Near-surface liquid water on Mars inferred from seasonal marsquakes

Domain: earth_science
Published: 2025-12-26
Editorial outcome: minor_revision

Abstract
Exploring the presence and behavior of water on Mars is critical for understanding the planet’s geological evolution, hydrological processes, and potential habitability, which has been a central objective of past and ongoing Martian exploration missions. Martian geomorphology provides evidence of ancient groundwater activity, but confirmation of present-day liquid water remains limited. Here, we infer near-surface brines confined to meter-scale depths in regions north of about 30° N in the northern hemisphere, based on the analysis of seasonal variations in marsquake seismicity and thermal modeling. The absence of seasonal marsquakes during colder periods and their abrupt resurgence in warmer seasons can be explained by ice-to-brine phase transitions. Our findings reveal a mechanism whereby seasonal melting of subsurface ice elevates pore pressure and lubricates faults, leading to a reduction in frictional strength, and ultimately inducing marsquakes. This mechanism accounts for the seasonal variation, clustering, high seismic b-values, and shallow focal depths of seasonal marsquakes. Additionally, we estimate that the melting point of briny ice on Mars is below approximately 250±13 K, advancing our understanding of Mars’ present-day brine cycle and near-surface hydrological processes.

Introduction
The presence of subsurface (meter-scale) liquid brines on present-day Mars is crucial for assessing the planet’s habitability, evolution, and the possibility of the existence of living or ever-lived organisms1,2. Water ice is widespread on Mars from the polar regions3,4, down to the subsurface of mid-latitudes5,6. The widespread ground ice distribution is supported by water ice exposed at recent craters and retreating scarps7,8, glacial landforms with viscous flow features9,10, dielectric properties consistent with water ice from radar images11,12, as well as the hydrogen-rich deposits poleward of latitudes ±60° identified from the Mars Odyssey Neutron Spectrometer13,14. Although current Mars surface conditions are not favorable for liquid water to persist, concentrated saline solutions, or liquid brines, are predicted to be stable under Martian conditions in theory15,16.

Recent discoveries of seasonal varying features on Mars (e.g., recurring slope lineae)17,18 suggest possibly indirect evidence of liquid brines that transiently flow on the Martian surface19, while their morphology does not allow the exclusion of the alternative possibility of granular flows20. Other geomorphological evidence, including gullies, slope streaks, and flow-like features in the polar regions, that might support the existence of liquid brines in the subsurface, also has alternative dry interpretations21. Radar data reveal strong reflectors at the base of Mars’ south polar cap that suggest the potential presence of a subglacial lake beneath the south pole22, but the typical local geothermal heat flux does not allow for the basal melting of polar ice caps23,24. Thermodynamic calculations suggest that groundwater may exist in Mars’ subsurface today, while it likely resides at kilometer-scale depths beyond the detection limits of orbital radar25. Thus, merely based on remote sensing observations, the existence and potential distribution of liquid brines in the subsurface of Mars remains ambiguous.

The physical properties of subsurface materials can provide inferences about the presence of liquid brines. Electrically, the presence of liquids increases the dielectric permittivity of the medium, detectable by orbital or in-situ radar on Mars22,26. Seismically, subsurface liquids alter the elastic and anelastic properties of the underground mass, which can be analyzed through detecting changes in seismic wave speeds and attenuation27. Analysis of crustal velocities derived from marsquake data recorded by the SEIS (Seismic Experiment for Interior Structure) seismometer aboard the InSight (Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport) lander28 reveals that the Martian upper crust, approximately 10 km thick below the landing site at Elysium Planitia (Fig. 1a), lacks a thick (i.e., kilometer-scale) cryosphere29. Beneath this region, the mid-crust (~10–20 km deep) is hypothesized to potentially host water or other volatiles based on the interpretation of seismic velocities with lithology models30. Conversely, coda wave analysis of marsquakes suggests that the crust traversed by the seismic waves may contain minimal amounts of volatiles and is predominantly dry31,32.

Variations in seismic activity can serve as a valuable proxy for detecting liquids in regions beyond the landing site of ground-penetrating radar or seismometer. On Earth, natural processes such as groundwater recharge from snowmelt during warm seasons often lead to increased seismicity in volcanic33,34 and glacial35 regions. In addition, artificial fluid injection into the subsurface, such as during industrial activities, also triggers seismic events, a phenomenon known as “induced seismicity”36. That is, an increase in seismicity may indicate groundwater recharge.

