Masakazu Ohtake

Source process of very long period seismic events associated with the 1998 activity of Iwate Volcano, northeastern Japan

We observed very long period seismic events that are associated with the 1998 activity of Iwate Volcano, northeast Japan. The events show a dominant period of 10 s and duration of 30–60 s, often with accompanying short-period waves at the beginning and at the end of the long-period signals. By analyzing the broadband seismograms we find that the source elongates in the east-west direction for ∼4 km at a depth of 2 km beneath the western part of Iwate Volcano. Results of moment tensor inversions show a source mechanism of mutual deflation and inflation of two chambers located at the western and eastern edges of the source region. The source region coincides with the low seismic velocity zone detected by seismic tomography and is very close to the locations of pressure sources estimated from crustal deformation data. On the basis of these results we infer that the very long period seismic events are generated by transportation and movement of magmatic fluid (hot water and/or magma) in a shallow part of the volcano. We further present a simple source model of very long period seismic events based on one-dimensional flow dynamics and propose a new parameter to characterize the size of very long period event: the energy flow rate, which is obtained by dividing the seismic moment by the dominant period. The energy flow rate was estimated as 3.1×1012 J/s for the event on July 29, 1998.

Temporal changes of the crustal structure associated with the M6.1 earthquake on September 3, 1998, and the volcanic activity of Mount Iwate, Japan

We detected temporal changes of crustal structure by cross spectrum analyses of seismic waves excited by two artificial explosions that were carried out about one month before and two months after a M6.1 shallow earthquake. Phase spectrum analysis shows that seismic velocity of the upper crust around the focal area of the M6.1 earthquake and Mount lwate decreased 0.3 - 1.0 % during the three months. Since lager decreases of the seismic velocity were observed at the region close to the focal area and a volcanic pressure source, we conclude that stress change in the upper crust is a most plausible candidate to cause the velocity changes among other candidates. We further apply a coherence analysis to the observed seismic coda to locate regions where short- wavelength heterogeneity temporally changed. The result indicates a possibility of temporal changes of the heterogeneity beneath the volcano, where a magmatic fluid intrusion is inferred.

Tidal triggering of earthquakes in Japan related to the regional tectonic stress

We observe a correlation between the Earth tide and earthquake occurrence that is closely related to the regional tectonic stress. We investigate the direction of the tidal compressional stress using shallow earthquakes occurring in 100 subregions of Japan for nearly five years. The azimuthal distribution of the compressional stress obtained for the observed earthquake data is compared with that synthesized for random earthquake occurrence. Statistical analysis confirms a significant difference between the observed and random catalogs for 13 subregions, which include the areas where unusual seismic activities took place recently, and where the possibility of future large earthquakes has been argued. For these subregions, earthquakes preferentially occur when the tidal compressional stress is near the dominant direction of P-axes of focal mechanisms obtained in the corresponding subregions. This suggests that the tidal stress may encourage earthquake occurrence when it acts in the direction to increase the regional tectonic stress.

Seismogram envelope inversion for the spatial distribution of high-frequency energy radiation from the earthquake fault : Application to the 1994 far east off Sanriku earthquake, Japan

Incoherency of high-frequency seismic waves leads to the idea to analyze seismogram envelopes disregarding phase information for the source study. We develop an inversion method for estimating high-frequency (above 1 Hz) energy radiation from the earthquake fault and site amplification factors from observed mean square S-wave seismogram envelopes, The inversion is executed by using the s-wave envelope Green function from the onset to coda for a point shear dislocation source in a scattering medium, which is formulated based on an extended version of the radiative transfer theory. The use of the envelope Green function enables us to estimate the spatial distribution of high-frequency energy radiation on a fault plane. This has been difficult by conventional waveform inversion methods. The rupture velocity and the duration time of energy radiation on each subfault are also estimated by grid search method. We apply this new method to the 1994 far east off Sanriku, northeastern Japan, earthquake (Mw 7.7), which ruptured a 90-km segment of the plate boundary along the Japan Trench. Inverting observed seismogram envelopes at 10 strong-motion stations in Japan, it is concluded that above 90% of the high-frequency energy was radiated from the western half of the fault with the largest energy radiation near the deep-side edge. The rupture velocity is estimated to be 2.7 km/s. The estimated site amplification factors, ranging between 0.3 and 15, are consistent with those independently estimated by the coda normalization method.

Simulating the Envelope of Scalar Waves in 2D Random Media Having Power-Law Spectra of Velocity Fluctuation

During propagation through random media, impulsive waves radiated from a point source decrease in amplitude and increase in duration with increasing travel distance. The excitation of coda waves is prominent at long lapse time. We use a finite-difference method to numerically simulate scalar waves that propagate through random media characterized by a von Karman autocorrelation function. The power spectral density function of fractional velocity fluctuation for κ -th order von Karman-type random media obeys a power law at large wavenumbers. Such media are considered to be appropriate models for the random component of the structure of the Earths lithosphere. The average of the square of numerically calculated wave traces over an ensemble of random media gives the reference envelope for the evaluation of envelope simulation methods. The Markov approximation method gives reliable quantitative predictions of the entire envelope for random media that are poor in short wavelength components of heterogeneity ( κ = 1.0), while it fails to predict the coda envelope for random media that have rich short-wavelength components ( κ = 0.1). The radiative-transfer theory reliably predicts the coda excitation for κ = 0.1 when the momentum-transfer scattering coefficient is used as the effective isotropic scattering coefficient. Replacing the direct term of the radiative-transfer solution with the envelope of the Markov approximation, we propose a new method for simulating the entire envelope from the direct arrival through the coda. The method quantitatively explains the whole envelope for κ = 0.1. For the case of κ = 0.5, however, our method predicts too much coda excitation. In such a case, the method can explain whole envelopes by using the effective scattering coefficient estimated from coda excitation. Manuscript received 10 April 2002.

