Methane in Izena Cauldron,Okinawa Trough

Methane in the deep water of Izena Cauldron (maximum depth: ca. 1650 m) at the east side of mid-Okinawa Trough was studied by casting a CTD system with 12 Niskin bottles for water sampling at 11 stations inside and outside the cauldron. The water contained much methane up to 706 nmoles/l. The depths of maximum concentration varied widely from station to station, indicating the existence of a considerable number of vents emitting methane and heat. The waters containing less methane formed a straight line in theT-S diagram, while those containing more methane were more largely deviated from the line. The temperature anomaly was virtually proportional to the methane concentration, suggesting that the oxidation rate of methane inside the cauldron is negligibly small and methane can be used as a tracer of the cauldron water. The relation and the estimated vertical diffusivity gave the following fluxes. The emissions of methane and heat out of the bottom below 1450 m turn out to be 1400 moles/day and 7×10¹⁰ cal/day, respectively. The total emission rates inside the cauldron are presumed to be about twice the above values. The turnover time of methane has been estimated to be 240 days, which is also that of heat generated from the bottom and probably that of the bottom water.     

Seismological evidence for a lithospheric normal faulting — the Sanriku earthquake of 1933

The focal process of the Sanriku earthquake of March 2, 1933, is discussed in relation to the bending mechanism of the lithosphere. On the basis of the P times obtained at more than 200 stations, it is confirmed that the hypocenter of this earthquake is within the lithosphere beneath the Japan trench. The P wave fault plane solution, the amplitude of long-period (100 s) Love and Rayleigh waves and two near-field observations suggest, almost definitely, that the Sanriku earthquake represents a predominantly normal faulting on a plane dipping 45° towards N 90° W. A fault size of 185 × 100 km², in agreement with the size of the aftershock area, is required to yield a slip dislocation of 3.3 m, a value consistent with the tsunami data. This result suggests that the fracture took place over the entire thickness of the lithosphere, thereby precluding the possibility that the Sanriku earthquake merely represents a surface tensile crack due to the bending of the lithosphere. This large scale lithospheric faulting is presumably due to a gravitational pull exerted by the cold sinking lithosphere. The fracture probably took place on an old fault plane which had once fractured and healed up. The existence of this fracture zone which decouples, to some extent, the oceanic lithosphere from the sinking lithosphere accounts for the sharp bend of the lithosphere beneath oceanic trenches and also the abrupt disappearance of seismic activity across oceanic trenches. The sharp bend of the lithosphere is therefore a result, not the cause, of great earthquakes beneath oceanic trenches.     

Magnitudes of great shallow earthquakes from 1953 to 1977

The surface-wavemagnitudes M_s are determined for 30 great shallow earthquakes that occurred during the period from 1953 to 1977. The determination is based on the amplitude and period data from all available station bulletins, and the same procedure as that employed in Gutenberg and Richter's “Seismicity of the Earth” is used. During this period, the Chilean earthquake of 1960 has the largest M_s, 8.5. The surface-wave magnitudes listed in “Earthquake Data Reports” are found to be higher than M_s on the average. By using the same method as that used by Gutenberg, the broad-band body-wave magnitudes m_B are determined for great shallow shocks for the period from 1953 to 1974. m_B is based on the amplitudes of P, PP and S waves which are measured on broadband instruments at periods of about 4–20 s. The 1-s body-wave magnitudes listed in “Bulletin of International Seismological Center” and “Earthquake Data Reports” are found to be much smaller than m_B on the average. Through the examination of Gutenberg and Richter's original worksheets, the relation between m_B and M_sis revised to m_B = 0.65 M_s+ 2.5 which well satisfies the mg and M_sdata for M_sbetween 5.2 and     

The rupture process and asperity distribution of three great earthquakes from long-period diffracted P-waves

