Masahiro Kosuga

Three‐stage rupture process of the 28 December 1994 Sanriku‐Oki Earthquake

We investigated the rupture process of the 28 December 1994 Sanriku-Oki earthquake (Mw = 7.7) using broadband seismograms recorded at local distances. The earthquake rupture nucleated at the eastern end of the aftershock area. As it propagated towards the west, a major subevent occurred near the center of the aftershock area about 26 s after the initial rupture, releasing most of the seismic energy of this earthquake. About 24 s later, another subevent followed at the western end of the aftershock area, emitting large high-frequency waves. The whole rupture time is about 55 sec. The three-stage rupture process is very similar to the earlier-stage rupture process of the 1968 Tokachi-Oki earthquake (Mw = 8.2). The difference is that the rupture of the Tokachi-Oki earthquake further propagated towards the north in the following stage instead of terminating at the western end. The occurrence of the Sanriku-Oki earthquake is critical to the validity of the characteristic earthquake so far believed to exist in this part of the plate boundary.

Spatial distribution of intermediate-depth earthquakes with horizontal or vertical nodal planes beneath northeastern Japan

Abstract The 1993 Kushiro-oki (off-Kushiro) earthquake ( M JMA = 7.8) is one of the largest intermediate-depth earthquakes that have occurred in northeastern Japan. This earthquake is a notable event that has a nearly horizontal fault plane extended from the lower seismic plane of the double-planed seismic zone toward the upper seismic plane. We have examined the spatial distribution of focal mechanism solutions of deep and intermediate-depth earthquakes beneath northeastern Japan concerning this peculiar event. Analyses of 127 new solutions along with existing solutions confirm the previous results on essential features of their spatial distribution. The predominant types of solutions are down-dip compression in the upper plane except beneath eastern Hokkaido, and down-dip extension in the lower plane. P-Axes in the upper seismic plane and T-axes in the lower plane are in the direction of plate convergence. Beneath Hokkaido, however, T-axes show significant deviation from the direction of plate convergence. The stress axes beneath the junction between the northeastern Japan arc and the Kuril arc exhibit slight deviation from those in the adjacent areas, suggesting slab distortion beneath the junction. We next investigate the spatial distribution of events with horizontal nodal planes as candidates for events with similar focal mechanisms to the Kushiro-oki earthquake. The activity of earthquakes with horizontal nodal planes is definitely high in the lower plane, especially beneath Hokkaido. Many of them have similar focal mechanisms to the Kushiro-oki earthquake, indicating that the Kushiro-oki earthquake is not exceptional but is a typical event in the region. However, the fault planes of these events are variable in space; some events are known to have vertical fault planes. There are shallow and deep depth bounds and a southwestern boundary for the events with horizontal fault planes. These facts suggest that the horizontal faulting is probably caused by some regional stress concentration owing to the unbending and the distortion of the segmented slab at the junction.

Tomographic inversion for P wave velocity structure beneath the northeastern Japan arc using local and teleseismic data

We have determined three-dimensional P wave velocity structure up to a depth of 296 km beneath northern Tohoku, Honshu, Japan, by inverting arrival times of local and teleseismic events simultaneously. Velocities in the upper crustal layers correlate well with the surface geology and Bouguer gravity anomalies. Low-velocity zones are ubiquitous in the crust and the uppermost mantle beneath the Quarternary volcanic areas. A well-known feature of a pronounced low-velocity mantle wedge underlain by the dipping high-velocity (high-V) Pacific plate is generally recovered over the study area. Strong heterogeneity within the high-velocity Pacific plate is suggested by the presence of a pronounced low-velocity anomaly confined to the depths of 130-200 km. The decreased seismicity of intermediate-focus events may be correlated with the low-velocity zone. A two-dimensional inversion scheme was invoked to determine stably the average mantle structure perpendicular to the island arc. In the depths of 50-150 km, the inclined high-V zone has a thickness of 80-100 km with its top coinciding with the upper plane of the double-planed deep seismic zone. Below the depth of 200 km, the high-V zone is shifted toward the bottom of the 80-km-thick reference slab. In the intermediate-depth layer (150-200 km), the high-V zone is not clearly visible within the 80-km-thick reference slab.

Pore pressure distribution in the focal region of the 2008 M7.2 Iwate-Miyagi Nairiku earthquake

The pore fluid pressure distribution in the focal region of the 2008 Iwate-Miyagi Nairiku earthquake was investigated through an analysis of the diversity of focal mechanisms. We inverted stress orientations and focal mechanisms directly from P-wave polarity data obtained from a dense aftershock observation network and other temporarily and routinely operated stations. The estimated stress orientation is consistent with that typical of NE Japan. Specifically, the σ 1 axis is oriented WNW-ESE nearly parallel to plate convergence, and the σ 3 axis is nearly vertical, consistent with a reverse-faulting stress regime, with some exceptions in the central part of the aftershock area. We obtained 2,497 well-determined focal mechanisms whose average number of P-wave polarity data is more than 70. The spatial distribution of pore fluid pressure was estimated by using the obtained orientations of the principal stresses and earthquake faults. The pore pressure ratio for each earthquake fault was calculated under the assumption that reduced frictional strength was caused by pore fluid pressure. The results showed that the diversity of the focal mechanisms here obtained requires high pore fluid pressures at many of the earthquake faults in the focal region. The spatial pattern of pore pressure ratio shows that areas of higher pore pressure ratio are distributed around the large coseismic slip area near Kurikoma volcano. Immediately beneath these high pore pressure areas, there exists a distinct seismic low-velocity zone that continues down to the mantle wedge below. These observations suggest that crustal fluids supplied from the mantle wedge have contributed to producing high pore pressures and to lowering the frictional strengths of those faults. Crustal fluids may also have contributed to generating the mainshock rupture.

