Earthquake seismograms that show Doppler effects due to crack propagation

Summary. Examples are presented of earthquake P-wave pulses seen on broadband seismograms, to show that on such recordings the pulse shapes are more clearly seen than on conventional short-period and long-period seismograms. Most of the broadband seismograms have been chosen because they show marked differences between the pulse lengths of P and those of the surface reflections. In addition some of the pulses appear to have smooth onsets and abrupt trailing edges so that the onset of the pulse is difficult to observe and the largest amplitude arrivals seen on the seismogram coincide not with the onset of motion but with the termination of motion: that is the large arrivals mark stopping phases of motion. We assume that the differences in pulse length are due to the effects of a moving source – that is a Doppler effect – and that the pulses with smooth onsets and abrupt trailing edges can be modelled simply by a source propagating on a line with low radiation amplitude at the start of motion. A trial and error method guided by a published fault plane solution is then used to obtain a fit between observed and computed seismograms for one of the earthquakes. This process leads to an estimate of the crack speed of about 1.4 times the 5-wave speed. The errors that may arise in estimating source depths and orientation, if stopping phases are not recognized as such, is discussed.

[1]  Robert Burridge,et al.  The effect of sonic rupture velocity on the ratio of S to P corner frequencies , 1975, Bulletin of the Seismological Society of America.

[2]  R. Pearce.  Fault plane solutions using relative amplitudes of P and pP , 1977 .

[3]  F. Press,et al.  Experimental determination of earthquake fault length and rupture velocity , 1961 .

[4]  J. Brune.  Tectonic stress and the spectra of seismic shear waves from earthquakes , 1970 .

[5]  Stewart w. Smith,et al.  SOURCE PARAMETERS OF EARTHQUAKES, AND DISCRIMINATION BETWEEN EARTHQUAKES AND NUCLEAR EXPLOSIONS. , 1968 .

[6]  C. Bufe,et al.  Directivity in the high-frequency radiation of small earthquakes , 1978, Bulletin of the Seismological Society of America.

[7]  Takashi Miyatake,et al.  Dynamical rupture process on a three‐dimensional fault with non‐uniform frictions and near‐field seismic waves , 1978 .

[8]  D. J. Andrews,et al.  Rupture velocity of plane strain shear cracks , 1976 .

[9]  J. Hudson,et al.  A Quantitative Evaluation of Seismic Signals at Teleseismic Distances—III Computed P and Rayleigh Wave Seismograms , 1972 .

[10]  L. B. Freund,et al.  The stability of a rapid mode II shear crack with finite cohesive traction , 1979 .

[11]  Tomowo Hirasawa,et al.  On the seismic body waves from a finite moving source , 1965 .

[12]  P. D. Marshall,et al.  Earthquakes that look like Explosions , 1974 .

[13]  T. Teng,et al.  A procedure for source studies from spectrums of long-period seismic body waves , 1965 .

[14]  P. Molnar,et al.  Fault plane solutions of shallow earthquakes and contemporary tectonics in Asia , 1973 .

[15]  K. Jacob,et al.  Synthetic seismic pulses from propagating faults , 1967 .

[16]  J. Hudson.  A Quantitative Evaluation of Seismic Signals at Teleseismic Distances—I Radiation from Point Sources , 1969 .

[17]  J. C. Savage.  Radiation from a realistic model of faulting , 1966 .

[18]  Robert Burridge,et al.  Admissible Speeds for Plane-Strain Self-Similar Shear Cracks with Friction but Lacking Cohesion , 1973 .

[19]  K. Khattri.  Body wave directivity functions for two‐dimensional fault model and kinematic parameters of a deep focus earthquake , 1972 .

[20]  T. V. McEvilly,et al.  Central U.S. crust—Upper mantle structure from Love and Rayleigh wave phase velocity inversion , 1964 .

[21]  G. Bollinger.  Determination of earthquake fault parameters from long-period P waves , 1968 .

[22]  Charles A. Langston,et al.  The February 9, 1971 San Fernando earthquake: A study of source finiteness in teleseismic body waves , 1978, Bulletin of the Seismological Society of America.

[23]  U. Chandra.  Seismicity, Earthquake Mechanisms and Tectonics of Burma, 20°N-28°N , 1975 .

[24]  G. Bollinger.  Fault length and fracture velocity for the Kyushu, Japan, earthquake of October 3, 1963 , 1970 .

[25]  J. Hudson.  A Quantitative Evaluation of Seismic Signals at Teleseismic Distances—II: Body Waves and Surface Waves from an Extended Source , 1969 .

[26]  R. Pearce.  Fault plane solutions using relative amplitudes of P and surface reflections: further studies , 1980 .

[27]  R. C. Lilwall.  Fault mechanisms and sub‐crustal seismic velocities on the Mid‐Atlantic Ridge , 1980 .