Maximum earthquake size varies considerably amongst the subduction zones. This has been interpreted as a variation in the seismic coupling, which is presumably related to the mechanical conditions of the fault zone. The rupture process of a great earthquake indicates the distribution of strong (asperities) and weak regions of the fault. The rupture process of three great earthquakes (1963 Kurile Islands, M_W = 8.5; 1965 Rat Islands, M_W = 8.7; 1964 Alaska, M_W = 9.2) are studied by using WWSSN stations in the core shadow zone. Diffraction around the core attenuates the P-wave amplitudes such that on-scale long-period P-waves are recorded. There are striking differences between the seismograms of the great earthquakes; the Alaskan earthquake has the largest amplitude and a very long-period nature, while the Kurile Islands earthquake appears to be a sequence of magnitude 7.5 events.The source time functions are deconvolved from the observed records. The Kurile Islands rupture process is characterized by the breaking of asperities with a length scale of 40–60 km, and for the Alaskan earthquake the dominant length scale in the epicentral region is 140–200 km. The variation of length scale and M_W suggests that larger asperities cause larger earthquakes. The source time function of the 1979 Colombia earthquake (M_W = 8.3) is also deconvolved. This earthquake is characterized by a single asperity of length scale 100–120 km, which is consistent with the above pattern, as the Colombia subduction zone was previously ruptured by a great (M_W = 8.8) earthquake in 1906.The main result is that maximum earthquake size is related to the asperity distribution on the fault. The subduction zones with the largest earthquakes have very large asperities (e.g. the Alaskan earthquake), while the zones with the smaller great earthquakes (e.g. Kurile Islands) have smaller scattered asperities.     

Earthquake doublets in the Solomon Islands

Large, shallow, thrust earthquakes in the Solomon Islands region tend to occur in closely related pairs. Two recent sequences are July 14, 1971 (M_S = 7.9) and July 26, 1971 M(_S = 7.9) and 14^h37^m, July 20, 1975 (M_S = 7.9) and 19^h54^m, July 20, 1975 (M_S = 7.7). The mechanism of these seismic doublets has important bearing on the triggering mechanism of earthquakes in subduction zones. Detailed analysis of the seismic body waves and surface waves were performed on the 1971, 1974, and 1975 doublets, providing a better understanding of: (1) the mechanics of seismic triggering, (2) the state of stress on the fault plane, and (3) the nature of subduction between the Pacific and Indian plates. The results indicate that although the geometry of the subduction zone in the Solomon Islands is complicated by the presence of several sub-plates, the slip direction of the Indian plate with respect to the Pacific plate is relatively uniform over the entire region. The large seismic moments of the 1971 sequence (1.2 · 10²⁸ and 1.8 · 10²⁸ dyne cm) indicate that these events directly represent the underthrusting of the Indian and Solomon plates beneath the Pacific plate. The body waves from these doublets, recorded on the WWSSN long-period seismograms, are remarkably impulsive and simple compared with those from events of comparable seismic moment in other subduction zones. In addition, the source dimensions of the body waves are 30–70 km in length, substantially smaller than the overall rupture surfaces radiating the surface waves which are 100–300 km in length. These facts suggest the existence of relatively large, isolated high-stress zones on the fault plane. This type of stress distribution is distinct from other regions which have more heterogeneous stress distribution on the fault plane, and this is proposed as the principal characteristic of this region responsible for the occurrence of the doublets and for the apparent efficiency of triggering in the Solomon trench. Prior to the 1971 sequence, similar sequences have occurred in the same area in 1919–1920 and 1945–1946. From the amount of slip (1.3 m) determined for the 1971 sequence and the apparent recurrence interval of 25 years, a seismic slip rate of 5 cm yr^?1 is determined. This value is a significant portion of the convergence rate between the Indian and Pacific plates indicating that the plate motion here is taken up largely by seismic slip.     