Seasonal variations of marsquake seismicity have been observed on Mars. In the marsquake catalogue compiled by the InSight Marsquake Service (MQS)37,38, tectonic marsquakes, classified as low-frequency and broadband events, are relatively well understood in terms of their both mechanism and location37,38, with their occurrence showing no significant temporal variation. In contrast, another group of marsquakes, often referred to as the 2.4-Hz and high-frequency events39,40, exhibits clear seasonal variations in occurrence rate (hereafter termed seasonal marsquakes). However, the driving mechanism behind seasonal marsquakes remains unclear37.

The seismicity of seasonal marsquakes predominantly occurs between solar longitudes Ls=33 and Ls=18740. Seasonal marsquakes were initially identified through seismicity analyses conducted during the first Martian year of the InSight mission (corresponding to MY35: Martian Year 35)40, and were subsequently corroborated using an expanded dataset from the second Martian year (corresponding to MY36: Martian Year 36)41,42. Previous studies have identified seasonal marsquakes as a response to variations in illumination conditions, rather than tidal forces and the carbon dioxide cycle40. Nevertheless, the fundamental question of how illumination modulates this seismic activity remains unknown.

In this study, we investigated the mechanism underlying the seismicity of seasonal marsquakes. Our analysis focused on the abrupt onset of seasonal marsquakes around early spring (Ls=25) in the northern hemisphere and the decline in their occurrence rate near the summer-fall transition (Ls=200). We also examined the spatial clustering characteristics and the Gutenberg-Richter seismic b-value43 of seasonal marsquakes. Based on these observations, we proposed that the seismic activity of seasonal marsquakes indicates the presence of briny ice and its phase transition to brines, initiating in the meter-scale subsurface and extending to greater depths on present-day Mars.

Results
Rapid change in occurrence rates of seasonal marsquakes
We focused on the seismic activity of seasonal marsquakes occurring during the early nighttime hours, specifically from local mean solar time (LMST) 17:00 to 24:00, the quietest period of a day at the InSight landing site on Mars (Fig. 2a). During the daytime, the strong wind noise dominates the recordings of the InSight SEIS seismometer, making the detection of seasonal marsquakes challenging (Fig. 2a).

We hypothesized that the seasonal marsquakes observed at night represent continuous processes initiated during the daytime, when illumination and temperature changes likely exert stronger control on seismic activity. In contrast, nighttime temperatures on Mars remain consistently and extremely low throughout the year, regardless of the season44, reducing the potential influence of illumination- or temperature-driven variations on seismic activity during this period.

The occurrence rate of seasonal marsquakes can be used to evaluate the potential presence of subsurface liquid brines. If seasonal temperature fluctuations were the sole driving factor, the seismicity rate would likely follow a gradual trend, reflecting the continuous nature of temperature changes. In contrast, the presence of subsurface liquid brines involves phase transitions between ice and brines, which would result in abrupt changes in seismic activities33–35,45.

Seasonal marsquakes are virtually absent in the northern early spring (Ls=0 to Ls=25), according to the official marsquake catalogue40,46. This absence cannot be explained by unfavorable noise conditions. Noise levels during this period (gray transparent boxes I and II in Fig. 2a) are comparable to, or even lower than, those observed during the spring-summer transition, when the seismicity of seasonal marsquakes reaches its peak (Supplementary Fig. 1). This contrast highlights that the paucity of seasonal marsquakes in early spring is a distinct phenomenon, emerging under optimal detection conditions.

To evaluate potential catalogue incompleteness, we applied template-matching analysis across a range of detection thresholds (Supplementary Text S1). The results confirm that no substantial number of detectable seasonal marsquakes was missed.

The observation is consistent across both the full dataset (Fig. 2) and the high-quality dataset, which includes only events above the completeness magnitude threshold (Supplementary Figs. 2 and 3). In contrast, tectonic marsquakes are more evenly distributed across spring and summer (Supplementary Fig. 4) and occur at a low and steady rate of approximately 0.1 events per Martian day (Supplementary Fig. 5). Additionally, we ruled out the possibility that tidal forces, one of the drivers of seismic activity on the Moon42,47, play any significant role in modulating seasonal marsquakes on Mars, as no clear time correlation between them has been observed and insufficient tide-induced stress to trigger seasonal marsquakes (Supplementary Text S2). We also excluded carbon dioxide sublimation as the primary mechanism triggering seasonal marsquakes because its annual pressure variations (below 250 Pa)48 are orders of magnitudes smaller than the thousands of pascals typically required to induce seismic activity49,50.

