Earthquake dynamics sustained by seismic CO_2

Domain: neuroscience
Published: 2026-02-14
Editorial outcome: major_revision

Abstract
Seismic faults paradoxically combine high frictional strength, which promotes stress accumulation and should hinder slip, with large seismic displacements. Among proposed dynamic weakening mechanisms, shear heating is crucial because it triggers decarbonation and CO₂ production. In carbonate faults, this process generates transient CO₂ pressurization that can modulate rupture dynamics and promote supershear propagation, yet the amount and pressure of produced CO₂ remain poorly quantified. We investigate carbonate faults in the Apennines, Italy, a region affected by Mw ≤ 7.1 earthquakes. Integrating nano-scale observations of fault surfaces with mineralogical and isotopic constraints, we develop a stoichiometric-thermodynamic model linking seismic decarbonation to Mw 5.9–6.5 earthquakes. Individual events can produce up to 12 tons of CO₂, generating quasi-lithostatic pressures (~196 MPa) under undrained confinement and supra-hydrostatic pressures (76–134 MPa) under drained conditions. Here, we conclude that seismic CO₂ pressurization can sustain dynamic slip and enhance the destructive potential of earthquakes in carbonate terrains.

Introduction
Understanding dynamic fault weakening processes is essential for assessing seismic hazard, as they govern earthquake rupture dynamics and can amplify destructive potential 1–6. Among the various dynamic weakening mechanisms, shear heating has been extensively documented in both natural faults and laboratory experiments 7–16. During shear heating, a portion of the mechanical energy released by fault slip is converted to frictional heat 17,18. In detail, laboratory-simulated earthquakes indicate that 68%-98% of the released energy is dissipated as heat 19. In carbonate-bearing rocks—whether of sedimentary or metamorphic origin—temperatures of ~600 °C (or lower) can trigger rapid decarbonation of CaCO₃, producing CO₂ within milliseconds to seconds 7,20. This reaction may generate sudden fluid overpressures, modulate dynamic stress along the fault surface, and influence rupture propagation 21–23.

Despite field observations and experimental studies of seismic decarbonation, the mass of CO₂ produced during natural earthquakes and its sudden pressure remain largely unconstrained. As a result, the reduction of effective normal stress in natural carbonate faults by generation of seismic CO₂—and thereby the enhancement of additional slip 17,24,25—remains largely unquantified.

Here, we address this gap by investigating recently exhumed normal fault surfaces within carbonate rocks from the seismogenic domain of the Apennines, Italy (Fig. 1). Micro- to nano-scale analyses reveal discrete microdomains of seismic decarbonation along natural slip surfaces. Combined X-ray diffraction and stable-isotope analyses provide further evidence of seismic decarbonation. We then integrate these natural-based constraints with stoichiometric and thermodynamic modeling to quantify the mass of seismic CO₂ and the associated transient pressure pulses. Our results indicate that Mw 5.9–6.5 earthquakes in carbonate fault zones can produce up to ~12 tons of CO₂ and generate pressures likely shifting from supra-hydrostatic to quasi-lithostatic depending on the drained vs. undrained conditions, respectively. Here, we show that these fluid pressures are capable of sustaining dynamic fault weakening and may enhance rupture propagation, providing insights into the mechanisms driving large, destructive earthquakes in carbonate seismogenic regions worldwide.

Results
Meso- to micro-scale analyses
We collected carbonate fault surfaces from the Mt. Vettore, Mt. Capo di Serre, and Tre Monti faults in the Apennines (Fig. 1c–e). These faults were selected due to their well-exposed, fresh principal slip surface (PSS) and their location within a seismogenic area that has experienced four earthquakes with Mw ≥ 5.9 since 2009, as well as numerous historical events up to Mw ≤ 7.1. Notably, two (Mw 6.0 and Mw 6.5) of the recent earthquakes were generated by the Mt. Vettore Fault.

The PSS of the Mt. Vettore Fault exhibits truncated clasts and comb fractures (Fig. 1c). Beneath the PSS, a 5 cm thick cataclasite contains fragments of host rock and calcite veins within a fine-grained matrix (Fig. 1c). Along the Mt. Capo di Serre Fault, centimetric comb and ring fractures are observed, along with a ~1 cm thick ultracataclasite layer composed of an extremely fine-grained matrix with millimetric to micrometric truncated clasts (Fig. 1d). The Tre Monti Fault features a convoluted ultracataclasite layer with a fine-grained matrix and truncated micrometric clasts (Fig. 1e).

Micro- to nano-scale analyses
Field emission scanning electron microscopy (FESEM) observations reveal 2–10 µm thick layers immediately beneath (in the footwall) all the investigated PSSs (Fig. 2). These layers are characterized by micrometric corroded carbonate grains with nanometer-scale decarbonation trails, along with rounded and bulbous pores and vesicles distributed within and between the grains (Fig. 2). These observations are consistent with features reported from high-velocity friction experiments 16,20 and from natural carbonate faults 26,27. A detailed correspondence between the nano-structures documented in this study and those described in previous works is summarized in Table 1, which links each feature observed here to specific figures in the literature.

The cataclasite beneath the PSS of the Mt. Vettore Fault contains carbonate nanocements indicating cementation processes (Fig. 3). Nanocements, which are less common in the other analyzed faults, consist of euhedral nanograins (<500 nm in diameter) located in the first 5–10 µm thick layer beneath the PSS, where they occur between grain boundaries and within micrometric cavities (Fig. 3).

Mineralogical analyses
We conducted X-ray diffraction (XRD) quantitative analyses on twelve samples: six samples were collected by scraping off the freshest and brightest thin portions of the PSSs and six other samples were obtained microdrilling the footwall rocks 6 mm below the PSS (Figs. 1c–e and 4a–c). The analyzed samples are composed of calcite (>80%), phyllosilicates, and minor quartz. Calcite content decreases with proximity to the PSS in all faults (Fig. 4a–c). The decrease is 3%, 4%, and 5–9% for the Mt. Vettore, Tre Monti, and Mt. Capo di Serre faults, respectively (Fig. 4a–c).

Isotopic analyses
We performed stable isotope (δ¹³C and δ¹⁸O) analyses on a total of 60 samples. For each fault, we selected five millimetric transects from the PSS to the footwall. For each transect, one sample was directly scraped from the PSS and three additional samples were drilled in the footwall at distances of 2, 4, and 6 mm (Fig. 4a–c). The Mt. Capo di Serre and Tre Monti Faults exhibit negative δ¹³C (−0.5‰ to −9‰) and positive δ¹⁸O (27.3‰ to 23.7‰). The Mt. Vettore Fault exhibits δ¹³C values between 2.3‰ and −0.1‰, and δ¹⁸O values between 28.4‰ and 26.3‰. The samples scraped directly from the PSS of the Mt. Vettore Fault display the highest δ¹³C and δ¹⁸O values (Fig. 4d).

Evidence of decarbonation
The meso-structures observed along the studied faults, including truncated clasts, ultracataclasite layers, and comb fractures (Fig. 1 and Supplementary Fig. 2), provide evidence for seismic slip. Similar structures documented in carbonate faults have been indeed interpreted as indicators of seismic slip 23,28–30.

The 2–10 µm thick layers containing decarbonation trails and rounded and bulbous pores and vesicles (Fig. 2) indicate decarbonation induced by seismic shear heating, consistent with findings from studies on natural 13,22,26,27,31 and experimental 7,10,14,16,20,32–34 faults (Fig. 2 and Table 1). Comparable features have not been observed in experiments simulating fault creep 12. The rounded and bulbous pores and vesicles are ductile structures, indicating that decarbonation occurred when the carbonate material had already entered a plastic regime. Moreover, no fractures associated with CO₂ escape are present. This nano-structural evidence suggests that CO₂ escape must have become efficient only after shear heating had elevated the temperature enough for the carbonate to behave plastically, allowing pore growth through viscous deformation instead of fracture-controlled pathways.

