Looking for Seismic Signatures in the Landscape: A Landslide-Based Record of Holocene Fault Ruptures in the Puget Lowland

Ozioko, Obinna; Booth, Adam; Duvall, Alison; Herzig, Erich

doi:10.5194/egusphere-2025-6555

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https://doi.org/10.5194/egusphere-2025-6555

Preprints

30 Mar 2026

| 30 Mar 2026

Looking for Seismic Signatures in the Landscape: A Landslide-Based Record of Holocene Fault Ruptures in the Puget Lowland

Obinna Ozioko, Adam Booth, Alison Duvall, and Erich Herzig

Abstract. Earthquake-triggered landslides pose a major hazard in tectonically active regions and may leave lasting imprints in the landscape that reflect past seismic activity. In the Puget Lowland of Washington State, an urban corridor transected by multiple Holocene-active crustal faults, we investigate whether deep-seated landslides retain a record of prehistoric earthquakes. Using a regional inventory of more than 2,000 deep-seated landslides mapped from high-resolution lidar, we reconstructed a 4,000-year landslide chronology by relating deposit surface roughness to age, calibrated with16 radiocarbon-dated landslides. Temporal clustering of landslides was assessed by identifying peaks in landslide frequency that exceeded a steady-state landslide production model. Those peaks were then compared to earthquake-based scenarios incorporating known ruptures on the Tacoma Fault Zone (TFZ), Seattle Fault Zone (SFZ), Southern Whidbey Island Fault Zone (SWIFZ), and Darrington-Devil’s Mountain Fault Zone (DDMFZ). Our reconstructed landslide history reveals clustering of landslides 1000, 1250, 1900, and 2800–3200 years before present (ybp), coinciding with the timing of major Holocene earthquakes around 1000, 2000, and 3000 ybp. A multi-fault earthquake model reproduces these elevated periods more closely than a steady-state scenario. Frequency Ratio (FR) analyses show persistent fault-proximal landslide clustering, particularly along the SFZ and TFZ, where FR decreases with distance from the fault during time intervals containing well-constrained surface rupturing earthquakes. Interpretation of landslide clustering on the DDMFZ is complicated by geomorphic predisposition; on the other hand, weaker signals near SWIFZ likely reflect preservation bias within the landslide record. Nonetheless, temporal and spatial patterns show that Holocene crustal earthquakes generally leave detectable signals in the landslide record of the Puget Lowland. These results show the usefulness of roughness-calibrated landslide chronology as an independent paleoseismic indicator. Overall, this study underscores the value of integrating paleolandslides in reconstructing past earthquake activity and refining hazard assessment in landslide-prone, seismically active landscapes.

Received: 30 Dec 2025 – Discussion started: 30 Mar 2026

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Obinna Ozioko, Adam Booth, Alison Duvall, and Erich Herzig

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-6555', Anonymous Referee #1, 22 Apr 2026

This manuscript presents a comprehensive and carefully executed analysis of whether Holocene crustal earthquakes leave a detectable imprint in the deep‑seated landslide record of the Puget Lowland. The authors compile an impressive regional landslide inventory, introduce new radiocarbon constraints, and combine roughness‑based age inference with preservation‑corrected frequency modeling and spatial frequency‑ratio analysis. The study is ambitious in scope, methodologically rigorous, and well grounded in existing work on landslide chronologies and paleoseismology. Overall, it represents a valuable contribution to ESurf.

I have very few substantive concerns about the scientific approach or the interpretations. For the most part, the authors are appropriately cautious in discussing uncertainty, preservation bias, and the challenges of attributing landslide clusters to specific fault ruptures, particularly in a setting where multiple faults have ruptured within overlapping time windows. The combination of temporal clustering and spatial proximity analysis is especially well conceived, and the results for the Seattle Fault Zone are convincing and consistent with independent paleoseismic constraints.

Primary comments

1. Interpretation of model performance: The comparison between the steady‑state, multi‑fault, and single‑fault landslide history models is thoughtful and clearly presented. However, the differences in RMSLE between models are very small, and in several cases the steady‑state or single‑fault simulations perform comparably to the multi‑fault model. While this is acknowledged in the text, I recommend emphasizing more explicitly that the preference for the multi‑fault scenario is guided primarily by tectonic plausibility and consistency with the regional earthquake chronology, rather than by statistically decisive differences in model fit. A brief clarifying statement would help avoid over‑interpretation of small performance differences.

2. Interpretation of roughness‑derived ages: The roughness–age calibration is carefully constructed and transparently documented, including the rationale for excluding problematic samples. While the limitations of roughness‑based dating are discussed later in the manuscript, it may help the reader to emphasize earlier, for example in the Results or opening of the Discussion, that the resulting landslide chronology is best interpreted as a regional signal capturing broad temporal clustering, rather than precise event‑scale timing. This would align well with the authors’ otherwise appropriately cautious tone.

3. Temporal binning sensitivity

The supplementary material demonstrates that the main clustering patterns persist across a range of bin widths. Given the importance of binning choices for peak identification, I suggest explicitly stating in the main text that the principal results are robust to reasonable changes in temporal binning, for instance 50 to 200 years, and that the interpretations do not rely on a single bin configuration.

Recommendation

This is a strong and timely study that demonstrates the potential of roughness‑calibrated landslide chronologies as independent indicators of Holocene seismicity in sediment‑filled forearc basins. With minor revisions to streamline presentation and further clarify interpretive emphasis, I believe the manuscript will be well suited for publication in Earth Surface Dynamics.

