Global survey of aftershock area expansion patterns

We developed an objective method to define the aftershock areas of large earthquakes as a function of time after the main shock. The definition is based upon the amount of energy released by aftershocks, the spatial distribution of the energy release is first determined and is contoured. The 1-day aftershock area is defined by a contour line corresponding to the energy release level of 10^15.6 ergs/(100 km² · day). The 10-day, 100-day and 1-y aftershock areas are similarly defined by contour lines corresponding to 10^14.8, 10^14.0, and 10^13.5 ergs/(100 km² · day), respectively. We also define the expansion ratios at time t by the ratio of the aftershock area at t to that at 1 day.Using this method we study the aftershock area expansion patterns of 44 large (M_s ? 7.5) and five moderate shallow earthquakes which occurred from 1963 to 1980. Each aftershock sequence is examined at four different times, i.e., 1 day, 10 days, 100 days, and 1 y after the main event. We define the aftershock area expansion ratios η and η_e by $\text{S(100)/S(}\text{1}\text{)}\text{and}\text{L(100)/L(1)}$, respectively: here S(t) and L(t) are the area and the length of the aftershock area, respectively, at time t. Our study suggests that a distinct regional variation of aftershock area expansion patterns is present; it is strongly correlated with the tectonic environment. In general, the subduction zones of the “Mariana” type have large expansion ratios, and those of the “Chilean” type have small expansion ratios. Some earthquakes that occurred in the areas of complex bathymetry such as aseismic ridges tend to have large expansion ratios.These results can be explained in terms of an asperity model of fault zones in which a fault plane is represented by a distribution of strong spots, called the asperities, and weak zones surrounding the asperities. The rupture immediately after the main shock mostly involves asperities. After the main rupture is completed, the stress change caused by the main shock gradually propagates outward into the surrounding weak zones. This stress propagation manifests itself as expansion of aftershock activity. In this simple picture, if the fault zone is represented by relatively large asperities separated by small weak zones (“Chilean” type), then little expansion of aftershock activity would be expected. On the other hand, if relatively small asperities are sparsely distributed (“Mariana” type), significant expansion occurs. The actual distribution of asperities is likely to be more complex than the two cases described above. However, we would expect that the expansion ratio is in general proportional to the spatial ratio of the total asperity area to the fault area.     

Horizontal strain and precision of EDM from repeated surveys of the Precise Geodetic Net of Japan

A simple statistical approach has been applied to the repeated electro-optical distance measurements (EDM) of 1,358 lines in the Tohoku district of Japan to obtain knowledge about the precision of EDM and the possible accumulation of strain. The average time interval between measurements is about seven or eight years. It is shown that the whole data of the difference between distance measurements repeated over a given lineD are interpreted in terms of EDM errors comprising distance proportional systematic errors and standard errors expressed by the usual form . The rate of horizontal deformation must therefore be much smaller than the strain rates of about 0.7 0.8 ppm over 7 to 8 years which have been hitherto expected.     

Carbon and Oxygen Isotopic Compositions of Carbonates from Precambrian Apatite-Bearing Carbonate Rocks of the Aldan Shield

Geostructural setting, as well as mineral and isotopic compositions, of separate apatite deposits and occurrences in the Aldan Shield composed of Precambrian metasedimentary apatite–carbonate rocks are considered. In terms of carbon and oxygen isotopic compositions, they differ from other carbonate rocks of the Aldan Shield, including carbonatites and Ca–Mg metasomatites, and resemble Phanerozoic and Precambrian metasedimentary carbonates. They formed in oxidizing conditions. The contribution of evaporite processes at different stages of their formation is supported by the carbonate enrichment in ¹³C and the presence of sulfates. It was established that apatite–carbonate rocks represent the product of complex alternation of sedimentary processes at different values of salinity in the basins occasionally characterized by the decomposition of older sediments under subaerial conditions.     

Sea-floor fissures, biological communities and sediment fatty acids of the Northern Okushiri Ridge, Japan Sea: Implications for possible methane seepage

A biological community was discovered in the Northern Okushiri Ridge, northeastern Japan Sea. The community was closely associated with sea-floor fissures, and presumed to be supported by methanotrophic and/or thiotrophic bacterial production. Sediments inside of and in the vicinity of the fissures were collected, and the short-chain (C_9–20) sediment fatty acids were analyzed for amounts and compositions. The fatty acid compositions were compared with those from a known methane seep and a submarine volcano in the Sagami Bay, central Japan, and from a whale skeleton at the Torishima Seamount, northwestern Pacific Ocean. As a result, a close relationship between the sediments from the Northern Okushiri Ridge, the known methane-seep, and the whale skeleton was found. This finding represents the first discovery of methane seepage and associated biological communities in the Japan Sea. This also supports the hypothesis that the eastern margin of the northern Japan Sea is at the early stage of new subduction.     

Graphite–sulfide deposits in Ronda and Beni Bousera peridotites (Spain and Morocco) and the origin of carbon in mantle-derived rocks

This paper describes unusual graphite–sulfide deposits in ultramafic rocks from the Serranía de Ronda (Spain) and Beni Bousera (Morocco). These deposits occur as veins, stockworks and irregular masses, ranging in size from some centimeters to a few meters in thickness. The primary mineral assemblage mainly consists of Fe–Ni–Cu sulfides (pyrrhotite, pentlandite, chalcopyrite and cubanite), graphite and chromite. Weathering occurs in some sulfide-poor deposits that consist of graphite (up to 90%), chromite and goethite. Texturally, graphite may occur as flakes or clusters of flakes and as rounded, nodule-like aggregates. Graphite is highly crystalline and shows light carbon isotopic signatures (δ¹³C≈− 15‰ to − 21‰). Occasionally, some nodule-like graphite aggregates display large isotopic zoning with heavier cubic forms (probably graphite pseudomorphs after diamond with δ¹³C up to − 3.3‰) coated by progressively lighter flakes outwards (δ¹³C up to − 15.2‰).Asthenospheric-derived melts originated the partial melting (and melt–rock reactions) of peridotites and pyroxenites generating residual melts from which the graphite–sulfide deposits were formed. These residual melts concentrated volatile components (mainly CO₂ and H₂O), as well as S, As, and chalcophile elements. Carbon was incorporated into the melts from the melt–rock reactions of graphite-bearing (formerly diamonds) garnet pyroxenites with infiltrated asthenospheric melts. Graphite-rich garnet pyroxenites formed through the UHP transformation of subducted kerogen-rich crustal material into the mantle. Thus, graphite in most of the studied occurrences has light (biogenic) carbon signatures. Locally, reaction of the light carbon in the melts with relicts of ¹³C-enriched graphitized diamonds (probably generated from hydrothermal calcite veins in the subducting oceanic crust) reacted with the partial melts to form isotopically zoned nodule-like graphite aggregates.