Large Inland Earthquakes Research Articles

Seismogenic layer, reflective lower crust, surface heat flow and large inland earthquakes

Regional variations in the cutoff depth of seismicity in southwest Japan are derived from more than 60,000 well-determined earthquakes. The thickness of the seismogenic layer is closely related to the strength of the crust and accordingly to the occurrence of large inland earthquakes, since the seismic–aseismic boundary is thought to be related to the brittle–ductile boundary in the crust. Large inland earthquakes are likely to occur in the area where the cutoff depth of seismicity changes abruptly. It is therefore important to survey the regional variations in this depth to evaluate the potential of large inland earthquakes. The cutoff depth of seismicity roughly coincides with a temperature range of 300–400°C in the crust. In addition, distinct S-wave reflectors as well as a reflective lower crust have generally been observed in some areas in Japan. The depths of the reflector and the top of the reflective lower crust lie several kilometers below the cutoff depth of seismicity and they seem to coincide with a temperature range of about 300–450°C. Therefore, in those areas where no earthquakes occur but a reflective layer exists, the cutoff depth can be mapped from a survey of reflectivity in the crust. The heterogeneity of the crust is very important for understanding the nature of the crust where large earthquakes are frequent. This paper proposes a model of crust inhomogeneity based on observation results.

西南日本のマイクロプレート・モデルと内陸大地震

西南日本のマイクロプレート・モデルと内陸大地震

A sequence of destructive earthquakes and the coupling of fault systems in central Japan

In the western part of central Japan, large successive inland earthquakes have occurred causing serious damage. The 1891 M=8.0 Nobi earthquake was followed by the 1909 M=6.8 Anegawa, the 1945 M=6.8 Mikawa, and the 1948 M=7.1 Fukui earthquakes. The two major fault systems, termed the Tsurugawan-Isewan tectonic line (TITL) and the Fukui-Neodani block boundary line (FNBB) have been identified as lines linking NW-SE-oriented active faults in the study area. An examination of the vertical displacement of ground surfaces indicates that tilting of the central part of the FNBB caused by the Nobi earthquake begins around the Sekigahara fault, on the central part of the TITL. Ruptures and vertical displacements can also be found along the Ogaki-Kisozaki fault close to the TITL. Therefore, it was concluded that the influence of the Nobi earthquake extended to a part of the TITL. Eighteen years after the Nobi earthquake, the Anegawa earthquake occurred along the Sekigahara fault. These features and the earthquake sequence can be explained by a block rotation model with strike-slip faulting. The Nobi earthquake created en echelon ruptures over an 80-km-long line with a left-lateral slip of up to 8 m along the Neodani fault. According to the model, this left-lateral slip must be accompanied by a clockwise rotation of the block between the FNBB and TITL. The block rotation should, in turn, induce a left-lateral slip on the TITL. Since the TITL exhibits a sharp bend along the Sekigahara fault, a transtensional region should appear along the fault owing to the left-lateral slip of the TITL. The expected transtensional region is consistent with areas damaged by the Anegawa earthquake, suggesting that the earthquake occurred in the transtensional regime. About 40 years after the Anegawa earthquake, the Mikawa and Fukui earthquakes were generated on the southern and northern ends of the FNBB, respectively. This implies that the fault movement of the Nobi and Anegawa earthquakes created a concentration of stress at both ends of the FNBB, resulting in the Mikawa and Fukui earthquakes. Approximately two years prior to the Nobi earthquake, seismic activity was initiated over a wide zone along the FNBB. Also, notable earthquakes aligned along the TITL were observed ten years prior to the Anegawa earthquake. The onset of seismicity along a fault system can be used in predicting destructive inland earthquakes which eventually occur along the fault.

Regional variations of the cutoff depth of seismicity in the crust and their relation to heat flow and large inland-earthquakes.

More than 8, 000 earthquakes have been relocated to derive regional variations of the seismic-aseismic boundary in the mid-crust of the northern Kinki district of Japan. The boundary depths are estimated as 13-15 and 18-20 km in the southwestern and northeastern parts of the study area, respectively. The relationship between the seismic cutoff depth and the cause of large intraplate earthquakes is studied, making use of the present observations and other available data of seismicity and surface heat-flow, on the basis of the brittle-ductile transition of rock deformation. The regional variations in the cutoff depth of seismicity appear to be well correlated with the thermal structure of the crust. The cutoff depths in various heat-flow provinces in Japan and other countries are found to be inversely proportional to the surface heat-flow values, with the depths roughly corresponding to isotherms of 200-400°C. The shape of the depth-frequency distribution of earthquakes calculated from high-quality data is quite similar to that of the shear resistance calculated using a simple brittle-ductile transition model. Large intraplate earthquakes appear to originate at the peak or just below the peak in the depth-frequency distribution, which also corresponds to the deepest portion of the seismogenic layer. Furthermore, in the source area of large earthquakes, rupture seems to start where sharp changes occur in the cutoff depth of seismicity. Thus, the seismic-aseismic boundary is closely related to large intraplate earthquakes and, in turn, to the tectonics of island arcs.

Recurrence of great earthquakes at subduction zones

Statistics of the recurrence times of great earthquakes at the Pacific subduction margins are made. The mean return period of great earthquakes is different from zone to zone, ranging from 27 to 117 years. The standard deviation of the return period proves to be very small, several years say, in some cases. The probabilities of a great earthquake recurring in each zone are estimated on the basis of Weibull distribution analysis. The mean return periods thus estimated are combined with the relative plate velocities at respective zones as obtained in the plate tectonics in order to estimate the ultimate displacement to rupture at the interface of the continental plate and the downgoing oceanic plate. It is presumed that great earthquakes at subduction zones occur as a result of a rebound of the continental plate at the time of rupture. The ultimate displacement thus estimated ranges from 2 to 8 m, and seems somewhat larger than that estimated on the basis of seismic observations, although the value of ultimate displacement seems to harmonize roughly with estimates based on geodetic observations on land. However, the ultimate displacement at the Aleutian—Alaska zone as estimated here seems much smaller than that estimated from actual observations. The ultimate strains, which are deduced from the displacements obtained on the assumption that the logarithmic extent of the deformed area is proportional to earthquake magnitude, are then calculated, and compared with those estimated for large inland earthquakes as revealed by repetition of geodetic surveys. The mean ultimate strain is estimated as 4.3 · 10 −5 for subduction-zone earthquakes while that for inland earthquakes has been estimated as 4.7 · 10 −5. As the agreement between both the ultimate strains is fairly good, it is tentatively concluded that the strength of the plate interface under the sea bottom is more or less the same as that in the crust on land.