Citation: https://doi.org/10.5194/egusphere-2025-6555-RC1
- AC1: 'Reply on RC1', Obinna Ozioko, 16 Jun 2026
  
  Response to Reviewers.
  We would like to sincerely thank Reviewer 1 for their time, careful evaluation of our manuscript, and their constructive comments and suggestions. We greatly appreciate the detailed feedback, which has helped improve the clarity, rigor, and overall quality of the manuscript.
  Point-by-point response to each comment is provided below. Reviewer comments are reproduced in black text, and our responses are provided immediately following each comment.
  Reviewer 1
  Interpretation of model performance: The comparison between the steady‑state, multi‑fault, and single‑fault landslide history models is thoughtful and clearly presented. However, the differences in RMSLE between models are very small, and in several cases the steady‑state or single‑fault simulations perform comparably to the multi‑fault model. While this is acknowledged in the text, I recommend emphasizing more explicitly that the preference for the multi‑fault scenario is guided primarily by tectonic plausibility and consistency with the regional earthquake chronology, rather than by statistically decisive differences in model fit. A brief clarifying statement would help avoid over‑interpretation of small performance differences.
  Response:
  Thank you for pointing this out. We agree that the differences in RMSLE among the steady-state, single-fault, and multi-fault scenarios are relatively small. To avoid over-interpreting these differences, we revised the text to emphasize that support for the multi-fault scenario is based primarily on its consistency with the observed temporal clustering and regional earthquake chronology, rather than on a statistically decisive improvement in model fit.
  Changes made in manuscript: Lines 614–634
  Interpretation of roughness‑derived ages: The roughness–age calibration is carefully constructed and transparently documented, including the rationale for excluding problematic samples. While the limitations of roughness‑based dating are discussed later in the manuscript, it may help the reader to emphasize earlier, for example in the Results or opening of the Discussion, that the resulting landslide chronology is best interpreted as a regional signal capturing broad temporal clustering, rather than precise event‑scale timing. This would align well with the authors’ otherwise appropriately cautious tone.
  Response:
  Thank you for this useful suggestion. We agree. The manuscript now explicitly states that the roughness-derived chronology is best interpreted as a regional record of broad landslide clustering rather than a precise reconstruction of individual landslide or earthquake events.
  Changes made in manuscript: Lines 558–561
  Temporal binning sensitivity: The supplementary material demonstrates that the main clustering patterns persist across a range of bin widths. Given the importance of binning choices for peak identification, I suggest explicitly stating in the main text that the principal results are robust to reasonable changes in temporal binning, for instance 50 to 200 years, and that the interpretations do not rely on a single bin configuration.
  Response:
  This is a useful suggestion. We added a sensitivity analysis statement showing that temporal bin widths ranging from 50 to 200 years produce similar clustering patterns and peak locations, indicating that the principal interpretations are not dependent on a single binning scheme.
  Changes made in manuscript: Lines 580–582
  
  Citation: https://doi.org/10.5194/egusphere-2025-6555-AC1
RC2:
'Comment on egusphere-2025-6555', Anonymous Referee #2, 15 May 2026
This is a review of “Looking for Seismic Signatures in the Landscape: A Landslide-based Record of Holocene Fault Ruptures in the Puget Lowland” by Ozioko and coauthors. This study modifies an available landslide inventory for the Puget Lowlands in Washington state and compares the temporal and spatial distribution of landslides with Holocene fault activity to assess if landslides could be coseismically triggered when compared to a steady-state landslide model. Landslide age was estimated using a surface-roughness model calibrated with limited radiocarbon ages. Fault activity was compiled from existing studies. The results indicate a broad temporal correlation with paleoearthquake timing and landslide-estimated ages, but some complications with spatial-temporal correlations of landslides for some of the fault zones. The conclusions will be of interest to a broad audience interested in cascading hazards, including landslide and earthquake hazards, in the Pacific Northwest and beyond. Overall, I found the study thoughtful with interesting implications, but the manuscript requires clarification for the reader to better understand the background, methods, and key interpretations. Below I outline several broader comments, followed by minor comments by line number and on the figures. I look forward to eventually seeing this work published.

Major comments:
Consider adding more descriptions of the fault zones, including the fault lengths, orientations, fault kinematics, and clearly stating any information on how the most recent event or paleoearthquake history is known, as this is important background information for the reader to evaluate the robustness of the temporal correlations and possible ground motions. I note below that several key references per fault, in addition to earthquake timing, are not fully mentioned or considered. In addition, which fault trace is actually used in the spatial analysis, as many of the faults are multi-stranded or represent secondary faults above deeper folds? And what is the source of the fault linework?

Throughout the manuscript, the authors use of Cal BP, cal BP, Cal YBP, and ybp to describes ages. I suggest choosing one and be consistent.

The methods section may benefit from some example figures of landslides that are smooth and older vs. rougher and younger, to demonstrate this metric to the reader. Consider adding a figure with a few of the dated landslides and a figure of the landslides with shaded relief or roughness to illustrate this.

The landslide database contains different types of mapped landslides and mass movements (L237-238). Are all of these landslide or mass movement types expected to result from ground-shaking? Consider adding a few sentences to discuss this limitation in the dataset if needed, and adding citations to support that these variable types of mass movement can result from coseismic shaking.

