Micro-seismicity in the southwestern Yukon, Canada




Meighan, Lindsey Nicole

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The objective of my research is to provide a better understanding of the relationship between the micro-seismicity, tectonics and crustal structure in southwest Yukon in order to improve seismic hazard assessments in this region. I used a combination of single event and multiple event location techniques to determine earthquake locations and depths. As well, frequency-magnitude statistics were calculated to analyze rates of seismicity and possible changes in the rates of seismicity. The addition of the YUK array in August 2010 has enabled location of smaller events and detection of a systematic northeast trend of earthquakes. Seismicity is concentrated in four main areas: 1) Yaktutat Block-Fairweather Fault, 2) Duke River Fault, 3) Denali Fault, and 4) a NE-trend. There was relatively little seismic activity during this period along the northern Denali Fault segment and only a small amount of activity along the southern portion of the Denali Fault. There is significantly more seismic activity along the Duke River Fault and NE-trend and a clear region of seismicity just west and parallel to the Alaska-Yukon border between the Duke River Fault and northern Denali Fault. Frequency-magnitude statistics and seismic hazard analyses for southwest Yukon were improved by decreasing the minimum magnitude of completeness from M3.0 to M1.0. Between September 2010 and November 2011, event magnitudes ranged from 0.2 to 4.7 and depths from 0 to 35 km. To address how the YUK array has improved single event locations and depths, we use a single-event location technique to monitor seismic activity. Only 37 of the 106 events detected for the Duke River Fault and NE-trend could potentially be located without the YUK array. When the Alaska Earthquake Information Center (AEIC) network was combined with the Canadian National Seismograph Network (CNSN), events within the NE-trend shift on average 6.6 km to the northeast and the depth increased on average 2.6 km. Within the Duke River and NE-trending clusters, there is an average maximum horizontal error of ±0.9 km and an average error in depth of ±3.2 km. Free depths in the Duke River and NE-trending clusters range from 0 to 20 km. These depths are not well-constrained as the closest station is more than 20 km away. Two events within the southern Denali Fault cluster have well-constrained depths of 4.8 km and 8.2 km at distance less than ~8 km from station YUK6, consistent with upper crust (2-10 km) focal depths. A Progressive Multiple Event Location technique (PMEL) was used to identify and better constrain spatial patterns along the Duke River Fault and NE-trend. Results clearly shows that events fall along the Duke River Fault and that the NE-trend events are located on a previously unidentified active fault. To determine rates of seismicity and possible changes in the rates of seismicity, I examine b-values from frequency-magnitude statistics for each cluster of earthquakes before and after the 2002 M7.9 Denali Fault earthquake. b-values increased from 0.81 ± 0.14 to 1.05 ± 0.22 , suggesting higher Coulomb stress and more frequent smaller earthquakes.



Yukon, b-values, Progressive Multiple Event Locations, seismic activity