Peer review of: Slip bursts during coalescence of slow slip events in Cascadia (2021)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
This is a well-written and illustrated paper that addresses one of the hottest topics today in 
Geophysics, namely the extent to which slow slip events act as pre-cursors to large and great 
subduction zone earthquakes, and the relevant underlying physics. The results described by the 
authors represent an important advance in our understanding of these complex events. In my 
opinion, it should be published. 
 
I have one minor point that the authors may wish to address. The basic thesis of the paper is that 
merging of SSEs leads to acceleration, which can lead to a large earthquake. A simplistic reading 
of the paper leads to the opposite conclusion - they have observed merging and acceleration, but a 
large earthquake did not occur. I am sympathetic to the authors' viewpoint that this is merely an 
example of a class of event that COULD lead to an earthquake, but some more nuanced discussion 
might be useful. For example, can they speculate on what the key threshold for an earthquake is 
based on their results? Perhaps it is peak slip rate, combined with region of the seismogenic zone 
being late in the earthquake cycle. If peak slip rate is the key, then merging of events may not be 
required - just acceleration (by any means) such some key threshold is crossed). From their 
figures it appears that peak slip rate was about 2 mm/day (the exact number should be referenced 
in the paper). Can they compare this value to other studies (where earthquakes did or did not 
occur). I believe there have been observations of this in both Central America and Japan. 
 
It may also be useful to plot maximum daily slip rate on Figure 3, it is presumably closely related 
to moment rate. 
 
END 
 
 
Reviewer #2 (Remarks to the Author): 
 
The paper highlights that slow slip fronts progressively merged together during a 2013 sequence 
along Cascadia subduction zone, and the slip rate temporally increased during the merging phases. 
Coalescence of rupture front has been proposed numerically and in laboratory experiments, but 
there has never been a field observation. If the coalescence of slow slip fronts is well constrained 
by geodetic data, the present result will be really exciting to the scientific community of 
seismology and geodesy. The manuscript is well organized and concise. However, there are some 
concerns/weakness related to the slip evolution inverted from the geodetic measurements as 
described below. Especially, the reliability and robustness of the inverted slip are not convincing to 
me. These points should be clarified and revised properly before the publication. 
 
1) To confirm the reliability of each slip patch, it is required to add maps showing displacement 
vectors at each time step (daily). Especially, the increment displacement vectors at each time step 
are essentially important before and after the merging phase of two different slip patches. The 
space and time evolution of horizontal displacement vectors are basic information to judge the 
reliability of slip distribution. 
 
2) I cannot understand well the method to invert daily slip amount on the plate interface. I guess 
that the authors inverted the cumulative displacement vectors on one day and the successive day, 
respectively, to obtain a spatial distribution of the cumulative slip on the plate interface. Then, the 
authors subtract the two cumulative slip amounts on the plate interface. Is it correct? More 
detailed explanation point should be added. And, estimation error of slip usually increases by 
subtraction. What is the minimum resolvable slip amount? Is a central slip-patch with small 
displacement on 26 September (Fig. 1) really constrained? 
3) The fitted curves to each displacement time series are very smooth, as shown from Figure S7 to 
S16. Why are these fitted curves so smooth? It seems to be quite strange. 
 
4) From Figure S7 to S16, many displacement signals are not well fitted by the predicted curves 
(e.g., CLRS, ALBH, PTAL, PTRF, SC02, SC04, P440, …). Why doesn’t it work so much? Especially, 
the predicted amplitude is often smaller than that from the observation. 
 
5) From Figure S7 to S16, there are offsets from the starting day as seen in many stations (e.g., 
P691, P441..). Why? The detrending does not work well, isn’t it? 
 
6) How did you account for common mode errors as well as seasonal variations? The explanation 
about this point should be added. 
 
7) Around merging area of two slow slip patches, there are several bore-hole strain meters. Can 
you find out the similar acceleration of slip rate from the bore-hole strain measurements at the 
same timing? The bore-hole data can strengthen your results. 
 
8) In Figure 3, the moment rate release has the second peak with a significant delay (3-4 days) 
after the merging of southern patch with northern one. Do you have an interpretation? 
 
9) In several references (Kaneko and Ampuero, 2011 
(http://dx.doi.org/10.1029/2011GL049953.); Fukuyama et al., 2018 
(https://doi.org/10.1016/j.tecto.2017.12.023))., coalescence of rupture front has been discussed 
numerically and in laboratory experiments. These papers have suggested the rupture front 
coalescence may initiate the unstable ruptures. 
 
 
 
 
Reviewer #3 (Remarks to the Author): 
 
Review Summary: 
 
The authors propose that if a slow slip event (SSE) nucleates at multiple locations or fronts on a 
subduction plate interface, then at the time of merging of these slip fronts into a single slip zone, 
the slip accelerates, thereby defining areas and periods of enhanced probability for the occurence 
of large earthquakes. This hypothesis is not novel, having been presented in a theoretical context 
by researchers such as J.H. Dieterich (Ref. #2). What is novel and interesting both to the seismic 
community and the wider field is this paper's detailed study of a SSE that occurred along the 
northern Cascadia Subduction Zone which demonstrates, for the first time, that such slip merging 
can indeed occur. 
 
