9:00 AM - 9:15 AM
[SCG52-06] Seismicity and velocity structure at the Kairei Hydrothermal Field at the Rodriguez Triple Junction in the Indian Ocean
Keywords:P, S wave velocity structure, Vp/Vs ratio, Kairei hydrothermal vent field, sepentinization, Indian ocean, seismicity
1. Introduction
The Kairei Hydrothermal Vent Field (KHF) is located at eastern area of the first segment of the Central Indian Ridge which is extended from the Rodriguez Triple Junction. The KHF extrudes fluid with rich H2 content. Although the KHF itself exists above a basaltic rock massif (Hakuho Knoll), gabbro and mafic rocks such as peridotite were discovered on the seafloor around the KHF. These deep rocks may move upward, may be serpentinized by sea water, and may contribute to the high H2 concentration of the Kairei vent fluid. To investigate seismic structure, we conducted a seismic survey with ocean bottom seismometers and an air-gun in 2013. Takata et al. (JpGU, 2015) reported a P wave velocity structure. They revealed high velocity areas with more than 6.0 km/s of the P wave velocity under 1-2 km from the
seafloor at the Hakuho Knoll and the Yokoniwa Rise which is located north of the Hakuho Knoll. This result indicated uplift of deep rocks. To investigate whether these rocks are serpentinized or not, it is important to obtain S wave velocity structure. This study reveals P and S wave velocity structures using both artificial (air gun) and natural (earthquakes) sources.
2. Data acquisition and analysis methods
We conducted a seismic refraction/reflection survey from January 27 to March 19 in 2013 using S/V Yokosuka of Jamstec (YK13-01, YK13-03). In the experiment, we used 19 OBSs, an air-gun (G I gun) and a single channel steamer cable. To make the S phase easer to see, we rotate the horizontal components of seismic sensors to the radial and transverse directions. We estimate horizontal direction of the sensors from trajectories of water waves. Using picking data of the S phase of the air gun, we construct a 3D S wave velocity structure by the FAST program (Zelt and Barton, 1998). Then, we construct 3D P and S velocity structures using natural sources by the tomoDD program (Zhang and Thurber, 2003). In this procedure, we used the estimated P and S wave structures as initial models. The relocation of earthquakes is also conducted by this program.
3. Results
From the result of the artificial sources, the high P wave velocity areas under the Hakuho Knoll and the Yokoniwa Rise have high Vp/Vs ratio with about 2.0. This ratio is higher than those for gabbro (about 1.9) and peridotite (about 1.7). Since the Vp/Vs ratio of serpentine is about 1.8-2.2, this result may indicate serpentinization of these areas. The result of the natural sources also shows high Vp/Vs ratio with about 2.0 at the KHF. A cluster of earthquakes was located a few km northwest of the KHF. This suggests that the faults of these earthquakes are the migration routes of water, and that the water reacts with peridotite to form serpentine. In the north of the first segment of the ridge axis, upper and lower event clusters were observed. The upper cluster occurred intermittently, and the lower one occurred almost continuously. This difference may represent different rising processes of magma.
Acknowledgment
We thank the captain and crew of S/V Yokosuka of JAMSTEC for their support. This work was supported by Grant-in-Aid for Scientific Research on Innovative Areas of the Ministry of Education, Culture, Sports, Science and Technology (Grant Number 20109002, TAIGA project).
The Kairei Hydrothermal Vent Field (KHF) is located at eastern area of the first segment of the Central Indian Ridge which is extended from the Rodriguez Triple Junction. The KHF extrudes fluid with rich H2 content. Although the KHF itself exists above a basaltic rock massif (Hakuho Knoll), gabbro and mafic rocks such as peridotite were discovered on the seafloor around the KHF. These deep rocks may move upward, may be serpentinized by sea water, and may contribute to the high H2 concentration of the Kairei vent fluid. To investigate seismic structure, we conducted a seismic survey with ocean bottom seismometers and an air-gun in 2013. Takata et al. (JpGU, 2015) reported a P wave velocity structure. They revealed high velocity areas with more than 6.0 km/s of the P wave velocity under 1-2 km from the
seafloor at the Hakuho Knoll and the Yokoniwa Rise which is located north of the Hakuho Knoll. This result indicated uplift of deep rocks. To investigate whether these rocks are serpentinized or not, it is important to obtain S wave velocity structure. This study reveals P and S wave velocity structures using both artificial (air gun) and natural (earthquakes) sources.
2. Data acquisition and analysis methods
We conducted a seismic refraction/reflection survey from January 27 to March 19 in 2013 using S/V Yokosuka of Jamstec (YK13-01, YK13-03). In the experiment, we used 19 OBSs, an air-gun (G I gun) and a single channel steamer cable. To make the S phase easer to see, we rotate the horizontal components of seismic sensors to the radial and transverse directions. We estimate horizontal direction of the sensors from trajectories of water waves. Using picking data of the S phase of the air gun, we construct a 3D S wave velocity structure by the FAST program (Zelt and Barton, 1998). Then, we construct 3D P and S velocity structures using natural sources by the tomoDD program (Zhang and Thurber, 2003). In this procedure, we used the estimated P and S wave structures as initial models. The relocation of earthquakes is also conducted by this program.
3. Results
From the result of the artificial sources, the high P wave velocity areas under the Hakuho Knoll and the Yokoniwa Rise have high Vp/Vs ratio with about 2.0. This ratio is higher than those for gabbro (about 1.9) and peridotite (about 1.7). Since the Vp/Vs ratio of serpentine is about 1.8-2.2, this result may indicate serpentinization of these areas. The result of the natural sources also shows high Vp/Vs ratio with about 2.0 at the KHF. A cluster of earthquakes was located a few km northwest of the KHF. This suggests that the faults of these earthquakes are the migration routes of water, and that the water reacts with peridotite to form serpentine. In the north of the first segment of the ridge axis, upper and lower event clusters were observed. The upper cluster occurred intermittently, and the lower one occurred almost continuously. This difference may represent different rising processes of magma.
Acknowledgment
We thank the captain and crew of S/V Yokosuka of JAMSTEC for their support. This work was supported by Grant-in-Aid for Scientific Research on Innovative Areas of the Ministry of Education, Culture, Sports, Science and Technology (Grant Number 20109002, TAIGA project).