The manuscript would really benefit from a broader discussion or comparison to other regions. The manuscript mentions some of these techniques have been used elsewhere – with the same or different success? Have these other studies been able to tie landslides to upper crustal earthquake histories? Expanding beyond a detailed discussion of these 4 fault zones in the Cascadia region may help broaden the interest of the manuscript to an international audience.

The writing in the manuscript can be tidied up a bit, as it is repetitive and at times contradictory. Please check for repetitiveness between the background, results, and discussion. In addition, some paragraphs address multiple topics and the main message isn’t quite clear – I suggest editing carefully for clarity and focusing on flow and organizing relevant details in the same parts of the text. I give a few examples below but this list is not exhaustive.

I struggled a little to follow the argument that the landslide age inventory agrees with the paleoearthquake histories but doesn’t always work out spatially – is a implication here that perhaps other (unknown or undated) faults may have similar paleoearthquake timing within uncertainty (as seen with the Seattle-Tacoma faults which ruptured at similar times) to explain this spatial disconnect? Or that other triggers could be responsible for the landslides? Along these lines, I think the manuscript would really benefit from a short discussion on the late Holocene climate as a possible triggering mechanism if not all landslides spatiotemporally align with faults that had earthquakes during a known time.

Minor comments:
L37-39: Consider adding the magnitude of these earthquakes.
L42: Define “shallow depths” here, as typical seismogenic crustal thicknesses are between 10-30 km or so.
L44: Define Mw at its first appearance in the manuscript.
L54: This example would benefit from more description to help the reader understand some of the differences between subduction zone and crustal fault earthquakes, such as the hypocentral depths of these earthquakes, whether or not they were onshore or offshore, etc. Also, some subduction events do create landslides of note – see Yin et al., 2026 and Grant and Collins 2025 for landslides during the 2025 M8.8 Kamchatka earthquake, but less than predicted by landslide models. Consider explaining some of these nuances in more detail.
L65-79: This paragraph would benefit from a better topic sentence or breaking it into 2 paragraphs.
L80-81: And perhaps more importantly, high-resolution satellite imagery, right?
L93: Consider editing to “timeline of prehistoric landslides”?
L93-98: What about cosmogenic exposure age dating, which has been successfully used to date some rockfalls and landslides? Consider adding a few sentences about this technique and how it fits into these issues with dating landslides.
L122: Consider changing “unobserved” to “prehistoric” or “Holocene”.
L140-141: This sentence is important information but feels out of place in a paragraph describing the geology. Perhaps this should go with the geochronology background/discussion?
L160: I suggest starting a new paragraph here as the focus shifts from the surficial geology and soil conditions to fault zones.
L167: What kind of fault is the Seattle Fault Zone? Did the entire fault zone rupture in this most recent event, or only one strand based on the available data (this may have implications for where ground-shaking occurred)? Also, this statement needs a citation (I think Bucknam et al., 1992 would be appropriate?)
L170-172: It may also be important to state that studies looking at the last 11 ka only determined 1 regional uplift event at 900-930 A.D. on the Seattle Fault (Sherrod et al., 2000; Davis et al., 2026), as this ties into the spatiotemporal analysis discussed later.
L171: Label Bainbridge Island to Figure 1.
L173: Consider also citing Kelsey et al., 2008, which also supports the uplift event on the Seattle fault zone ~1.1 ka.
L184: What type of fault is the Tacoma Fault Zone? How was the most recent event on this fault dated?
L194: What type of fault is the Southern Whidbey Island Fault zone? Note that Kelsey et al. (2004) only provides evidence of the 2800-3200 earthquakes and limited evidence of 1 or 2 earthquakes on a different strand – please add a citation for the statement that the fault has hosted 4 earthquakes in the Holocene. It looks like this may come from Sherrod et al. 2008, who note 4 earthquakes since ~16.4 ka, and that the most recent earthquake may be younger than 2.7 ka. Please consider these details in your analysis.
L198: Please add more details on the Darrington-Devils Mountain Fault zone, including type of fault, length, orientation, etc.
L216-218: Consider expanding this discussion, as it seems relevant. How does this study differ from the two mentioned here?
L252: Consider adding a citation to support this statement.
L337-344: It is great to discuss this limitation of the alternative triggering mechanisms, but it’s unclear how this was discussed or considered in the interpretation? Are there any paleoclimate datasets that may help determine periods of higher or more intense precipitation events in the past? Consider expanding this discussion.
L445-456: this paragraph switches from using ybp to cal BP? I suggest using one and being consistent.
L535: What is “rmsle”? Please define.
L545-547: This statement is hard to evaluate given the overlapping lines and confidence intervals in Figure 4. I suggest making two separate parts for this figure, one to illustrate the steady-state model vs. landslide inventory and the other for the earthquake model and landslide inventory.
L553-554: The timing may agree, but do the landslides with the appropriate surface roughness correspond spatially to these fault zones?
L568-574: I don’t quite follow this argument on the individual fault zones sometimes agreeing with the landslide inventories?
L688-690: This statement is a little misleading because the spatial-temporal model doesn’t align, so the landslides near these faults don’t have the “right timing” to be associated with the paleoearthquakes on these faults. Consider clarifying.
L737-739: I suggest adding more details for these USGS ShakeMap scenarios, such as the magnitude of the earthquake being modeled.
L768-770: I would urge caution at interpreting these landslides that occur hundreds of years after the earthquake as a post-seismic response, especially drawing to the comparison of post-seismic landslides on an annual to decadal scale. Has this been observed elsewhere? It seems that at these timescales following an earthquake, other triggering mechanisms could be plausible as well.
L773: If one is invoking high-intensity precipitation events, why is that not the trigger? Why must the trigger be an earthquake hundreds of years earlier?
L873-875: Consider adding some citations to support this statement.
Figure 1: Cite the source for the fault traces. Consider also showing their fault kinematics. Add a citation for the lidar dataset. Check the basemap carefully, we there are duplicate city labels in the background. Also consider adding a white box to outline the legend I the lower left, it’s hard to read some of the text because it’s overlapping the DEM and other words. Add a citation for the Hamma Hamma landslide, if possible. Consider also adding ages for the new landslides, this could be done by color coding the symbol for a specific age range. If possible, add an inset that shows this site in the larger context of the entire US west coast or North America, to provide a better reference for an international audience.
Figure 2: Add scale to field photographs (how long is the shovel?) Also consider adding latitude and longitude coordinates to the map.
Figure 3: this exponential relationship seems largely dependent on the 2014 Oso landslide data. Consider performing a sensitivity test to assess how much this one datapoint affects the modeled line.
Figure 4: these lines all overlap and it’s hard to tell when something goes above the steady-state model in the background. Consider making this figure 2 panels.
Table 1 was hard to read in the current PDF format for review, but I assume it will be reformatted in any final publication.
Citation: https://doi.org/10.5194/egusphere-2025-6555-RC2
- AC2:
  'Reply on RC2', Obinna Ozioko, 16 Jun 2026
  Response to Reviewer 2.
  We would like to sincerely thank Reviewer 2 for their time, careful evaluation of our manuscript, and their constructive comments and suggestions. We greatly appreciate the detailed feedback, which has helped improve the clarity, rigor, and overall quality of the manuscript.
  In response to the reviewers’ comments, we have substantially revised the manuscript. Major revisions include expanding the descriptions of the Seattle, Tacoma, Southern Whidbey Island, and Darrington–Devils Mountain fault zones to provide additional information on fault geometry, kinematics, lengths, and paleoseismic histories. We also clarified the fault datasets and methods used in the spatial analyses. Throughout the manuscript, terminology and age notation were standardized, additional background was provided on earthquake-triggered landslides and landslide dating methods, and several sections were reorganized to improve clarity, readability, and logical flow.
  We expanded discussions of uncertainty and alternative interpretations, including the limitations of roughness-derived ages, the influence of climatic forcing and precipitation-driven landsliding, and the distinction between temporal and spatial evidence for coseismic landsliding. Additional figures, figure revisions, and supplementary analyses were incorporated to improve transparency and support interpretation of the results. These include new examples illustrating landslide roughness and age relationships, revised figure captions and map documentation, and a sensitivity analysis evaluating the influence of the 2014 Oso landslide on the age–roughness calibration.
  We carefully considered every comment and suggestion provided by the reviewers. In several cases, we incorporated the recommended revisions directly into the manuscript; in others, we clarified our rationale where we elected to retain the original approach. We believe these revisions have significantly strengthened the manuscript and improved its accessibility to a broader audience.
  A detailed, point-by-point response to each comment is provided below. Reviewer comments are reproduced in black text, and our responses are provided immediately following each comment.
  