However, the authors' contention that this merging of slip results in an observed increase in slip 
rate is open to question. Referring to Figure 3, the authors argue that the dramatic increase in 
moment rate release implies an increase in average slip rate, which I interpret to refer to a 
temporal change in the slip velocity. Is it possible that changes in moment rate release reflect 
changes in the area of the fault that is active at a given time? Authors need to discuss the 
contributions of changes in slip velocity versus the contributions of changes in slip area to the daily 
moment rate release. If the total area of active fault slip on a given day is the main contributer to 
variations in moment rate release, then authors should also keep in mind that low-amplitude, 
distributed slip would not be well resolved, especially in mid- to northern Vancouver Island where 
the spatial coverage of GPS stations is sparse. 
 
In addition, the authors themselves point out that the slip velocities must be much greater (closer 
to seismic velocities) in order to trigger seismic rupture. If such high slip velocities are required to increase the likelihood of an earthquake, does it not render the (relatively) modest proposed 
change in slip velocity caused by merging of slips far less significant in determining earthquake 
probability? Authors need to address this in order to justify the speculation that it is the 
coalescence of slip fronts that is the significant factor. 
 
 
More detailed comments: 
 
Abstract L1/2 
Replace "earthquake ruptures are preceded" with "earthquake ruptures can be preceded" 
 
Abstract L10 
"slip drastically accelerates" See "Review Summary" 
 
Page 1 L1/2 
insert "northern" before "Cascadia subduction fault". The southern portion of Cascadia has 
different return periods for SSE (Brudzinski & Allen, Geology (2007) 35 (10): 907-910) 
 
Page 2 L16/17 
"Both slip and tremor locations indicate a southeastward ...migration" 
This migration is not apparent for the slip. On Figure S4,Sep.16 to Sep.24, the small patch of 
northern slip looks essentially stationary. This is likely controlled by the fact that there is only a 
single GPS station (NTKA) in this area. 
 
Page 2 L24/25 
"...meaning that the average slip rate increased by a factor > 5" 
What is meant here by "average slip rate"? The total area identified to be involved in slip appears 
to have increased by roughly a factor of 5 from Sep.26 to Sep.28. Can this be the the main cause 
of the sharp increase in moment rate release shown in Figure 3? How are changes in moment rate 
release due to slip area variations seperated from changes due to slip velocity variations? 
 
Page 3 L9 
"Although such [coalescence] episodes have not been observed during other SSE's in Cascadia" 
Have the authors themselves examined other SSE's in Cascadia or are they stating that no-one 
else has pointed them out? (which should not be surprising since the concept of actual observed 
SSE slip coelescence is the novel aspect of this paper). The authors are referred to a JGR paper by 
Honn et al., 2009, Northern Cascadia episodic tremor and slip: A decade of tremor observations 
from 1997 to 2007,J. Geophys. Res.,114, B00A12, doi:10.1029/2008JB006046. This paper 
proposes a classification scheme for northern Cascadia ETS (Episodic Tremor and Slip) episodes 
and points out that "Halting and jumping are very common in ETS migration patterns, and along-
strike migration can happen in both directions." An examination of the slip behaviour over a 
decade of observations presented by Kao et al., in the context of slip coalescence, may provide 
additional supporting data. 
 
Page 4 L18/19 
"The scale and duration of the possible micro-events composing SSE's..." By pointing out that 
there could exist local and episodic slip rates much larger than the resolved daily average slip 
rates, are the authors implying that slip rates closer to seismic slip rates are required for an SSE to 
trigger an earthquake? If so, then the changes in slip rate resulting from the convergence of SSE's 
is not a key factor in determining the probability of the occurence of a large earthquake. Instead, 
it would be the generation of high-amplitude, short-duration micro-slip events which would govern 
earthquake likelihood. If this is the case, then the authors need to show that the coalescence of 
slow slip fronts results in a greater number of possibly critical micro-slips. 
 
Supplementary Materials 

Figure S1 
GPS coverage in central to northern Vancouver Island is sparse, limiting the resolution of the slip 
inversion. There are 4 GPS stations in this region that, unfortunately, are not included in the 
nominal UNAVCO analyses (see edited version of Figure S1). This raises the question: Does the 
mismatch in spatial sampling density between the northern and southern areas present any 
intrinsic bias in defining the extent and development of the slip zones? 
 
Figure S18 
In evaluating the resolution power of their approach it should be pointed out that the question is 
not the fidelity of reproducing "target" slips but one of uniqueness. How many other model slip 
distributions will adequately replicate the target slips within nominal observational error?