Hiroe Miyake

Source Characterization for Broadband Ground-Motion Simulation: Kinematic Heterogeneous Source Model and Strong Motion Generation Area

We estimate strong motion generation areas that reproduce near-source ground motions in a broadband frequency range (0.2-10 Hz) using the empirical Greens function technique. The strong motion generation areas are defined as extended areas with relatively large slip velocities within a total rupture area. Four M 6 class (the 1997 Kagoshima on March and May, 1997 Yamaguchi, and 1998 Iwate) and several moderate-size earthquakes in Japan were analyzed. We examine the relationship between the strong motion generation area and the area of asperities, which is characterized based on heterogeneous slip distributions estimated from low-frequency (<1 Hz) waveform inversions (Somerville et al. , 1999). We performed waveform fitting in acceleration, velocity, and displacement, then obtained the strong motion generation area occupying about a quarter of the total rupture area. The size and position of the strong motion generation area coincide with those of characterized asperities. We find self-similar scaling of seismic moment to both the size of the strong motion generation area and the rise time for a magnitude range of analyzed earthquakes. Based on our results, we propose a characterized source model for the broadband ground-motion simulation, which consists of strong motion generation areas with large slip velocities and a background slip area with a small slip velocity. Waveform modeling of the characterized source model suggests that the strong motion generation areas play important roles in simulating broadband ground motions and have the potential to be an about 10-MPa stress-release area on the fault. Manuscript received 22 August 2002.

Mapping pressurized volcanic fluids from induced crustal seismic velocity drops

Seismic noise reveals volcanic plumbing Monitoring the way in which seismic noise passes through Earths crust after a large earthquake can clarify how volcanoes erupt. Japan has the highest-density seismic network in the world. Brenguier et al. observed reductions in seismic velocity below volcanic regions of Japan from before, to the weeks and months after the 2011 Tohoku-Oki earthquake (see the Perspective by Prejean and Haney). This indicates that pressurized fluids below volcanoes can weaken in response to dynamic stress perturbations. Science, this issue p. 80; see also p. 39 The stress response from a large earthquake can identify crustal areas with pressurized volcanic fluids. [Also see Perspective by Prejean and Haney] Volcanic eruptions are caused by the release of pressure that has accumulated due to hot volcanic fluids at depth. Here, we show that the extent of the regions affected by pressurized fluids can be imaged through the measurement of their response to transient stress perturbations. We used records of seismic noise from the Japanese Hi-net seismic network to measure the crustal seismic velocity changes below volcanic regions caused by the 2011 moment magnitude (Mw) 9.0 Tohoku-Oki earthquake. We interpret coseismic crustal seismic velocity reductions as related to the mechanical weakening of the pressurized crust by the dynamic stress associated with the seismic waves. We suggest, therefore, that mapping seismic velocity susceptibility to dynamic stress perturbations can be used for the imaging and characterization of volcanic systems.

Long-period ground motions from a large offshore earthquake: The case of the 2004 off the Kii peninsula earthquake, Japan

The 2004 off the Kii Peninsula earthquake excited long-period ground motions over a wide area of Honshu Island of Japan. This remarkable excitation was observed in the Osaka, Nobi, and Kanto basins as well as in the Omaezaki region. The record section indicates two types of developed long-period motions by the basin surface waves, either by the source or passage effect of the shallow and large offshore earthquake. Their combination resulted in the well-developed long-period ground motions observed within the distant basins as those during the 1985 Michoacan and 2003 Tokachi-oki earthquakes did. The distributions of pseudo-velocity response spectra confirmed this development at periods of 5–7 s in the Osaka and Nobi basins and of 7–10 s in the Kanto basin. The comparison of the distributions with the thicknesses of the sediments and the S-wave velocities of the surface layers shows that these characteristics of the long-period ground motions are closely related to the structures of the basins. The earthquake provided a timely warning of damaging long-period ground motions from future megathrust events in the Tonankai, Nankai, and Tokai regions.

