Florian Schmid, Vera Schlindwein
AG Seismologie 2014
Seismicity of an amagmatic Southwest Indian Ridge segment
First results of a combined long term OBS experiment
Objectives
Contact: fschmid@awi.de
For the first time an array of 10 ocean bottom seismometers (OBS) has been installed in late 2012 at an amagmatic segment of the Southwest Indian Ridge (SWIR), in the furious fifties latitudes. During the multidisci- plinary RV Polarstern cruise ANT-29/8
(1)in autumn 2013 five short wi- de-angle seismic profiles were shot across the array and 9 instruments could be recovered afterwards. To date various aspects of the formation of new oceanic crust at the SWIR are only partly understood. Local seis- micity at mid ocean ridges settings reflects tectonic, volcanic and hydro- thermal activity and will be exploited in this study to shed some light on the processes leading to the amagmatic creation of new oceanic crust at the SWIR.
20˚W
0˚ 10˚E
20˚E 30˚E
60˚S 50˚S 40˚S
30˚S Africa
Antarctica
South-
west Indian
Ridge experiment
location
Acknowledgements: We thank the masters and crews of RV Polarstern for the invaluable support during the deployment and recovery cruises ANT-29/2 & ANT-29/8. The AWI DEPAS instrument pool kindly provided the instruments.
Wide-angle seismic experiment Preliminary earthquake epi- and hypocenters
Next steps
For the wide-ang- le seismic survey a cluster of four G-gun type air-
guns was used as
seismic sources and an average shot point spacing
of 150m was achieved. Results of forward mode- ling and inversion of refracted crustal (Pg) and mantle (Pn) phases and
Moho reflected (PmP) phases pro-
vide a 2D image of crustal and upper
mantle P-wave velocities. High amplitude crustal refractions indica- te steep velocity gradients near the
A cross-section along ridge axis B C cross-section D
across ridge axis
−40
−30
−20
−10 0
Depth below sea level [km]
0 20 40 60 80 100
Distance along profile [km]
all hypocenter− and station positions are projected on the cross−section
−40
−30
−20
−10 0
Depth below sea level [km]
0 5 10 15 20 25 30 35
Distance along profile [km]
legend
depth inverted hypocenters with depth errors hypocenters with fixed depth
OBS station positions
Moho, as of 1D input velocity model
12˚48'E 13˚00'E 13˚12'E 13˚24'E 13˚36'E 13˚48'E 14˚00'E 14˚12'E
52˚36'S 52˚36'S
52˚24'S 52˚24'S
52˚12'S 52˚12'S
52˚00'S 52˚00'S
A
B C
D
20 km
40 km
60 km
80 km
100 km
S02
S03
S05
S06
S07 S08
S09 S10
0 10 20
km
S
E N
W
legend
epicenters with error ellipses OBS station positions cross−section endpoints wide−angle seismic profile
−6000 −5000 −4000 −3000 −2000 Water depth [m]
A
0
2
4
6
8
10
Depth [km]
0
2
4
6
8
10
Depth [km]
0 10 20 30 40 50 60 70 80 90 100
Distance [km]
2 2
2
2 2 4
4 4
4 4
6 6
6 6 6
0
10
0
10
0 50 100
7
S02 S03
Moho
0 1 2 3 4 5 6 7 8 9 P−wave velocity [km/s]
0
2
4
6
8
10
Depth [km]
0
2
4
6
8
10
Depth [km]
0 10 20 30 40 50 60 70 80 90 100
Distance [km]
2 2
2
2 2 4
4 4
4 4
6 6
6 6 6
0
10
0
10
0 50 100
7
S02 S03
Moho
0
2
4
6
Time−Offset/7 [s]
0
2
4
6
Time−Offset/7 [s]
−40 −30 −20 −10 0 10 20 30 40
Offset [km]
S03 Hydrophone
NE
NE SW
SW
SW NE
Pn PmP Pg Pn
Pg PmP Pw
The continuous dataset of 11 month recording was scanned with a STA/LTA trigger, retur- ning a total of 2697 de- tections. For 1449 of these events P- and S-phase onsets could be picked manually at 3 or more stations. The HY- POSAT(2) algorithm, used for the localisation was able to relocate 1404 events of the given data- set.
Preliminary results indicate that the local seismicity is strongly constrained to the rift valley while only odd events are found beyond the axial valley flanks.
sea floor and decreased velocity gradients in the lower crust. Moho re- flections indicate a crustal thickness in the range of 3.0-4.5 km along the modeled profile section. Mantle refractions are observed up to 45 km offset, providing a good constraint on upper mantle P-wave velo- cities.
Four vertical velocity profiles were ext- racted from the modeled profile section for the region of highest ray coverage (see grey dashed lines in figure above and graphic to the right). A 1D input velocity model for the earthquake locali- sation was constructed, based on P-wave velocities and average seismic layer thicknesses of the four profiles.
The hypocenter depth inversion returned a stable solution for 314 events. Hypocenter depths were projected on cross-sections along and across the ridge axis. The majority of depth inver- ted hypocenters form a cluster situated in the center of the rift valley in the range of 50 - 75 km along cross-section AB. The cluster ranges at depths of 6 - 12 km below the sea floor. Scat- tered events are found at greater depth. However, explanation and interpretation of hypocen- ters deeper than 15 km below the sea floor at mid ocean ridge settings is somewhat intricate from a physical point of view, due to the rheological properties of the mantle. Possibly, deep hypocenters here are a result of falsely identified S-phases.
+ detailed quality control of localisation results, in particular for deep hypocenter events
+ estimation of body wave magnitudes
+ establishing an event catalogue suitable to serve as basis for a local tomography study
gales are very frequent in the furious fifties
OBS deployment
0 2 4 6 8 10 12 14
Depth below sea floor [km]
1 2 3 4 5 6 7 8 9
P−wave velocity [km/s]
km 40 km 50 km 60 km 70
1D input velocity model for earthquake localisation
profiles extracted from 2D section
+ set up a benchmark test and model parameters for a combined local earthquake and active source
tomography study
+ run the tomography model and do an integrated interpretation of all results
References
(1) Schlindwein, V. (Ed.) (2014), The expedition of the Research Vessel „Polarstern“ to the Antarctic in 2013 (ANT-XXVIII/8), 111 pp., Alfred-Wegener-Institut,
Bremerhaven.
(2) Schweitzer, J. (2001), HYPOSAT - An enhanced routine to locate seismic events, Pure Appl. Geophys., 158, 277-289