Periodic ripple of coda envelope observed in northeastern Japan

Abstract We have investigated the SH envelopes (bandpass filtered and mean squared seismograms) in the early coda of local and regional earthquakes observed at five stations in northeastern Japan. The most remarkable feature we have found is a periodic increase in coda amplitude with nearly a constant time interval of several seconds. We call this phenomena ‘ripple’ of coda envelope. We examined four possible origins of periodic ripple: an artifact due to inadequate data handling, repeated source radiation, site effect, and path-originated phase. The possibility of artifact due to inadequate filtering or smoothing is low because the ripple is visible in both unfiltered seismograms and filtered traces with various filtering parameters. Repeated source radiation is not responsible for the ripple, because the ripple has longer time duration than that of source time function of the investigated events, and because the appearance of ripple at five stations is not synchronous. Local site amplification factors determined by the spectral ratio of horizontal to vertical motions suggest the contribution of surficial low velocity layers (LVL) to the periodic ripple. The events with prominent ripple are predominantly distributed in shallow range off the Pacific coast and intermediate-depth range along the volcanic front of the Japanese Islands. This trend is commonly observed for all the investigated stations having different site amplification factors. This observation is suggestive of a path-dependent origin such as trapped waves in the LVL in the upper part of the subducting slab. Thus the guided waves trapped in the LVL are the probable origin of the periodic ripple. The LVL is either the surficial sedimentary layers under the recording site or the subducting oceanic crust.

Prestate of Stress and Fault Behavior During the 2016 Kumamoto Earthquake (M7.3)

Fault behavior during an earthquake is controlled by the state of stress on the fault. Complex coseismic fault slip on large earthquake faults has recently been observed by dense seismic networks, which complicates strong motion evaluations for potential faults. Here we show the three-dimensional prestress field related to the 2016 Kumamoto earthquake. The estimated stress field reveals a spatially variable state of stress that forced the fault to slip in a direction predicted by the “Wallace and Bott Hypothesis.” The stress field also exposes the pre-condition of pore fluid pressure on the fault. Large coseismic slip occurred in the low-pressure part of the fault. However, areas with highly pressured fluid also showed large displacement, indicating that the seismic moment of the earthquake was magnified by fluid pressure. These prerupture data could contribute to improved seismic hazard evaluations. Plain Language Summary The three-dimensional prestress field around the 2016 Kumamoto earthquake controlled fault behavior of the earthquake. The estimated heterogeneous state of stress on the fault forced the fault to slip in the direction predicted. The stress field also exposed the precondition of pore fluid pressure on the fault. Large coseismic slip occurred not only at the low-pressure part of the fault but also highly pressured part. It indicates that the seismic moment of the earthquake was magnified by fluid pressure. These prerupture data could contribute to upgrading seismic hazard evaluation.

Focal mechanisms and stress field in the Nobi fault area, central Japan

In this study, we obtained 728 focal mechanisms of small earthquakes with depths shallower than 20 km that occurred from May 2009 to May 2013 in the Nobi fault area in central Japan. The averages of the azimuths of the P- and T-axes were N97° ± 23° E and N6° ± 32° E, and the averages of the dips of the P- and T-axes were 11° ± 10° and 32° ± 25°, respectively. These variations in the P- and T-axes come from variation of the focal mechanisms; both strike-slip and reverse fault earthquakes were observed in the study area. A stress tensor inversion method was applied to the focal mechanisms, and we obtained and characterized the spatial pattern of the tectonic stress. We found that the maximum principal stress (σ1) is oriented E–W over almost the entire study area. The stress ratio R, which is defined as R = (σ1 – σ2)/(σ1 – σ3), ranges from 0.65 to 0.98, and the average R over the entire study area is 0.82. The average stress ratio is close to unity, indicating σ2 ≈ σ3, and thus the dominant stress in this region is a uniaxial compression in the direction of σ1. The direction of the σ1-axis fluctuates locally at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake. This fluctuation is limited to within a very narrow zone across the seismic fault in the upper crust shallower than approximately 10 km, suggesting that most of the deviatoric stress at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake was not released.

Random Inhomogeneities in the Lithosphere Inferred from Coda Analysis

Random inhomogeneities in the lithosphere have been investigated through the analysis of coda waves that are thought to be generated by the inhomogeneities. The power spectral density of upper crustal inhomogeneities measured directly using borehole logs obeys power law for a wide range of wavenumber, which is the characteristics of the exponential or the von K~rman autocorrelation function. The methods preferentially used for the coda analysis are phenomenological modeling based on the energy transport theory or stochastic simulation of wave propagation based on the Born approximation in random media. The parameters estimated from the methods are the scattering coefficient and coda attenuation (Q-1c) of scattering media or the fractional fluctuation of seismic wave velocity and the correlation length of random inhomogeneities. Although the frequency and lapse time dependence of Q-1c has been widely recognized from the numerous works conducted in the last two decades, the physical meaning of Q-1c and its lapse time dependence have not yet been clarified completely. Some recent works tried to understand it by considering layered structures with different values of scattering coefficient and intrinsic attenuation. The estimated range of fractional velocity fluctuation and correlation length is 1-8% and 0.15-1.0 [km] in the crust and 2-8% and 1-20 [km] in the upper mantle, respectively. The magnitude of fractional fluctuation shows little difference between the crust and the upper mantle, however, the correlation distance exhibits large difference that reaches one order of magnitude. In addition, the horizontal correlation length is much longer than the vertical one, forming the anisotropic inhomogeneities in the lithosphere. The recent observations of high-frequency teleseismic Pn gave an additional evidence of random inhomogeneities in the upper mantle. Thus the coda methods have great advantage to get stochastic insight into realistic image of the lithosphere.