Seismic radiation by magma injection: An anomalous seismic event near Tori Shima, Japan

The earthquake with a bodywave magnitude m_b=5.5, which occurred near Tori Shima, Japan, on June 13, 1984 (origin time: 0229:25.3 UT, 31.448°N, 140.036°E, depth of 10 km, m_b =5.5, M_S=5.5) is anomalous because it generated tsunamis which are disproportionately large for the magnitude of the earthquake. At Hachijo Island, 150 km from the epicenter, tsunamis were visually observed with peak-to-peak amplitude of 130 to 150 cm. Long-period seismic radiation is also anomalous. Love waves are almost absent, and Rayleigh waves are radiated with equal amplitude and phase in all directions. A simple double-couple model cannot explain these observations. With the assumption of no net volume change at the source, these data can be best explained with a compensated linear vector dipole (CLVD) with the principal tensional dipole in the vertical direction. The scalar moment of this dipole is 4×10^(24) dyn cm. Moment tensor inversions of long-period body waves and surface waves yield an almost identical solution. This CLVD source can be interpreted as horizontal fluid injection. The location of the event is in the Smith depression which is one of the nascent back arc basins just behind the Bonin arc. These basins are filled with thick sediments, and numerous young volcanoes are found near this site. Magmatic injection is most likely to occur in this tectonic environment. However, the time scale of the seismic event seems too short for magma injection to occur. A more likely mechanism involves water-magma interaction. The injection may be viewed as hydrofracturing driven by supercritical water heated by injected magma. The estimated volume of injected water is about 0.018 km^3 and that of basaltic magma is about 10% of this. This type of deformation is more efficient for tsunami generation than faulting with the same scalar moment

Tsunami generation of the 1993 Hokkaido Nansei-Oki earthquake

Heterogeneous fault motion of the 1993 Hokkaido Nansei-Oki earthquake is studied by using seismic, geodetic and tsunami data, and the tsunami generation from the fault model is examined. Seismological analyses indicate that the focal mechanism of the first 10 s, when about a third of the total moment was released, is different from the overall focal mechanism. A joint inversion of geodetic data on Okushiri Island and the tide gauge records in Japan and Korea indicates that the largest slip, about 6 m, occurred in a small area just south of the epicenter. This corresponds to the initial rupture on a fault plane dipping shallowly to the west. The slip on the northernmost subfault, which is dipping to the east, is about 2 m, while the slips on the southern subfaults, which are steeply dipping to the west, are more than 3 m. Tsunami heights around Okushiri Island are calculated from the heterogeneous fault model using different grid sizes. Computation on the smaller grids produces large tsunami height that are closer to the observed tsunami runup heights. Tsunami propagation in the nearly closed Japan Sea is examined as the free oscillation of the Japan Sea. The excitation of the free oscillation by this earthquake is smaller than that by the 1964 Niigata or 1983 Japan Sea earthquake.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43222/1/24_2004_Article_BF00874395.pd

Rupture process of large earthquakes in the northern Mexico subduction zone

The Cocos plate subducts beneath North America at the Mexico trench. The northernmost segment of this trench, between the Orozco and Rivera fracture zones, has ruptured in a sequence of five large earthquakes from 1973 to 1985; the Jan. 30, 1973 Colima event ( M s 7.5) at the northern end of the segment near Rivera fracture zone; the Mar. 14, 1979 Petatlan event ( M s 7.6) at the southern end of the segment on the Orozco fracture zone; the Oct. 25, 1981 Playa Azul event ( M s 7.3) in the middle of the Michoacan “gap”; the Sept. 19, 1985 Michoacan mainshock ( M s 8.1); and the Sept. 21, 1985 Michoacan aftershock ( M s 7.6) that reruptured part of the Petatlan zone. Body wave inversion for the rupture process of these earthquakes finds the best: earthquake depth; focal mechanism; overall source time function; and seismic moment, for each earthquake. In addition, we have determined spatial concentrations of seismic moment release for the Colima earthquake, and the Michoacan mainshock and aftershock. These spatial concentrations of slip are interpreted as asperities; and the resultant asperity distribution for Mexico is compared to other subduction zones. The body wave inversion technique also determines the Moment Tensor Rate Functions ; but there is no evidence for statistically significant changes in the moment tensor during rupture for any of the five earthquakes. An appendix describes the Moment Tensor Rate Functions methodology in detail.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43169/1/24_2004_Article_BF00875970.pd

Combination treatment of parotid salivary gland cancer

Background: Cancer of the parotid salivary gland (PSG) accounts for more than a half of all cases of salivary gland malignancies. Its treatment strategy remains a matter of debate.Aim: To identify factors significantly associated with the 3-year survival of patients with PSG cancer after combination treatment.Materials and methods: Thirty nine patients with morphologically confirmed PSG cancer (T1-4N0) were recruited into the study. Surgery (partial or total parotidectomy without the cervical lymphatic node dissection) was performed in 32 patients. The most frequent diagnosis (n = 10; 25.6% of all cases) was adenocarcinoma. Radiation to PSG and the regional lymphatic nodes was used in 15 patients as neoadjuvant and in 24 as adjuvant regimen. Three-year survival rates were analyzed in 36 patients by the Kaplan-Meier method, with consideration of their sex, age and the sequence of surgical and radiation treatment.Results: The 3-year survival after combination treatment of PSG cancer patients was 82.7%. Women, patients above 60 years of age and those who received adjuvant radiation therapy demonstrated a trend towards better 3-year survival, although it was non-significant (p > 0.05).Conclusion: Combination strategy remains an effective approach to PSG cancer irrespective of age and sex of patients. The use of radiation therapy as a single modality is possible only in exceptional cases