Shin Aoi

Trampoline Effect in Extreme Ground Motion

In earthquake hazard assessment studies, the focus is usually on horizontal ground motion. However, records from the 14 June 2008 Iwate-Miyagi earthquake in Japan, a crustal event with a moment magnitude of 6.9, revealed an unprecedented vertical surface acceleration of nearly four times gravity, more than twice its horizontal counterpart. The vertical acceleration was distinctly asymmetric; the waveform envelope was about 1.6 times as large in the upward direction as in the downward direction, which is not explained by existing models of the soil response. We present a simple model of a mass bouncing on a trampoline to account for this asymmetry and the large vertical amplitude. The finding of a hitherto-unknown mode of strong ground motion may prompt major progress in near-source shaking assessments.

Recurrent slow slip event likely hastened by the 2011 Tohoku earthquake

Slow slip events (SSEs) are another mode of fault deformation than the fast faulting of regular earthquakes. Such transient episodes have been observed at plate boundaries in a number of subduction zones around the globe. The SSEs near the Boso Peninsula, central Japan, are among the most documented SSEs, with the longest repeating history, of almost 30 y, and have a recurrence interval of 5 to 7 y. A remarkable characteristic of the slow slip episodes is the accompanying earthquake swarm activity. Our stable, long-term seismic observations enable us to detect SSEs using the recorded earthquake catalog, by considering an earthquake swarm as a proxy for a slow slip episode. Six recurrent episodes are identified in this way since 1982. The average duration of the SSE interoccurrence interval is 68 mo; however, there are significant fluctuations from this mean. While a regular cycle can be explained using a simple physical model, the mechanisms that are responsible for the observed fluctuations are poorly known. Here we show that the latest SSE in the Boso Peninsula was likely hastened by the stress transfer from the March 11, 2011 great Tohoku earthquake. Moreover, a similar mechanism accounts for the delay of an SSE in 1990 by a nearby earthquake. The low stress buildups and drops during the SSE cycle can explain the strong sensitivity of these SSEs to stress transfer from external sources.

Ground motion and rupture process of the 2004 Mid Niigata Prefecture earthquake obtained from strong motion data of K-NET and KiK-net

The 2004 Mid Niigata Prefecture earthquake (37.289°N, 138.870°E, 13.1 km, MJMA 6.8; JMA), also known as the 2004 Niigata Prefecture Chuetsu earthquake, was a thrust type earthquake that occurred on October 23, 2004 at 17:56 (JST). Strong ground motions of PGA 800-1700 cm/s2 and PGV 60-130 cm/s were observed at stations located immediately above the source region. We deduced the rupture process of this earthquake with a multi-time-window linear waveform inversion procedure. We used near-fault strong ground motion data observed at nine K-NET and KiK-net stations within 50 km from the epicenter. In order to obtain appropriate Green’s functions for the waveform inversion, we constructed two velocity structure models for stations on the hanging wall and one structure model for stations on the footwall. The estimated total slip distribution contains three asperities: (a) around the hypocenter, (b) in the upper-middle section of the fault plane, and (c) southwest of the hypocenter. The maximum slip is 3.8 m at the hypocenter and the total seismic moment is 1.2 × 1019 Nm, which corresponds to Mw=6.7. The moment rate functions in asperities (a) and (c) have a short rise time, while those in asperity (b) have a longer rise time.

Source process of the 2007 Niigata-ken Chuetsu-oki earthquake derived from near-fault strong motion data

The 2007 Niigata-ken Chuetsu-oki earthquake generated strong ground motions in Kashiwazaki and Kariwa, where the world largest nuclear power plant was in operation. Due to the complexity of the aftershock distribution, activation of the northwest-dipping fault and/or the southeast-dipping fault is proposed. To explore the fault geometry and source process of the earthquake, we performed multi-time window linear waveform inversions for both the fault planes from near-fault strong motion data. A fault plane model of 30 km in length by 24 km in width was set to cover the region of aftershock distribution within 24 h of the mainshock. Both inverted slip models provided moment magnitudes of 6.7 with a small asperity near the rupture starting point, and a large asperity approximately 10 km southwest of the rupture initiation, which is located in the region of relatively sparse aftershock distribution. Both the small and large asperities are located near the intersection between the two conjugate fault plane models, and the asperities of both models have similar radiation patterns. Therefore, the difference of the residuals between the observed and synthetic waveforms for both models was not significant, indicating that it is difficult to conclude which fault is the rupture.

Strong localized asperity of the 2011 Tohoku-Oki earthquake, Japan

Our moment tensor inversion of the 2011 Tohoku-Oki earthquake, using regional broadband strong-motion waveforms, indicates that the earthquake can be approximated by a point source. The amplitude spectra of the observed displacement seismograms were fitted by the omega-square model, which resulted in the corner frequency of around 0.016 Hz. This implies a large slip over a circular fault having a radius of 70 km, with a rupture duration of about 40 s. The moment-rate function estimated from the inversion shows a large impulse of similar duration. We interpret this impulse to correspond to the rupture estimated from the corner frequency. From the seismic moment released during the impulse, we have estimated the average slip and stress drop over the fault to be 50 m and 40 MPa, respectively. This stress drop corresponds to an effective normal stress larger than 200 MPa, indicating that a strong localized asperity (mega asperity) was ruptured during the earthquake. Previous simulation studies suggested the importance of a large effective normal stress at a shallow plate interface, which was explained by a pore pressure distribution along the plate. We have explored the possibility of a subducted seamount to be the origin of the mega asperity.

