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Volume 66, Issue 4
Displaying 1-7 of 7 articles from this issue
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ARTICLES
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Hiroyuki FUJIWARA2014 Volume 66 Issue 4 Pages 67-71
Published: March 25, 2014
Released on J-STAGE: May 20, 2014
JOURNAL FREE ACCESSThere is a similarity between the distribution of prime numbers and the pattern of earthquake occurrence. Earthquakes occur in a discrete manner in time and space. When viewed as a whole, however, we find some laws, such as Gutenberg-Richter law, that govern the entire earthquakes that seem to be individually independent. A similar phenomenon can be observed also in the world of number. The most basic example is the distribution of the prime numbers in integers. We consider a correspondence between earthquakes and prime numbers. We parameterize occurrence time of earthquakes as the prime numbers and magnitude of earthquakes as the interval of prime numbers. Then we obtain a relationship similar to Gutenberg-Richter law. We call the model obtained by this correspondence as “arithmetic seismic activity model”. If we can parameterize earthquakes using prime numbers, knowledge that has been cultivated in the number theory can be used for understanding of earthquakes. The distribution of prime numbers is related to the distribution of zeros of Riemann zeta function. Researches are in progress to understand the zeros of the Riemann zeta function as an eigenvalue problem of quantum dynamical system. Earthquake may be modeled as a phenomenon corresponding to a change in the energy level of a quantum dynamical system associated with prime numbers.View full abstractDownload PDF (335K) -
Yuichi NAMEGAYA, Toshifumi YATA2014 Volume 66 Issue 4 Pages 73-81
Published: March 25, 2014
Released on J-STAGE: May 20, 2014
JOURNAL FREE ACCESSTo find tsunamis that affected the Pacific coast of eastern Japan during period between A.D. 869 and the 1600s, we searched a historical earthquake and tsunami database for tsunami descriptions. We examined reliability of the descriptions considering the written dates and independence of documents. As a result, at least two events were found. One is the Oei tsunami which occurred on September 1st, 1420 on the Julian calendar (the 7th month 23rd day, 27th year of Oei era on the Japanese calendar). The tsunami affected Kawarago and Aiga villages in Hitachi city, Ibaraki prefecture. Sea receded 9 times during 4 hours, exposing the coastal seafloor, and many fish were carried to the land. No description of earthquake ground shaking was found. The other is the Kyotoku earthquake tsunami which occurred on December 12th, 1454 (11th month 23rd day, 3rd year of Kyotoku era). An earthquake occurred in night and the tsunami widely inundated the coastal area of “Oshu” (the old name of province from Aomori to Fukushima prefectures). Many people were killed in the tsunami. Another entry in the database on December 31st, 1455 (11th month 23rd day, first year of Kosho era) is likely to be a fake event.View full abstractDownload PDF (607K) -
Junji KOYAMA, Motohiro TSUZUKI2014 Volume 66 Issue 4 Pages 83-95
Published: March 25, 2014
Released on J-STAGE: May 20, 2014
JOURNAL FREE ACCESSAfter the 2011 Tohoku-oki megathrust earthquake of Mw 9.0, we observed tens of thousands of inland and volcanic earthquakes all over the Japan Islands, which are not confined within the aftershock area of off the Pacific coast of Tohoku region. This reminds us to evaluate the earthquake activity in a much wider sense, discarding an ordinary idea of the foreshock-mainshock-aftershock activity within a limited aftershock area of a particular earthquake. There occurred several megathrust earthquakes worldwide in the last one hundred years. We have studied their significant events before and after the earthquakes based on our new hypothesis on the earthquake generating zone with the distinct difference between Along-dip Double Segmentation (ADDS) and Along-strike Single Segmentation (ASSS). In summary, some of significant aftershocks (larger than Mw 7.5) of ADDS megathrusts are those (Type I) in and near outer-rises, where some are dip-slip normal faultings and some are strike-slip faultings within subducting oceanic plates with component of normal fault. These outer-rise earthquakes are considered to be controlled by the slab-pull of descending slabs of oceanic plates induced by the reduction of plate couplings by megathrusts. In addition, some are also significant (Type II) along the plate interfaces within aftershock areas and those extending aftershock areas similarly to the corresponding megathrusts. On the other hand, aftershocks of ASSS occurred in and near corresponding aftershock areas and significant and or disastrous outer-rise events are rare. Some large (not significant) inland earthquakes are also known for both ADDS and ASSS megathrusts. All these results are not dependent on whether the subduction geometry is oblique or orthogonal.View full abstractDownload PDF (4653K) -
