/cm³の花崗岩粉末を比較測定して後者の²¹⁴Pb 352 keVにおける計数効率を決定した。今回は花崗岩粉末と活性炭粉末から密度1.0 g/cm³の媒質を調整し，得られた計数効率は前回とよく一致した。この方法を用いて花崗岩の少ない地層である広島県備北地域の17カ所とこれまで測定データの少ない瀬戸内海島嶼部の21カ所を中心に地下水のラドン濃度を測定した結果を報告する。

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Seismic activity in Hiroshima and Shimane prefectures and surrounding areas was investigated on the basis of the observational data obtained by the Shiraki Microearthquake Observatory and its substations.
Since May 1968, locations of microearthquakes of magnitude down to 1.0 have been capable from data obtained by the Shiraki observation net only. Data from other sources, for example JMA stations and other microearthquake stations, are used if necessary and available.
About 1100 epicenters were determined during the period from May, 1968, to June, 1970. Epicentral distribution thus obtained shows features as follows: a seismic zone parallel to the coast line of the Sea of Japan in the direction NE-SW, a seismic zone perpendicular to the former one in the east of Hiroshima prefecture, several aseismic regions, one of which is adjacent to the most activee seismic region with a sharp boundary.
In the spring of 1970, there occurred a very active seismic swarm near Kutsugahara station. The rise and fall of the activity were observed and investigated in detail. Some 2500 earthquakes of magnitude down to 0.0 were recorded, of which about 260 larger shocks were located.
The maximum earthquake in this sequence was of magnitude 4.6 and after that a great many earthquakes including many felt shocks were observed. The activity declined slowly with several recurrences including two earthquakes of magnitude 4.3. On the contrary, microearthquake activity was not higher than usual immediately before the maximum earthquake.
Epicenters in this sequence are concentrated in a region 3km in width and 7km in length. Focal depths were about 15km at first and became gradually shallow to a few kilometers for following ten days.
Thirteen days before the maximum earthquake, there occurred a shock of magnitude 2.6 followed by many aftershocks. This aftershock activity was reduced to the normal level for twenty or thirty hours. Epicenters in this activity were located a few kilometers northwest to the epicentral area of the activity thirteen days later. The former activity may be regarded as foreshocks in a broad sense of the word.
Moreover several seismic activities were observed and most of them are of swarm type, that is, regarded as superpositions of swarms and/or foreshock-mainshock-aftershock sequences.

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