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.     

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.     

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.     

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.     

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.     

Isotopic equilibration ages for the Miyanohara tonalite from the Higo metamorphic belt in central Kyushu, Southwest Japan: Implications for the tectonic setting during the Triassic

Abstract Miyanohara tonalite occurs in the middle part of the Higo metamorphic belt in the central Kyushu, Southwest Japan. This tonalite intrudes into early Permian Ryuhozan metamorphic rocks in the south and is intruded by Cretaceous Shiraishino granodiorite in the north. The Miyanohara tonalite yielded three mineral ages: (i) 110–100 Ma for Sm–Nd and Rb–Sr internal isochrons and for K–Ar hornblende; (ii) 183 Ma for Sm–Nd internal isochron; and (iii) 211 Ma for Sm–Nd internal isochron. The ages of 110–100 Ma may indicate cooling age due to the thermal effect of the Shiraishino granodiorite intrusion. The ages of 183 Ma and 211 Ma are consistent with timing of intrusion of the Miyanohara tonalite based on geologic constraints. The hornblende in the sample which gave 183 Ma shows discontinuous zoning under microscope, whereas the one which gave 211 Ma does not show zonal structure. These mineralogical features suggest that the 183 Ma sample has suffered severely from later tectonothermal effect compared with the 211 Ma sample. Therefore, the age of 211 Ma is regarded as near crystallization age for the Miyanohara tonalite. The magmatic process, geochronology and initial Sr and Nd isotope ratios for the Miyanohara tonalite are similar to those of early Jurassic granites from the Outer Plutonic Zone of the Hida belt that constitutes a marginal part of east Asia before the opening of the Japan Sea. Intrusion of the Miyanohara tonalite is considered to have taken place in the active continental margin during the late Triassic.     

Pb, Nd, and Sr isotopic constraints on the origin of Miocene basaltic rocks from northeast Hokkaido, Japan: Implications for opening of the Kurile back-arc basin

Late Miocene (7–9 Ma) basaltic rocks from the Monbetsu‐Kamishihoro graben in northeast Hokkaido have chemical affinities to certain back‐arc basin basalts (referred to herein as Hokkaido BABB). Pb‐, Nd‐ and Sr‐isotopic compositions of the Hokkaido BABB and arc‐type volcanic rocks (11–13 Ma and 4–4.5 Ma) from the nearby region indicate mixing between the depleted mantle and an EM II‐like enriched component (e.g. subducted pelagic sediment) in the magma generation. At a given ⁸⁷Sr/⁸⁶Sr, Hokkaido BABB have slightly lower ¹⁴³Nd/¹⁴⁴Nd and slightly less radiogenic ²⁰⁶Pb/²⁰⁴Pb compared with associated arc‐type lavas, but both these suites are difficult to distinguish solely on the basis of isotopic compositions. These isotopic data indicate that while generation of the Hokkaido BABB involves smaller amounts of the EM II‐like enriched component than do associated arc lavas, Hokkaido BABB are isotopically distinct from basalts produced at normal back‐arc basin spreading centers. Instead, northeast Hokkaido BABB are more similar to basalts erupted during the initial rifting stage of back‐arc basins. The Monbetsu‐Kamishihoro graben may have developed in association with extension that formed the Kurile Basin, suggesting that opening of the basin continued until late Miocene (7–9 Ma).