Detection and location of earthquakes in the Canadian Rocky Mountain
Trench by kurtosis and Bayesian sampling in the presence of strong
cultural noise
Abstract
To improve our understanding of the Canoe Reach Geothermal Field in the
Rocky Mountain Trench of western Canada, we examine the distribution of
local earthquakes using a network of 10 broadband seismometers deployed
over a 40 by 60 km area across the trench. The Canoe Reach area exhibits
strong cultural noise from communities, roads and trains that makes
detecting earthquake signals challenging. We propose detecting
earthquakes in the area of the trench by measuring the kurtosis of the
seismic signal, which is a statistical moment representing the
distribution tail and is insensitive to emerging signals but more
sensitive to impulsive earthquake onsets. Examining the kurtosis of the
three-component seismograms for four months of data, we identified eight
local earthquakes. An earthquake catalog produced by STA/LTA detections
found 11 events for the same four-month period, four of which were
detected through our kurtosis approach. By further exploring the
kurtosis detections, we are refining our catalog to identify the source
of discrepancies between it and the STA/LTA catalog. We then estimated
locations of our detected events, and the uncertainties of those
locations, through nonlinear Bayesian sampling. This method treats the
origin times, half-space velocities, and the picking noise for P and S
arrivals as unknowns. We employed this parameterization to test whether
Bayesian sampling could account for the challenging noise environment.
Locating our detected events found that five events occurred outside the
seismic network and three events occurred inside. The average horizontal
and vertical uncertainty is 28 and 19 km respectively for the outside
events. These uncertainties are lower at 7 and 9 km for the inside
events. While the inside events exhibit lower spatial uncertainties than
the outside events, their uncertainties remain large. We then examined
whether the uncertainties could be further improved by jointly locating
multiple events. Jointly inverting two of the events from within the
array decreased their average horizontal uncertainty from 6.5 to 2.5 km
and the vertical from 14 to 7 km. Reducing uncertainties in the
locations of the events in this manner will clarify their distribution
and all for an improved understanding of the seismicity and structure of
the Rocky Mountain Trench.