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[SCG53-P02] Generation Requirements of PL Waves for Shallow Earthquakes around Japan-Possibility of Immediate Tsunami Prediction
Keywords:PL Waves, Seismic Wave Propagation, Difference Method, Immediate Tsunami Prediction
PL is a long period phase with a period of several seconds to several tens of seconds, which is observed between P wave and S wave. P-waves emitted from shallow epicenters interfere with each other as they propagate through the crust by multiple wide-angle reflections, forming long-period wave groups. Sommville (1930) named this wave PL, meaning P-long wave, which was later interpreted by Oliver and Major (1960) as the leaking mode of Rayleigh waves. A longer-period (100-1000 s) phase generated by a similar mechanism in the upper mantle was named W-phase by Kanamori (1993) and is widely used for CMT analysis of large earthquakes and immediate tsunami prediction. Recently, W-phase analysis has also been applied to wave groups with periods of tens of seconds or less (i.e., PL waves) in near-field strong-motion records (e.g., Duputel et al., 2012; Hayes et al., 2009; Usui and Yamauchi, 2013). In this study, in order to utilize the PL waves in the near-field wavefield as an indicator to immediately determine the possibility of tsunami generation, the characteristics of PL waves in large trench earthquakes and their generation and propagation requirements were investigated based on seismic wave propagation simulations.
As an example of PL waves observation, Figure 1a shows the vertical motion velocity record of K-NET during the foreshock of the 2009/3/9 off the Pacific coast of the Eastern Japan Earthquake (M7.3, 8 km depth). At the AOM014 (K-NET Nenokuchi) station, located at the epicentral distance of 310 km, a wave group of PL waves with a period of about 20 s is clearly observed between the P and S phases of the Radial (R) and Vertical (Z) components. It is possible to evaluate the size and depth of the earthquake, namely the possibility of tsunami generation, from the seismic amplitude of PL waves in the near-field strong-motion records.
In order to confirm the generation and propagation characteristics of PL waves caused by shallow, large trench earthquakes, we performed seismic wave propagation simulations using the two-dimensional finite difference method. We selected a cross-sectional area from off Miyagi to northeast Japan (Fig. 2a), set up sedimentary layers, crust and mantle structures using the JIVSM (Koketsu et al., 2012) model, and calculated seismic wave propagation. The epicenter was set as a point source for simplicity, and seismic waves were radiated using a triangular source-time function with a period of 20 seconds based on the GCMT analysis results of the 2009/3/9 earthquake. The record section of the Z-component velocity waveform is shown in Figure 2b. The record section of the Z-component velocity waveform is shown in Figure 2b, and the waveforms at the epicenter distance of 310 km for different source depths of 1, 2, 4, 8, 16, 32, and 64 km are compared in Figure 2c. As a result, it is confirmed that PL waves with the same amplitude as S waves or Rayleigh waves are generated in earthquakes shallower than 8 km depth, and the amplitude weakens with depth.
Observations of PL waves with periods of about 20 s in large (>M7) and shallow (< 40 km) epicentral earthquakes occurring in the ocean may provide a simple assessment of tsunami generation potential prior to CMT inversion analysis of W-phase (PL waves) using far- and near-field waveform records. Earthquakes that occurred inland and propagated overland are dominated by short-period PL waves with periods around 7 s (Furumura and Kennett, 2018), which can be distinguished from earthquakes that occurred in the ocean. In this study, we focused on the Miyagi-oki earthquake. However, the propagation characteristics and prevailing period of PL waves are considered to be affected by deep structures such as the thickness of the crust, as shown by the difference in PL waves between land and sea propagation paths. It is necessary to confirm the regional characteristics of PL wave propagation by analyzing observed waveforms and simulating seismic wave propagation in other regions, such as along the Nankai Trough and the eastern margin of the Japan Sea.
As an example of PL waves observation, Figure 1a shows the vertical motion velocity record of K-NET during the foreshock of the 2009/3/9 off the Pacific coast of the Eastern Japan Earthquake (M7.3, 8 km depth). At the AOM014 (K-NET Nenokuchi) station, located at the epicentral distance of 310 km, a wave group of PL waves with a period of about 20 s is clearly observed between the P and S phases of the Radial (R) and Vertical (Z) components. It is possible to evaluate the size and depth of the earthquake, namely the possibility of tsunami generation, from the seismic amplitude of PL waves in the near-field strong-motion records.
In order to confirm the generation and propagation characteristics of PL waves caused by shallow, large trench earthquakes, we performed seismic wave propagation simulations using the two-dimensional finite difference method. We selected a cross-sectional area from off Miyagi to northeast Japan (Fig. 2a), set up sedimentary layers, crust and mantle structures using the JIVSM (Koketsu et al., 2012) model, and calculated seismic wave propagation. The epicenter was set as a point source for simplicity, and seismic waves were radiated using a triangular source-time function with a period of 20 seconds based on the GCMT analysis results of the 2009/3/9 earthquake. The record section of the Z-component velocity waveform is shown in Figure 2b. The record section of the Z-component velocity waveform is shown in Figure 2b, and the waveforms at the epicenter distance of 310 km for different source depths of 1, 2, 4, 8, 16, 32, and 64 km are compared in Figure 2c. As a result, it is confirmed that PL waves with the same amplitude as S waves or Rayleigh waves are generated in earthquakes shallower than 8 km depth, and the amplitude weakens with depth.
Observations of PL waves with periods of about 20 s in large (>M7) and shallow (< 40 km) epicentral earthquakes occurring in the ocean may provide a simple assessment of tsunami generation potential prior to CMT inversion analysis of W-phase (PL waves) using far- and near-field waveform records. Earthquakes that occurred inland and propagated overland are dominated by short-period PL waves with periods around 7 s (Furumura and Kennett, 2018), which can be distinguished from earthquakes that occurred in the ocean. In this study, we focused on the Miyagi-oki earthquake. However, the propagation characteristics and prevailing period of PL waves are considered to be affected by deep structures such as the thickness of the crust, as shown by the difference in PL waves between land and sea propagation paths. It is necessary to confirm the regional characteristics of PL wave propagation by analyzing observed waveforms and simulating seismic wave propagation in other regions, such as along the Nankai Trough and the eastern margin of the Japan Sea.