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[SSS09-P10] Source-region dependence and causes of sP waves from deep- and intermediate-depth earthquakes
Keywords:deep earthquakes, sP wave, reflect, convert
Various converted waves appear clearly in the seismograms of deep earthquakes. These converted waves provide important information about the Earth’s internal structure. In this study, we focused on sP wave, which are converted to P waves at ground surface from S wave radiated upward from the epicenter. We found that the appearance of sP waves varies significantly; In some cases, sP waves are more prominent than direct S wave, while in other cases they are hardly identifiable. In this study, we organized characteristics of sP waves observed at broadband stations all over the Japanese Islands from many deep- and intermediate-depth earthquakes around Japan.
In this study, we investigated deep and intermediate-depth earthquakes occurring between 123E and 150E and between 25N and 50N, and deeper than 100 km. We analyzed earthquakes whose magnitudes are equal or larger than five occurred from April 2004 to March 2023. Distribution of these events is nearly parallel to the Chishima Trench in northern Japan and the Izu-Ogasawara Trench in southern Honshu, respectively. For these earthquakes, we used seismic waveforms of the vertical component of all stations in F-net, a broad-band seismograph network in Japan. We tried to identify the appearance of sP-waves from record sections in various frequency range within 0.05-0.8 Hz. If seismic phases that appeared over a wide range of epicentral distances within a 15-second time window starting five seconds before theoretical sP travel time, calculated using the iasp91 model, we identified them as sP waves. The identification of the sP waves was assisted by comparing the amplitude of sP waves with that of the direct S wave, and with noise level before P wave arrival.
The results revealed that the existence of sP waves exhibits strong regional characteristics related to hypocenter locations. The sP waves were clearly observed only in earthquakes that occurred two regions: one off the coast of northern Japan at the depth greater than 200 km, and the other around the Nansei Islands.
We are investigating the causes for this regional characteristic. First, we investigated significance of sP waves by numerical simulation of 2-D P-SV seismic wave propagation. To minimize influences of the radiation pattern from the epicenter, we radiated isotropic S waves. The simulation shows clear sP waves regardless of the depth of the epicenter by 1D seismic velocity structure model. However, the amplitude of sP waves was always much smaller than S waves. The characteristics differed from observations. Second, we performed a numerical simulation assuming a double-couple source model with changing the dip angle of the source. As a result, the amplitudes of sP waves were very small at wide range of epicentral distances only when the epicenter depth was shallower than 600 km and the dip angle was about 10 degrees.
The radiation pattern of S waves might be a key factor in explaining the absence of sP-waves originated from the source at specific regions. For direct waves, the radiation pattern from the source is expected to cause the variation of S-wave amplitude varying with epicentral distance, as the radiation angle of seismic waves from the source generally changes with epicentral distance. On the other hand, ray-tracing results suggest that the radiating angles of sP waves remain nearly constant over a wide range of epicentral distances. Consequently, it is possible that sP waves are not observed over a wide range of epicentral distances at a specific source radiation pattern. This hypothesis should be tested through 3D ray tracing and numerical simulations of wave propagations in the future studies.
In this study, we investigated deep and intermediate-depth earthquakes occurring between 123E and 150E and between 25N and 50N, and deeper than 100 km. We analyzed earthquakes whose magnitudes are equal or larger than five occurred from April 2004 to March 2023. Distribution of these events is nearly parallel to the Chishima Trench in northern Japan and the Izu-Ogasawara Trench in southern Honshu, respectively. For these earthquakes, we used seismic waveforms of the vertical component of all stations in F-net, a broad-band seismograph network in Japan. We tried to identify the appearance of sP-waves from record sections in various frequency range within 0.05-0.8 Hz. If seismic phases that appeared over a wide range of epicentral distances within a 15-second time window starting five seconds before theoretical sP travel time, calculated using the iasp91 model, we identified them as sP waves. The identification of the sP waves was assisted by comparing the amplitude of sP waves with that of the direct S wave, and with noise level before P wave arrival.
The results revealed that the existence of sP waves exhibits strong regional characteristics related to hypocenter locations. The sP waves were clearly observed only in earthquakes that occurred two regions: one off the coast of northern Japan at the depth greater than 200 km, and the other around the Nansei Islands.
We are investigating the causes for this regional characteristic. First, we investigated significance of sP waves by numerical simulation of 2-D P-SV seismic wave propagation. To minimize influences of the radiation pattern from the epicenter, we radiated isotropic S waves. The simulation shows clear sP waves regardless of the depth of the epicenter by 1D seismic velocity structure model. However, the amplitude of sP waves was always much smaller than S waves. The characteristics differed from observations. Second, we performed a numerical simulation assuming a double-couple source model with changing the dip angle of the source. As a result, the amplitudes of sP waves were very small at wide range of epicentral distances only when the epicenter depth was shallower than 600 km and the dip angle was about 10 degrees.
The radiation pattern of S waves might be a key factor in explaining the absence of sP-waves originated from the source at specific regions. For direct waves, the radiation pattern from the source is expected to cause the variation of S-wave amplitude varying with epicentral distance, as the radiation angle of seismic waves from the source generally changes with epicentral distance. On the other hand, ray-tracing results suggest that the radiating angles of sP waves remain nearly constant over a wide range of epicentral distances. Consequently, it is possible that sP waves are not observed over a wide range of epicentral distances at a specific source radiation pattern. This hypothesis should be tested through 3D ray tracing and numerical simulations of wave propagations in the future studies.