  Reviewer 2:
  Major Comment 1: Consider adding more descriptions of the fault zones, including the fault lengths, orientations, fault kinematics, and clearly stating any information on how the most recent event or paleoearthquake history is known, as this is important background information for the reader to evaluate the robustness of the temporal correlations and possible ground motions. I note below that several key references per fault, in addition to earthquake timing, are not fully mentioned or considered. In addition, which fault trace is actually used in the spatial analysis, as many of the faults are multi-stranded or represent secondary faults above deeper folds? And what is the source of the fault linework?
  Response:
  We thank the reviewer for this detailed and helpful comment. We agree that additional information on the fault zones and the spatial analysis methodology improves the context and transparency of the study. In response, we expanded the geologic setting section to provide additional details on the geometry, kinematics, fault lengths, and paleoearthquake histories of the Seattle, Tacoma, Southern Whidbey Island, and Darrington–Devils Mountain fault zones. We also clarified the source of the fault linework used in the analyses and specified how fault traces were represented in the distance-to-fault calculations.
  Revision: Lines 173 – 190, 201 -215, 216-233, 234 – 244.
  Major Comment 2: Throughout the manuscript, the authors use of Cal BP, cal BP, Cal YBP, and ybp to describes ages. I suggest choosing one and be consistent.
  Response:
  We agree that the age notation was inconsistent throughout the manuscript. In response, we standardized the terminology and now use cal yr BP consistently throughout the text when referring to calibrated radiocarbon ages before present (AD 1950). We also added a definition of this terminology in the geochronology section to improve clarity and consistency.
  Revision: Manuscript-wide revision; terminology standardized throughout the text, with definition added in the geochronology section.
  Major Comment 3: The methods section may benefit from some example figures of landslides that are smooth and older vs. rougher and younger, to demonstrate this metric to the reader. Consider adding a figure with a few of the dated landslides and a figure of the landslides with shaded relief or roughness to illustrate this.
  Response:
  Thank you for the observation and suggestion. We have added a new figure showing representative examples of an older, smoother landslide and a younger, rougher landslide, together with their associated surface roughness characteristics. This figure provides a visual demonstration of the geomorphic degradation captured by the roughness metric and the rationale underlying the age–roughness calibration.
  Revision: Surface Roughness–Age Model, Figure 3, Lines 349 - 356
  Major Comment 4: The landslide database contains different types of mapped landslides and mass movements (L237-238). Are all of these landslide or mass movement types expected to result from ground-shaking? Consider adding a few sentences to discuss this limitation in the dataset if needed, and adding citations to support that these variable types of mass movement can result from coseismic shaking.
  Response:
  Thank you for this suggestion. We have added additional citations to show that each of these failure types has been documented in association with strong earthquake ground shaking. We also note that hydrologic conditions may precondition slopes for failure and contribute to later reactivation, making it difficult to attribute individual landslides to a single triggering mechanism.
  Revision: Methodology – Landslide Mapping, Lines 288-296
  Major Comment 5: The manuscript would really benefit from a broader discussion or comparison to other regions. The manuscript mentions some of these techniques have been used elsewhere – with the same or different success? Have these other studies been able to tie landslides to upper crustal earthquake histories? Expanding beyond a detailed discussion of these 4 fault zones in the Cascadia region may help broaden the interest of the manuscript to an international audience.
  Response:
  The authors thank the reviewer for this thoughtful suggestion. The Discussion was expanded to place the results in a broader context and highlight how landslide chronologies have been used in other settings to investigate past earthquake activity. We added comparisons with recent studies from the Seattle Fault Zone and Boulder Creek Fault and expanded the discussion to emphasize the potential application of regional landslide chronologies in tectonically active regions where paleoseismic records are sparse. This broader perspective helps place the findings within the growing use of landslide records as complementary paleoseismic indicators.
  Revision: Discussion session, Lines 903 – 915.
  Major Comment 6: The writing in the manuscript can be tidied up a bit, as it is repetitive and at times contradictory. Please check for repetitiveness between the background, results, and discussion. In addition, some paragraphs address multiple topics and the main message isn’t quite clear – I suggest editing carefully for clarity and focusing on flow and organizing relevant details in the same parts of the text. I give a few examples below but this list is not exhaustive.
  Response:
  We thank the reviewer for this keen observation and suggestion. The manuscript has been carefully revised for clarity, organization, and readability. Repetitive text was reduced, several paragraphs were reorganized to improve logical flow, and sections that overlapped between the Introduction, Results, and Discussion were streamlined. We also revised a number of multi-topic paragraphs to better separate methods, results, and interpretation, improving the overall clarity of the manuscript.
  Revision: Manuscript-wide revision:
  Methods: Radiocarbon sampling and dating – lines 311 to 321, Surface Roughness Age-model – Lines 368 to 372, landslide History model – lines 401 to 405.
  
