10:45 AM - 11:00 AM
[STT36-06] Estimation of the focal mechanism of earthquakes beneath optic fiber cable with DAS
Keywords:DAS, focal mechanism
Distributed-Acoustic-Sensing (DAS) is a new technology of seismology, which observes the axial strain or strain-rate of optic-fiber cable by analyzing the phase of backscattered light passing through it. However, obtaining the information of earthquake sources from DAS records is challenging, because DAS only measures the component in the direction along the cable.
Here, we will show one of the first attempts at estimating focal mechanisms from DAS. Four earthquakes of M2.2, M2.3, M3.2 and M3.4 in the Kyoto Nishiyama Fault Zone on October 19, 2022, were recorded by the DAS along Route 9. First, we re-determined the hypocenter of these events using DAS records. Then, we utilized cross-correlation and deconvolution for each event pair to estimate the focal mechanism of these events. The P-wave records along the cable, the partial derivative of displacement of ordinary seismograms with respect to the axial direction, depends on the angle between the direction of the P-wave and the cable. As we multiply (cross-correlate) or divide (deconvolve) between neighboring events on each channel, we can remove the effect of cable geometry because we can assume that the angle is the same on each event. As a result, we succeeded in obtaining the relative polarity for almost all of the event pairs in the far-field channels, while we could not determine the relative polarity in some pairs for some channels, which locate close to the source. This result is likely attributed to the fact that these channels are near the source, rendering the assumption, "the angle is the same", invalid. We will discuss the estimated focal mechanisms by inverting the polarity of channels obtained from the DAS records together with conventional seismograms.
Here, we will show one of the first attempts at estimating focal mechanisms from DAS. Four earthquakes of M2.2, M2.3, M3.2 and M3.4 in the Kyoto Nishiyama Fault Zone on October 19, 2022, were recorded by the DAS along Route 9. First, we re-determined the hypocenter of these events using DAS records. Then, we utilized cross-correlation and deconvolution for each event pair to estimate the focal mechanism of these events. The P-wave records along the cable, the partial derivative of displacement of ordinary seismograms with respect to the axial direction, depends on the angle between the direction of the P-wave and the cable. As we multiply (cross-correlate) or divide (deconvolve) between neighboring events on each channel, we can remove the effect of cable geometry because we can assume that the angle is the same on each event. As a result, we succeeded in obtaining the relative polarity for almost all of the event pairs in the far-field channels, while we could not determine the relative polarity in some pairs for some channels, which locate close to the source. This result is likely attributed to the fact that these channels are near the source, rendering the assumption, "the angle is the same", invalid. We will discuss the estimated focal mechanisms by inverting the polarity of channels obtained from the DAS records together with conventional seismograms.