Mitsuyuki Hoshiba

Earthquake Early Warning Starts Nationwide in Japan

When an earthquake occurs, a certain amount of time elapses before destructive seismic energy hits nearby population centers. Though this time is measured on the order of seconds, depending on the proximity of the rupture to a given city or town, a new public safety program in Japan is taking advantage of the fact that seismic energy travels slower than electronic communication. In this program, the Japan Meteorological Agency (JMA) rapidly determines the hypocenter (earthquake epicenter and focal depth) and magnitude of the earthquake by using real-time data from stations near the hypocenter. The distribution of strong ground shaking is anticipated quickly, and then the information is delivered immediately to government officials, representatives from various industries, members of the news media, and individuals before strong ground shaking reaches them. For example, on receiving the warning, the control room of a railway company can send an emergency notice to all train drivers to stop their trains immediately, elevators in buildings can be triggered to stop at the nearest floor and open their doors automatically, and surgeons can temporarily suspend their surgical operations to avoid risk to patients on operating tables.

Initial results from WING, the continuous GPS network in the western Pacific area

To investigate tectonic deformation in the western Pacific, a continuous GPS tracking network has been established, and named the Western Pacific Integrated Network of GPS (WING). Between 1995 and March 1997 we establised ten new sites. Data for the period July 1995 to October 1996 were analyzed, together with data from International GPS Service for Geodynamics (IGS) global sites, to estimate daily coordinates. A fiducial-free approach was used to obtain the most accurate baseline estimates. To fix the estimated coordinates to the terrestrial reference frame, the Tsukuba IGS site is assumed to be moving westward relative to the stable Eurasian continent at ∼2 cm/yr according to Hekis [1996] estimate. We find that: (1) velocities of sites well within oceanic plates are in good agreement with rigid plate motion models; (2) sites close to plate boundaries are all affected by the deformation at those boundaries, among which back-arc rifting (spreading) is clearly visible at the Mariana and Okinawa troughs; (3) sites in eastern Asia are moving east to east-southeast relative to the stable Eurasian continent, suggesting long distance effects of the northward collision of India with Asia.

Simulation of coda wave envelope in depth dependent scattering and absorption structure

Coda waves are considered to be composed of scattered waves. For simplicity, previous models of coda wave generation have often assumed spatial uniformity of scattering and absorption. To account for possible depth dependent attenuation, the energy density of coda waves are synthesized using the Monte Carlo simulation method with multiple isotropic scattering assumption. Uniform velocity structure is assumed and impulsive point source is used. In depth dependent structures given in this paper, the synthesized envelopes give coda Q−1 decreasing with increasing lapse time, which has been observationally known well but has not been adequately explained with uniform models. The energy density becomes asymptotically close to a common decay curve irrespective of epicentral distances as the lapse time increases. However, the amount of the energy density from two sources is different at the same receiver at even the same lapse time for the same radiation energy when the focal depths are different. Theoretical basis of the conventional coda normalization method is examined in cases of the strong depth dependent attenuation structure.

GPS seismometers with up to 20 Hz sampling rate

The large near-field displacements before and during an earthquake are invaluable information for earthquake source study and for the detection of slow/silent quakes or pre-seismic crustal deformation events. However due to bandwidth limitations and saturation current seismometers cannot measure many of these displacements directly. In a joint experiment between the University of New South Wales (UNSW) and the Meteorological Research Institute (MRI), two Trimble MS750 GPS receivers were used in the Real-Time Kinematic (RTK) mode with a fast sampling rate of up to 20 Hz to test the feasibility of a “GPS seismometer” in measuring displacements directly. The GPS antenna, an accelerometer, and a velocimeter were installed on the roof of an earthquake shake-simulator truck. The simulated seismic waveforms resolved from the RTK time series are in very good agreement with the results from the accelerometer and the velocimeter, after integrating twice and once respectively. Moreover, more displacement information are revealed in the GPS RTK results although they are noisier.

Determination of the location of faulting beneath Kobe During the 1995 Hyogo‐Ken Nanbu, Japan, Earthquake from near‐source particle motion

We constrained the location of faulting beneath Kobe during the 1995 Hyogo-ken Nanbu (Kobe), Japan, earthquake by investigating the particle motion of strong motion records from sites close to the causative fault. The intersection of the earths surface and the extension of buried fault plane is consistent with known active fault lines but not with the severely damaged zone. The intersection line also agrees with geodetic data.

Earthquake early warning for the 2016 Kumamoto earthquake: performance evaluation of the current system and the next-generation methods of the Japan Meteorological Agency

