Research
Temporal Variations of S-wave Velocity beneath Axial Seamount
S-wave velocity variation is key parameter to illustrate the magma transportation in a volcanic eruption cycle. Use a ocean bottom seismometer (OBS) and a bottom pressure recorder (BPR) to calculate the admittance between vertical displacement and pressure (D/P Ratio Method, Ruan et al., JGR, 2014). Obtain the S-wave velocity by inverting the observed admittance through synthetic admittance grid search.
Axial Seamount Volcano
Axial Seamount is a very active submarine volcano located at the intersection of the Juan de Fuca Ridge (JdFR) and Cobb-Eickelburg seamount chain. It’s most recent eruption at April 24, 2015, are well-recorded by the OOI cabled array, providing an unprecedented chance to study the process of the eruption (Kelley et al, Marine Geology, 2014).
With the continuous observation from bottom pressure recorders, inflation-deflation model of caldera elevation was treated as the representation of magma supply and eruption (Nooner et al., Science, 2016). In addition, seismicity variations before, during and after eruption revealed another critical trace of the eruption precursor (Wilcock et al, Science, 2016). Under the full-configured observation condition, a preliminarily temporal changes of S-wave structure beneath Axial Seamount was been seen (Doran & Crawford, Geology. 2020).
D/P Ratio Methods
Displacement-pressure ratio (D/P-Ratio) is the transfer function (also called Admittance) between vertical displacement and pressure in frequency domain (Ruan et al., JGR, 2014). The D/P ratio method uses the microseismic noises, which are dominated by the fundamental-mode Rayleigh wave in the period band of 0.16 - 0.22 Hz, as indicated by the coherence between displacement and pressure data.
Theoretical calculations using Rayleigh-wave mode method shows that the D/P-Ratio is sensitive to the mid (major magma reservoir) and lower oceanic crust (1.5-7 km), therefore can be used to invert for the shear-wave velocity in magma reservoir and lower crust.
D/P Ratio Results
The almost 6 years of continuous records of vertical displacement and pressure data to calculate the D/P-Ratio, the results showed an abrupt increase right on the eruption date (red dashed line) in station AXCC1.
In the AXEC2, a dramatic increase came weeks before the eruption and halfway back to the baseline level when the eruption occurred. The different behaviours at AXCC1 and AXEC2 indicated a distinguishing pattern of magma migration beneath the central and eastern caldera.
D/P ratio results were affected during the summer probably due to lower level of microseismic noises, similar to the reducing energy of infragravity wave in summer (Doran & Crawford, 2020).
Weekly & Daily S-wave velocity Variations
AXCC1
In the lower crust: S-wave velocity drop occurred on the day of eruption
In the magma reservoir: A relatively stable velocity
AXEC2
In the lower crust: S-wave velocity dramatically declined more than a month earlier than the eruption. Besides, a fast secondary velocity drop was seen after the eruption.
In the magma reservoir: A permanent decrease happened after the eruption.
Weekly S-wave velocity variations in 6 years beneath AXCC1 and AXEC2. Red dash lines are the eruptive date.
Daily S-wave velocity variations before and after eruption beneath AXCC1 and AXEC2. Gray bars are hourly seismicity obtained from the result in Wilcock et al., 2016.
Seismic Airgun Application
Use multiple excitation signals of the seismic airgun, a high signal-noise-ratio (SNR) waveform can be derived by signal stacking. High SNR waveforms make us able to pick an accurate arrival time. After excitations spanning different times, the arrival time difference can be used to measure the crustal velocity change by cross-correlation.