Takahiro KUNITOMO2014 Volume 66 Issue 4 Pages 97-112
Published: March 25, 2014
Released on J-STAGE: May 20, 2014
JOURNAL FREE ACCESSNIED Hi-net is the nation-wide high sensitivity seismograph network which has been constructed by the National Research Institute for Earth Science and Disaster Prevention. The high quality observation data of the Hi-net brought us outstanding contribution to seismology. Recently, the detection of minute changes of seismic travel times with the use of the Hi-net data is tried by various methods such as seismic ACROSS (Accurately Controlled, Routinely Operated, Signal System), seismic interferometry and so on. Although time accuracy is essential for precise measurement of temporal variations of seismic travel times, reliability evaluation of the sampling timing of the Hi-net data has been not always sufficient so far. This paper reports the temporal variations of the sampling timings at the twenty Hi-net stations for the eight years from 2004 to 2012 and proposes an approach to detect minute changes in seismic travel times in sub-millisecond order. The sensor check signal which is usually used to calibrate the seismometer, is enabled on 1PPS of GPS receiver independently of the sampling clock. Therefore the sampling clock can be calibrated by the GPS clock using the delay time of the sensor check signal. The delay times are usually stable, but stepwise changes were observed at restarts of ADC (Analog-to-Digital Converter). This method was applied to the TOKI seismic ACROSS signals observed by Hi-net Yaotsu (11.3 km distance from TOKI). Consequently, sub-millisecond to several milliseconds stepwise travel time changes were removed, and travel time changes owing to large earthquakes, and annual and secular travel time changes became clear. This correction method is useful to not only the seismic source-receiver system such as ACROSS but also other methods to detect seismic travel time changes of utilizing arrival time difference between observation stations.View full abstractDownload PDF (2386K) -
Shigeo KINOSHITA2014 Volume 66 Issue 4 Pages 113-125
Published: March 25, 2014
Released on J-STAGE: May 20, 2014
JOURNAL FREE ACCESSThis article proposes a design method of state feedback sensor (SF sensor) equipped with acceleration, velocity and displacement output channels as a regulator system and shows a method for compensating the frequency response of each channel of SF sensor using complementary filtering. Three output channels of SF sensor have the same frequency response characteristics in the acceleration, velocity and displacement domains. It is revealed that one output signal of SF sensor fuses with the complementary signal produced by filtering another output signal. Compared with the real channel of SF sensor, as a result, apparent channel generating fused-signal broadens measurable bandwidth. This technique of data-fusion is also possible to apply to the outputs of two sensors whose sensitivities and frequency responses are different. A simulation of such a data-fusion using equivalent signals of L-4C short period and STS-2 broadband seismometers produces reasonable result.View full abstractDownload PDF (709K) -
—Application of Seismic Interferometry to Hi-net Continuous Waveform Data—Takumi HAYASHIDA, Masayuki YOSHIMI, Haruo HORIKAWA2014 Volume 66 Issue 4 Pages 127-145
Published: March 25, 2014
Released on J-STAGE: May 20, 2014
JOURNAL FREE ACCESSWe applied seismic interferometry to microtremor data recorded in the Chukyo area, for the evaluation of spatial accuracy of existing three-dimensional S-wave velocity models. We used continuous data from the Hig-sensitivity seismograph network (Hi-net) to obtain cross-correlation functions (CCFs) between the selected station pairs in and around the sedimentary basin. We found that the derived CCFs for selected Hi-net station pairs are comparable with those derived from neighboring broadband seismic stations, by deconvolving the instrument response from the short period data in the frequency range between 0.05 and 0.5 Hz. The derived CCFs for 101 station-pairs showed clear and coherent wavetrains that correspond to surface wave propagation. The derived group velocities of the wavetrains are variable in the higher frequency range (>0.2 Hz), reflecting complicated velocity structure beneath the area. The comparisons between derived group velocities and theoretical group-velocity dispersion curves clarify that systematic differences between the expected structure model from the CCFs and existing models are detected in the west side of the Ise Bay region, while the agreements are generally good for many station pairs. This indicates that the CCFs from the Hi-net data provide valuable information for validation and improvement of a velocity structure model in this region.View full abstractDownload PDF (6358K)
LETTER
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Yuji YAGI2014 Volume 66 Issue 4 Pages 147-149
Published: March 25, 2014
Released on J-STAGE: May 20, 2014
JOURNAL FREE ACCESSDownload PDF (180K)
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