Surface warming fuels seasonal marsquakes
The abrupt changes in the seismicity of seasonal marsquakes (Fig. 2) suggest that the events are likely triggered with a threshold limit. Although seasonal marsquakes in the northern hemisphere coincide with warmer periods, their abrupt onset cannot be explained by temperature or illumination alone. If either factor were the sole driver force, the occurrence rate would vary gradually, mirroring the smooth seasonal evolution of these parameters. While diurnal temperature variations can trigger thermal marsquakes (i.e., super-high-frequency events)51, these are small in magnitude, last only a few tens of seconds, and are confined to the vicinity of the InSight lander41,51. In contrast, seasonal marsquakes are larger in magnitude up to Mw ~3.037, last for hundreds of seconds (Fig. 3b), and occur at epicentral distances beyond 20° (~1200 km)39,41,52.

Furthermore, seasonal marsquakes can be detected at night, indicating that illumination alone is unlikely to sustain this seismic pattern. Illumination-driven effects, such as thermal expansion and contraction, are transient by nature and dissipate quickly after sunset, corresponding to the occurrence of thermal marsquakes41,44,51. In contrast, if groundwater processes are the driver, the movement of liquid brines would facilitate gradual subsurface pressure redistribution and sustained fault instability, extending seismic activity beyond the window of direct solar heating.

To explain the rapid and distinct variations in seasonal marsquakes, we proposed a model where the melting of ice triggers these events (Fig. 3). During cold seasons (Fig. 3a), pore spaces are ice-cemented, resulting in relatively stronger faults that are less prone to slip. In warm seasons (Fig. 3b), however, the melting of ice into brine elevates pore pressures and lubricates faults53,54, a process that reduces the effective normal stress and friction coefficient, thereby promoting slip and inducing marsquakes. In this model, surface warming drives seismic activity by promoting the phase transitions from ice to brine. Consequently, higher temperatures lead to the increased melt production, thereby promoting seismicity. This mechanism explains why the peak occurrence of seasonal marsquakes aligns with the warm season in the northern hemisphere.

By analogy, seasonal marsquakes would also be expected in the southern hemisphere warm season (i.e., northern fall and winter) if substantial subsurface ice exists in the southern hemisphere. However, the seismic catalogue reveals virtually no events during this period40,46, owing to high wind-generated noise (Fig. 2a), which severely impedes detection even with template-matching approaches. Crucially, this observational gap does not weaken our model, as the conclusive evidence comes from the low-noise windows of the northern early spring (gray transparent boxes I and II in Fig. 2a). The near-absence of marsquakes in early spring, followed by a sharp surge at the spring-summer transition, robustly supports a phenomenon concentrated in the northern hemisphere and driven by temperature-dependent ice melting (Fig. 3). The application of novel detection techniques to the existing dataset, as well as future missions targeting the southern hemisphere or operating under quieter conditions, will provide critical opportunities to test and refine our model.

Marsquake clustering linked to localized brines activity
The spatial clustering of seasonal marsquakes reinforces the hypothesis of subsurface brines on Mars. If temperature or illumination were the sole drivers of seasonal marsquakes, their effects would likely influence a broad spatial region, resulting in a diffuse distribution of events. However, the observed clustering of seasonal marsquakes is inconsistent with a solely temperature- or illumination-driven mechanism.

Two key observations underscore the spatial clustering of seasonal marsquakes. First, these events are clustered in terms of epicentral distance. For most seasonal marsquakes, the arrival time differences between the direct P and S waves are consistently around 249±24 s (Supplementary Fig. 6 and refs. 39,41,55). Second, the scattering and attenuation properties of the medium along the propagation paths of seasonal marsquakes also exhibit clustering. The P- and S-wave rise times, which reflect the medium scattering properties56,57, are concentrated around 80±16 s and 106±18 s, respectively (Supplementary Fig. 7a, b). Additionally, the consistency in quality factors of coda waves (3300±980) further confirms the clustering (Supplementary Fig. 7c).

