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Session: Earthquake Characterization Using Fiber-optic Cables

Type: Oral

Date: 10/8/2024

Time: 12:00 PM

Room: Stanley Park Ballroom

10-m-deep Earthquake Swarms (Mw -2) Near the Milun Fault in Hualien, Taiwan, Detected by the MiDAS Seismic Monitoring System

After the 2018 M6.4 Hualien earthquake in Taiwan, the Milun fault Drilling and All-inclusive Sensing (MiDAS) project drilled two scientific boreholes on the hanging (Hole A) and foot walls (Hole B) of the Milun fault, which had 0.5-m co-seismic slip during the earthquake and caused serious damage in Hualien City. The project deployed a 7.5-km-long optical-fiber cable perpendicular to the Milun fault, passing through Hole A and B (from top to bottom) as well as the main fault zone on the surface. The downhole triaxial seismometer arrays were installed in the boreholes. Both distributed acoustic sensing (DAS) and traditional seismometer systems operate continuously in earthquake monitoring. In the present study, we manually detect earthquakes from the continuous seismic records in a period of 2023/3/16-5/31 (76 days) and identify two local seismic swarms occurring on 4/7 and 5/2. It is worth noting that the swarm on 5/2, including 75 events within 5 hours, has an extremely small ts-tp arrival time of ~0.15 s in the sensors in Hole B, suggesting that the events were very close to the borehole. Benefited from the combined seismic monitoring system of the DAS and downhole seismometers, we determine (i) the DAS node with the first arrival time of S wave in Hole B to estimate the event depth, as well as (ii) the P-wave polarization and amplitude in ground velocity from the downhole seismometers to obtain the horizontal location and magnitude. In the end, we recognize that these earthquakes occurred at the depths of ~10 m and at the locations ~50 m away from the Hole B wellhead with the magnitudes approximating to Mw -2. The ultra-shallow microearthquakes observed in the present study are perhaps a record value of natural earthquakes, which can further contribute to the seismological studies, such as, the earthquake initial-phase and finite-dimension issues, as well as temporal variation of physical parameters (e.g., rigidity and wave velocity) in the ultra-shallow structures.

Presenting Author: Yen-Yu

Additional Authors