A preliminary stable isotope study of volcanic ashes discharged by the 1979 eruption of Ontake Volcano,Nagano, Japan

Sulphur isotopic compositions of pyrite, anhydrite and native sulphur in volcanic ashes discharged by the 1979 eruption of Ontake volcano, Nagano, Japan were determined. The isotopic data indicate that sulphate in anhydrite and a part of native sulphur were produced by the disproportionation reaction of sulphite formed by dissolution of SO₂ in volcanic gases into water which filled a mud reservoir probably located just below the crater zone. Some part of H₂S in volcanic gases was fixed as pyrite and some was oxidised to form native sulphur. Hydrothermal alteration of country rocks to form pyrite, anhydrite and clay minerals had proceeded in the mud reservoir before eruption at temperatures ranging from 110° to 185°C which were estimated by oxygen isotopic fractionation between anhydrite and water.

     

Focal process of the great Chilean earthquake May 22, 1960

Long-period strain seismogram recorded at Pasadena is used to determine the focal process of the 1960 Chilean earthquake. Synthetic seismograms computed for various fault models are matched with the observed strain seismogram to determine the fault parameters. A low-angle (～ 10°) thrust model with rupture length of 800 km and rupture velocity of 3.5 km/sec is consistent with the observed Rayleigh/Love wave ratio and the radiation asymmetry. A seismic moment of 2.7 · 10³⁰ dyn · cm is obtained for the main shock. This value, together with the estimated fault area of 1.6 · 10⁵ km², gives an average dislocation of 24 m. The strain seismogram clearly shows unusually long-period (300–600 sec) wave arriving at the P time of a large foreshock which occurred about 15 minutes before the main shock, suggesting a large slow deformation in the epicentral area prior to the major failure. A simple dislocation model shows that a dislocation of 30 m, having a time constant of 300–600 sec, over a fault plane of 800 × 200 km² is required to explain this precursory displacement. The entire focal process may be envisaged in terms of a large-scale deformation which started rather gradually and eventually triggered the foreshocks and the “main” shock. This mechanism may explain the large premonitory deformations documented, but not recorded instrumentally, for several Japanese earthquakes. The moments of the main shock and the precursor add to 6 · 10³⁰ dyn · cm which is large enough to affect the earth's polar motion.     

Petrochemistry of Late Miocene to Quaternary Igneous Rocks and Metallogenesis in Southwest Hokkaido, Japan

Abstract: Southwest Hokkaido is largely covered by Late Miocene to Quaternary igneous rocks, and has a large number of gold veins and base-metal veins of the same age. Investigation of the silica-normalized concentration of elements has revealed regional petrochemical zoning; large ion lithophile elements (LILE) and K₂O/(Na₂O+K₂O) of the rocks increase toward Japan Sea, whereas total FeO, CaO, and ⁸⁷Sr/⁸⁶Sr decrease. Mapped concentration isoplethes of these elements are not ideally parallel to the volcanic front, but protrude to the west at Funka Bay, and to the northwest at Matsumae Peninsula. Isoplethes of ⁸⁷Sr/⁸⁶Sr show similar patterns and two more northwestward protrusions in the northeast (Jozankei block) of southwest Hokkaido. Contrary to the general petrochemical trend, both high– and low-LILE volcanic rocks occur in the Jozankei block. The ore deposits are distributed in four metallogenic zones; manganese–base–metal zone on the Japan Sea side, pyrite-limonite zone mainly along the volcanic front, gold zone in the middle, and two units of gold–base–metal zone. The northern unit of this zone is in the Jozankei block, and seems a part of the gold zone overlapped by the manganese–base–metal zone. Thus, as a rule, pyrite–limonite, gold, and base-metal deposits accompany low–, intermediate–, and high-LILE igneous rocks, respectively. Individual deposits and volcanic rocks make chains oblique to the zones and the volcanic front. The majority of the ore deposits are distributed along ridges of Bouguer anomalies overlapped by the volcanic chains, which apparently control the patterns of the petrochemical isoplethes. This is typical for two volcanic chains to the north and south of Funka Bay, where the petrochemical isoplethes protrude to the west. This indicates that both the igneous activity and the mineralization have been under the control of tectonic fractures at the roots of the volcanic chains. The geological, petrochemical and metallogenic data support the idea that the chemical characteristics of the deposits are correlated mainly with the chemistry of the associated magmas, and partly with that of the host rocks.