Takashi Kunugi

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.

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.

Underdamped responses of a well to nearby swarm earthquakes off the coast of Ito City, central Japan, 1995

We observed coseismic and noncoseismic water level changes of an underdamped well-aquifer system at epicentral distances of ∼5 km during the 1995 earthquake swarm activity off the coast of Ito City, Shizuoka Prefecture, central Japan. Each water level change consists of a static offset with a linear trend component followed by a damped harmonic oscillation. The characteristic frequency and decay parameter of these oscillations are 18.3 s and 0.024 s−1, respectively. We detected 14 such events within 8 days during the activity. The offset values are mostly 2 mm or less, but there were two exceptionally large offsets of 4 and 6 mm. The observed waveforms are consistent with an analytical expression for the theoretical well level response to step-like pressure head change in an underdamped well-aquifer system. This consistency allowed us to estimate the parameters of the well-aquifer system and the offset component of pressure head changes from the observed hydrographs. The seismic records at the nearest station indicate that among the 14 events only seven, including the two exceptionally large events, were apparently synchronous with swarm earthquakes. Among these seven events the largest two are quantitatively explained as the consequence of the coseismic volumetric strain changes. The other five are interpreted as either the effect of coseismic strain change near a node of the volumetric strain field or some other coseismic effects. The seven well events not synchronous with earthquakes indicate the occurrence of abrupt local pressure disturbances in the aquifer within a distance of a few tens of meters from the well.

Iodine-stabilized Nd:YAG laser applied to a long-baseline interferometer for wideband earth strain observations

We stabilized a frequency-doubled Nd:YAG laser (λ=532 nm) with reference to an iodine absorption line, and applied it to a long-baseline interferometer for earth strain observations. To obtain unmodulated light, saturated absorption signals of an external iodine cell were detected by the modulation transfer technique using an acousto-optic modulator working as both amplitude and frequency modulators. Two feedback loops, which could control the laser frequency by a piezo-electric actuator and a thermal actuator, realized fast and wide-range frequency stabilization, and ensured long-term stable operation. From a beat-note measurement between two identical systems, we obtained a frequency stability (in Allan variance) of ≲2×10−13 for time intervals of 10 to 1000 s. By applying the stabilized Nd:YAG laser to a light source of a 10 m interferometer, we successfully observed earth tides and earthquakes in strain variations. Other geophysical signals, detectable by this strainmeter, are also discussed.

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.

Rapid estimation of tsunami source centroid location using a dense offshore observation network

This paper proposes a rapid method of estimating tsunami source locations using real-time ocean-bottom hydrostatic pressure data from a dense offshore observation network. We defined two characteristic locations representing the real-time tsunami disturbance and the initial sea surface height distribution. First, we defined the tsunami centroid location (TCL), which is the centroid location of the maximum absolute amplitude of the real-time ocean-bottom hydrostatic pressure changes. Second, we defined the centroid location of the absolute values of the initial sea surface height displacements. To determine whether the TCL can approximate the centroid location of the tsunami source, we examined approximately 1000 near-field synthetic tsunami scenarios and a realistic tsunami scenario of the 2011 Tohoku earthquake in the Japan Trench. From these examinations, it was confirmed that in most scenarios, the TCLs obtained within a few minutes after the occurrence of an earthquake were close to the actual corresponding tsunami source locations.

Multi-index method using offshore ocean-bottom pressure data for real-time tsunami forecast

We developed a real-time tsunami forecast method using only pressure data collected from the bottom of the ocean via a dense offshore observation network. The key feature of the method is rapid matching between offshore tsunami observations and pre-calculated offshore tsunami spatial distributions. We first calculate the tsunami waveforms at offshore stations and the maximum coastal tsunami heights from any possible tsunami source model and register them in the proposed Tsunami Scenario Bank (TSB). When a tsunami occurs, we use multiple indices to quickly select dozens of appropriate tsunami scenarios that can explain the offshore observations. At the same time, the maximum coastal tsunami heights coupled with the selected tsunami scenarios are forecast. We apply three indices, which are the correlation coefficient and two kinds of variance reductions normalized by the L2-norm of either the observation or calculation, to match the observed spatial distributions with the pre-calculated spatial distributions in the TSB. We examine the ability of our method to select appropriate tsunami scenarios by conducting synthetic tests using a scenario based on “pseudo-observations.” For these tests, we construct a tentative TSB, which contains tsunami waveforms at locations in the Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench and maximum coastal tsunami heights, using about 2000 tsunami source models along the Japan Trench. Based on the test results, we confirm that the method can select appropriate tsunami scenarios within a certain precision by using the two kinds of variance reductions, which are sensitive to the tsunami size, and the correlation coefficient, which is sensitive to the tsunami source location. In this paper, we present the results and discuss the characteristics and behavior of the multi-index method. The addition of tsunami inundation components to the TSB is expected to enable the application of this method to real-time tsunami inundation forecasts in the near future.

Source Process of the 2011 Off the Pacific Coast of Tohoku Earthquake

The kinematic source process of the 2011 Off the Pacific Coast of Tohoku earthquake is studied using strong motion data both in low- and high-frequency ranges. The slip distribution is estimated by the waveform inversion analysis using velocity waveforms in the frequency range from 0.01 to 0.125 Hz at strong motion stations along the Pacific coast. The strong motion generation area (SMGA) is estimated by the strong ground motion simulation in 0.1–10 Hz using the empirical Green’s function method. The slip distribution is characterized by a large asperity with peak slip of 48 m which is imaged in the shallower portion of the source fault near the Japan Trench. Four SMGAs are identified in the deeper portion of the source fault. Unlike the past M7–8 subduction-zone plate-boundary events, the SMGAs and the asperity seem to be complementary in space. But the rupture time of each SMGA matches the timing of slip in each area. The total size of SMGAs is much smaller than the asperity area. This event coincides with empirical scaling relationships between total rupture area, asperity area, SMGA, and its seismic moment proposed for subduction-zone plate-boundary earthquakes by previous papers although the asperity abstracted for this event may have different nature from those of past smaller earthquakes.