Yo Fukushima

Laboratory Study on Scattering Characteristics of Shear Waves in Rock Samples

To understand the scattering characteristics of seismic shear waves in the Earth, a laboratory physical model experiment in ultrasonic frequency range was performed. By analyzing waveform envelopes and particle motions of shear waves in media with different correlation distance and fluctuation intensity, we studied the strength of scattered wave excitation and the distortion of shear-wave polarization as a function of ka (wavenumber times the correlation distance). We used gabbro and granite as media having different small-scale random heterogeneities. The granite we used contained preferred-oriented thin microcracks. The correlation distance a and the standard deviation of the fractional fluctuation of the shear-wave velocity ϵ were estimated by fitting exponential-type autocorrelation functions. The estimated values were a = 0.84 mm and ϵ = 8.1% for the gabbro, and a = 0.39 mm and ϵ = 17.0% for the granite. Waveform measurements were performed in the frequency range 0.25-1 MHz, which roughly corresponds to ka as 0.25-1. Envelope broadening appeared when ka exceeds 0.4 for the granite and 1.4 for the gabbro. Shear-wave polarizations were distorted by scattering when ka exceeds 0.2 for the granite and 0.7 for the gabbro. The results on envelope broadening suggest the importance of large-angle scattering due to small-scale random heterogeneities and cracks in the envelope broadening, in addition to diffraction effects due to large-scale heterogeneities. The effect of aligned microcracks on scattering was also examined. It was found that scattering was prominent when shear wave propagates perpendicular to the aligned crack plane. Manuscript received 1 March 2002.

Japanese L-Band Radar Improves Surface Deformation Monitoring

The termination of the Japanese Earth Resources Satellite (JERS 1) in October 1998 and the subsequent delay of the launch of its successor, the Advanced Land Observing Satellite (ALOS), caused a troublesome interruption in the availability of L-band synthetic aperture radar (SAR) data to the several research institutions and government agencies in Japan that had been studying surface deformation by SAR techniques since the Japan Aerospace Exploration Agency (JAXA, formerly the National Space Development Agency of Japan) launched JERS 1 in 1992. SAR is an all-weather type of sensor that is suitable for the derivation of disaster mitigation information, and—as shown, for example, by Massonnet and Feigl [1998]—interferometric SAR (InSAR) techniques can be used to derive precise ground surface deformation maps associated with earthquakes, volcanism, landslides, and slow tectonic motions, as well as with oil pumping and water withdrawal, with fine spatial accuracy. The electromagnetic similarity (coherence) between two SAR images acquired over the same area before and after an event—for instance, an earthquake—is measured, and where appropriately high coherence is achieved, the change in the distance between SAR and the ground target can be derived with fine (centimeter) precision. Both airborne and spaceborne SAR systems are commonly used, although the latter often are preferred due to their long-term monitoring capacity and global coverage.

Fault model of the 2007 Noto Hanto earthquake estimated from PALSAR radar interferometry and GPS data

The 2007 Noto Hanto earthquake (Mw 6.6) is the first major earthquake in inland Japan since the launch of the Japanese ALOS satellite. Interferometric synthetic aperture radar successfully mapped the coseismic displacements from both ascending and descending orbits in and around the earthquake epicentral region. The line-of-sight displacements toward the satellite from the ascending and descending orbits amount to about 50 and 20 cm, respectively. The interferograms as well as the GPS data of the GEONET network are inverted in order to determine the slip distribution on the source fault. The dip and strike angles of the preferred model are 48° and N51°E, respectively, and its fault slip area reaches the seafloor. The dip angle is not well constrained in our tested range between 40° and 60°. The dominant slip area is located just above the hypocenter, at depths between 4 and 10 km. Many aftershocks seem to have occurred on and around the area of dominant slip.

Relationship between Fluctuations of Arrival Time and Energy of Seismic Waves and Scale Length of Heterogeneity: An Inference from Experimental Study

In order to study the effect of heterogeneity on the arrival time and energy fluctuations of seismic waves, we made laboratory-scale physical model experiments by observing elastic waves propagating through crystalline rock samples having different scales of heterogeneity. A laser Doppler vibrometer was used to measure the waveforms. About 100 waveforms were obtained over a square grid with a spacing of 1 mm in linear profiles for Westerly and Oshima granite samples, which correspond to small and large scales of heterogeneity, respectively. The same measurement was done for steel block, which is a homogeneous medium. Characteristic scale length of heterogeneity of Westerly and Oshima granites are found to be 0.22 mm and 0.46 mm, respectively; the values were obtained from spatial autocorrelation function of velocity fluctuation based on microstructure photo image. The arrival time of P wave was determined by searching the best-fit point that divides the waveform into two different autoregressive (AR) models based on Akaike information criterion (AIC). This method automatically yields the arrival time of P waves, and the fluctuation of arrival time can be studied as a statistical distribution. We also calculated logarithmic values of the wave energy and studied their fluctuation. The observed fluctuations of the arrival time and the log of wave energy show nonsymmetric distributions. Both the variances of arrival time and log-energy distributions increase with increasing scale length of heterogeneity. This indicates a positive correlation between the variances of the fluctuation and the scale length of heterogeneity. The study also reveals that the correlation coefficient between fluctuations of arrival time and energy of seismic wave increases with increasing length of heterogeneity.