We cannot directly determine whether the decarbonated layers observed along the PSSs originated from a single seismic event or multiple events. However, building on analogies with the ~10 µm thick decarbonated layers produced during single laboratory-simulated earthquakes 16,34, we infer that the 2–10 µm thick layers observed in our samples (Fig. 2) likely formed during a single seismic event.

XRD analyses further support decarbonation, revealing a 3–9% decrease in calcite content over the last 6 mm from the footwall to the PSS (Fig. 4a–c), consistent with seismic carbonate breakdown. However, the presence of post-decarbonation nanocements along the PSSs (Fig. 3) suggests that the measured calcite content near the PSSs may include secondary calcite possibly precipitated from fluids that flowed along the fault after decarbonation. As a result, the actual decrease in calcite resulting from decarbonation may be greater than indicated by XRD results.

Stable isotope data reveal the complexity of the decarbonation process. These data align along a direct correlation trend between δ¹³C and δ¹⁸O, with δ¹³C ranging from 2.3‰ to −9‰ and δ¹⁸O from 28.5‰ to 23.7‰ (Fig. 4d). In detail, the isotope graph highlights a major linear pattern that can be read along two opposite trends, one leftward and one rightward (Fig. 4d). A decrease in δ¹³C and δ¹⁸O values (leftward trend) is indicative of fault rock decarbonation 35,36, whereas an increase (rightward trend) is associated with post-decarbonation cementation 37, a process highlighted by our FESEM observations (Fig. 3). The decarbonation trend is characterized by a significant δ¹³C depletion relative to typical carbonate host rocks of the Apennines, reflecting an enrichment of ¹²C in the residual carbonate. This trend is associated with isotopic fractionation between the residual carbonate and the produced CO₂ 35,36 (Fig. 4d). Consistently, shear heating is indicated by the modest δ¹⁸O decrease measured from the footwall to the PSS, suggesting reduced oxygen isotopic fractionation.

Notably, the decarbonation trend does not consistently start at the typical isotopic composition of the Apennine carbonate host rocks 38 and no systematic relationship is observed between δ¹³C values and the sample distance from PSSs. Moreover, some samples collected as distant as 6 mm from the PSSs exhibit negative δ¹³C values (Fig. 4d). This suggests that the isotopic signature within the 6 mm from the PSS records the cumulative effect of multiple decarbonation episodes, associated isotopic fractionation, and subsequent cementation, now interleaved in the rock record. Alternatively, the volume of decarbonated material may have been substantially underestimated, implying a greater extent of decarbonation.

At the Monte Vettore Fault, the opposite trend – where PSS samples display the highest δ¹³C values (Fig. 4d) – is interpreted as evidence for carbonate cementation driven by post-decarbonation fluid ingress from the surrounding host rock 39. At least in this case (Monte Vettore), in fact, cementation was most intense along the PSS, as indicated by the presence of nanocements within microcavities and along grain boundaries (Fig. 3). The extent of this cementation was likely controlled by the distribution of seismic damage and the associated porosity generated during seismic rupture.

CO₂ mass modeling
To model the mass of CO₂ produced by seismic decarbonation, we first consider the sources of uncertainty affecting our estimates of decarbonated material. Both the thickness of the decarbonated layer (2–10 µm; Fig. 2) and the percentage of carbonate breakdown (3–9%; Fig. 4a–c) likely represent minimum values. Several lines of evidence support this interpretation.

First, as mentioned above, at the high temperatures reached during seismic shear heating, carbonate enters a plastic regime 40,41. The rounded and bulbous pores and vesicles generated by decarbonation (Fig. 2) attest to this viscous behaviour, implying that part of the transient porosity produced during CO₂ production may have been rapidly reabsorbed during slip. Such plastic compaction would partially erase the true thickness of the decarbonated layer, indicating also that the measured percentage of carbonate breakdown captures only a fraction of the total extent of decarbonation.

Second, laboratory simulations of seismic slip in carbonate rocks show that a substantial fraction of the fault surface is mechanically consumed during seismic sliding, producing measurable mass loss 16,33,42. This mechanical abrasion further removes the nano-scale structures diagnostic of decarbonation, thereby contributing to an underestimation of the total volume of carbonate that underwent breakdown.

Taken together, these considerations indicate that the CO₂ masses estimated here should be regarded as conservative lower-bound values.

In light of these considerations, to model the mass of CO₂ produced, we first calculate the volume of decarbonated fault rocks. We multiply the conservative 2 µm thickness of the decarbonated fault rock layer by the area of seismic slip patches associated with recent earthquakes in the Apennines (Methods). The slip patches associated with the recent earthquakes in the Apennines were derived from seismological models for the 2009 Mw 6.3 L’Aquila 43, 2016 Mw 6.0 Amatrice, 2016 Mw 5.9 Visso, and 2016 Mw 6.5 Norcia earthquakes 44,45 and have average areas between 85 km² (Visso earthquake) and 170 km² (L’Aquila earthquake). Assuming that only 3%—corresponding to the minimum extent of carbonate breakdown constrained by XRD analysis—of this volume underwent decarbonation (Methods), CO₂ production due to single seismic decarbonation events ranges from 6 to 12 tons, for these two end members (Fig. 5). The related decarbonated volumes are 5.1 and 10.2 m³ , respectively (Supplementary Table 4). For comparison, Fig. 5 shows CO₂ mass estimates based on seismic slip patches derived from the magnitude–rupture size empirical relationship 46 for the same four earthquakes. Results from both approaches are similar, with slightly higher CO₂ masses estimated using the empirical relationship.

CO₂ pressure modeling
To evaluate the impact of decarbonation on earthquake dynamics, we calculate the seismic CO₂ pressure along the PSS. We explore and explain two geologically plausible scenarios: undrained and drained conditions (Fig. 6).

Undrained condition: The undrained scenario reflects the extremely low permeability of the fault core and slip surfaces 47,48 and the impulsive nature of seismic decarbonation 7,14,20,49. Under these conditions, CO₂ is generated too rapidly to escape during the earliest stages of slip 50,51, favouring the development of highly pressurized systems. This scenario is further supported by the marked reduction in carbonate permeability with increasing temperature 40,41.

To model the CO₂ pressure in undrained conditions, we employ the ideal and two CO₂-dedicated real gas (Peng and Robinson and Span and Wagner) equations of state (EOS) 52,53, assuming that the produced CO₂ remains confined, at the instant of its formation, in the decarbonated rock volume (Methods). We predict CO₂ theoretical pressures of 196, 634, and 821 MPa using the three aforementioned EOS, respectively (Fig. 6a). The pressure obtained from these equations remains constant, irrespective of the produced CO₂ mass. This derives from the assumption that, at the activation instant of shear heating-induced CO₂ production, CO₂ remains confined within the volume in which it is generated. Consequently, the number of CO₂ moles produced is directly proportional to the volume it occupies, implying a constant CO₂ molar volume (Methods). This assumption is supported by laboratory studies on small carbonate samples that reveal that even trace amounts of seismic CO₂ (in the order of ppm) generated during the first few microseconds to seconds of seismic slip, can produce substantial pore pressure increase and related weakening 7,10,14,20, thus implying CO₂ sudden confinement during its onset of production. In summary, the theoretical CO₂ pressures obtained under undrained conditions are 196 MPa for the real gas EOS, 634 MPa for the Peng and Robinson EOS, and 821 MPa for the Span and Wagner EOS.