  Results: 3.2 Age-Roughness Model and Inferred Age – lines 553 to 559.
  Major Comment 7: I struggled a little to follow the argument that the landslide age inventory agrees with the paleoearthquake histories but doesn’t always work out spatially – is a implication here that perhaps other (unknown or undated) faults may have similar paleoearthquake timing within uncertainty (as seen with the Seattle-Tacoma faults which ruptured at similar times) to explain this spatial disconnect? Or that other triggers could be responsible for the landslides? Along these lines, I think the manuscript would really benefit from a short discussion on the late Holocene climate as a possible triggering mechanism if not all landslides spatiotemporally align with faults that had earthquakes during a known time.
  Response:
  We thank the reviewer for this insightful observation. The Discussion was expanded to consider climatic forcing as an alternative explanation for some landslide clusters. We added discussion noting that prolonged wet periods and major storm events in the Pacific Northwest can produce widespread landsliding and potentially generate temporal clusters that resemble earthquake-triggered signals. This addition further emphasizes the importance of combining temporal patterns with spatial analyses and independent paleoseismic constraints when interpreting potential coseismic landslide records.
  Revision: Discussion – lines 806 to 816.
  
  Minor Comments
  L37-39: Consider adding the magnitude of these earthquakes.
  
  Response 1:
  We thank the reviewer for this suggestion. The estimated magnitudes of the example earthquakes were added to provide additional context on the scale of shaking and its potential to generate widespread coseismic landsliding.
  Revision: Introduction – lines 36 to 37.
  L42: Define “shallow depths” here, as typical seismogenic crustal thicknesses are between 10-30 km or so.
  
  Response 2.
  We thank the reviewer for this helpful suggestion. We clarified the meaning of shallow crustal earthquakes by specifying the typical depth range associated with seismogenic crust in the introduction. Line 42.
  L44: Define Mw at its first appearance in the manuscript.
  
  Response 3
  We thank the reviewer for noting this oversight. Mw (moment magnitude) is now defined at its first occurrence in the manuscript. Line 37.
  L54: This example would benefit from more description to help the reader understand some of the differences between subduction zone and crustal fault earthquakes, such as the hypocentral depths of these earthquakes, whether or not they were onshore or offshore, etc. Also, some subduction events do create landslides of note – see Yin et al., 2026 and Grant and Collins 2025 for landslides during the 2025 M8.8 Kamchatka earthquake, but less than predicted by landslide models. Consider explaining some of these nuances in more detail.
  
