Source model for strong ground motion generation in the frequency range 0.1–10 Hz during the 2011 Tohoku earthquake

The source model of the 2011 Tohoku earthquake, which is composed of four strong motion generation areas (SMGAs), is estimated based on the broadband strong ground motion simulations in the frequency range 0.1–10 Hz using the empirical Green’s function method. Two strong motion generation areas are identified in the Miyagi-oki region west of the hypocenter. Another two strong motion generation areas are located in the Fukushima-oki region southwest of the hypocenter. The strong ground motions in the frequency range 0.1–10 Hz along the Pacific coast are mainly controlled by these SMGAs. All the strong motion generation areas exist in the deeper portion of the source fault plane. The stress drops of the four SMGAs range from 6.6 to 27.8 MPa, which are similar to estimations for past M 7-class events occurring in this region. Compared with the slip models and aftershock distributions of past interplate earthquakes in the Miyagi-oki and Fukushima-oki regions since the 1930s, the SMGAs of the 2011 Tohoku earthquake spatially correspond to the asperities of M 7-class events in 1930s. In terms of broadband strong ground motions, the 2011 Tohoku earthquake is not only a tsunamigenic event with a huge coseismic slip near the trench but is also a complex event simultaneously rupturing pre-existing asperities

Improving the Retrieval of Offshore-Onshore Correlation Functions With Machine Learning

The retrieval of reliable offshore‐onshore correlation functions is critical to improve our ability to predict long‐period ground motions from megathrust earthquakes. However, localized ambient seismic field sources between offshore and onshore stations can bias correlation functions and generate nonphysical arrivals. We present a two‐step method based on unsupervised learning to improve the quality of correlation functions calculated with the deconvolution technique (e.g., deconvolution functions, DFs). For a DF data set calculated between two stations over a long time period, we first reduce the data set dimensions using the principal component analysis and cluster the features of the low‐dimensional space with a Gaussian mixture model. We then stack the DFs belonging to each cluster together and select the best stacked DF. We apply our technique to DFs calculated every 30 min between an offshore station located on top of the Nankai Trough, Japan, and 78 onshore receivers. Our method removes spurious arrivals and improves the signal‐to‐noise ratio of DFs. Most 30‐min DFs selected by our clustering method are generated during extreme meteorological events such as typhoons. To demonstrate that the DFs obtained with our method contain reliable phases and amplitudes, we use them to simulate the long‐period ground motions from a Mw 5.8 earthquake, which occurred near the offshore station. Results show that the earthquake long‐period ground motions are accurately simulated. Our method can easily be used as an additional processing step when calculating offshore‐onshore DFs and offers a new way to improve the prediction of long‐period ground motions from potential megathrust earthquakes

Structural Features Along the Median Tectonic Line in Southwest Japan: An Example of Multiphase Deformation on an Arc‐Bisecting Fault

A geological survey for the Late Cretaceous Izumi Group distributed on the Median Tectonic Line (MTL) active fault system in the central part of southwestern Japan has revealed varied deformation styles. Among the confined deformation zones found in the western and central parts of the study area, some are located far from the active trace of the MTL (Negoro Fault), at distances of up to 300–350 m. Such kink zones may have been generated during a contraction phase of the MTL from the end of the Pliocene to the early Pleistocene. We identified clear active foldings in a narrow zone sandwiched between a north dextral and a south reverse active fault. Western and eastern upheavals of the crustal sliver show ridge and domal active morphologies, respectively. Structural analysis was extended to the north of the MTL, where the Izumi Group has suffered multiphase deformation since the Cretaceous. The phase stripping method was introduced to extract the neotectonic trend, which successfully delineated complicated deformation zones related to the morphological divergence of the MTL active fault system

Numerical Analysis of Trampoline Effect in Extreme Ground Motion

Very large vertical surface acceleration of nearly four times gravity was measured at a strong motion observation station in Iwate Prefecture during the 2008 Iwate-Miyagi Inland, Japan, earthquake (Mw 6.9). The station is located about 3 km southwest of the epicenter and equipped with three-component accelerometers, installed at both the free surface and the bottom of a 260-m borehole. The wave form of the vertical acceleration shows a clearly asymmetric form with large amplitude in the upward direction. Aoi et al. (2008) reported and qualitatively explained the mechanism of this phenomenon by the analogy of bouncing a piece of matter on a trampoline, and thus they called it the “trampoline effect.” To simulate this recently discovered nonlinear behavior of the surface ground motion, numerical analysis with a finite-element method has been employed with parameters derived from the borehole data at the station. The analysis successfully simulates the asymmetric vertical motion. Results indicate that the asymmetric motion may be characterized by the existence of a lower bound of negative acceleration, which in most cases corresponds to the acceleration of gravity, and high positive pulses caused by the compression stress of the disturbed surface ground material

Three-Dimensional Architecture of the Median Tectonic Line in Southwest Japan Based on Detailed Reflection Seismic and Drilling Surveys