Drained condition: In the drained condition, we simulate scenarios in which CO₂ generated by seismic decarbonation progressively escapes from the fault zone. This view is consistent with previous studies 8,21 and with our FESEM observations of decarbonation textures (Fig. 2), revealing nano-structural evidence for CO₂ escape pathways. We employ the ideal and one CO₂-dedicated real gas (Saxena and Fei) EOS 54. This configuration, adapted from the analytical model of Famin et al. 21 (Methods), reproduces the transient pressure increase followed by partial dissipation through the fault permeability. We examine pressure evolution at two end member hypocentral depths—9.2 km (Norcia earthquake) and 7.5 km (Visso earthquake) (Fig. 6b and Supplementary Fig. 8). For each depth, we explore three permeability configurations: a reference value calibrated from Famin et al. 21, and two end-member cases obtained by varying this parameter by one order of magnitude (k×10 and k×10⁻¹)(Methods).

The drained scenario predicts a non-linear increase in fluid pressure as CO₂ mass grows (Fig. 6b). Using the previously estimated CO₂ masses normalized to rupture areas derived from seismological models (2.5 g m⁻²; Methods; Supplementary Table 4), the modeled CO₂ pressure at 9.2 km depth rises from an initial hydrostatic value of ~90 MPa (zero CO₂ production) to 112 MPa using the Saxena and Fei EOS and to 119 MPa using the ideal gas EOS. A one-order-of-magnitude change in permeability yields pressures of 128 MPa (k×10⁻¹) and 100 MPa (k×10) for the Saxena and Fei EOS, and 134 MPa (k×10⁻¹) and 106 MPa (k×10) for the ideal gas EOS (Fig. 6b).

At 7.5 km depth, pressure increases from ~75 MPa (zero CO₂ production) to 80 MPa using the Saxena and Fei EOS and to 86 MPa using the ideal gas EOS. Varying permeability by one order of magnitude produces CO₂ pressures of 89 MPa (k×10⁻¹) and 76 MPa (k×10) for the Saxena and Fei EOS, and 98 MPa (k×10⁻¹) and 79 MPa (k×10) for the ideal gas EOS (Fig. 6b).

In summary, the theoretical CO₂ pressures obtained under drained conditions span 100–128 MPa (Saxena and Fei EOS) and 106–134 MPa (ideal gas EOS) at 9.2 km, and 76–89 MPa (Saxena and Fei EOS) and 79–98 MPa (ideal-gas EOS) at 7.5 km.

Methods
Setting, field analysis, and sampling
Field mesostructural analyses were conducted at selected sampling sites along the slip surfaces of the Mt. Vettore, Mt. Capo di Serre, and Tre Monti normal faults (Fig. 1; Supplementary Figs. 1 and 2). These faults are exposed within the seismically active, post-orogenic extensional domain of the Apennines fold-and-thrust belt. The investigated areas were exhumed from a maximum depth of approximately 3 km since the mid-Pliocene 62. Accordingly, the maximum ambient thermobaric conditions experienced by the studied faults are estimated at ~75 °C and ~80 MPa.

Sampling locations and coordinates are provided in Supplementary Fig. 1. Fault rock samples were analyzed at hand scale by cutting them parallel to the slip direction and perpendicular to the slip surface, polishing the freshly cut surfaces and scanning them at high resolution using an Epson Perfection V370 Photo scanner at Sapienza University of Rome, Italy. These observations focused on identifying centimeter- to millimeter-scale primary structures indicative of seismic slip, guiding the selection of suitable samples for thin-section preparation.

Optical microscopy and FESEM analyses
Optical microscopy on rock thin sections was conducted using a Zeiss Axioskop microscope with a Pentax K1 II camera at the Microanalysis and Electron Microscopy Laboratory of CNR-IGAG in Rome. The analysis focused on the principal slip surfaces to identify areas for further investigation using a field emission scanning electron microscope (FESEM). Areas with seismic slip indicators (e.g., ultracataclasites and truncated clasts; Supplementary Fig. 3) were selected for FESEM analyses.

FESEM analyses were performed on a Merlin Zeiss system at the CERTEMA Multidisciplinary Technological Laboratory in Grosseto, Italy. This analysis provided detailed measurements of the fault rock thickness, with evidence of carbonate breakdown linked to decarbonation, including pores, vesicles, and decarbonation trails (Fig. 2). Moreover, FESEM observations allowed us to identify secondary cements along the analyzed faults (Fig. 3). These analyses were performed at an accelerating voltage of 10 kV and a probe current of 2 nA. The working distance for backscattered electron imaging was set to 6.5 mm. To avoid any external carbon contamination, the samples were coated with a 5 nm thick chromium layer.

X-ray diffraction
X-ray diffraction (XRD) measurements were performed using a Bruker D8 Advance system (CuKα radiation) at 40 kV and 30 mA in the X-ray Laboratory of Sapienza University of Rome. These analyses were conducted on six samples obtained from the principal slip surfaces by scraping off its freshest and most polished portions, while six samples were extracted from the footwalls by microdrilling at a depth of 6 mm below the principal slip surfaces (Fig. 4a–c). A microdrill equipped with drill bits as small as 0.3 mm in diameter ensured high precision in sample collection. Whole-rock powders were analyzed over a 2θ range of 2°–70°, with a step size of 0.02° 2θ and a 1-s counting time per step, on a Si-wafer to enhance the signal to noise ratio. The X-ray quantitative analysis was performed by Profex-BGMN Rietveld refinement software 63. The estimated quantification error is greater than ±0.6% (Supplementary Figs. 4–8 and Supplementary Table 1). XRD analyses allowed the quantification of decarbonation-related carbonate breakdown. Specifically, by comparing carbonate content between samples taken from the slip surface and those located 6 mm below it, we determined the extent of carbonate loss due to decarbonation (Fig. 4a–c).

Stable isotopes
The carbon (δ¹³C) and oxygen (δ¹⁸O) isotopic composition of bulk carbonate was measured using a GasBench II coupled to a Delta Plus mass spectrometer (ThermoFisher Scientific). Calibration was performed using internal standards MS (δ¹³C=+2.13‰, δ¹⁸O=−1.73‰), (δ¹³C=2.53‰, δ¹⁸O=−4.73‰) and NEW12 (δ¹³C=−3.83‰, δ¹⁸O=−9.59‰), which were linked to international reference materials NBS 19 (δ¹³C=+1.95‰, δ¹⁸O=−2.2‰) and NBS 18 (δ¹³C=−5.01‰, δ¹⁸O=−23.00‰). These analyses were conducted at the Stable Isotope Laboratory of CNR-IGG in Pisa, Italy, to isotopically validate the occurrence of decarbonation along faults. Five microtransects orthogonal to the principal slip surfaces were selected from each fault (Fig. 4a–c), resulting in a suite of 60 samples. For each transect, one sample was carefully collected from the freshest and most polished portion of the principal slip surface. Additionally, one sample was extracted at each of the following distances from the slip surface within the footwall: 2, 4, and 6 mm (Fig. 4a–c). These extractions were performed using a microdrill equipped with drill bits as small as 0.3 mm in diameter, ensuring high precision. Isotopic results are reported in V-PDB and V-SMOW notations (Fig. 4d; Supplementary Table 2).