  Response 4
  We appreciate this suggestion. The Introduction was expanded to provide additional context on the differences between crustal and subduction-zone earthquakes, including their typical rupture depths, source locations, and expected landslide responses.
  Revision: Introduction – lines 52 to 58.
  L65-79: This paragraph would benefit from a better topic sentence or breaking it into 2 paragraphs.
  
  Response 5:
  We thank the reviewer for this suggestion. The paragraph was revised and divided into two paragraphs to improve organization, readability, and logical flow. Revision: Lines 69 to 83
  L80-81: And perhaps more importantly, high-resolution satellite imagery, right?
  
  Response 6
  We thank the reviewer for this suggestion. Statement was updated to include high resolution satellite imagery. Line 87.
  L93: Consider editing to “timeline of prehistoric landslides”?
  
  Response 7
  We appreciate the reviewer’s suggestion. The text was revised to use “prehistoric landslides” for improved clarity and consistency with the study objectives.. Line 99-100
  L93-98: What about cosmogenic exposure age dating, which has been successfully used to date some rockfalls and landslides? Consider adding a few sentences about this technique and how it fits into these issues with dating landslides.
  
  Response 8
  We thank the reviewer for this helpful suggestion. The paragraph was revised to include cosmogenic exposure dating as an additional technique for constraining landslide ages. We briefly discuss its application to rockfalls and landslides, while noting the practical limitations associated with applying the method to large regional inventories.
  Revision: Lines 103 to 107.
  L122: Consider changing “unobserved” to “prehistoric” or “Holocene”.
  
  Response 9
  We thank the reviewer for this suggestion. We have replaced “unobserved with prehistoric as suggested. Line 132.
  L140-141: This sentence is important information but feels out of place in a paragraph describing the geology. Perhaps this should go with the geochronology background/discussion?
  
  Response 10
  We thank the reviewer for this observation and suggestion. We agree that this statement was better suited to the geochronology discussion than the geologic setting description. The sentence was relocated to the Radiocarbon Sampling and Dating section, where it now provides additional context regarding the suitability of radiocarbon dating in the Puget Lowland. Lines 328 - 330.
  L160: I suggest starting a new paragraph here as the focus shifts from the surficial geology and soil conditions to fault zones.
  
  Response 11
  We appreciate the reviewer’s suggestion. A new paragraph was started at this location to improve the organization of the section and provide a clearer transition from surficial geology and soil conditions to the discussion of active fault zones. Line 169
  L167: What kind of fault is the Seattle Fault Zone? Did the entire fault zone rupture in this most recent event, or only one strand based on the available data (this may have implications for where ground-shaking occurred)? Also, this statement needs a citation (I think Bucknam et al., 1992 would be appropriate?)
  
  Response 12
  Thank you for highlighting this point. Additional information was added describing the geometry, kinematics, and paleoseismic history of the Seattle Fault Zone. The revised text clarifies that the Seattle Fault Zone is a south-dipping reverse fault system, summarizes evidence for its most recent earthquake, and includes additional references supporting the timing and characteristics of the rupture.
  Revision: Lines 176 to 190
  L170-172: It may also be important to state that studies looking at the last 11 ka only determined 1 regional uplift event at 900-930 A.D. on the Seattle Fault (Sherrod et al., 2000; Davis et al., 2026), as this ties into the spatiotemporal analysis discussed later.
  
  Response 13
  This is a helpful observation. The Seattle Fault Zone description was expanded to include additional paleoseismic context. Lines 176 to 190
  L171: Label Bainbridge Island to Figure 1.
  
  Response 14
  
  Thank you for pointing this out. Bainbridge Island has been added to Figure 1 to improve geographic reference and figure readability.
  L173: Consider also citing Kelsey et al., 2008, which also supports the uplift event on the Seattle fault zone ~1.1 ka.
  
  Response 15
  Response 15:
  We appreciate the recommendation. Kelsey et al. (2008) has been added to the discussion of the Seattle Fault Zone and is now cited alongside other studies supporting the ~1.1 ka uplift event.
  Revision: Study Area – Line 184 - 185
  
  L184: What type of fault is the Tacoma Fault Zone? How was the most recent event on this fault dated?
  
  Response 16
  We appreciate the reviewer’s comment. Additional information was added describing the Tacoma Fault Zone as a north-dipping reverse fault system and summarizing the evidence used to constrain the timing of its most recent earthquake.
  Revision: Lines 201 to 205.
  L194: What type of fault is the Southern Whidbey Island Fault zone? Note that Kelsey et al. (2004) only provides evidence of the 2800-3200 earthquakes and limited evidence of 1 or 2 earthquakes on a different strand – please add a citation for the statement that the fault has hosted 4 earthquakes in the Holocene. It looks like this may come from Sherrod et al. 2008, who note 4 earthquakes since ~16.4 ka, and that the most recent earthquake may be younger than 2.7 ka. Please consider these details in your analysis.
  
  Response 17
  Thank you for this careful review and suggestions. The Southern Whidbey Island Fault Zone description has been revised to provide additional detail on fault kinematics and paleoseismic history. We clarified the evidence for the ~2800–3200 yr BP earthquake(s), revised the discussion of Holocene earthquake recurrence to better reflect the available paleoseismic constraints, and added the appropriate references supporting the interpretation. The revised text also acknowledges the uncertainty associated with the timing and number of past ruptures on different strands of the fault system.
  Revision: Lines 216 to 233
  L198: Please add more details on the Darrington-Devils Mountain Fault zone, including type of fault, length, orientation, etc.
  