Array Back-Projection Imaging of the 2007 Niigataken Chuetsu-oki Earthquake Striking the World’s Largest Nuclear Power Plant

The 2007 Niigataken Chuetsu-oki earthquake occurred near the Kashiwazaki–Kariwa nuclear power plant in Japan, the largest in the world. The strong motions were recorded by seven seismometers installed at the foundation slab (base-mat) of the plant and exceeded the design level of the ground motion for the plant. The strong motion observed by the seismographs in and around the plant show high coherency with three significant pulses. In order to understand the cause of these pulses, the rupture process of the earthquake was estimated using these seismograms. The seismograph network was taken into account as a dense array and semblance-enhanced waveform stacking was performed. By projecting the power of the stacked waveforms onto the fault plane, the asperities that generated significant pulses were successfully separated. The first and third pulses were generated at the hypocenter and the southwest edge of the rupture zone, respectively. The rupture propagated toward the southwest and terminated offshore from the power plant. The overall pattern of the imaged asperities coincides well with the slip distribution determined by conventional waveform inversions.

Rupture process of the 2005 West Off Fukuoka Prefecture earthquake obtained from strong motion data of K-NET and KiK-net

We have investigated the rupture process of the 2005 West Off Fukuoka Prefecture earthquake by the multitime- window linear waveform inversion method using the strong ground motion data recorded at 11 K-NET and KiK-net stations. From the waveforms of the P-wave portion, it is indicated that the energy release in the first few seconds was markedly lower than the subsequent part, and this causes difficulty in identifying onset of the S-wave. To decide an appropriate time window for the waveform inversion, we estimate the S-wave onset using aftershock records. The inverted slip distribution shows a single asperity of 8 km × 6 km and its center located 8 km to the southeast and 6 km above the hypocenter. The asperity explains most of the large-amplitude signals in the observed waveforms. The turning point from the initial low-energy-release rupture to the main high-energyrelease rupture is estimated from the spatial variation of the observed initial rupture phase. It is found 3.3 s after the initiation of the rupture at about 4 km to the southeast of the hypocenter. Stress drop during the initial rupture is estimated to be in the same order of those of moderate size aftershocks, which indicates that the initial rupture is an ordinary dynamic rupture.

Source estimates of the May 2006 Java earthquake

In the early morning of 27 May 2006, local time, central Java was jolted by strong seismic ground motion. In spite of the moderate size of the earthquake (Mw = 6.4), it caused severe damage nearYogyakarta city. According to a report from the Social Affairs Ministry of Indonesia, more than 5700 people were killed, 38,000 injured, and 423,000 evacuated. As a result of the shaking, more than 126,000 buildings collapsed, and more than 392,000—including those of the famous Prambanan temple complex, located about 17 kilometers east of Yogyakarta—were severely damaged (Figure 1).

Rupture process of the 2007 Notohanto earthquake by using an isochrones back-projection method and K-NET/KiK-net data

We estimated the source process of the 2007/3/25 Notohanto earthquake using a new method for source imaging based on an “isochrones-backprojection” of observed seismograms in the source region (IBM). The IBM differs from conventional earthquake source modeling approaches in that no inversion procedures are required. The idea of IBM is to directly back-project amplitudes of seismogram envelopes around the source into a space image of the earthquake rupture. The method requires the calculation of isochrones times at every station used for source imaging, for a set of grids points distributed within the source fault plane. Total grid “brightness” is calculated by adding all observed waveform envelope amplitudes at every station, for every isochrone line crossing the grid, in order to produce an image of the total fault plane brightness distribution. Our source imaging results of the Notohanto earthquake show two large brightness regions; the first region is located 10 km above the hypocenter, and the second region is located at the bottom of the northern end of the fault plane. These regions approximately correspond to large slip areas obtained by a conventional inversion approach. Our method has the capability to quickly map asperities of large earthquakes using observed strong motion data.

Boundary Shape Waveform Inversion for Estimating the Depth of Three-Dimensional Basin Structures

I propose a waveform inversion scheme that estimates three-dimensional basin structure, in particular, the depth of the boundary between sediment and bedrock. This study is an extension of the boundary shape waveform inversion scheme (Aoi et al. , 1995, 1997) to a three-dimensional structure. The idea is to directly parameterize the basin topography and invert it by using long-period strong ground motions including basin-induced waves and direct waves. The depth of the topography is represented by a series of model parameters and basis functions that are rectangular (boxcar) functions. By this choice of basis functions, the inversion determines the average depth of the bedrock inside rectangles with the constraint that the observation equation, which is nonlinear in the model parameters, be satisfied best in the sense of least squares. The observation equation is linearized by omitting higher-order terms and is solved iteratively by singular value decomposition. To solve this equation, sensitivity functions (differential seismograms) are required, as well as the synthetic waveforms themselves. In this study, the finite-difference method (FDM) is used to calculate waveforms. Sensitivity functions are obtained numerically by taking the difference of waveforms from perturbed and unperturbed models. The positions of the discontinuity of the medium are required to lie on, not between, the finite-difference (FD) grid points to maintain the accuracy of FD calculation. The basis functions having rectangular domain are chosen to overcome this limitation. Since the basis functions take a constant value in a particular rectangle, inside the rectangle the perturbed model is just one grid deeper than the unperturbed model. The inverted correction values of the parameters for each iteration are rounded to the nearest multiple of the grid spacing of the FD calculation. I show the formulation of the proposed scheme and demonstrate its validity by performing a numerical experiment.