The 2016 Kumamoto earthquake (Kumamoto earthquake sequence) is an extremely high-seismicity event that has been occurring across Kumamoto and Oita Prefectures in Japan since April 14, 2016 (JST). The earthquake early warning system of the Japan Meteorological Agency (JMA) issued warnings for 19 events in the Kumamoto earthquake sequence from April 14 to 19, under some of the heaviest loading conditions since the system began operating in 2007. We analyzed the system performance for cases where a warning was issued and/or strong motion was actually observed. The results indicated that the system exhibited remarkable performance, especially for the most destructive earthquakes in the Kumamoto earthquake sequence. In addition, the system did not miss or seriously under-predict strong motion of any large earthquake from April 14 to 30. However, in four cases, the system issued over-predicted warnings due to the simultaneous occurrence of small earthquakes within a short distance, which implies a fundamental obstacle in trigger-data classifications based solely on arrival time. We also performed simulations using the integrated particle filter (IPF) and propagation of local undamped motion (PLUM) methods, which JMA plans to implement to address over-prediction for multiple simultaneous earthquakes and under-prediction for massive earthquakes with large rupture zones. The simulation results of the IPF method indicated that the IPF method is highly effective at minimizing over-prediction even for multiple simultaneous earthquakes within a short distance, since it adopts a trigger-data classification using velocity amplitude and hypocenter determinations using not-yet-arrived data. The simulation results of the PLUM method demonstrated that the PLUM method is capable of issuing warnings for destructive inland earthquakes more rapidly than the current system owing to the use of additional seismometers that can only be incorporated by this method.Graphical abstract.

Real-time seismic intensity prediction using frequency-dependent site amplification factors

A promising approach for the next generation of earthquake early warning system is based on predicting ground motion directly from observed ground motion, without any information of hypocenter. In this study, we predicted seismic intensity at the target stations from the observed ground motion at adjacent stations, employing two different methods of correction for site amplification factors. The first method was frequency-dependent correction prediction, in which we used a digital causal filter to correct the site amplification for the observed waveform in the time domain. The second method was scalar correction, in which we used average differences in seismic intensity between two stations for the site amplification correction. Results from thousands of station pairs that covered almost all of Japan showed that seismic intensity prediction with frequency-dependent correction prediction was more accurate than prediction with scalar correction. Frequency-dependent correction for site amplification in the time domain may lead to more accurate prediction of ground motion in real time.

Observation of Local and Distant Earthquakes and Tsunami Warning System

In Japan, Japan Meteorological Agency, JMA, has the official responsibility for the public for tsunami forecasts and issuance of tsunami information. For the tsunami forecast the first step is precise and prompt estimation of the location and magnitude of the earthquake because generation of tsunami is strongly controlled by the focal depth and the magnitude of the earthquake. In case that the location is at sea, focal depth is shallow and magnitude is large, we should expect the tsunami generation. For local tsunami, since JMA started its tsunami forecast in 1952, constant efforts have been made for sophistication and speedup of the forecast for over 50 years. The continuous improvements enable us to issue a forecast 3 to 4 minutes at the quickest after the occurrence of the earthquake. For distant tsunami such as ocean wide destructive one, international cooperation of information exchange is important. It is necessary to collect seismic waveform data in quasi-real-time from foreign countries for the precise forecast, and sea level data for tsunami observation. For both local and distant cases, it is important to keep the seismic observation network and analyzing system well maintained for reliable tsunami forecast. Biography Dr. Mitsuyuki Hoshiba joined Japan Meteorological Agency, JMA, after graduation from Kyoto University, Japan, and since then he has worked as a seismologist. After spending 17 months as an observer of earthquakes and volcanoes at Fukuoka branch office of JMA, he moved to Meteorological Research Institute, MRI, where he worked for 15 years as a researcher on seismic wave propagation. He received a doctorate in geophysics in 1993 from Kyoto University with a dissertation on multiple scattered seismic waves and excitation of seismic coda. He stayed for 6 months at University of Southern California for a cooperative research in 1993, and spent a year as visiting scientist at University of Munich in 1996. Since two years ago, he has worked at headquarter of JMA as a deputy director of Earthquake Prediction Information Division. When a large earthquake occurs anywhere in Japan, he rushes to his office and studies the data.

Description of Coda Wave Envelope Using Radiative Transfer Theory and Monte Carlo Simulation

It is widely accepted that coda waves are composed of incoherent scattered waves for high frequencies. Using the random phase approximation, approaches based on the radiative transfer theory (energy transport theory) have been adopted instead of wave propagation theory itself for the construction of models which describe the envelopes of coda waves. The radiative transfer theory deals directly with the transport of energy through a medium containing scatterers. Since the first paper in 1960s on the single scattering model for cases of weak scattering and that on the diffusion model for cases of strong scattering, a number of improved models have been proposed to expand these models into more complicated cases, e.g., non isotropy in scattering, non spherical source radiation, and the case of separated locations of source and receiver. The single scattering model satisfies causality but does not energy conservation law; on the other hand the diffusion model satisfies the energy conservation law but does not causality. Multiple scattering models have been proposed in 1980s to consider an interpolation between the weak and strong scattering cases. The multiple scattering model can be applicable without assuming the strength of scattering, and satisfies both energy conservation law and causality, but has no simple analytical expression in 3 dimensional space. In parallel with the analytical development, several approaches using Monte Carlo technique were conducted to simulate multiple scattering processes and synthesize the coda wave envelope. Using Monte Carlo simulation the coda wave envelope can be synthesized even in the case of complicated structure having depth dependence on both scattering strength and velocity, beyond mathematical difficulties in analytical studies. The radiative transfer approach has provided a framework for measurements of scattering attenuation and intrinsic absorption, and the comparison of the theory with observation has led us to understand the stochastic inhomogeneity in the real Earth.