  Response 18
  Thank you for the helpful suggestions. We have updated the description of the Darrington–Devils Mountain Fault Zone to include additional information on its geometry, orientation, kinematics, and regional extent.
  Revision: Lines 234 to 236, 242to 244.
  L216-218: Consider expanding this discussion, as it seems relevant. How does this study differ from the two mentioned here?
  
  Response 19
  This is a valuable point. The discussion was expanded to more clearly distinguish this study from previous work. We now explicitly note that earlier studies focused on landslide responses associated with individual fault systems, whereas this study evaluates landslide chronologies across multiple active crustal fault zones within the Puget Lowland.
  Revision: Lines 258 to 264.
  L252: Consider adding a citation to support this statement.
  
  Response 20
  
  We appreciate the reviewer’s observation. The line under radiocarbon sampling was extensively revised (lines 311 to 312). The statement was substantially change and no additional citation was added.
  L337-344: It is great to discuss this limitation of the alternative triggering mechanisms, but it’s unclear how this was discussed or considered in the interpretation? Are there any paleoclimate datasets that may help determine periods of higher or more intense precipitation events in the past? Consider expanding this discussion.
  
  Response 21.
  We appreciate the reviewer’s thoughtful suggestion. The discussion of climatic forcing was expanded to better explain how precipitation-driven landsliding may influence the reconstructed chronology. We note that major storm events and prolonged wet intervals in the Pacific Northwest can generate widespread landsliding and potentially produce temporal clusters that resemble earthquake-triggered signals. This limitation is discussed as an alternative explanation for some landslide peaks and reinforces the importance of integrating spatial analyses and independent paleoseismic constraints when interpreting potential coseismic signals.
  Revision: Discussion – Lines 806 to 814
  L445-456: this paragraph switches from using ybp to cal BP? I suggest using one and being consistent.
  
  Response 22:
  Thank you for identifying this inconsistency. Age notation was standardized throughout the manuscript to improve consistency and readability. All references to calibrated radiocarbon ages now use a single convention, cal yr BP.
  L535: What is “rmsle”? Please define.
  
  Response 23
  We thank the reviewer for catching this omission. The Root Mean Square Logarithmic Error (RSMLE) has been defined accordingly. Lines 593-594.
  L545-547: This statement is hard to evaluate given the overlapping lines and confidence intervals in Figure 4. I suggest making two separate parts for this figure, one to illustrate the steady-state model vs. landslide inventory and the other for the earthquake model and landslide inventory.
  
  Response 24
  We appreciate the reviewer’s suggestion. We considered separating the steady-state and earthquake-model results into two figures. However, we chose to retain the combined figure because the primary objective is to facilitate direct comparison between the observed landslide chronology and both model scenarios within a single visual framework. Presenting the models together allows readers to more readily evaluate similarities and differences in model performance and assess the extent to which each reproduces the observed temporal patterns.
  L553-554: The timing may agree, but do the landslides with the appropriate surface roughness correspond spatially to these fault zones?
  
  Response 25
  Thank you for raising this important point. This issue is addressed in the spatiotemporal clustering analysis presented later in the manuscript. While the temporal results identify periods of elevated landslide activity that coincide with known earthquake chronologies, the subsequent spatial analysis evaluates whether those landslides are preferentially concentrated near the corresponding fault zones
  L568-574: I don’t quite follow this argument on the individual fault zones sometimes agreeing with the landslide inventories?
  
  Response 26
  We appreciate the reviewer’s comment. The text was revised to clarify how the individual fault simulations compare with the reconstructed landslide chronology. We now explicitly distinguish between agreement with individual landslide clusters and agreement with the overall landslide history. The revised discussion explains that each single-fault model reproduces only the cluster associated with its respective earthquake timing, whereas the multi-fault model reproduces the broader pattern of elevated landslide activity observed across the inventory.
  Revision: Lines 620 – 632.
  L688-690: This statement is a little misleading because the spatial-temporal model doesn’t align, so the landslides near these faults don’t have the “right timing” to be associated with the paleoearthquakes on these faults. Consider clarifying
  
  Response 27
  Thank you for pointing out the ambiguity. We agree that the original wording could be interpreted as implying a direct spatiotemporal correspondence between fault-proximal landslides and the most recent earthquakes on the Darrington–Devils Mountain and Southern Whidbey Island fault zones. However, this paragraph was intended only to summarize the temporal clustering results. Our point was that the regional landslide chronology contains peaks near ~2000 and ~3000 ybp that broadly overlap the independently constrained timing of the most recent earthquakes on these fault systems. We were not asserting that landslides located proximal to these faults necessarily exhibit corresponding ages. To avoid this ambiguity, we revised the text to clarify that the statement refers to temporal overlap within the regional inventory, while spatial correspondence is evaluated separately in the subsequent sections.
  Revision: Line 743
  L737-739: I suggest adding more details for these USGS ShakeMap scenarios, such as the magnitude of the earthquake being modeled.
  
  Response 28
  
  We appreciate this suggestion. We revised the text to include the magnitudes of the modeled earthquakes associated with the USGS and Washington Geological Survey ShakeMap scenarios used in our interpretation
  Revision: line 797-798
  L768-770: I would urge caution at interpreting these landslides that occur hundreds of years after the earthquake as a post-seismic response, especially drawing to the comparison of post-seismic landslides on an annual to decadal scale. Has this been observed elsewhere? It seems that at these timescales following an earthquake, other triggering mechanisms could be plausible as well.
  
