Seasonality in particle motion of microseisms

SUMMARY Microseisms are a continuous source of seismic signal which mainly consist of Rayleigh waves but are known to contain some other types of seismic waves. We developed a simple processing procedure for single-station three-component seismic data which allows us to select Rayleigh-wave dominated portions. Application of this procedure to data from Southern California led us to confirm that excitation of the secondary-peak microseisms (the predominant energy at about 0.15 Hz) occurs in coastal regions; the source directions, viewed at each station, do not change very much throughout the year. We also discovered that the ratio of the horizontal to vertical amplitudes in Rayleigh waves, termed H/Z in this paper, is shown to have seasonal variations. The H/Z estimates typically reach their maximum in winter and their minimum in summer. Seasonal variations are observed at most stations but peak-to-peak amplitudes of seasonal variations vary greatly from station to station, ranging between 0 and 40 per cent. Two hypotheses were examined to explain this phenomenon; the first hypothesis is that it is caused by seasonal changes in seismic velocities in a layer between the surface and the groundwater level. The second hypothesis is that it is caused by seasonal changes in relative excitation of higher modes compared to dominant fundamental-mode Rayleigh waves. The first hypothesis is not likely, because predicted amplitudes of seasonal variations in H/Z are too small to explain observed variations. The second hypothesis seems quite plausible if source regions move seasonally among regions with different ocean depths. The technique developed in this paper, however, is not sufficient to answer this question conclusively.

[1]  Richard G. Derwent,et al.  Radiative forcing in the 21st century due to ozone changes in the troposphere and the lower stratosphere , 2003 .

[2]  H.-H. Essen,et al.  Microseismological evidence for a changing wave climate in the northeast Atlantic Ocean , 2000, Nature.

[3]  J. Capon.  Long-period signal processing results for LASA, NORSAR, and ALPA. , 1972 .

[4]  Peter D. Bromirski,et al.  Vibrations from the “Perfect Storm” , 2001 .

[5]  Bernard Budiansky,et al.  Seismic velocities in dry and saturated cracked solids , 1974 .

[6]  A. W. Lee,et al.  On the Direction of Approach of Microseismic Waves , 1935 .

[7]  R. T. Lacoss,et al.  ESTIMATION OF SEISMIC NOISE STRUCTURE USING ARRAYS , 1969 .

[8]  Nicholas E. Graham,et al.  Ocean wave height determined from inland seismometer data: Implications for investigating wave climate changes in the NE Pacific , 1999 .

[9]  L. Rivera,et al.  Prograde Rayleigh wave particle motion , 2005 .

[10]  M. Longuet-Higgins.  A theory of the origin of microseisms , 1950, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[11]  Y Nakamura,et al.  A METHOD FOR DYNAMIC CHARACTERISTICS ESTIMATION OF SUBSURFACE USING MICROTREMOR ON THE GROUND SURFACE , 1989 .

[12]  Keiiti Aki,et al.  A NOTE ON THE USE OF MICROSEISMS IN DETERMINING THE SHALLOW STRUCTURES OF THE EARTH’S CRUST , 1965 .

[13]  D. L. Anderson,et al.  Preliminary reference earth model , 1981 .

[14]  Keiiti Aki,et al.  Possibilities of seismology in the 1980's , 1980 .

[15]  Frank L. Vernon,et al.  Strong directivity of ocean‐generated seismic noise , 2004 .

[16]  J. Tromp,et al.  Theoretical Global Seismology , 1998 .

[17]  Keiiti Aki,et al.  Space and Time Spectra of Stationary Stochastic Waves, with Special Reference to Microtremors , 1957 .

[18]  Keith McCamy,et al.  Microseisms: Coastal and pelagic sources , 1969 .

[19]  H. Essen,et al.  On the generation of secondary microseisms observed in northern and central Europe , 2003 .

[20]  W. Munk,et al.  Comparative spectra of microseisms and swell , 1963 .

[21]  M. Ohori,et al.  A Comparison of ESAC and FK Methods of Estimating Phase Velocity Using Arbitrarily Shaped Microtremor Arrays , 2002 .

[22]  Frank Krüger,et al.  Ocean-generated microseismic noise located with the Gräfenberg array , 1998 .

[23]  Masanori Horike,et al.  INVERSION OF PHASE VELOCITY OF LONG-PERIOD MICROTREMORS TO THE S-WAVE-VELOCITY STRUCTURE DOWN TO THE BASEMENT IN URBANIZED AREAS , 1985 .