Scaling of characterized slip models for plate-boundary earthquakes

We characterized source rupture models with heterogeneous slip of plate-boundary earthquakes in the Japan region. The slip models are inferred from strong-motion, teleseismic, geodetic, or tsunami records. For the identification of asperities in the slip models, we found that the area of subfaults retrieved with slips of >1.5 times the total average slip provides a size approximately equivalent to the characterized asperity by Somerville et al. (1999). We then carried out regression analyses of the size and slip for the rupture area and asperity. The obtained scaling relationship to the seismic moment indicates that rupture area S, average slip D, and combined area of asperities Sa are 1.4, 0.4, and 1.2 times larger, respectively, than those of crustal earthquakes. In contrast, the ratios of the size and slip between the asperities and rupture area (Sa/S and Da′/D) are the same for plateboundary earthquakes as for crustal earthquakes. The above analyses indicate that plate-boundary and crustal earthquakes share similar source characteristics.

Surface Rupturing and Buried Dynamic-Rupture Models Calibrated with Statistical Observations of Past Earthquakes

In the context of the slip-weakening friction model and simplified asper- ity models for stress state, we calibrate dynamic rupture models for buried and surface-rupturing earthquakes constrained with statistical observations of past earth- quakes. These observations are the kinematic source models derived from source in- versions of ground-motion and empirical source models of seismic moment and rupture area. The calibrated parameters are the stress-drop distribution on the fault and average stress drop. We develop a set of dynamic rupture models that consist of asperities and surrounding background areas. The distribution of dynamic stress drop outside the asperity is characterized by a fraction of the stress drop on the as- perity. From this set of models, we identify dynamic fault models with defined stress- drop characteristics that satisfy the observations. The selected dynamic fault models show that surface-rupturing earthquakes are characterized by a large area of negative stress-drop surrounding the asperities, while buried earthquakes present positive or zero stress drop. In addition, the calibrated fault models that match the observations show that the average stress drop is independent of earthquake size for buried earth- quakes, but scale dependent for surface-rupturing earthquakes. This suggests that, in the context of our parameterization, buried earthquakes follow self-similarity scaling, and surface-rupturing earthquakes break this self-similarity. We apply the calibrated dynamic models to simulate near-source ground motion consistent with observations that suggest that buried earthquakes generate stronger ground motion than surface- rupturing earthquakes at high frequency. We propose possible mechanisms that satisfy this observation, as follows: buried rupture has a hypocenter location below the as- perity; this can produce strong directivity of the slip velocity function toward the free surface. That effect, in addition to a reduced fault area and low fracture energy during rupture, may be significant in enhancing high-frequency ground motion. On the other hand, surface-rupturing earthquakes have a shallow hypocenter, large fracture energy on the asperities, and enhanced energy absorption due to large areas of negative stress drop in the background area. These characteristics of large earthquakes inhibit severe directivity effects on the slip velocity function directly toward the free surface, redu- cing the high-frequency ground motion.

Estimation of rupture propagation direction and strong motion generation area from azimuth and distance dependence of source amplitude spectra

Strong motion generation areas which reproduce ground motions in 0.2 to 10Hz were estimated using the empirical Greens function method. This strong motion generation area was somewhat smaller than the total rupture area, and coincident with the area of asperities derived from heterogeneous slip distributions estimated by waveform inversions using lower frequencies (<1Hz). We confirmed that the azimuth and distance dependence of observed source amplitude spectra in the near-source area, i.e. rupture directivity effects, were controlled by rupture propagation style and size of the strong motion generation area. We found that the source displacement spectra at stations in forward rupture propagation directions had higher corner frequencies and steeper high-frequency decays, compared with stations in sideways directions. Stations in backward directions had opposite tendencies. Different relationships between size and average corner frequencies of the strong motion generation area were proposed for unilateral and bilateral ruptures with radial propagation.