Estimates of CO₂ mass produced by decarbonation during shear heating
The mass of CO₂ produced by decarbonation was quantified assuming a dry environment 64 as follows:
(1) Calculation of seismic slip patch areas. The empirical rupture areas for four earthquakes (Mw 5.9 Visso, Mw 6.0 Amatrice, Mw 6.3 L’Aquila, and Mw 6.5 Norcia) were estimated using the empirical equation A = 10^(M−4) 46, where A represents the rupture area in km² and M is the earthquake magnitude (see blue dots in Fig. 5). Rupture areas for the same earthquakes were obtained also from geophysical models reported in previous studies 43–45 (see red dots in Fig. 5). These patches were digitized within the QGIS software environment, considering only regions where seismic slip exceeded 10 cm. The software was then used to automatically calculate their areas. Both empirical and seismologically modeled seismic patch areas are reported in Supplementary Table 4.
(2) Calculation of decarbonated fault rock volume. The volume of decarbonated fault rock was determined by multiplying the seismic slip patch areas by the measured thickness of decarbonated layers, constrained through FESEM observations. Decarbonation was identified in layers ranging from 2 to 10 µm in thickness (Fig. 2). To ensure a conservative estimate, a thickness of only 2 µm was adopted, despite some fault layers exhibited thicknesses of up to 10 µm (Supplementary Table 4). We then considered the lowest decrease in calcite content (3%) within these layers due to decarbonation, on the basis of XRD analysis (Fig. 4a–c). For conservative purposes, also in this case, we used the lower value, i.e., 3% (Supplementary Table 4). The wt % of minerals was converted to vol % (Supplementary Table 1) by using density values of 2.71 g/cm³ for calcite, 2.65 g/cm³ for quartz, and 2.70 g/cm³ for phyllosilicates.
(3) Determination of the decarbonated CaCO₃ mass. The decarbonated CaCO₃ volume was multiplied by the density of CaCO₃ (2.71 g/cm³), assuming it consists entirely of pure calcite. The resulting mass was converted to moles by dividing the mass by the molar mass of CaCO₃ (100.09 g/mol).
(4) Calculation of produced CO₂ moles. The moles of decarbonated CaCO₃ were then used to determine the moles of produced CO₂, based on the stoichiometric assumption that each mole of CaCO₃ produces one mole of CO₂ during decarbonation. Finally, the mass of produced CO₂ was obtained by multiplying the moles of CO₂ by its molar mass (44.01 g/mol) (Supplementary Table 4).
(5) Normalization of produced CO₂ mass to the rupture areas derived from seismological models, the resulting coseismic CO₂ production is 2.5 g m⁻² (Supplementary Table 4).

Estimates of CO₂ pressure during shear heating
To estimate the pressure of CO₂ during shear heating, we considered two end-member conditions: undrained and drained (Fig. 6). The undrained case reflects the extremely low permeability of the fault zone and the impulsive character of seismic decarbonation in the initial stages of slip, during which CO₂ is produced too rapidly to escape the system. Conversely, the drained case represents a condition in which the produced CO₂ can progressively migrate out of the decarbonated zone, in agreement with previous experimental and analytical models 8,21.

For undrained conditions, various equation of state (EOS) can be applied. Above its critical temperature (31 °C) and pressure (7.4 MPa), CO₂ exists in a supercritical state. The two most used real gas EOS for describing CO₂ under supercritical conditions are those of Peng-Robinson 52 and Span and Wagner 53. The Span and Wagner EOS can be applied for pressure <800 MPa and temperature <825 °C 53, whereas the applicability ranges for the Peng-Robinson EOS were not provided by the authors 52. Some studies 65,66, however, suggest that CO₂, even if in a supercritical state, behaves as an ideal gas above approximately 300 °C. Therefore, the CO₂ produced at ~600 °C by shear heating could also be theoretically described using the ideal gas EOS. Although this behavior may be inhibited by the confined volume in which CO₂ is initially trapped, it could still be justified given the instantaneous (from a few microseconds to seconds) 7,20 nature of the decarbonation process linked to earthquakes.

We calculated CO₂ pressure using the three aforementioned EOS:
(1) Peng-Robinson EOS:
P = RT / (Vm − b) − a / (Vm² + 2bVm − b²)
a = 0.45724 * R²Tc² / Pc
b = 0.07780 * RTc / Pc
a = [1 + k(1 − √(T/Tc))]²
k = 0.37464 + 1.54226ω − 0.26992ω²
where R is the universal gas constant (8.314 J/(mol K), T is the temperature, Vm is the molar volume, a and b are parameters related to the critical temperature (Tc) and critical pressure (Pc) of CO₂, k is a constant, and ω is the acentric factor (0.224) 52. Pressure computation applying this EOS was performed using the WolframAlpha online resource, available at https://www.wolframalpha.com/input?i=Peng-Robinson+equation+of+state.
(2) Span and Wagner EOS;
P = ρRT [1 + δ * (∂ϕr / ∂δ)]
where ρ is the density, R is the universal gas constant (8.314 J/(mol K), T is the temperature, δ is the reduced density normalized to the critical density of CO₂ (467.6 kg/m³), ϕr is the residual part of the Helmholtz free energy function, and (∂ϕr / ∂δ) is the partial derivate of ϕr with respect to δ 53. Pressure computation applying this EOS was performed using the open source LONER18 software 67, available at https://fluids.unileoben.ac.at/Computer.html.
(3) Ideal gas EOS:
P = RT / Vm
where Vm is the molar volume, T is the temperature, and R is the universal gas constant (8.314 J/(mol K).

Under undrained conditions, we assumed that, at the first instant of shear heating-related CO₂ production, CO₂ remains confined within the volume in which it is generated. Consequently, in this stage, the volume assumed in Eqs. (1)–(3) is the decarbonated volume (see above).

This assumption implies that the number of CO₂ moles produced and the volume scale proportionally, thus the molar volume remains constant. Therefore, the variables in the three EOSs are only P and T. As a result, for a given T each EOS yields a single pressure value for all the analyzed earthquakes, irrespective of their magnitude (Fig. 6a and Supplementary Table 5).

It is important to emphasize that in our model, we extrapolate data collected from faults exhumed from a maximum depth of 3 km (see above) to hypocentral depths ranging between 7.5 and 9.2 km (Fig. 6a). Specifically, we extrapolate the thickness of seismically decarbonated rocks, observed via FESEM along fault surfaces (Fig. 2), and the corresponding decarbonation volume (estimated at 3%; Fig. 4a–c) to greater depths. To minimize uncertainties, we adopt conservative values (minimum observed thickness and calcite loss) in our model. Given the tight conditions of the fault surface in the pre- to early-seismic instants, pressure calculations are performed under the assumption of an initial dry system 64.

A key parameter in our calculations is the activation temperature of decarbonation during shear heating. Experimental studies indicate that decarbonation take place at approximately 600 °C 7,16,22,23,31,68, which we adopted for our CO₂ pressure calculations (Supplementary Table 4). However, shear heating-induced decarbonation has also been reported at temperatures as low as ~450 °C 14,33. Using this lower temperature value would reduce the estimated CO₂ pressures by a maximum of 19% compared to those calculated at 600 °C (Supplementary Table 5). Therefore, also considering an activation temperature of 450 °C, given that the calculated pressures remain, in many cases, significantly higher than lithostatic pressure at hypocentral depths (Fig. 6a and Supplementary Table 5), this reduction does not affect the conclusions of our study.

For drained conditions, we adopted the thermodynamic model proposed in Famin et al. 21. In this model, the produced CO₂ migrates within the fault core according to the permeability, storage capacity, and compressibility of the fault material. CO₂ pressure for drained conditions was calculated using the Python code provided in the Supplementary Information. The use of the code requires the previous installation of the Numpy, Matplotlib and Scipy libraries.

