Variations in slow slip moment rate associated with rapid tremor reversals in Cascadia

Abstract

During large slow slip events, tremor sometimes propagates in the reverse along‐strike direction for a few hours, at speeds 10 to 40 times faster than the forward propagation. We examine the aseismic slip that underlies this rapidly propagating tremor. We use PBO (Plate Boundary Observatory) borehole strainmeter data to search for variations in the slow slip moment rate during 35 rapid tremor reversals (RTRs) that occurred beneath Vancouver Island. The strain records reveal that, on average, the strain rate increases by about 100% ( ±30%) during RTRs. Given the Green's functions expected for slip in the RTR locations, these strain rate increases imply 50 to 130% increases in the aseismic moment rate. The median moment released per RTR is between 8 and 21% of the daily slow slip moment, equivalent to that of a MW 5.0 to 5.1 earthquake. By combining the RTR moments with the spatial extents suggested by tremor, we estimate that a typical RTR has peak slip of roughly one‐sixth of the peak slip in the main slow slip event, near‐front slip rate of a few to ten times the main front slip rate, stress drop around half the main event stress drop, and strain energy release rate around one‐tenth that of the main front. Our observations support a picture of RTRs as aseismic subevents with high slip rates but modest strain energy release. RTRs appear to contribute to but not dominate the overall slow slip moment, though they may accommodate most of the slip in certain locations.

Insights into the causal relationship between slow slip and tectonic tremor in Guerrero, Mexico

“…For slow dislocations such as shown in Figure or those observed in Cascadia during the ETS, the length of the slip front, W , is much larger than the distance between the front and the place where the slip has reached its final value, L (i.e., W ≫ L ). In these cases, α = 1/ π is the more appropriate choice for a uniform stress drop distribution [ Rubin and Ampuero , ; Hawthorne et al ., ]. Assuming a shear modulus of 18 GPa [ Royer et al ., ] and a constant SSE propagation speed of 0.8 km/d [ Radiguet et al ., ], by taking the average of the maximum slip rates as 1 and 0.45 mm/d for the transient zone and the sweet spot (see Figures c and d), respectively, the corresponding stress drops for these regions are ~7 and ~3 kPa.…”

Seismic Moment, Seismic Energy, and Source Duration of Slow Earthquakes: Application of Brownian slow earthquake model to three major subduction zones

“…Hawthorne et al (2016) estimated the seismic moment of SSEs during rapid tremor reversal of 2-8 hr for M w 5.1 events, which equates to a moment rate of 2.8 × 10 12 Nm/s for T = 5 hr. Using highly sensitive strainmeters, Itaba and Ando (2011) found a M w 5.3 SSE with a duration of 1.5 days and a moment rate of 0.9 × 10 12 Nm/s in Kii, Japan.…”

“…The energy rate increases with the moment rate, and the ratios of the former to the latter, that is, the scaled energy, lie between 3 × 10 À10 and 10 À9 , yielding almost constant values, as expected from equation (9). The pink and blue diamonds are the estimates of Itaba and Ando (2011) and Hawthorne et al (2016), respectively. Comparison between the model and observations.…”

Using Low‐Frequency Earthquake Families on the San Andreas Fault as Deep Creepmeters

“…Bartlow et al, 2014;Bletery et al, 2017;Bostock et al, 2015;Brodsky & Mori, 2007;Hawthorne et al, 2016;Ide et al, 2007;Ito & Obara, 2006; long timescale) Episodic (short timescale) Continuous (short timescale) (a) versus median slip per episode. (b) versus median slip between episodes.…”