| 08 May 2026
The impact of pre-existing weaknesses on strike-slip fault evolution: insight into strain partitioning of the 2019 Ridgecrest earthquake
Abstract. The 2019 Ridgecrest mainshock produced a complex rupture pattern at the northwest end of the dextral rupture where a set of disconnected pre-existing faults that trend perpendicular to the strike of the mainshock fault had sinistral slip. This highly segmented geometry of active faulting may reflect immature faulting that provides insights into the early development of strike-slip faults. Physical experiments that simulate upper crustal deformation using scaled analogue materials, such as wet kaolin, allow us to control loading and material rheology, and directly document the complete evolution of strike-slip fault systems that grow in material with pre-existing weaknesses. To assess the impact of pre-existing weaknesses on strike-slip fault evolution we vary initial orientation and spacing of the vertical surfaces, and nature of basal shear loading (localized and distributed). Weaknesses oriented 60˚ and 90˚ from the applied dextral loading showed negligible slip while weaknesses oriented 120˚ developed sinistral slip and weaknesses oriented 150˚ had dextral slip. Experiments that developed sinistral slip along cross-faults (120˚) also showed distributed dextral strain between the faults that contributed to significant rotation of material, including the cross-faults, within the shear zone. When the sinistral faults were rotated to orientations unfavourable for continued slip, new dextral faults developed. This finding suggests that strike-slip systems with active cross faults represent immature stages of evolution that will become reorganized upon further strain accumulation. The amount of off-fault deformation and shear zone width depends on the presence of pre-existing weaknesses (even if they had low slip) and the persistence of fault irregularities that arose from slip along and interaction of new faults with the pre-existing weaknesses. Understanding how the orientation of pre-existing weaknesses influences the early evolution of strike-slip faults and strain localization over geologic time scales can inform future seismic hazard assessments of regions with pre-existing structures.
The authors present analogue “claybox” models of strike-slip fault evolution in the upper crust testing localized vs distributed basal shear, uncut vs precut weaknesses in the crust, and varying the angle and spacing of the pre-cut weaknesses relative to the primary active fault. Their results show that the angle of pre-cut weaknesses (a proxy for pre-existing faults) controls if slip occurs on those weaknesses in the early stages of fault evolution, and if the slip is sympathetic or antithetic to slip on the primary fault. The presence or absence of pre-existing weaknesses also affects the geometric complexity of the final throughgoing rupture. The authors link their results to observed deformation in the Mw 7.1 Ridgecrest earthquake, though these results could allow geologists to gain insights from surface rupture patterns on many continental strike-slip faults. The results could also impact fault displacement hazard analysis near immature strike-slip faults (recommending wider deformation zones), though this is not mentioned in the text.
In general, this study is easy to read and mostly logically organized. The figures are informative and illustrate the story well. Although there are some minor grammatical or typographic errors in the text (see minor comments), I recommend only minor edits prior to publication. Nice work!
General Comments:
Minor Comments:
Line 6 – Add Mw 7.1 to the description of the Ridgecrest mainshock
19 – rephrase “if they had” to “those with”
22 – Might consider rephrasing the final sentence of the abstract as the pre-existing structures may be unknown prior to rupture.
24-25 – Name the historical earthquakes with complex fault networks that are being referred to here so that the “these recent earthquakes” and “these ruptures” in lines 26 and 28 are referring back to actual earthquakes.
Figure 1 caption - States the maps are “taken” from Rosa et al., 2024, and Milliner et al., 2021. Are the data from those publications or the figures themselves? If the figures themselves, make sure proper permissions are acquired and state instead “figure from”. If just the data, then state “data from”.
62 – Add a comma between “clay, which”
86 – delete the “to” between “Fault systems evolve”
92 – Put the “in the crust” after “weaknesses” instead of after “bedding planes”
95 – Consider adding an introductory sentence to this paragraph something along the lines of “We test four cases of strike-slip fault evolution: with and without basal shear and with and without pre-existing weaknesses”. Then go ingo the details of the claybox model. Also, instead of “All strike-slip fault experiments…” consider “Our strike-slip” or “All our…”.
121-122 – The sentence “We expects…dextral slip (Figure 3)” is more of a hypothesis than methods. Consider rephrase.
130 – “several other” is vague. Are there more relevant details?
149 – Add a “the” in “impact the number of detected faults”
151 and 153 – “5x5 mm^2” and “(area <6 mm ^2)” both are areas already, and do not need the ^2.
176 – It seems a word is missing after “basal”
177 – It seems 1 or 2 is missing after “stage”
185 – Weaknesses (plural)
91 – in the title: “…with 2 cm weakness spacing”
202 – “experiment with 90 degree weaknesses”
Figure 4 – consider adding an arrow or few to point out the pre-cut weaknesses in one of the panels
Figure 5 caption – The x axis in the figure is “displacement” but the figure caption calls it “timing”. I understand how they are linked, but consider using “displacement” in the caption as the figure does not have a time component labelled.
238 – update to “This reorganization of the active FAULT system marks the START of stage…”
247 – add a reference to Figure 6 somewhere in the first sentence of this paragraph.
260 – Add a comma between “…faults, none…”
267 and 290 – sinistral and dextral are misspelled
274, 281, and throughout – use “less” instead of “lesser”
284 – what kind of fault irregularities - geometrical?
295-296 – update to: “…as the applied loading was accommodated as DISTRIBUTED deformation and kinematic efficient increased as the faults evolved and accommodated MORE LOCALIZED slip”
Figure 8 – what do the light tan and dark tan regions represent in panel A? How are the onset of each stage defined – is it manual my inspecting the deformation patterns, or is there a quantitative threshold?
315 – replace “that” with “than”
334 – Add a comma between “displacement, slip”
341 – Add a comma after “stage 2,”
353 – missing final ) after “stage 1”
378 – add a comma after “During stage 2,”
393 – “irregular” rather than “irregularly”
400 – add a comma after “pre-cut surfaces,”
426 – “rheometer” what a great word!
432/Figure 10 caption – Where do the data for part B, “real-world orientations” come from?
437-440 – Completely agree!
454 – “…the shear zone that resembles the pattern…”
464 – “contribute TO curved cross-faults”
474 – “production OF simultaneously active…”
484 – “this early fault irregularity IMPACTS”
495 – irregular rather than irregularly
500 – less rather than lesser
505 – update to “increasingly unfavorable”
507 – consider adding a sentence or half a sentence about how these results might also inform PFDHA around immature faults – e.g., the results could support recommending wider deformation zones around immature faults.