Natural surface rebound of the Bangkok plain and aquifer characterization by persistent scatterer interferometry

We estimated recent surface displacements around Bangkok by means of persistent scatterer interferometry with ALOS/PALSAR images acquired from November 2007 to December 2010. Land subsidence due to excessive groundwater pumping has been reported in this region. However, we detected ground surface uplift around the mega-city, along with seasonal surface displacement, with high spatial resolution. We then discriminated long-term natural rebound and seasonal displacement by fitting exponential and sinusoidal functions to displacement time-series, and mapped their spatial distributions. This mapping allowed us to infer that the second and third shallowest aquifers are laterally continuous, whereas the shallowest aquifer has lateral discontinuities. The temporal decay rate of the long-term rebound might reflect spatial changes of the Chao Phraya River watershed or the magnitude of the preceding groundwater extraction. We demonstrated that our method of decomposing the displacement time series into different spatial and temporal patterns is useful for understanding aquifer connectivity and the elastic response pattern in an aquifer system.

Postseismic displacements following the 2007 Noto peninsula earthquake detected by dense GPS observation

We have been conducting dense GPS observation in and around the epicentral region of the 2007 Noto peninsula earthquake since March 25, 2007, in order to detect postseismic displacements. Continuous observation has been underway at 12 sites to fill the gap of GEONET. Preliminary analysis of data up to early May shows that initial postseismic displacement rapidly decayed within 20 days after the occurrence of the mainshock. Horizontal displacements do not exceed 20 mm even at sites above the aftershock zone for this period. We also found a maximum uplift of about 20 mm there. Inversion of postseismic displacements with the variable slip model suggests a nearly right-lateral afterslip of less than 5 cm on the shallow portion of the source fault. Fitting a theoretical function to a time series of coordinate changes also suggests that the observed postseismic displacements might have been generated by afterslip.

A performance study of a laser Doppler vibrometer for measuring waveforms from piezoelectric transducers

The stresses at transducer contacts were estimated from accurate particle velocity measurements by using a laser Doppler vibrometer (LDV). We then evaluated the performance of LDV for ultrasonic waveform measurements in physical model experiments that are employed for modeling seismic observations. For such experiments, the characteristics of the source and detector should be exactly known. Discshaped compression and shear-mode piezoelectric transducers were attached on a flat surface of a steel semicircular column, and ultrasonic waves were excited by single-shot sine waves with 0.25, 0.5, and 1 MHz frequencies. Radial and transverse components were measured by LDV at a distance of 150 mm from the source. The maximum amplitudes of waves with respect to radiation angle give a radiation pattern of a transducer. Each observed radiation pattern was fitted to the calculated radiation pattern by assuming a harmonically oscillating stress distributed uniformly on a flat circular area. The observed radiation patterns show fairly good agreement with the calculated radiation patterns for both radial and transverse components when the source frequencies are 0.25 and 0.5 MHz. Because the best-fit stress values were independently estimated from the radial and the transverse radiation patterns, the 2 stress values should be equal for each source and frequency. The discrepancy between the estimated radial and transverse stress values becomes larger as the source frequency increases. Provided that coincidence of the 2 stress values indicates the validity of waveform measurements, the results suggest that LDV is applicable for measuring the 3-D particle-velocity at frequencies up to 0.5 MHz.

Chapter 8 Laboratory Experiments of Seismic Wave Propagation in Random Heterogeneous Media

Abstract Subsurface structures contain small‐scale random heterogeneities which generate scattered waves. Considerable fluctuations appear in seismic waveform when heterogeneity scales are comparable or a little smaller than the dominant seismic wavelengths. Waveform fluctuation by scattering can be examined in laboratory experiments by using random heterogeneous materials as scale models. A laser Doppler vibrometer is used to accurately record waveforms propagating through a model specimen. By taking advantages of laboratory experiments, one can reveal some quantitative relationships between wave fluctuations and the intensity or the characteristic scale length of random heterogeneity. Variations in travel times, fluctuations of amplitude, phase, and particle‐motion, as well as envelope formation are examined with respect to the statistical properties of random heterogeneities. The variations can be characterized in terms of the scale‐invariant values, ka and kL, the wavelength‐normalized values for the characteristic scale length of heterogeneity and the wave travel distance, respectively. On the basis of experimental results, we obtain the boundary between equivalent homogeneous media approach and scattering random media approach. The two different approaches come from two different properties of the same medium in wave propagation problems, depending on the values of ka and kL. The boundary is critical in seismic imaging techniques, because strong scattered waves degrade the seismic signals used for imaging and deteriorate the image quality.

Coseismic Deformations of the 2011 Tohoku, Japan, Earthquake and Triggered Events Derived from ALOS/PALSAR

The Tohoku earthquake on March 11, 2011, caused a remarkably large deformation on the island of Honshu, Japan. By analyzing ALOS/PALSAR data, a range increase of up to 3.6 m at the tip of the Oshika Peninsula, the closest point to the epicenter, was detected from ascending orbits. Combining ascending and descending interferograms, this peninsula was confirmed to have subsided and shifted eastward. This deformation may have been caused by a huge reverse slip on the plate interface near the trench axis.