Wednesday, April 18th, 2018

Wednesday Seminar

4:10 PM, 55 Roessler
Tea and cookies at 3:45 in the aviary - (2110 EPS)

“Imaging Northern Cascadia slow slip on scales from seconds to weeks and 100 m to 300 km”

      – by Dr. Kenneth Creager, University of Washington

Using the 200-seismometer Array of Arrays we have identified 34,000 low-frequency earthquakes (LFEs) on the plate interface below the Olympic Peninsula, Washington and determined their seismic moments. The LFEs spatially cluster into 45 families with horizontal dimensions of hundreds of meters and separated by gaps ranging from 1 to 20 km. Individual LFEs vary from Mw 0.7 to 2.1 and unlike regular earthquakes, follow an exponential magnitude-frequency distribution with a characteristic moment of 2.0x10^11 N-m (Mw=1.5). An exponential moment-frequency distribution implies a scale-limited source process. Seismic Moment equals the shear modulus (which we know) * area (A) * average slip (s). Thus, we have a constraint on the characteristic (mean) value of A * s for individual LFEs within each family. We do not have any direct constraints on A or s for individual LFEs, but we have geodetic constraints on the total slip during Episodic Tremor and Slip (ETS) events (S_ETS) and we have an estimate of the area of a family patch based on the distribution of individual LFE locations. We consider two endmember models: (1) connected patch model in which individual LFEs rupture different portions of the LFE family patch, but when summed over all LFEs every part of the patch slips the same amount. (2) For the ductile matrix model the patch is divided into regions with no LFE slip, and regions with LFE slip summing to S_ETS. We will explore the implications of these models in terms of stress drop and geologic observations. We find that 0.2% of the geodetically inferred slip is accommodated by LFEs. Down-dip LFE slip occurs through a larger number (800-1,200) of small LFEs, while updip LFE slip occurs primarily during ETS events through a smaller number (200-600) of larger LFEs. This could indicate that the plate interface is stronger and has a higher stress threshold updip. A detailed analysis of tremor migration and amplitudes suggests that ETS slip can be broken into a nucleation phase which starts down dip at low amplitude and small area and propagates updip increasing linearly in amplitude and area for about a week before filling the width of the tremor-zone. It then enters a propagation phase in which it propagates linearly along strike at 8-10 km/day for up to 4 weeks.

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