We have measured the shift and width of the kaonic hydrogen 1s state due to the KN strong interaction. We have observed, for the first time, distinct K-series kaonic hydrogen x rays with good signal-to-noise ratio in the energy spectrum. The measured energy shift and width were determined to be DE͑1s͒ 2323 6 63͑stat͒ 6 11͑syst͒ eV (repulsive) and G͑1s͒ 407 6 208͑stat͒ 6 100͑syst͒ eV, respectively. [S0031-9007(97)02992-X] PACS numbers: 13.75. Jz, 25.80.Nv, 29.30.Kv, 36.10.Gv The determination of the strong-interaction energy level shift and width of the kaonic hydrogen x rays is one of the most important subjects for the understanding of the KN interaction. It is strongly affected by the presence of the L͑1405͒ subthreshold resonance. The study of the KN interaction is also relevant to the important question of K 2 condensation in dense matter [1,2].The observation of the shift and width of the kaonic hydrogen K a ͑2p ! 1s͒ x rays gives direct information about the KN s-wave interaction at the K 2 p threshold energy in a fairly model independent way [3]. The status of the study was quite puzzling due to the contradiction between the signs of the scattering lengths obtained by the previous x-ray measurements [4-6] and those extracted from the analyses of the low energy KN data, e.g., , as shown in Fig. 1. This contradiction is known to be almost impossible to reconcile within the conventional theoretical framework. Moreover, the x-ray signals of the previous experiments are very difficult to identify in their spectra. Therefore, a definitive experiment has been long awaited.We accumulated data for 760 hours at KEK-PS K3. A detailed description of our experimental setup is given in a separate paper [10]. Here we present a short summary.Optimization of the target density is quite important for this experiment. As a compromise between kaon stopping yield and kaon loss during the atomic cascade due to the Stark effect, we chose to operate the hydrogen FIG. 1. The energy shift and width of 1s state. One-standarddeviation region of shift and width of the previous experiments are plotted together with theoretical calculations. The present result is shown in bold.

Detection of shallow slow earthquakes offers insight into the near-trench part of the subduction interface, an important region in the development of great earthquake ruptures and tsunami generation. Ocean-bottom monitoring of offshore seismicity off southern Kyushu, Japan, recorded a complete episode of low-frequency tremor, lasting for 1 month, that was associated with very-low-frequency earthquake (VLFE) activity in the shallow plate interface. The shallow tremor episode exhibited two migration modes reminiscent of deep tremor down-dip of the seismogenic zone in some other subduction zones: a large-scale slower propagation mode and a rapid reversal mode. These similarities in migration properties and the association with VLFEs strongly suggest that both the shallow and deep tremor and VLFE may be triggered by the migration of episodic slow slip events.

We investigated the spatial variation in the stress fields of Kyushu Island, southwestern Japan. Kyushu Island is characterized by active volcanoes (Aso, Unzen, Kirishima, and Sakurajima) and a shear zone (western extension of the median tectonic line). Shallow earthquakes frequently occur not only along active faults but also in the central region of the island, which is characterized by active volcanoes. We evaluated the focal mechanisms of the shallow earthquakes on Kyushu Island to determine the relative deviatoric stress field. Generally, the stress field was estimated by using the method proposed by Hardebeck and Michael (2006) for the strike-slip regime in this area. The minimum principal compression stress (σ3), with its near north-south trend, is dominant throughout the entire region. However, the σ 3 axes around the shear zone are rotated normal to the zone. This result is indicative of shear stress reduction at the zone and is consistent with the right-lateral fault behavior along the zone detected by a strain-rate field analysis with global positioning system data. Conversely, the stress field of the normal fault is dominant in the Beppu-Shimabara area, which is located in the central part of the island. This result and the direction of σ3 are consistent with the formation of a graben structure in the area.

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