We sincerely thank you for the efforts you have made to review our manuscript. 

We have addressed all of the review comments in the following paragraphs.

[Comment]

L54 ‘…ratio to outside of the valley.’

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

L77-79 This sentence needs to be improved

[Reply]

We will modify as following sentence.

“Furthermore, considering the morphological conditions of the drainage basin forming debris flow fans as the conditions generating debris flows, the risk of debris flow occurrence can be evaluated even in basins where debris flow fans are disappeared owing to the effects of erosion by mainstream rivers and artificial land formation.”

[Comment]

117 & 119 (and other places) use the term ‘rock’ in these cases, not rocks

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

L124 between the two sites

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

Paragraph beginning on L145 state when the field surveys were conducted

[Reply]

We will add date of the field surveys as following sentence.

“Field surveys were conducted in 18 and eight basins in the (distribution) areas of Paleogene trench-fill turbidites (date of the field surveys :17-18 November 2021) and Neogene marine sedimentary rocks (date of the field surveys :15-16 November 2021), respectively, to validate the extraction of debris flow fans from topographic maps.”

[Comment]

L212 (and other places) it seems like here, and maybe in some other places, you are confusing alluvial fans with debris fans. Many people do this, but it is my sense that fan formation dominated by debris flows should be called debris fans (or debris flow fans) and those dominated by water flooding should be called alluvial fans. I know these terminologies have been extensively discussed and debated by geomorphologists and I do not wish to add to that debate which has largely been confusing, but it seems to me that when alluvial processes (floods) dominate fan development the fan should logically be called an alluvial fan and when debris flow processes dominate the fan should be called a debris fan. Of course, there is some continuum between these two dominating processes, but I think fans can be characterized by some of the defining characteristics – e.g., for debris fans, steep gradients, poorly sorted materials and for alluvial fans, gentler gradients and more well-sorted materials in the fan. I don’t think the authors need to go into any detailed explanation of this, but they just need to use consistent and appropriate terminology.

[Reply]

Thank you for your comment. In this study, we have focused on fans formed by debris flows.

Therefore, we decided not to use the term alluvial fan.

[Comment]

L221-223 Can you provide any insights into why these fans were wrongly identified?

[Reply]

These fans were wrongly identified because fluvial processes partially eroded the debris flow fans.

[Comment]

L251 You say ‘only a few gravels’, this would better read “only a little gravel”; but did you not find coarser materials (e.g., boulders) on the surface?

[Reply]

We will modify as pointed out by the reviewer.

I confirmed that there were cobble gravels on the surface of debris flow fans.

[Comment]

L258 gravel instead of granules?

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

L323 You could also cite some of Lee Benda’s early research here.

[Reply]

Thank you for your comment.

We will cite some of Lee Benda’s early research here.

[Comment]

L359 again, maybe you mean ‘debris fans’?

[Reply]

Saito (1998) was defined it as debris flow and alluvial fans.

Therefore, we will modify it like that.

[Comment]

L370 Here and in at least one other place, try to avoid using one-sentence paragraphs.

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

Conclusions: It might be worth mentioning in which cases it would be most desirable to conduct some field checking of debris fan conditions.

[Reply]

We will think of inserting the following sentence on line 405（before “As a result”）.

Debris flow fans and the boundaries of drainage basins at the study sites were extracted using topographic maps and DEMs. Field surveys were conducted to validate the extraction of debris flow fans from topographic maps and to check current activity of the debris flow in selected basins. After calculating the morphological parameters in each basin, a decision tree analysis was performed using the presence or absence of debris flow fans as the objective variable and the morphological variables of the drainage basin as the explanatory variables.

We are looking forward to receive review comments from the other reviewer.

Thank you again for your review.

We sincerely thank you for the efforts you have made to review our manuscript. 

We have addressed all of the review comments in the following paragraphs.

[Comment]

The title refers to rock strength, yet the authors did not directly measure the strength of the base rocks at their study sites; they provided only supporting evidence of difference in rock strength. I recommend modifying the title to reflect geological differences rather than rock strength differences.

[Reply]

Thank you for your advice.

After submitting this paper, we measured the rock strength in the research areas using a Schmidt rock hammer. We will add those values to the section of study site.

Therefore, we would like to keep the title as it is.

[Comment]

l. 60-61 “distinguishing between debris flows and debris floods” might be more precisely described as “distinguishing between debris flow deposits and debris flood deposits.”

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l.63 “discriminant function” might be more accurately described as “discriminant analysis,” since discriminant function is used in discriminant analysis.

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 82-83 “geological sedimentary processes” might be more accurately described as “(rock) formation processes.” 

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 84-85 “many debris flow fans have been formed” might be better stated as “many debris flow fans remain.”

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l.109-111 The authors used “ha” here but “m2” elsewhere. Consistency in units for the same quantity is recommended.

[Reply]

We will modify as pointed out by the reviewer.

We would like to use m² consistently.

[Comment]

l. 125-126 The authors did not describe the geological and geomorphological features related to landform development during the Quaternary period except for uplift rates. However, understanding the Quaternary landform development of the valley-fill plains, where debris fan distribution was investigated, is crucial. Are the valley-fill plains in this study composed of Holocene strata? Are there any (old) debris fans formed on Pleistocene terraces away from the present riverbeds? Introducing the Quaternary geology and geomorphology characteristics of your study sites is important for understanding the relationship between current sediment productivity and debris fan development, and consequently, the sediment transport process within the basin and the debris-flow hazard risk at the basin outlet.

