太平洋プレートの海溝型地震の広帯域震源モデルに基づく微視的断層パラメータの特性

Abstract

It has been pointed out that large amounts of slip areas for large interplate earthquakes are different from strong motion generation areas SMGAs. It has been also shown that the combined areas of asperities which are defined as large slip areas extracted from slip models by waveform inversion estimated from records in the period longer than about 2 s are larger than the combined areas of SMGAs estimated from strong motion records in the period range of about 0.1 to 10 s. However, the strong motion recipe for interplate earthquakes is established mainly based on asperities of previous large earthquakes. In addition, the characteristics of each SMGA as well as total SMGAs is important to strong motion prediction since SMGAs of large interplate earthquakes are located at spatially separated positions and ruptured with time intervals. Therefore we examine the characteristics of inner fault parameters based on 19 previous broadband source models composed of SMGAs estimated by the empirical or stochastic Green's function method for five interplate and five intraplate earthquakes from MW 6.8 to 9.0 in Japan. Nine earthquakes of the Pacific plate from MW 7.0 to 9.0 out of ten are used for the development of scaling relations. In order to examine the time-dependent source characteristics of each SMGA we define acceleration envelope source time functions of SMGA as AENVSMGA by applying the scaling relation of the rms value of acceleration in the source region by Hanks (1979) and the envelope model by Boore (1983) to element faults of SMGAs. For interplate earthquakes the power coefficient for seismic moment M0 is less than 1/3 for the maximum value of A of SMGAs because the number of SMGAs increases with M0. The rupture starting points of all SMGAs are located at deeper portion than the hypocenter and the ruptures tend to progress to deeper zone. The maximum value of AENVSMGA for each earthquake is 13.5×1010[Nm/s2] on average with the standard deviation of a factor of 1.26 and independent of MW from 7.2 to 9.0. The maximum values of each AENVSMGA are ranging from 4×1010～20×1010[Nm/s2]. These features are qualitatively consistent with the saturation effects of peak ground accelerations to MW pointed out by the attenuation characteristics of interplate earthquakes. The maximum value of AENVSMGA for the first ruptured SMGA tends to be a little larger than those of the other SMGAs located at deeper portion. For intraplate earthquake, the maximum value of AENVSMGA for the first ruptured SMGA tends to be smaller than those of the other SMGAs. We found that the combined area of SMGAs for both interplate and intraplate earthquakes can be modeled by the same scaling relation to M02/3. The combined area of SMGAs is 0.20 times of combined area of asperities by Murotani et al. (2013) for interplate earthquakes and 0.54 times of those by Iwata and Asano (2011) for intraplate earthquakes. The difference between the combined areas of SMGAs and asperities for subduction-zone earthquakes is larger than crustal earthquakes, that is, 0.8 times (Satoh, 2010). The duration Td from the origin time to the end of the last ruptured SMGA for both interplate and intraplate earthquakes is also modeled by the same scaling relation to M01/3 and the centroid depth of SMGAs. The duration Td decreases with the centroid depth. The ratio of combined duration of SMGAs to Td is larger than one for intraplate earthquakes and less than one for interplate earthquakes. This result means that SMGAs for interplate earthquakes rupture with longer time intervals than intraplate earthquakes.