The subsurface morphology of an arc-bisecting tectonic zone has been unraveled by means of well-organized seismic investigation with the aid of borehole stratigraphic control. The Median Tectonic Line (MTL) active fault system in southwestern Japan, one of the world’s largest intraplate transcurrent faults, is driven by the recent oblique subduction of the Philippine Sea Plate. Six tied seismic profiles covering the mountainous range, the southern foothill of which is truncated by the MTL, were used to visualize the Quaternary basins on both feet of the Izumi Mountains. North- and east-trending basement deformation was confirmed on the northern and southern sides of the watershed, respectively; this deformation reflects the spatial diversity in tectonic stress. Seismic data on the southern Izumi flank revealed a low-angle fault parallel to the MTL active fault system; this fault may be interpreted as a dormant structure that developed from 6 to 2 Ma under the intermittent increases of the compressive regime. A kink zone in the upthrown block of the thrust was identified on seismic profiles and continuously traced through field geological survey. This zone confirms the prevailing contractional phase related to the transient convergence mode of the oceanic plate

Source-rupture process of the 2011 Ibaraki-oki, Japan, earthquake (M-w 7.9) estimated from the joint inversion of strong-motion and GPS Data: Relationship with seamount and Philippine Sea Plate

The source-rupture process of the 2011 Ibaraki-oki earthquake was estimated from the joint inversion of the strong-motion and global positioning system (GPS) data. The estimated seismic moment and maximum slip are 7.8 × 1020 Nm (Mw 7.9) and 6.3 m, respectively. The total rupture duration is approximately 30 s. The derived source model has one large slip area, which is surrounded by the subducted seamount and the northeastern edge of the Philippine Sea plate. From this model, we concluded that the rupture propagation of the 2011 Ibaraki-oki earthquake was stopped by the seamount and the Philippine Sea plate. We also showed the possibility of the rupture of this event being the reactivation of the preexisting asperity of an event that occurred in 1923

Along‐dip variation in seismic radiation of the 2011 Ibaraki‐oki, Japan, earthquake ( M w 7.9) inferred using a multiple‐period‐band source inversion approach

To elucidate the spatial variation in period‐dependent seismic radiation for the 2011 Ibaraki‐oki earthquake (Mw 7.9) in Japan, we applied a multiple‐period‐band source inversion approach to near‐source strong‐motion waveforms of this earthquake. We estimated source models of this earthquake in three successive period bands (5–10, 10–25, and 25–50 s) using strong‐motion data and Green's functions based on a 3‐D velocity structure model. The source models in the period bands of 10–25 and 25–50 s had large slips in the area to the south and southeast of the hypocenter in the depth range of 23–35 km, while the large slip area for the source model in the period band of 5–10 s was located in the deeper region ~30 km west of the hypocenter in the depth range of 35–45 km. These results indicate that long‐period (10–25 and 25–50 s) and short‐period (5–10 s) seismic waves were predominantly radiated from these different regions along the dip direction during the 2011 Ibaraki‐oki earthquake. This along‐dip variation in the dominant period of seismic radiation can be explained by the variation in scale in unstable sliding patches according to depth

Source rupture processes of the foreshock and mainshock in the 2016 Kumamoto earthquake sequence estimated from the kinematic waveform inversion of strong motion data

The 2016 Kumamoto earthquake sequence started with an MJMA 6.5 foreshock occurring along the northern part of the Hinagu fault, central Kyushu, Japan, and the MJMA 7.3 mainshock occurred just 28 h after the foreshock. We analyzed the source rupture processes of the foreshock and mainshock by using the kinematic waveform inversion technique on strong motion data. The foreshock was characterized by right-lateral strike-slip occurring on a nearly vertical fault plane along the northern part of the Hinagu fault, and it had two large-slip areas: one near the hypocenter and another at a shallow depth. The rupture of the mainshock started from the deep portion of a northwest-dipping fault plane along the northern part of the Hinagu fault, then continued to transfer to the Futagawa fault. Most of the significant slip occurred on the Futagawa fault, and the shallow portion of the Hinagu fault also had a relatively large slip. The slip amount on the shallowest subfaults along the Futagawa fault was approximately 1–4 m, which is consistent with the emergence of surface breaks associated with this earthquake. Right-lateral strike-slip dominated on the fault segment along the Hinagu fault, but normal-slip components were estimated to make a significant contribution on the fault segment along the Futagawa fault. The large fault-parallel displacements recorded at two near-fault strong motion stations coincided with the spatiotemporal pattern of the fault slip history during the mainshock. The spatial relationship between the rupture areas of the foreshock and mainshock implies a complex fault structure in this region

Fission Track Thermochronology of Late Cretaceous Sandstones of the Izumi Group Adjacent to the Median Tectonic Line Active Fault System in Southwest Japan

Fission track (FT) thermochronology was applied to the Late Cretaceous turbidite sandstones of the Izumi Group adjacent to the Median Tectonic Line active fault system in southwest Japan. Apatite FT analyses revealed the following three stages of cooling (uplift) events: 95–78 Ma (Cenomanian–Campanian) from >130°C, 74–46 Ma (Campanian–middle Eocene) from approximately 100°C, and 27–7 Ma (late Oligocene–late Miocene) from approximately 70°C. By contrast, zircon FT analysis indicated cooling from >300°C at ca. 70 Ma. Apparent discrepancies between the cooling initiation times obtained using the two analytical methods indicate the distinct provenances of tuffaceous sandstones of the Izumi Group. The second episode is likely related to regional exhumation events on the eastern Eurasian margin. The latest event, which terminated by the end of the Miocene, appears to have been manifested in the strong deformation of the arc under a compressive stress provoked by the resumed subduction of the Philippine Sea Plate