  Response 29
  Thank you for making this point. The discussion was revised to avoid attributing landslides occurring centuries after an earthquake to a direct post-seismic response. We now acknowledge that such timescales likely reflect the cumulative effects of multiple conditioning and triggering mechanisms. The revised discussion instead suggests that repeated earthquake shaking may contribute to long-term hillslope weakening, increasing susceptibility to failure when combined with subsequent climatic or geomorphic triggers.
  Revision: Discussion section, lines 836 – 846.
  L773: If one is invoking high-intensity precipitation events, why is that not the trigger? Why must the trigger be an earthquake hundreds of years earlier?
  
  Response 30
  
  We thank the reviewer for highlighting this distinction. The discussion was revised to reflect this as stated in response 29 above.
  L873-875: Consider adding some citations to support this statement.
  
  Response 31
  We appreciate the reviewer’s suggestion. This statement is intended as a synthesis of results and interpretations developed earlier in the manuscript rather than the introduction of a new concept. The supporting evidence is presented throughout the Results and Discussion sections, including the temporal clustering analyses, spatial frequency-ratio analyses, and comparison with ShakeMap scenarios. We therefore retained the statement in the conclusion as a summary of the study's findings rather than adding additional citations.
  
  32. Figure 1: Cite the source for the fault traces. Consider also showing their fault kinematics. Add a citation for the lidar dataset. Check the basemap carefully, we there are duplicate city labels in the background. Also consider adding a white box to outline the legend I the lower left, it’s hard to read some of the text because it’s overlapping the DEM and other words. Add a citation for the Hamma Hamma landslide, if possible. Consider also adding ages for the new landslides, this could be done by color coding the symbol for a specific age range. If possible, add an inset that shows this site in the larger context of the entire US west coast or North America, to provide a better reference for an international audience.
  Response 32
  Thank you for these detailed suggestions. Figure 1 was revised to improve clarity and documentation. Citations were added for the fault trace data, lidar dataset, and Hamma Hamma landslide. The fault strands used in the spatial analyses are shown as thick red lines for clarity. The figure layout and legend were also modified to improve readability. Inset map was broadened to provide regional context of the study area. We considered the suggested additions of age-coded landslide symbols; however, these were not incorporated because the primary purpose of Figure 1 is to provide geographic and tectonic context for the study area, while landslide age distributions are presented and discussed elsewhere in the manuscript.
  Revision: Figure 1.
  
  Figure 2: Add scale to field photographs (how long is the shovel?) Also consider adding latitude and longitude coordinates to the map.
  
  Response 33
  We appreciate the reviewer’s suggestions. Figure 2 was revised to provide additional context for the field photographs and site location. The caption now includes the length of the shovel used for scale, and geographic coordinates were added to the map to facilitate site identification and comparison with other studies.
  Revision: Figure 2.
  Figure 3: this exponential relationship seems largely dependent on the 2014 Oso landslide data. Consider performing a sensitivity test to assess how much this one datapoint affects the modeled line.
  
  Response 34.
  
  Thank you for this insightful suggestion. We performed a sensitivity analysis excluding the 2014 Oso landslide from the age–roughness calibration. The resulting exponential relationship was nearly identical to the preferred model, indicating that the calibration is not dependent on the Oso landslide despite it representing the youngest and roughest calibration point. The fitted model including Oso was Age = 183409.2e^(-276.126R), with R² = 0.8766. Excluding Oso yielded Age = 183400.1e^(-276.122R), with R² = 0.8735. Because the fitted parameters and goodness-of-fit changed negligibly, we retained Oso in the calibration dataset.
  Revision: Supplementary material (S5-S6)
  Figure 4: these lines all overlap and it’s hard to tell when something goes above the steady-state model in the background. Consider making this figure 2 panels.
  
  Response 35:
  We appreciate the reviewer’s suggestion. We considered separating the steady-state and earthquake-model results into two figures. However, we elected to retain the combined figure because its primary purpose is to facilitate direct comparison between the observed landslide chronology and both model scenarios within a single visual framework. Presenting the datasets together allows readers to more readily evaluate similarities and differences in model performance and assess the extent to which each reproduces the observed temporal patterns.
  Revision: No change made.
  
  Table 1 was hard to read in the current PDF format for review, but I assume it will be reformatted in any final publication.
  
  Response 36.
  
  Response 36.
  Thank you for this comment. We agree that the table formatting in the review PDF reduced readability. We anticipate that final journal production and typesetting will further improve the presentation and readability of Table 1.
  Revision: No change made.
  
  Citation: https://doi.org/10.5194/egusphere-2025-6555-AC2

Obinna Ozioko, Adam Booth, Alison Duvall, and Erich Herzig

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https://doi.org/10.5194/egusphere-2025-6555-supplement

Obinna Ozioko, Adam Booth, Alison Duvall, and Erich Herzig

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Short summary

Landslides triggered by recent earthquakes are well known, but how far shaking spread during prehistoric earthquakes is unclear. We studied thousands of landslides in western Washington to test whether they record past earthquakes. We inferred landslide ages from their roughness and compared them with known ancient earthquakes. Our results show that large prehistoric earthquakes caused widespread landsliding over the past 4000 years, improving understanding of earthquake hazards and future risk.


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