Crustal Earthquakes in Kyushu Toshiki KAKUTA, Hiroki MIYAMACHI, Hiroshi KUBOTA and Akio TAKAGI Department of Earth Sciences, Faculty of Science, Kagoshima University, Korimoto 1-21-35, Kagoshima 890, Japan (Received April 25, 1992; Accepted July 24, 1992) A notable WNW-ESE alignment of crustal earthquakes, which have been active since the latter half of 1989, was observed along a narrow zone from Amakusa-nada through Izumi City to Kakuto area; it nearly corresponds to a discontinuity presumed in the subducting plate. Focal mechanisms of some events suggested left lateral movements with T-axes of north or northwest trends. Though not comparable in length with the above, alignments of similar trends are also inferred in Central Kyushu, where seismic activities are mainly characterized by NE-SW tectonic lines such as Beppu-Shimabara and Amakusa-nada grabens and Futagawa-Hinagu active fault system. In a rather small area west off Satsuma peninsula, South Kyushu, crustal earthquakes also swarmed. The seismic zone was composed of two narrow planes steeply dipping and intersecting with each other. The events along the WNW-ESE alignment mentioned above also swarmed especially at the places where the alignment crosses Amakusa-nada graben, NE-SW fault systems in the northwest of Kagoshima prefecture and a graben-like structure trending NE-SW at Kakuto area. Generally speaking, high seismic activity is observed in an area where tectonic lines or alignments of different trends intersect. Sporadic shallow earthquakes were noticed along "Nobeoka" tectonic line geologically inactive. Key words: Crustal earthquake, Alignment of seismic events, Active fault system, Tectonic structures intersecting with different trends.
Fig. 1. Observation stations and geological structures in and around Kyushu. The squares refer to KGU stations and the circles to FMO stations. BSG, Beppu-Shimabara graben; AMG, Amakusa-nada graben; UYL, Usuki-Yatsushiro tectonic line; BTL, Butsuzo tectonic line; NTL, Nobeoka tectonic line; Hfs, Futagawa- Hinagu fault system; Ifs, Izumi fault system; Nfs, Nishiyama fault syatem; If, Ichino fault; Mf, Mekuradake fault; Sf, Sendai river fault; Ff, Futagawa fault; As, Aso caldera; Kt, Kakuto caldera; Ai, Aira caldera; At, Ata caldera; Ki, Kikai caldera; 1, Tsurumi-lake; 2, Kuju-zan; 3, Aso-san; 4, Kirishima-yams; 5, Sakurajima; 6, Kaimon-dake; 7, Satsuma-Iwojima; 8, Kuchinoerabu-jima; 9, Unzen-dake.
Fig. 2. Relocated epicenters of crustal earthquakes plotted on the figure of "summit level and submarine topography in and around Kyushu" by the Research Group for Active Tectonics in Kyushu (1989). The shaded area corresponds to a high land surrounded with a localized summit level.
Ho<35<60<m<100<e<150<x<300 M23-456-7S Fig. 3. Relocated epicenters of shallow earthquakes plotted on the geological structures in and around Kyushu. Active volcanoes referring to closed triangles are the same as those in Fig. 1.
Fig. 4. Crustal earthquakes occurring in a narrow zone of WNW-ESE trend from Amakusa-nada through Izumi City to Kakuto area: epicenters (upper) and a vertical section projected on a line parallel to the trend (lower).
Fig. 5. A possible mechanism solution composed for the events occurring in a northwestern part of Izumi City on July 24, 1990. The closed and open circles refer to P initial motions of compression and dilatation, respectively: equal area projection on the lower hemisphere. Fig. 6. Relocated epicenters for the earthquake swarm west off Satsuma peninsula. The cross refers to the event of M=5.5 on August 15, 1984. Note that the seismic zone is composed of two narrow planes intersecting with each other.
Fig. 7. Vertical sections for the events in the planes trending E-W (upper) and NNE-SSW (lower) shown in Fig. 6. Fig. 9. Frequenciy distributions of S-P times: yearly numbers at KOSK (left) and monthly numbers at AKNE in 1990 (right). Take note of a rather long time of no observation due to a system trouble at KOSK in 1990. (A) February 18 1990 (B) August 15 1984 Fig. 8. Possible mechanism solutions for the event of M=4.0 occurring in the E-W plane on February 18, 1990 (a) and that of M=5.5 likely occurring in the NNE-SSW plane on August 15, 1984 (b). See caption for Fig. 5.