A High Quality Digital Network for Microearthquake and Ground Tilt Observations in the Kanto-Tokai Area, Japan

The National Reseach Center for Disaster Prevention (NRCDP) has completed a high quality, digital network for microearthquake and ground tilt observations in the Kanto-Tokai area, Japan. There are 71 stations, including the three deep borehole observatories with depths of from 2300 to 3500 m which the NRCDP completed in 1980 to overcome artificial noise at the surface in and around Tokyo.

Scale Dependence of Apparent Stress for Earthquakes along the Subducting Pacific Plate in Northeastern Honshu, Japan

Apparent stress, which quantifies the ratio of high-frequency spectral amplitude to the direct current level of source displacement spectra, is known to reflect the source rupture process. Whether this parameter depends on the seismic moment has not yet been well established because of the difficulties in the reliable estimation of high-frequency source spectra. To overcome this problem, we estimate S -wave source spectra by using site amplification factors and attenuation factor ![Graphic][1] , which are systematically measured by the coda normalization method. Analyzing horizontal components of velocity seismograms recorded at 70-borehole seismic stations of Hi-net (National Research Institute for Earth Science and Disaster Prevention) in northeastern Honshu, Japan, we estimate site factors and ![Graphic][2] in the 0.5–32 Hz frequency band. By using these factors, we evaluate the source spectra of 225 small to moderate earthquakes that occurred in and around the subducting Pacific plate. Site-amplification factors we obtained strongly depend on frequency, and the frequency dependence clearly changes with the lithology and geologic age of rock. Obtained ![Graphic][3] values decrease with frequency in proportion to the reciprocal of frequency. The apparent stresses estimated from S -wave spectra clearly increase from 104 to 107 Pa with the seismic moment increasing, which cannot be attributed to the limited-frequency band and other artificial causes. The power of scale dependence is estimated as 0.39–0.44 for the seismic moment range from 1011 to 1017 N m. Some of the scale dependence of apparent stress is attributed to the scale dependence of Brune’s stress drop. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif

SYNTHESIS OF SCATTERED ENERGY DENSITY FOR NONSPHERICAL RADIATION FROM A POINT SHEAR-DISLOCATION SOURCE BASED ON THE RADIATIVE TRANSFER THEORY

Abstract It is well known that S -coda wave amplitudes of local earthquakes smoothly decay with increasing lapse time and asymptotically converge to a common master curve irrespective of their epicentral distances. This property is thought to be the conceptual basis for the conventional coda normalization method, which has often been used for the estimation of amplitude attenuation and site amplification factors around the world. We note that it has also been widely accepted that the common decay curve is independent of the radiation pattern especially at long lapse times. However, there have been few theoretical attempts to clarify the way how S -coda wave envelopes neglect the radiation pattern of the source with increasing lapse time. Focusing on this problem, we try to formulate the multiple scattering process in a scattering medium for an impulsive radiation from a point-shear dislocation source on the basis of the radiative transfer theory. We assume that the inhomogeneous medium in a 3-D space is represented by a homogeneously random distribution of point-like scatterers and, for simplicity, that scattering is isotropic. The master equation is written in the form of a convolution integral equation for the energy density, which corresponds to the mean square seismogram amplitude. Introducing the spherical harmonics expansion for nonspherical radiation from the source, in addition to the Fourier transform in space and the Laplace transform in time, we are able to solve the integral equation in each spherical harmonics mode analytically. The inverse Fourier-Laplace transformation gives the spatiotemporal change in energy density for each spherical harmonics mode. As the order of spherical harmonics mode increases, the corresponding energy density decreases more rapidly with increasing lapse time. The energy density faithfully reflects the nonspherical radiation pattern of a point-shear dislocation immediately after the direct wave arrival. However, the direction dependence diminishes as lapse time increases, and the energy density asymptotically converges to that for a spherically symmetric radiation, which corresponds to the lowest spherical harmonics mode. At a distance of the mean free path from the source, the difference between the maximum energy density and the minimum energy density at the same lapse time becomes less than 3% when the lapse time exceeds twice the travel time of the direct wave.

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.

Broadband Source Process of the 1998 Iwate Prefecture, Japan, Earthquake as Revealed from Inversion Analyses of Seismic Waveforms and Envelopes

An earthquake of M 6.1 occurred on 3 September 1998, along an active fault at the southwestern foot of Mt. Iwate, a volcano in northeastern Japan. Acceleration records of this earthquake were obtained at seven stations within 40 km of the epicenter. In order to investigate the source process of this earthquake in a broad frequency range, we simultaneously conducted inversion analyses of low-frequency seismic waveforms and high-frequency seismogram envelopes. First, executing the envelope inversion by using the envelope Green function derived from the radiative-transfer theory in the high-frequency band of 2-16 Hz, we estimated the spatial distribution of seismic-wave energy radiation on the fault plane of 10 km × 10 km. We found that seismic-wave energy was strongly radiated from the southwestern deeper part of the fault plane. By using data from the same stations and using the same fault geometry as the high-frequency analysis, we applied a waveform-inversion method to three-component displacement records in the low-frequency band of 0.1-0.33 Hz. This result showed that seismic moment was mainly released at the shallow part of the fault. Comparing these results, we found that high-frequency energy was strongly radiated from the deepest periphery of the region, where seismic moment was mainly released. This result implies that the radiation of high-frequency energy was associated with the arrest of rupture for this earthquake. Manuscript received 21 April 2001.