Following Famin et al. 21, the code computes the CO₂ produced using the equation of state:
P = (m / MCO₂) * ZRT / ω
which yields the fluid pressure increase (Pf / P0):
Pf = (P0 * ϕf * Zf * m) / (ϕ0 * Z0 * m0)
where the subscripts 0 and f refer to quantities before and after seismic CO₂ production, respectively. R, T, ϕ and MCO₂ are the gas constant, the temperature, porosity of the fault zone (0.03; Famin et al.) 21, and the molar mass of CO₂, respectively.
Z, which is the compressibility factor of CO₂ 21,54, is calculated using:
Z = 1 − 0.5917 * (T/Tc) + 0.09122 * (T/Tc)² + 1.4164x10⁻⁴ * (T/Tc)³ − 2.8349x10⁻⁶ * ln(P/Pc)
where Pc and Tc are the pressure and temperature at the critical point of CO₂. m0 and mf are the masses of CO₂ present before and after seismic decarbonation in a volume of fault zone equal to 1×1×ω m³. m0 is calculated assuming hydrostatic pressure in Eq. (A). mf is equal to (m0 + me), where me is the mass of CO₂ produced by a 1 mm thick layer of fault surface. For conservative purposes, also in this case, we used the lower value, i.e., 3% on the basis of XRD analysis. ω is the maximum distance of seismic CO₂ outflow in the direction perpendicular to the fault surface, taken as the 2 µm thick of decarbonated layer plus twice the hydraulic diffusion length scale of CO₂ (Lf) given by:
Lf = √(Df * ψ)
where ψ is the timescale of seismic decarbonation, set at 5 s, consistent with experimental seismic decarbonation and CO₂ production in laboratory carbonate faults 7,20. Df is the hydraulic diffusivity after seismic CO₂ production and is given by:
Df = k / (η * βf)
where kf is the final permeability of the fault, βf its storage capacity, and η the viscosity of supercritical CO₂ (4E−05 Pas) 69. βf is computed from:
βf = βb − βs + ϕf * (βCO₂ − βs)
where βs is the compressibility of carbonate mineral grains (1.25E−11 Pa⁻¹) 8, βb is the bulk rock compressibility, and βCO₂ is the compressibility of supercritical CO₂, given by:
βCO₂ = −(1/P) * (∂Z / ∂P) * T=cte

To calculate the pressure-dependent permeability of the fault (kf), we use the empiric law derived from Famin et al. 21 as follow:
k = 3x10⁻¹⁷ * e^(−6.14x10⁻⁸ * (Prock − P))
where (Prock − P) is the effective pressure.

In the absence of specific laws of pressure dependence for ϕ and βb, and following the method adopted in Famin et al. 21, we calculated:
ϕ = ϕ0 * e^(−2.97x10⁻⁸ * (Prock − P))
βb = 2.5x10⁻¹⁰ * e^(−1.38x10⁻¹³ * (Prock − P))
using ϕ0 = 0.03 (Famin et al.) 21.

We modelled the evolution of CO₂ pressure at two representative hypocentral depths: 9.2 km, corresponding to the Norcia earthquake, and 7.5 km, corresponding to the Visso earthquake. For each depth, we explored three permeability configurations: a reference value calibrated from Famin et al. 21, and two end-member cases obtained by varying this parameter by one order of magnitude (k×10 and k×10⁻¹). The resulting pressure–permeability relationships are presented in Fig. 6b, Supplementary Fig. 8 and detailed in Supplementary Table 6, which also lists the input parameters used in the simulations.

