A Catalog of Low Frequency Earthquakes at the Southern Edge of Cascadia Subduction

A Catalog of Low-Frequency Earthquakes at the Southern Edge of Cascadia Subduction David R. Shelly1 1 U.S. Geological Survey, Geological Hazards Science Center, Golden, CO 80401 This catalog contains 61,441 detections from 27 low-frequency earthquake (LFE) families located in an isolated zone offset to the west from the southern edge of the main band of Cascadia tremor. The included detections span from January 1, 2018, through November 30, 2024. Details of the investigation are published in: Shelly, D. R., Goldberg, D. E., Wech, A. G., & Thomas, A. M. (2025). A northeast-dipping zone of low-frequency earthquakes at the southern edge of Cascadia subduction. Geophysical Research Letters, 52, e2025GL116116. https://doi.org/10.1029/2025GL116116 As presented here, the catalog integrates detections from the 27 LFE families. A threshold of 9 times the daily median absolute deviation (MAD) of the correlation sum across all channels is applied. Additionally, we require the mean correlation for all channels with data to be at least 0.12. This provides more robust detection than the initially applied threshold of 8 times MAD. Potential duplicate detections were removed, retaining only the detection with the highest correlation sum in any 4.0 s window. Please see publication above for further details. LFE family locations have been estimated as follows, with columns of identification number, latitude (degrees), longitude (degrees), and depth (km below mean surface): 1 40.063867 -123.597624 28.189 2 40.069832 -123.632715 27.623 3 40.036072 -123.684090 22.192 4 40.070557 -123.619751 28.199 5 40.063883 -123.593148 28.578 6 40.063208 -123.593563 28.223 7 40.070020 -123.630452 27.747 8 40.088265 -123.709562 24.216 9 40.069922 -123.621134 28.134 10 40.069836 -123.627124 27.834 11 40.040125 -123.675838 22.449 12 40.068445 -123.626107 27.775 13 40.069963 -123.623511 27.960 14 40.036499 -123.668848 23.295 15 40.069820 -123.632642 27.698 16 40.064124 -123.599935 28.170 17 40.070052 -123.620215 28.126 18 40.067786 -123.610710 28.209 19 40.072070 -123.686711 25.676 20 40.064315 -123.601424 28.162 21 40.027307 -123.606934 23.712 22 40.073580 -123.644653 27.497 23 40.053434 -123.556055 27.045 24 40.064563 -123.601245 28.189 25 40.069836 -123.619857 28.164 26 40.072095 -123.712524 24.127 27 40.036267 -123.532186 25.939 The LFE family locations may be updated in the future. Therefore they are given here separately from the catalog file. The LFE catalog (LFEcat_MTJ_2018-2024.csv) fields are as follows: year, month, day, hour, minute, second: Event time. Time given is template start time in UTC, starting 1 s prior to estimated S-wave arrival time at first station. Seconds are reported to 0.01s precision. s_of_day: Event time, given as second of the day (i.e. 0-86400 s). Time given is template start time in UTC, starting 1 s prior to estimated S-wave arrival time at first station. ccsum: correlation sum across all stations thresh: initial detection threshold of 8 times the daily median absolute deviation of the correlation sum for that day cc_div_thr: ccsum divided by thresh meancc: mean correlation among channels with data nchan: number of data channels existing for the event (defined by having a non-zero correlation value) ID: full reference ID of family idnum: number associated with the family ID (1-27) Note on Family IDs: Each family has an associated identification code, which is a number followed by 1-2 ‘s’. The family IDs are almost meaningless and are simply used as unique identifiers. Originally the numeric code was taken from the timing of the original source event. The number of ‘s’ indicates the number of iterations of stacking and cross-correlation that were applied to derive the template waveforms. The ‘z’ designator in family #17 (23012031243zs) indicates the correction of data on BK.BRIC, due to a polarity reversal in the original source event data.

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