17:15 〜 19:15
[SIT20-P09] Low seismic velocity anomaly in the lowermost mantle beneath the Western Pacific Ocean
In the lowermost mantle of the Earth, there exists a region called the large low-shear-velocity province (LLSVP), where the S wave velocity is slower than that in the surrounding area. Furthermore, the existence of an ultra-low velocity zone (ULVZ), where the slow anomaly is orders of magnitude larger than that of the LLSVP, has also been suggested just above the core-mantle boundary (CMB). However, the detailed distribution of this zone is still not well understood. Obayashi et al. (2017) found that the ScS-S travel time difference anomaly was much larger than the SKS-S travel time difference anomaly, suggesting the existence of a localized low-velocity anomaly at the bottom of the mantle. This study aims to verify their results using six events that occurred in the deep slab beneath Fiji and to estimate seismic structure above the CMB beneath the western Pacific Ocean.
To investigate the western edge of the Pacific LLSVP, we analyze seismic waveforms of earthquakes that occurred in Fiji and were observed in Japan. We collected waveform data from the J-Array at the Earthquake Research Institute of the University of Tokyo for deep earthquakes with focal depths greater than 500 km. We used six events: two of the earthquakes used by Obayashi et al. (2017) and four others that they did not use. First, the east-west and north-south components of the velocity waveform were converted into radial and lateral components, and then the differential times between the peaks of S and ScS or S and SKS were measured on the radial waveforms, which were low-pass filtered with a corner frequency of 0.2 Hz (Figure 1).
Figure 2 shows that anomalies of the differential times of ScS-S and SKS-S calculated by subtracting the theoretical traveltime difference estimated from a reference model IASP91 (Kennett and Engdahl, 1991) from the observed value. The ScS delay relative to the S arrival time is very large (7.189 [s] on average), while SKS delay is small (0.843 [s] on average), which is consistent with the result of Obayashi et al. (2017) who reported that there are localized low velocity anomalies above the CMB. The magnitude of the S-wave low velocity anomalies is estimated by assuming that the low velocity anomalies exist within the epicenter distance range where the SKS waves propagate through the core. We will present estimated amplitudes of the low velocity anomalies.
Figure1: Radial component velocity waveforms of J-array stations aligned by epicentral distance for the Mw 6.9 event on May 16th, 1998. The waveforms were low-pass filtered with a corner frequency of 0.2 Hz. Crosses indicate the S (black), SKS (green), and ScS (red) peaks. Theoretical traveltimes of S, SKS and ScS waves estimated from IASP91 (Kennett and Engdahl, 1991) are represented by black, green and red lines, respectively.
Figure2: Travel time difference anomalies for ScS-S (red) and SKS-S (green) as a function of azimuth for all six investigated events. Travel time difference anomalies are by subtracting the theoretical travel time difference from the observed values.
To investigate the western edge of the Pacific LLSVP, we analyze seismic waveforms of earthquakes that occurred in Fiji and were observed in Japan. We collected waveform data from the J-Array at the Earthquake Research Institute of the University of Tokyo for deep earthquakes with focal depths greater than 500 km. We used six events: two of the earthquakes used by Obayashi et al. (2017) and four others that they did not use. First, the east-west and north-south components of the velocity waveform were converted into radial and lateral components, and then the differential times between the peaks of S and ScS or S and SKS were measured on the radial waveforms, which were low-pass filtered with a corner frequency of 0.2 Hz (Figure 1).
Figure 2 shows that anomalies of the differential times of ScS-S and SKS-S calculated by subtracting the theoretical traveltime difference estimated from a reference model IASP91 (Kennett and Engdahl, 1991) from the observed value. The ScS delay relative to the S arrival time is very large (7.189 [s] on average), while SKS delay is small (0.843 [s] on average), which is consistent with the result of Obayashi et al. (2017) who reported that there are localized low velocity anomalies above the CMB. The magnitude of the S-wave low velocity anomalies is estimated by assuming that the low velocity anomalies exist within the epicenter distance range where the SKS waves propagate through the core. We will present estimated amplitudes of the low velocity anomalies.
Figure1: Radial component velocity waveforms of J-array stations aligned by epicentral distance for the Mw 6.9 event on May 16th, 1998. The waveforms were low-pass filtered with a corner frequency of 0.2 Hz. Crosses indicate the S (black), SKS (green), and ScS (red) peaks. Theoretical traveltimes of S, SKS and ScS waves estimated from IASP91 (Kennett and Engdahl, 1991) are represented by black, green and red lines, respectively.
Figure2: Travel time difference anomalies for ScS-S (red) and SKS-S (green) as a function of azimuth for all six investigated events. Travel time difference anomalies are by subtracting the theoretical travel time difference from the observed values.