On Scaling of Fracture Energy and Stress Drop in Dynamic Rupture Models: Consequences for Near‐Source Ground‐Motions

We calculate spontaneous dynamic rupture models for several well-recorded moderate to large earthquakes and analyze the scaling properties of fracture energy and stress drop. Among the set of 12 source models for 9 different earthquakes, the large events did break the surface while the moderate-size events occurred as completely buried ruptures (i.e. no surface faulting). We find that dynamic and static stress drop differ by only about 10%. Fault-averaged stress drop increases with increasing earthquake magnitude, while also fault-averaged (or maximum) fracture energy grows with magnitude. The scaling of fracture energy with the stress intensity factor appears to be sensitive to whether or not the earthquake rupture broke the surface, indicating that large earthquakes consume more fracture energy as the rupture expands and reaches the surface. This scaling of fracture energy may shed light on the recent observation that large, surface breaking earthquakes apparently generate lower near-source ground motions than buried ruptures in a certain period range of engineering interest. The derived empirical scaling relations for fracture energy may help to constrain the initial conditions for future dynamic rupture modeling, but can also be used in physics-based source characterization for near-source ground-motion calculations.

Long‐period seismic amplification in the Kanto Basin from the ambient seismic field

Tokyo, like many seismically threatened cities, is situated atop a sedimentary basin that has the potential to trap and amplify seismic waves from earthquakes. We study amplification in the Kanto Basin by exploiting the information carried by the ambient seismic field. We use 375 seismic stations from the high sensitivity seismograph network across central Honshu as virtual sources and 296 seismic stations of the Metropolitan Seismic Observation network shallow borehole seismometers within the basin as receivers to map the basin response. We find a linear relationship between ground motion and basin depth at periods of 2–10 s that could be used to represent 3-D basin effects in ground motion prediction equations. We also find that the strength of basin seismic amplification depends strongly on the direction of illumination by seismic waves.

Widespread ground motion distribution caused by rupture directivity during the 2015 Gorkha, Nepal earthquake

The ground motion and damage caused by the 2015 Gorkha, Nepal earthquake can be characterized by their widespread distributions to the east. Evidence from strong ground motions, regional acceleration duration, and teleseismic waveforms indicate that rupture directivity contributed significantly to these distributions. This phenomenon has been thought to occur only if a strike-slip or dip-slip rupture propagates to a site in the along-strike or updip direction, respectively. However, even though the earthquake was a dip-slip faulting event and its source fault strike was nearly eastward, evidence for rupture directivity is found in the eastward direction. Here, we explore the reasons for this apparent inconsistency by performing a joint source inversion of seismic and geodetic datasets, and conducting ground motion simulations. The results indicate that the earthquake occurred on the underthrusting Indian lithosphere, with a low dip angle, and that the fault rupture propagated in the along-strike direction at a velocity just slightly below the S-wave velocity. This low dip angle and fast rupture velocity produced rupture directivity in the along-strike direction, which caused widespread ground motion distribution and significant damage extending far eastwards, from central Nepal to Mount Everest.

Long-period ground motion simulation of a subduction earthquake using the offshore-onshore ambient seismic field

Large earthquakes that occur in subduction zones are likely to generate long-period ground motions that can cause severe damage even at great distances from the epicenter. We extracted surface-to-surface impulse response functions from the ambient seismic field recorded by offshore ocean bottom seismometers located atop the Nankai subduction zone and onshore stations. We showed that these offshore-onshore impulse response functions can be used to accurately simulate the long-period ground motions generated by an offshore moderate subduction earthquake. Moreover, we also found that the distributions of the earthquake and impulse response function pseudovelocity response spectra have similar maximum amplifications in the same area close to the earthquake epicenter. This suggests that the ambient seismic field recorded by the increasing number of ocean bottom seismometers around the world can be used to assess seismic hazard related to offshore subduction earthquakes without prior knowledge of the velocity structure.