To estimate the drained pressure conditions, we used a CO₂ mass of 2.5 g m⁻² (green line in Fig. 6b and Supplementary Fig. 8), obtained by dividing the previously calculated total CO₂ mass by the surface area of the seismic patch affected by decarbonation—assumed to represent 3% of the total fault area, in agreement with XRD results (Supplementary Table 6). To model the drained scenario using the ideal-gas formulation, we set Z = 1 and its derivative equal to zero in Eq. (C).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript uses microstructures observation, XRD analyzes, and oxygen-carbon isotope geochemistry in principal slip surfaces of carbonate faults to quantify decarbonation during earthquakes. Their conclusion is that decarbonation is a signification mechanism in coseismic fault weakening.\nWhile I acknowledge that the approach is interesting and that the results deserve publication, I have noticed several issues in the present manuscript:\n1. I disagree with the general statement that coseismic CO2 production in faults has never been quantified (lines 21-22, 42). This overstatement becomes particularly problematic in the conclusion, lines 186-187: \"in this work, we provide the first quantification of the mass and pressure of seismically generated CO2\". Several studies, some of them cited in this manuscript, have quantified coseismic CO2 emission in faults by means of modeling or geochemical analyses (e.g. Kim et al., Geology, 2021, Sulem and Famin, JGR, 2009, Famin et al., EPSL, 2008). I understand the need of authors to emphasize the impact of their discovery, but this goes too far.\n2. I am not convinced by the interpretation of FESEM pictures as microstructural evidence of decarbonation. What is displayed is evidence of carbonate damaging, but this can be done seismically, or without carbon loss.\n3. I am neither convinced by the isotopic evidence of decarbonation. In a previous publication (Curzi et al., Earth Sci Rev. 2024), the first author interpreted d18O and d13C fractionation in the same faults as evidence of meteoric fluid circulation in the fault zones. So, how can the same fractionation be now considered as unequivocal evidence of coseismic decarbonation.\n4. Finally, I am not convinced by the quantification of coseismic decarbonation, because 1/ uncertainties of XRD estimates of calcite loss are not provided; 2/ there is no evidence that the 3wt% carbonate loss is as \"conservative\" as asserted by the authors line 114. Indeed, this loss may be the result of multiple seismic events, or even of aseismic creep.\nDetailed comments justifying the four points above are available in the annotated manuscript attached to my review. For all these reasons, I cannot recommend publication of this manuscript at the present stage. I encourage authors to sharpen their arguments before resubmitting this work.", "reviewer_2": "Review of manuscript submitted for publication to Nature Communications\nManuscript title: Earthquake dynamics sustained by seismic CO2\nAuthors: Manuel Curzi, Andrea Billi, Luca Aldega, Ilaria Baneschi, Chiara Boschi, Antonio Caracausi, Andrea Cavallo, Giovanni Ruggieri, and Eugenio Carminati\nThis paper investigates the critical role of seismic decarbonation in earthquake dynamics, focusing on carbonate faults in the Apennines, Italy. It addresses the apparent paradox of fault slip occurring despite high frictional strength by linking shear heating-induced CO₂ production to dynamic weakening mechanisms. Using nanoscale observations, mineralogical, and isotopic analyses, the study quantifies CO₂ production during earthquakes, revealing up to 12 tons of CO₂ generated at quasi- to supra-lithostatic pressures. These pressures can amplify seismic slip, potentially triggering supershear rupture and increasing destructive potential. By providing the first quantification of seismic CO₂ mass and pressure, the research highlights the overlooked impact of fluid overpressure dynamics on earthquake physics and hazard assessment, while also suggesting carbonate faults as transient CO₂ reservoirs influencing subsurface carbon cycling. This work enhances understanding of fault mechanics and underscores the importance of integrating fluid dynamics into seismic models.\nIn the following I offer suggestions to the authors to improve their manuscript, first by highlighting a few areas where the significance of the manuscript could be enhanced and then a series of detailed comments on minor issues. Overall, this manuscript is creative and advances our knowledge of rupture in carbonate-hosted rocks.\n1. Micro- to Nano-Scale Evidence of Decarbonation\nThe section describes microstructural features such as truncated clasts, ultracataclasite layers, and comb fractures, which are interpreted as seismic slip indicators. The interpretation of these features as seismic slip indicators is appropriate but could be separated from the raw data presentation. For example, the description of the features (e.g., \"2 to 10 µm thick layers containing pores and vesicles\") could be presented first, followed by a separate discussion on their significance as seismic markers.\n2. Mineralogical Evidence of Decarbonation\nThe decrease in calcite content is presented alongside the interpretation that this reduction is due to seismic carbonate breakdown. While the interpretation appears valid, it could be separated from data presentation. For instance, the calcite content decrease (3–9%) could be presented first, followed by a discussion on how this aligns with seismic decarbonation processes.\n3. Isotopic Evidence of Decarbonation\nThe isotopic trends (δ¹³C and δ¹⁸O values) are presented alongside interpretations of decarbonation and post-decarbonation cementation. The data presentation (e.g., δ¹³C ranging from 2.3‰ to -9‰) could be separated from the interpretation of trends (e.g., leftward trend indicating decarbonation). This would make the section clearer and allow readers to distinguish between raw data and its implications.\n4. Seismic Decarbonation\nThe section presents evidence of decarbonated layers and calcite reduction, alongside interpretations about whether these layers formed during single or multiple seismic events. The data (e.g., \"2–10 µm thick layers containing pores and vesicles\") could be presented first, followed by a separate discussion on the possibility of single versus multiple seismic events. This would help avoid conflating observations with interpretations.\n5. Seismic CO₂ and Earthquakes\nThe section presents CO₂ mass estimates and pressure calculations, alongside interpretations about their impact on fault slip dynamics. The quantitative data (e.g., \"CO₂ production ranges from 6 to 12 tons\") could be presented separately from the interpretation (e.g., \"pressures are theoretically sufficient to uplift the fault hanging wall\"). This would make the section more structured and easier to follow.\nAlso, the authors may consider comparing their results with recent results on another carbonate-hosted seismogenic fault in the same area, the Venere Fault (Zamani et al., 2025b).\nDetailed comments:\nLine 18. \"… with meter-scale displacements” Conversely, large elastic deformation would not be able to accumulate in low frictional strength materials.\nLine 20. This sentence is cast as a general statement applicable to all faults yet decarbonation is specific to carbonate-hosted faults … please rephrase …\nLine 20. The term instantaneous is ambiguous because ultimately it does take some time to generate heat and break carbonates down. The term is relative to the scale of observation … rephrase.\nLine 38. Perhaps rephrase as “In carbonate-bearing rocks of sedimentary, metamorphic or crystalline origin, …”\nLine 51. This mass needs to be relative to a given slip surface, I.e., in tons per m3 to be useful …\nLine 56. This section title is an interpretation and perhaps should remain a factual statement of observations\nLine 64. It seems that the sentence was truncated after with.\nLine 70. Spell out this acronym the first time it is used …\nLine 70. Avoid using subjective statements or qualifiers …\nLine 79. This section title is an interpretation and perhaps should remain a factual statement of observations\nLine 80. Spell out this acronym the first time it is used …\nLine 87. This section title is an interpretation and perhaps should remain a factual statement of observations\nLine 108. This is acceptable as long as the slip distances in experiments and nature are similar.\nLine 212. Karst-like features in the hanging wall were also observed in samples from the Venere Fault; our further analysis shows that the vugs are filled with microorganisms.\nLine 369. Please update the reference as “Zamani, N., Murphy, M.A., Ferré, E.C. and Barou, F., 2025. Unraveling carbonate fault dynamics, from friction to decarbonation, through the 1959 Mw 7.2 earthquake in Montana. Scientific Reports, 15(1), p.8625.”\nAdditional reference:\nZamani, N., Satolli, S., Murphy, M., Demory, F., Pace, B., Gattacceca, J., Kaňuk, J., Nováková, M., Gottardi, R. and Ferré, E.C., 2025. Record of seismic slip in carbonates: Insights from the Venere Fault during the 1915 Avezzano earthquake (Mw 7.0), Central Italy. Journal of Structural Geology, 197, p.105421.\nNina Zamani", "reviewer_3": "Review of manuscript NCOMMS-25-27311\nTitle: \"Earthquake dynamics sustained by seismic CO₂\"\nAuthors: Curzi, Billi, Aldega, Baneschi, Boschi, Caracausi, Cavallo, Ruggieri and Carminati.\nThis manuscript addresses the question of seismogenic deformation in carbonate faults and more specifically how frictional heating causes decarbonation, generating large volume of CO₂. In turns, this pressurized CO₂ weakens faults dynamically, which may trigger supershear slip and thus amplify shaking. I find this article very well written, clear, concise, of major importance and I believe that it should be published after some minor issues are addressed by the co-authors.\nIn the following, I present the five areas where the manuscript could be improved.\n1. Extrapolation of surface data to hypocentral depths\nThe authors base their model on observations from faults exhumed from about 3 km depth, but seem to apply their results to earthquakes with hypocenters at ~7-10 km depth. Although the authors acknowledge this extrapolation (e.g., p.14), but the physical conditions (temperature, pressure, rock permeability) at depth will differ significantly and this could affect decarbonation. Further discussion or justification seems warranted.\n2. Assumption of single seismic event for decarbonation layers\nThe 2–10 µm thick decarbonated layers are assumed to originate from a single seismic event based on analogies to lab experiments. However, the paper itself notes possible multi-event or polyphase origins (e.g., p.3). Most of the time, as shown by recent studies on carbonate-hosted seismogenic faults (e.g., Tesei et al., 2013; Bhowmick et al., 2025; Zamani et al, 2025a, b) seismogenic faults experience more than one event and this simplification does affect the CO₂ production estimates.