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.