Haruo Horikawa

Rupture process of the 2005 West Off Fukuoka Prefecture, Japan, earthquake

The 2005 West off Fukuoka Prefecture, Japan, earthquake (MS 6.7) is a moderate-size crustal event. The rupture process of this earthquake was inferred from strong motion data. It is observed regardless of epicentral distance and azimuth that the initial P-wave with a small amplitude continued for 2.5–4 s, suggesting prominent initial rupture. The duration of the initial rupture was estimated to be 3.3 s from travel time analysis. Inversion analysis shows that the initial rupture continued for 3.5 s and the overall rupture finished within 10 s. An area of large slip with two peaks of slip amount appeared to the southeast of the hypocenter, and the location of the large-slip area is consistent with the origin of the main rupture inferred from travel time analysis. Small amount of slip was found at and around the hypocenter, and the rupture velocity was slow there. The seismic moment of this earthquake was estimated to be 5.7 × 1018 Nm (Mw 6.4). The maximum stress drop calculated from the derived slip distribution exceeded 10 MPa. An area of negative stress drop expanded, corresponding to the distribution of small slip. Forward modeling of the observed waveforms suggests that the negative stress drop is not fully attributable to poor resolution of inversion analysis for small amount of slip. Since dynamic stress drop should be positive around the hypocenter so that rupture propagates over the entire fault, it is plausible that the negative stress drop appeared after healing.

Characterization of the 2007 Noto Hanto, Japan, earthquake

Characterization of the 2007 Noto Hanto, Japan, earthquake (MS 6.9), a moderate-size crustal event, was performed. The rupture process was firstly inferred from strong motion data. Inversion analysis revealed that the overall rupture finished within 6 s, and the seismic moment of this earthquake was estimated to be 1.1×1019 N m (Mw 6.6). Two areas of large slip and stress drop (asperities) were inferred on the fault plane. The maximum static stress drop calculated from the derived slip distribution exceeded 16 MPa for the major asperity, and the minor asperity has a similar maximum value. An area of negative stress drop corresponding to the distribution of small slip exists between the two asperities. This strongly suggests that the fault is segmented. A boundary between surface faults was located above the major asperity, but an area of negative stress drop appeared between the asperity and fault boundary. This suggests that the configuration of the surface faults reflects only a shallow part of the causative fault. The ratio of radiated energy to seismic moment was also estimated for the purpose of estimation of fault activity. Comparison of the derived value with those of other Japanese earthquakes suggests that the causative fault of the Noto Hanto earthquake is not very active.

Coastal deformation associated with the 2007 Noto Hanto earthquake, central Japan, estimated from uplifted and subsided intertidal organisms

The March 25, 2007 Noto Hanto earthquake (Mj = 6.9, Mw = 6.7) generated vertical crustal movement along the northwestern coast of the Noto Peninsula, central Japan. Soon after the event, we estimated the pattern and amount of coseismic coastal movement based on uplifted and subsided intertidal sessile organisms. Our observations reveal a broad 20-km-wide asymmetric zone of surficial deformation above and across the south-dipping source fault, with a steep north-facing frontal limb and a gentle south-facing back limb. The maximum coseismic uplift was approximately 40 cm at the crest of the zone of deformation. The result of forward modeling suggests that the top of the south-dipping source fault is buried at a depth of approximately 2 km, and that 1.2 m of slip on the fault provides the best fit to our surface observations. Our results demonstrate that traditional field investigations should be combined with modern instrumental observations such as GPS and InSAR to obtain the most effective and reliable spatio-temporal estimates of crustal movement associated with large earthquakes.

Modeling slip processes at the deeper part of the seismogenic zone using a constitutive law combining friction and flow laws

The fault zone in the earth’s crust is thought to consist of several regions from top to bottom: the upper frictional region, the brittle-ductile transition zone and the ductile region. The upper frictional region consists of the unstable frictional zone, the unstable-stable transition zone, and the stable frictional zone. Recent geological observations of fault rock suggest that at the deeper part of the seismogenic zone, co-seismic frictional slip coexists with interseismic flow processes. We propose a possible model for slip processes at the deeper part of the seismogenic zone in which the frictional slip and flow processes are connected in series. In this model, in the ductile region, power law creep is dominant. Around the unstable-stable transition zone, we assume that co-seismic frictional slip coexists with aseismic flow processes. We investigate simple 1-D and 2-D models where rate- and state-dependent friction coexists with power law creep that has a threshold stress. The results of numerical simulations show that the amount of slip during the interseismic period is greater in the case where friction coexists with power law creep than it is when only friction is at work. It is also found that, for the case where friction coexists with power law creep, frictional slip is largely inhibited in the ductile region.

Fault geometry and slip distribution of the 2003 Tokachi-oki earthquake as deduced from teleseismic body waves

I have analyzed teleseismic body waves from the 2003 Tokachi-oki earthquake (Mw 8.1), and inferred the slip distribution. Two simple fault models are assumed for estimating the effect of fault geometry on derived slip distributions. One is a single planar fault with a dip of 20° and the other is a compound fault having a shallow plane with a dip of 5° and deeper, landward plane with a dip of 20°. The compound-fault model is preferable because it explains the initial part of the observed P-waves better. It is found that the planar fault has one asperity (patch of large slip) near the hypocenter and the other asperity to the landward side of the hypocenter. The compound-fault model shares the landward asperity with the planar-fault model, but does not have the asperity near the hypocenter. The other asperity on the compound fault is found far from the hypocenter. This difference of the slip distributions suggests the importance of accurate modeling of the fault dip angle when deducing the slip distribution from teleseismic body waves.

Special issue “2016 Kumamoto earthquake sequence and its impact on earthquake science and hazard assessment”

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