\n3. Simplified modeling of CO₂ pressure\nThe pressure model assumes that CO₂ is instantly confined to the volume in which it was formed, however, real-time fracture propagation and fluid escape during slip (e.g., p.5) most likely occur. The pressure is calculated assuming ideal and real gas equations but is simplified by neglecting heterogeneous rupture or time-dependent gas expansion potentially leading to an overestimate.\n4. Alternative weakening mechanisms\nThe paper focuses on CO₂-induced weakening while probably underestimating other mechanisms including thermal pressurization by water or flash heating. These mechanisms may be important or co-occurring in carbonate faults. Hence, the role of CO₂ might be slightly overstated.\n5. Isotopic interpretations\nThe δ¹³C and δ¹⁸O data used to infer decarbonation vs. cementation show non-systematic trends, including anomalies such as footwall samples with decarbonation-like signals (e.g., p.3). The dual trends (leftward and rightward) are ambiguous, and the interpretations seem to rely on indirect reasoning hence weakening the conclusions.\nReferences\nBhowmick, S., Levi, T., Boneh, Y., Marco, S., Yang, T., Meher, B. and Weinberger, R., 2025. Magnetic and microstructural perspectives on faulting in carbonate rocks, northern Israel. Journal of Structural Geology, p.105436.\nTesei, T., Collettini, C., Viti, C. and Barchi, M.R., 2013. Fault architecture and deformation mechanisms in exhumed analogues of seismogenic carbonate-bearing thrusts. Journal of Structural Geology, 55, pp.167-181.\nZamani, N., Murphy, M.A., Ferré, E.C. and Barou, F., 2025. Unraveling carbonate fault dynamics, from friction to decarbonation, through the 1959 Mw 7.2 earthquake in Montana. Scientific Reports, 15(1), p.8625.\nZamani, N., Satolli, S., Murphy, M., Demory, F., Pace, B., Gattacceca, J., Kaňuk, J., Nováková, M., Gottardi, R. and Ferré, E.C., 2025. Record of seismic slip in carbonates: Insights from the Venere Fault during the 1915 Avezzano earthquake (Mw 7.0), Central Italy. Journal of Structural Geology, 197, p.105421.\nEric C. Ferré (NMSU, 07/01/2025)"}, "author_response": "Dear Reviewers,\nPlease find enclosed the revised version of our manuscript, “Earthquake dynamics sustained by\nseismic CO₂,” by Manuel Curzi and co-authors.\nWe would like to express our sincere gratitude to you and the reviewers for the time and effort\ninvested in evaluating our work. We believe that the manuscript has significantly benefited from\nyour comments and those of the reviewers.\nThe main comments from the reviewers concerned:\n1. The need to more thoroughly support the micro- to nano-scale evidence of decarbonation;\n2. The organization of the manuscript, with a clearer separation of results from interpretations,\nmodeling, and discussion;\n3. The necessity to more clearly highlight the novelty of our work compared to the existing\nliterature;\n4. The quantification of uncertainties associated with the XRD analyses;\n5. Clarification of the interpretation of the isotopic data;\n6. The consideration of more conservative decarbonation scenarios than those originally\nassumed in our modeling;\n7. Stronger support for the extrapolation of our results to hypocentral depths.\nIn response to these concerns, we have made the following changes:\n1. We conducted a literature review comparing our micro- to nano-scale observations with\ndecarbonation structures reported from laboratory experiments simulating seismic slip and\nnatural faults. This led to the inclusion of two new figures (Figs. 3 and 4) and supporting text\n(lines 80-85, 114-117), which strengthen the interpretation of our FESEM observations.\n2. We reorganized the manuscript into distinct sections—Results, Interpretations, Modeling,\nand Discussion—to improve the logical flow and ensure a clear distinction between\nobservational data and their interpretations (see lines 61, 109, 156, and 220).\n3. We clarified the novelty of our work. Previous approaches were based on experiments or\ntheoretical models. We now explicitly state that our model integrates meso- to nano-\nstructural observations from natural carbonate faults, X-ray diffraction, stable isotope\nanalysis with stoichiometric and thermodynamic calculations. This is detailed at lines 47, 51,\nand 53–55.\n4. We quantified XRD uncertainties by Rietveld refinement. As described in the Methods\nsection (lines 510-512) and shown in Supplementary Figure 7, the calculated errors are\nnegligible: <0.6% for calcite, <0.2% for quartz, and <0.5% for phyllosilicate minerals. The low\nerror in calcite content does not affect our calculations for coseismic decarbonation.\n5. We removed the citation Curzi et al. (2024), which may have caused confusion, and replaced\nit with Baldermann et al. (2020) and Sharp et al. (2003), which more directly support our\ninterpretation of the type of isotopic fractionation discussed in the text. We also emphasized\nthat our samples represent residual fault rocks subjected to seismic frictional processes,\nrather than mineralized veins as analyzed in Curzi et al. (2024) (see lines 136-137).\n6. We performed additional calculations using lower decarbonation percentages (2.5%, 2%,\n1.5%, and 1%), and found that, even under these conservative scenarios, CO₂ pressures\nremain largely unaffected due to the confined-volume model assumption (lines 176–182).\nThis modeling approach is now explicitly supported by experimental results (Han et al., 2007;\nViolay et al., 2013; Kim et al., 2021), as stated in lines 193–197 and 225-233.\n7. We expanded the discussion on extrapolating our data to hypocentral depths, emphasizing\nthat higher temperatures and lower permeability at such depths may enhance both\ndecarbonation and CO₂ confinement, potentially resulting in CO₂ pressure exceeding our\nconservative estimates (lines 203–207).\nIn summary, the main numerical changes are as follow:\n- Main text length increased from c. 2400 to 2900 words (Methods section also expanded)\n- References increased from 49 to 56 (main text + Methods)\n- Figures increased from 4 to 7.\n- Supplementary figures increased from 6 to 7.\nWe have carefully addressed all reviewer comments and revised the manuscript and supplementary\nmaterials accordingly. Below, please find our point-by-point responses to the reviewers’ comments\n(in blue), with the original comments in black.\nAll major revisions are highlighted in yellow in the manuscript.\nWe thank you again for the opportunity to revise and resubmit this manuscript.\nBest regards,\nManuel Curzi (on behalf of all co-authors)"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Second review of the manuscript «Earthquake dynamics sustained by seismic CO2 »\nBy Curzi et al.\nThis first round of corrections has clearly improved the quality and the readability of the manuscript. However, I find that some comments raised in my first review have not been fully/properly addressed, and would need a second round of polishing. Also, the modelling part, as it is presented, raises some new questions. Here is why:\nFirstly, I appreciate the effort of the authors to cite previous works, including my own. But I still feel uncomfortable with overstatements in the narrative, for instance «To our knowledge, these [CO2] estimates, based on data from natural carbonate faults, represent the first of their kind in the literature, and no direct comparison can be done » (lines 171-172) or « We present, to our knowledge, the first quantification of the mass and pressure of CO2 generated during seismic slip using data from natural carbonate fault surfaces » (lines 221-222).\nI disagree with these assertions, because such quantifications do exist. For instance, I published mass and pressure estimates of CO2 released during seismic slip based on the observation of a natural fault (Famin et al., 2008, cited in this manuscript), and I am pretty sure that other case studies exist as well. However, my work focused on a granitic fault releasing CO2 by friction melting, whereas the present paper quantifies CO2 released by decarbonation in carbonate faults. There are thus similarities (quantification of coseismic CO2 release based on natural fault observation) and differences (granitic vs carbonate faults and melting vs decarbonation) between the two studies. This new quantification is nevertheless very valuable as it enables a comparison of CO2 emission in granitic and carbonate faults. So, why do the authors choose to omit previous quantifications instead of using them for comparison?\nFor the record, I estimated a granitic fault to release 1.8 – 3.4x10^3 tons of CO2 during a M7.2 earthquake, reaching pressures of 50 – 110 MPa (abstract and Fig. 6 in my paper). The CO2 mass released in the present manuscript is two orders of magnitude lower (62 – 90 t CO2 for 6.8 – 7.1 EQ, Fig. 7a), and yet generates much higher pressures (196 – 821 MPa). I provide an explanation to this difference in my second point. In any case, I would recommend the authors to elaborate on these differences rather than claiming to be the first.\nSecondly, I am also sceptical about the hypotheses used for CO2 mass and pressure quantification. In particular, I think that the assumption of CO2 remaining confined within the decarbonated volume is unrealistic. There are two reasons for that: 1) because textures in Figs 2b-c, f, h, and 3a show an interconnected pore network, suggesting some permeability between the decarbonated pockets during seismic slip. 2) Because the Clapeyron slope of the decarbonation reaction is positive, meaning that as soon as decarbonation starts, the increased pressure requires a higher temperature for decarbonation to continue. In other words, the reaction is self-limiting and should stop before reaching pressures of hundreds of MPa. Fixing temperature to a given value (600°C or 450°C in this study) does not allow pressure and temperature to increase jointly along the Clapeyron slope, yielding unrealistically high pressures values.\nAgain, I proposed a more realistic approach in my paper, by considering a non-zero coseismic permeability, thus allowing CO2 to escape the principal fault zone. Coseismic CO2 pressure is therein computed as a function of rock permeability (pressure-dependent), rock compressibility, and CO2 compressibility. This permeability limits the pore pressure rise, and allows the reaction to proceed. This is the reason why the authors obtain huge (and overestimated) CO2 pressures despite the low masses and volumes of CO2 released by coseismic decarbonation, compared to friction melting in a granitic fault. I strongly encourage the authors to consider a non-zero principal fault zone permeability, even during the “decarbonation seismic instant” of their Figure 7c. My paper used a very simple permeability-pressure dependency, while Sulem and Famin (2009, also cited in the ms) used a more sophisticated mathematical treatment, specifically designed for the decarbonation of carbonate faults. Other coseismic permeability estimates from natural examples can be found elsewhere (e.g. Wibberley and Shimamoto, 2005). The authors could easily adapt any of these developments to their own equations of state.