For outliers in the clusters and a few events occurring within the event gap shown in Fig. 2a, they may arise from the classification methodology employed in the official marsquake catalogue37,38. Notably, all marsquakes analyzed in this study are categorized as either 2.4-Hz events or high-frequency events in the catalogue. While these two categories generally exhibit seasonal variation and clustering, they should not be strictly equated with seasonal marsquakes. The current classification method relies on seismogram spectra rather than underlying mechanisms, which may result in events from different processes (e.g., meteorite impact) being grouped together. Nonetheless, these outliers do not affect our hypothesis of brine-related processes, as most events show clear seasonal variation, clustering, and high occurrence rates.

High seismic b-value for seasonal marsquakes
A high seismic b-value (2.81) has been observed for seasonal marsquakes58. However, the underlying cause of this observation is unexplored. Here, we suggested that the elevated b-value may serve as another independent indicator of subsurface liquid brines.

The seismic b-value, as defined in the Gutenberg-Richter law43, characterizes the magnitude-frequency relationship of seismic events, with higher b-values indicating a greater proportion of smaller magnitude events. Terrestrial observations and laboratory experiments have shown that the seismic b-value is inversely related to the differential stress59–61. The presence of fluids can increase pore pressures, thereby reducing effective normal stress. This reduction in effective normal stress facilitates fault slip at lower differential stresses, resulting in higher b-values62.

For tectonic marsquakes, which exhibit no seasonal variation, the seismic b-value is approximately 1.31±0.18 (see “Methods,” and also refs. 58,63), similar to that of tectonic earthquakes on Earth60. In contrast, the b-value for seasonal marsquakes is significantly higher, even considering uncertainties (2.42±0.10 in this study; see “Methods”; 2.81 in ref. 58). This notably elevated seismic b-value for seasonal marsquakes provides additional evidence for the presence of liquids on Mars, consistent with observations from terrestrial analogs such as magma chambers in volcanic regions64, fluid-injection wells62, and snowmelt zones65.

Shallow brine reservoir and melting dynamics
The temperature dependence of seasonal marsquakes constrains the depth range of subsurface liquid brines to the conductive zone influenced by temperature. To estimate this depth, we modeled the subsurface annual temperature profile using the heat conduction equation. The input surface temperature was derived from the Mars Climate Database66, where we extracted the daily maximum surface temperature curve (Supplementary Fig. 8a) over a Martian year at the InSight landing site. Thermal diffusivity of the subsurface material may carry significant uncertainties; thus, we adopted two distinct values of thermal diffusivity: 5.1×10−8 m2 s−1 for the Martian soil measured at the InSight44,67, and 7×10−7 m2 s−1 for buried rocks44,68.

Our calculation indicates that the thermally affected depth ranges from approximately 1.3 to 3.5 m (Supplementary Fig. 8b, c), consistent with previous estimates44. While this result is specific to the InSight landing site, similar depths would be expected in regions with comparable thermal properties. The meter-scale thickness of this conductive zone suggests that seasonal temperature variations are limited to shallow depths (~2.0 m). If present, any ice within this narrow layer could therefore sustain liquid brines during periods when temperatures rise above the eutectic point.

Notably, although the bottom of the conductive layer (Fig. 3a) lies at about 2.0 m depth, the total amount of melted liquid is not confined to this shallow zone. This depth marks the boundary for heat conduction (Fig. 4a), yet salinity-driven convection (Fig. 4b) can transport heat and salts downward, enabling the melting of deeper ice. Here, convection denotes the compositional process arising from brine-ice interactions, initiated by salinity gradients, rather than bulk fluid circulation. More specifically, during warm seasons, surface heating raises the temperature within the shallow conductive layer above the eutectic point of water and common salts (e.g., perchlorates, chlorates, and sulfates)69, producing a partially molten mixture of ice and brine (Fig. 3b). The brines, which are denser than ice, gas, and low-salinity water, migrate downward under gravity through pore spaces. Driven by salinity gradients, this meltwater sinks, pushing the brine front deeper than would be allowed by heat conduction alone. As melting proceeds, the latent heat consumed gradually cools the brine, which keeps thermally driven melting relatively shallow. In turn, salinity-driven convection becomes the dominant force driving ice melting at greater depths.

To test whether salinity-driven convection can melt the underlying ice under isothermal conditions with brine and ice at the same temperature, we performed controlled laboratory experiments (see Methods). The results consistently show that once formed, the brine readily melts the ice underneath (Supplementary Fig. 9 and Fig. 4c and Supplementary Movies 1–5), as the melting point of ice is depressed by dissolved salts. While variables like temperature, acrylic content (analogous to Martian rock fragments), and salt concentration affect the percolation rate and depth, melting still occurs. Besides, the salinity-driven convection is also confirmed by recent experimental, theoretical, and numerical studies on permafrost thawing70.

