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Abstract Distributed Acoustic Sensing (DAS) is an emerging technology that converts optical fibers into dense arrays of strainmeters, significantly enhancing our understanding of earthquake physics and Earth's structure. While most past DAS studies have focused primarily on seismic wave phase information, accurate measurements of true ground motion amplitudes are crucial for comprehensive future analyses. However, amplitudes in DAS recordings, especially for pre‐existing telecommunication cables with uncertain fiber‐ground coupling, have not been fully quantified. By calibrating three DAS arrays with co‐located seismometers, we systematically evaluate DAS amplitudes. Our results indicate that the average DAS amplitude of earthquake signals closely matches that of co‐located seismometer data across frequencies from 0.01 to 10 Hz. The noise floor of DAS is comparable to that of strong‐motion stations but higher than that of broadband stations. The saturation amplitude of DAS is adjustable by modifying the pulse repetition rate and gauge length. We also demonstrate how our findings enhance the understanding of fiber‐optic seismology and its implications for natural hazard mitigation and Earth structure imaging and monitoring. Specifically, our results suggest that with proper settings, DAS can detectP‐waves from an M6+ earthquake occurring 10 km from the cable without saturation, indicating its viability for earthquake early warning. Through quantitative comparison and analysis, we also find that local ambient traffic noise levels strongly affect the quality of seismic interferometry measurement, which is a powerful tool for near‐surface imaging and monitoring. Our methodology and findings are valuable for future DAS experiments that require precise seismic amplitude measurements. 

Abstract We present a real-data test for offshore earthquake early warning (EEW) with distributed acoustic sensing (DAS) by transforming submarine fiber-optic cable into a dense seismic array. First, we constrain earthquake locations using the arrival-time information recorded by the DAS array. Second, with site effects along the cable calibrated using an independent earthquake, we estimate earthquake magnitudes directly from strain rate amplitudes by applying a scaling relation transferred from onshore DAS arrays. Our results indicate that using this single 50 km offshore DAS array can offer ∼3 s improvement in the alert time of EEW compared to onshore seismic stations. Furthermore, we simulate and demonstrate that multiple DAS arrays extending toward the trench placed along the coast can uniformly improve alert times along a subduction zone by more than 5 s. 

ABSTRACT In September 2017, over 450 lives were lost in Mexico as a result of two unusual, large-magnitude, normal earthquakes. On 7 September, an M 8.2 earthquake occurred offshore of the State of Oaxaca in the Gulf of Tehuantepec, one of the largest extensional earthquakes to have occurred in a subduction zone. Twelve days later on 19 September an M 7.1 damaging earthquake struck near Puebla and Morelos, over 600 km away. Both earthquakes occurred in the downgoing Cocos plate, which is subducting beneath the North American plate. The first large event was followed on 23 September by a shallow M 6.1 extensional earthquake near Juchitán de Zaragoza, Oaxaca. Researchers from Mexico and the United States collaborated to deploy a temporary seismic network to study the aftershocks of the M 8.2 Tehuantepec, Mexico, earthquake, which included a three-week deployment of 51 Magseis Fairfield Z-Land 5-Hz three-component nodal seismometers (“nodes”) near Juchitán and a 6-month deployment of 10 Nanometrics Trillium 120PA broadband seismometers with Reftek RT130 dataloggers for 6 months. In this article, we analyze the capabilities of the nodes to calculate the horizontal/vertical (H/V) spectral ratio and relative amplification using both microtremors and earthquakes and validate the results calculated with the nodes using data from broadband stations from this and previous deployments in the area. We create maps showing a correlation of the distribution of the fundamental frequency and relative amplification of the soil and compare them with the geology and the damage caused by the September 2017 earthquakes. There is a lack of public awareness and discrepancies in the construction procedures in the region, and we find that the majority of damaged houses in the area of study followed the location of river beds and tended to be in places with low resonance frequencies despite being in a low amplification zone.