\nI would like to conclude on a positive note. This manuscript has potential for becoming a major contribution to the quantification of coseismic CO2 release in carbonate faults. For that, it needs a thorough comparison with other natural case studies, and more realistic assumptions concerning CO2 quantification. I suspect that taking CO2 loss into account will drastically reduce the coseismic pressures attained in their carbonate faults, which might turn down the conclusions of the paper. Therefore, my recommendation goes from moderate revision (if the conclusions are not altered) to major revision (if they are).\nVincent Famin\nWibberley CA, Shimamoto T. Earthquake slip weakening and asperities explained by thermal pressurization. Nature. 2005 Aug 4;436(7051):689-92. doi: 10.1038/nature03901. PMID: 16079843.\nSpecific comments :\nFigure 5 missing.\nLines 143-146: “This [the absence of d13C variation] suggests that the 2-10 μm thick decarbonated layers along the PSSs likely formed during a single seismic event, while the isotopic signature within the 6 mm from the PSS records the cumulative effect of multiple decarbonation (with associated fractionation) and post-decarbonation cementation episodes now interleaved in the rock record. ». I disagree with this statement. The absence of systematic d13C variation with distance from the fault indeed suggests that the isotopic signature reflects cumulative episodes of decarbonation/cementation. The inference that the decarbonated layers formed during a single event comes from a separate argument of lab/nature comparison detailed above (lines 118-122). Please make two separate sentence to make this clearer.\nLine 125 : again, Figure 5 is called but there is not Figure 5.\nLines 203-204 : « Extrapolating our data derived from exhumed fault rocks to hypocentral depths, where higher temperatures and lower permeability prevail, suggest enhanced decarbonation and CO2 confinement. ». If my understanding is correct, the permeability is considered to be 0, since the CO2 has no time to escape the decarbonated volume. If so, how lower permeability would enhance CO2 overpressure?\nLines 224-225 : « The minimum pressure predicted by the ideal gas equation of state (190 MPa; Fig. 7b) aligns with both previous numerical and experimental findings. ». What about natural case studies ? Contrary to the above « to our knowledge… » statements, these studies do exist.\nLine 226 : « …large CO2 volumes (900 t/m^2)… ». the volume is expressed in a mass unit. Either change the unit or change to « large CO2 masses »", "reviewer_2": "The authors have addressed most of the reviewers comments adequately, although the microstructural evidence presented to directly support the decarbonation interpretation remains ambiguous. Considering the complexity and lack of reproducibility in natural systems, this is most likely the best explanation we can have. Therefore, I recommend publication of this manuscript.", "reviewer_3": "The co-authors have genuinely taken care of the reviewer comments to the best of their ability and have substantially improved the manuscript in multiple areas."}, "author_response": "Thank you for your constructive and stimulating comments.\nWe have carefully considered the publications you suggested and revised the manuscript accordingly. In particular:\n- We removed the following sentences from the previous version of the manuscript:\n“To our knowledge, these CO₂ estimates, based on data from natural carbonate faults, represent the first of their kind in the literature, and no direct comparison can be done” and\n“We present, to our knowledge, the first quantification of the mass and pressure of CO₂ generated during seismic slip using data from natural carbonate fault surfaces”. We agree that these statements were too categorical and have now been omitted to reflect a more realistic perspective.\n- We expanded the text by providing a direct comparison (in the text and in Fig. 7) between our results and the seismic CO₂ production estimates reported in Famin et al. (2008), Han et al. (2007), Violay et al. (2013), and Spagnuolo et al. (2015), as suggested. This comparison, which now contextualizes our results within previously published data, has been integrated into the revised manuscript at lines 248-262:\n“To contextualize the CO₂ masses derived from our model (Fig. 7), we compare and discuss them with independent estimates from both experimental and natural systems.\nA first benchmark comes from seismic CO₂ production associated with pseudotachylyte formation in carbonate-bearing granites and scaled to the Mw 7.2 Kobe earthquake21. Those estimates exceed ours by nearly two orders of magnitude (Fig. 7). The discrepancy is explained by the very different extents of decarbonation involved: pseudotachylyte formation requires almost complete carbonate breakdown (≈99 wt% of the wall-rock CO₂ content21), whereas our calculations use a conservative 3 wt%. As noted above, this 3 wt% likely represents an underestimated value.\nA second comparison comes from laboratory earthquakes on carbonate faults, where CO₂ concentrations ranging from ~400 to ~29,000 ppm have been measured in experiments involving small samples7,20,49. However, these values cannot be directly compared with our mass estimates, because the measured CO₂ reflects volatile accumulation within small experimental volumes rather than the integrated CO₂ production normalized to the rupture area of natural faults. As such, laboratory CO₂ captures local, transient decarbonation at the scale of individual slip patches, whereas our estimates describe the integrated mass balance associated with seismic slip along natural faults.”.\nWe thank the Reviewer for this constructive and insightful comment. We fully agree with the Reviewer that assuming complete confinement of the produced CO₂ is unrealistic, and we have substantially revised the manuscript to address this point in depth.\nBelow we summarize the key areas in which the manuscript has been substantially improved in response to this comment.\n1. Implementation of a non-zero seismic permeability (drained condition)\nFollowing the reviewer’s suggestion, we incorporated the analytical framework developed by Famin et al. (2008) to compute CO₂ pressure under drained conditions, explicitly accounting for pressure-dependent permeability, rock compressibility, pore compressibility, and progressive CO₂ outflow. The revised Methods section (lines 851-924) now describes this implementation in detail.\nThe resulting pressure–permeability relationships are presented in Figure 8b and Supplementary Figure 8, and the model parameters are reported in Supplementary Table 6.\n2. CO₂ pressures under drained vs undrained conditions\nThe new simulations show that, under drained conditions, seismic CO₂ pressures reach supra-hydrostatic values (76–134 MPa) at hypocentral depths of 7.5–9.2 km. These results confirm that, even when CO₂ drainage is allowed, the fluid remains significantly pressurized during slip, in agreement with both earlier models and experimental observations on carbonate faults.\nIn parallel, we retain the undrained scenario as a theoretical end-member representing the earliest, low-permeability instants of slip. We now explicitly state in the Discussion (lines 285-288) that the theoretical pressures predicted by real-gas EOS (up to 634–821 MPa) are unrealistically high. We therefore adopt the ideal-gas estimate (196 MPa) as the only physically plausible value for the undrained peak.\n3. Transitions between undrained and drained states\nBuilding on the reviewer’s suggestion and on geological and experimental evidence (i.e., rheology of carbonate rocks, CO pressurization, and decarbonation-related nano-structures along faults), we now propose that the fault may evolve from undrained to drained conditions.\nIn the revised Discussion (lines 295-307), we link this transition to:\n- Transient confinement and CO pressure peak (undrained conditions, quasi-lithostatic, ~196 MPa),\n- Permeability increase driven by plastic deformation of carbonate at high temperatures (shear heating),\n- Development of decarbonation-related bulbous and rounded pores and vesicles that can promote CO dissipation from the fault (drained conditions, supra-hydrostatic, maximum 76–134 MPa).\nThis revised interpretation provides a physically and geologically realistic mechanism for pressurization during earthquakes, consistent with fault mechanics, theoretical models and our FESEM observations. In the new version, the Discussion explicitly integrates our FESEM observations (showing rounded, bulbous, viscous-flow pores and vesicles), which appear only after carbonate plasticization and therefore represent CO₂ escape pathways. This supports the idea that full confinement cannot be sustained for the whole duration of slip and validates the drained modelling approach we now adopt.\nIn summary, the new modelling demonstrates that supra-hydrostatic pressures persist even under drained conditions, undrained peaks remain physically plausible only at the onset of slip, the combined effect produces a pressurized system throughout most of the earthquake, and CO₂-induced weakening remains significant in both conditions.\nWe therefore conclude that incorporating non-zero permeability, as suggested by the reviewer, strengthens the physical plausibility of our model and improves the robustness of the manuscript’s conclusions.\nAll corresponding changes are highlighted in yellow in the updated manuscript and occur in the Abstract (lines 26-30), Introduction (lines 55-57), Interpretation (lines 118-123), Modelling (lines 164-181, 199-205, 221-245), Discussion (lines 248-262, 267-278, 285-316), Methods (lines 851-924), and Figures (updated Figures 7, 8, 9, and Supplementary Figure 8).\nWe thank the reviewer again for this highly constructive comment, which significantly improved the robustness of our model."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors’ responses adequately address all issues raised in my review, and the manuscript has been improved accordingly. I have no further suggestions, apart from minor typographical or editorial comments in the attached PDF, which can be addressed during proof editing. I congratulate the authors on their thorough revision."}, "author_response": ""}]

Decision letter