[Reply]

Thank you for your important comment. 

We would like to modify it as follows.

Most of the debris flow fans have been formed on valley plains or Holocene terraces in the studied area. In other words, the target debris flow fans have been constructed during the Holocene with warm and wet climates similar to the current conditions.

[Comment]

l. 139 “extracted” could be replaced with “delineated.”

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 139 “using topographic maps and DEMs” might be more specific as “using published topographic maps and digital elevation models (DEMs).”

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 147 “such as grain size, timing of debris flows, and morphology of debris flow fans” might be more accurately described as “such as grain size distribution, debris flow history, and morphology of fan deposits.” 

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 216-217 This part is unclear. Pebble represents the 4-64 mm class, but a grain size of 24 cm falls into the Cobble class.

[Reply]

This is a mistake in the Cobble class. Furthermore, Line 257 was also classified as a pebble even though its particle size was 14 cm. This will also be revised to the Cobble class.

[Comment]

l. 218 “which continue to be a source of sediment” might be better as “which continue to produce (or generate) sediments.”

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 223 Specify “fluvial processes” as either those of the main river or those of the tributaries.

[Reply]

Part of the debris flow fans had disappeared due to erosion from the main river.

[Comment]

l. 260 “topographic map analysis” could be more accurately described as “topographic map interpretation.”

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 318-400 Consider discussing why certain morphological features did not contribute to the formation of debris fans in your study sites, despite their importance in previous studies. Adding discussion about features not selected as significant determinants in the decision tree analysis would be beneficial (though not a requirement for acceptance).

[Reply]

Thank you for your important comment. 

We would like to write the following sentence in place of the current sentence in Line 370.

Basin area and basin relief, which characterize the entire basin, were not selected as factors determining the decision tree in both of geologic units. These morphological variables are affected by areas that does not directly contribute occurrences of debris flows. Consequently, regardless of the geology, the morphological factors that control sediment supply activities and the ratio of sediments reaching the valley mouth as debris flow are more important for the formation of debris flow fans than morphological features of the entire basin.

[Comment]

l. 326 “still exists” might be better as “still remains.”

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 331-332 “Jakob (2021) defined such basin as watershed supply-limited basins.” Is the term “watershed” necessary here?

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 332-333 “once a debris flow occurs because of a landslide” could be replaced with “once a landslide-initiated debris flow occurs.”

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 333 “are formed” could be “were formed” or simply “exist.”

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 335 “missing” might be better as “absence of.”

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 347 “the knick line” should be replaced with “the convex slope break.”

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 375 Specify “valley side” as “valley side slopes.”

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 376 “bed sediments” might be more accurate as “bed materials.”

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 379 “classified” might be better as “determine.”

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 386-393 It’s unclear why the cited previous studies support the authors’ hypothesis. If the authors aim to argue this point, they need to explain which type of rock in which regions has lower strength and produces finer-grained sediments in the referenced studies, and vice versa.

[Reply] 

Thank you for your important comment.

We would like to modify it as follows.

Insert the following sentence after line 384 (Sed Group 4).

Field surveys showed that the debris (gravelly clasts) on debris flow fans derived from the weathered Neogene sedimentary rocks, which are relatively soft rocks with fractures densely developed at 1-15 cm intervals, were maximum particle size of 14 cm. Whereas, in the Paleogene accretionary prism site, which is underlain relatively hard rocks with sparsely developed joints at 5-24 cm intervals, the debris were maximum particle size of 24 cm. Thus, the debris (gravelly clasts) in the region of the Neogene sedimentary rocks can be easily transported downstream even with low relief ratios. Previous studies reported that lower limits of relief ratio for forming debris flow fans varies depending on geology (Wilford et al., 2004; De Scally et al., 2010; Ilinca, 2021). 

Additionally, we will replace the sentence “These results...” on line 391 with the following sentence.

“Our study implies that a difference in the threshold of relief ratio reflects difference of rock strength and a gran-size composition of produced debris among the studied sites.”

In addition, Wildford et al., 2004 was a mistake in Wilford et al., 2004. We will correct the relevant parts.

Finally, we would also like to add a table that summarizing the results for each region.

[Comment]

l. 391-393 Is the grain size at the former site (the Paleogene accretionary prism site) coarser than at the latter site (the Neogene sedimentary rock site)?

[Reply]

The grain size derived from the Paleogene accretionary prism is coarser than that from the Neogene sedimentary rocks. We will modify the sentence as follows.

These results support our hypothesis that the smaller threshold of relief ratio in the Neogene sedimentary rocks (soft rock) site than in the Paleogene accretionary prism (hard rock) site is due to the smaller grain size in the former. 

[Comment]

l. 396 The authors use expressions like “terrains (basins) that have not experienced significant dissection” in several places. A more concise term like “poorly-dissected basins” could be used. Conversely, “well-dissected basins” can describe the opposite.

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 412 “no formation” might be clearer as “absence.”

[Reply]

We will modify as pointed out by the reviewer.

[Comment]

l. 417 “classified” might be better as “determine.”

[Reply]

We will modify as pointed out by the reviewer.

Thank you again for your review.