Peer review of: Were changes in stress state responsible for the 2019 Ridgecrest, California, earthquakes? (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
This is a very readable paper, linking the local and time dependent variation in seismic b-value 
before and after the Ridgecrest earthquakes with estimates of the stress changes as computed 
from the program Coulomb. The results indicate that there is predictive information in the b-value 
obtained which are the only actual observables that one has. 
 
A few observations. I would be curious to understand how the magnitude cutoff plays into the 
conclusions. That is, whether the information is contained in the very smallest earthquakes, or in 
the intermediate magnitude events. I would also curious to know how the events of Ridgecrest are 
related to events earlier in time, including the 1999 Hector Mine sequence and the 1992 Landers 
event. But possibly this is a calculation for another paper. In any event, I think this paper can be 
published substantially as is. The English is good with the exception of a few minor and obvious 
corrections needed. 
 
 
Reviewer #2 (Remarks to the Author): 
 
Summary: 
 
This study by K. Nanjo investigates earthquake triggering and characteristics of seismicity before, 
during, and after the 2019 Ridgecrest earthquakes. Nanjo focuses in particular on determining 
maps of “b-values” for different time periods, and shows how the nucleation area for the Mw 6.4 
foreshock and Mw 7.1 mainshock both had low b-values before these events occurred, and mid-to-
high b-values afterward. Nanjo shows that the b-value map correlates well with the slip 
distribution of the mainshock. I think the main conclusions are that the b-values provide insights 
into the state of stress in the fault zone, which is likely closely related to the nucleation and 
evolution of earthquakes in the sequence. I think this is an interesting paper and has a generally 
compelling analysis. I think the topic is appropriate for the journal. 
 
Questions/comments: 
 
1) The author refers to the supplementary material frequently, and since there are only 3 main 
text figures, I feel like some of these should be moved from the supplement to the main text. My 
understanding is that the format of this journal can accommodate more figures. 
 
2) Lines 134-139 discuss a supplementary figure, which relies on a metric that is not commonly 
used in earthquake science (if ever). The figure and text do not make sense without digging into 
the methods to figure out what you are talking about. 
 
3) The quantity phi defined by the author in the Spatial Organization section should be reported 
with uncertainties. More discussion is needed about this, why it was chosen. 
 
4) The points about the Garlock fault and aseismic behavior are probably not relevant here. For 
one, none of the seismicity you are looking at is on the Garlock fault where it was creeping. Also, 
this creep is only at very shallow depths, compared to any seismicity in the area, which is much 
deeper. Finally the argument about low b-values reflecting aseismic potential of the fault I think is 
incorrect, as this ignores the frictional stability (a-b) of the fault itself. You could have a rate 
strengthening fault with high stress and the fault would still creep. 
 
5) The slip model for the mainshock is purely teleseismic and therefore does not include any of the 
high quality near-field data. This is important since you are calculating Coulomb stresses in the 
near-field. Also, rather than using a slip model for the Mw 6.4, the author instead seems to take a 
moment tensor from the SCSN catalog, and somehow apply this to a discretized grid. This makes a 
lot of assumptions and is problematic. The analysis should be repeated using one of the slip 
models already published for both events. 
 
6) I’m not sure why the Coulomb analysis was only done for the NW part of the rupture. There is 
definitely slip on the SE part. 
 
7) The mainshock location used by Nanjo is not the best hypocenter provided by the SCSN. This 
location is just using a 1D velocity model. I would recommend using the location provided here by 
Hauksson: https://scedc.caltech.edu/research-tools/alt-2011-dd-hauksson-yang-shearer.html . It 
looks generally like the mainshock depth is very shallow, around 2 km. This seems to make your 
story more consistent, actually. 
 
8) Los Angeles is spelled incorrectly in the figure 1 caption 
 
9) Subfigure letters e.g. (A) should be in bold on figures to ease visibility and match the captions.