Having established the viability of ice melting, a key requirement for our conceptual model of Mars (Fig. 3) is to explain how brines can penetrate to depths of hundreds or thousands of meters to reach the bedrock with faults. The experiment (Supplementary Movie 1) reveals a mechanism capable of enabling this rapid and deep percolation: channelization. We observed that the development of channels during percolation (Fig. 4c–e) enhances downward flow rates by up to orders of magnitude. On Earth, injection-induced earthquakes demonstrate that fluid conduits facilitate rapid transport, greatly reducing the time for fluids to reach seismogenic faults and induce slip71. On Mars, comparable pathways such as fractures and other heterogeneities may serve as natural conduits. Therefore, the channelization observed in our experiments offers a compelling mechanism for how brines exploit these subsurface features to achieve the necessary penetration depths.

Our experiments confirm that brine percolation is viable under isothermal conditions. Although conducted at the centimeter scale, the governing physical factors, such as permeability, brine supply, and fracture connectivity, are scale-independent, implying that the qualitative behavior may extend to Martian environments. Quantitative extrapolation to relevant field scales, however, must account for coupled nonlinear dynamics across meter to hectometer depths. These dynamics include interactions among temperature evolution, freeze-thaw cycles, heterogeneous ice saturation, and evolving permeability. Critically, certain Martian conditions, such as seasonal thermal gradients, freeze-front propagation rates, capillary forces, and reduced gravity, cannot be fully reproduced in laboratory experiments. While these factors limit precise quantitative projection, they do not diminish the fundamental physical plausibility of brine infiltration on Mars.

The infiltration depth, probably extending to hundreds of meters to a few kilometers, aligns with the shallow crustal origin inferred for seasonal marsquakes39. For example, waveform comparisons show that seasonal marsquakes resemble impact events (focal depth: 0 km) more closely than tectonic marsquakes (focal depth >10 km) (e.g., Supplementary Fig. 10). This similarity supports a shallow origin of seasonal marsquakes. Additionally, the depth range matches the slip or rupture lengths estimated from the seismic magnitudes (Mw 1.2–3.0) of seasonal marsquakes37,38. The rupture sizes range from 47 to 373 m in ice, 31 to 249 m in frozen soil, and 22 to 173 m in crustal rock (see Methods).

The shallow heat conduction boundary rules out the possibility that seasonal marsquakes are triggered by the freezing of aquifers and the associated volumetric expansion during colder seasons (in the southern hemisphere), a mechanism proposed prior to the landing of the InSight mission72. This is because, unlike the warming mechanism, where both conduction and convection processes are involved, the cooling mechanism relies solely on heat conduction, which affects only shallow depths on the meter scale. As a result, an approximately 10% volumetric expansion within the top few meters, associated with the brine-to-ice phase transition, is likely insufficient to generate seasonal marsquakes with seismic magnitudes ranging from Mw 1.2 to 3.037,38. However, while we excluded freezing and volume expansion as mechanisms for the seasonal marsquakes recorded by the InSight, we did not rule out the possibility of such events occurring in regions beyond the sensitivity of the InSight seismometer. Future seismological missions can help validate this freezing and volume expansion mechanism.

Where to find near-surface brines
The location of the brine reservoir responsible for seasonal marsquakes must meet two key requirements. First, daily midday temperatures at this location need to rise continuously after Ls=25, corresponding to the onset of seasonal marsquakes in the northern early spring (Fig. 2a). Second, these elevated temperatures need to persist long enough (e.g., from Ls=25 to Ls=200) to sustain conditions favorable for seasonal marsquakes (Fig. 2b).

Regions north of about 30 °N satisfy both requirements and emerge as the most plausible source for near-surface brines (Fig. 1b and Supplementary Fig. 11c). In contrast, low-latitude regions either exhibit a temperature decrease after Ls=25 (Supplementary Fig. 11a) or show a significantly delayed temperature peak (Supplementary Fig. 11b). The inferred brine source locations are consistent with previous studies, including the water-ice consistency map73, which indicates subsurface ice at depths of 1–5 m across the mid-latitudes (30 °N–60 °N) (Fig. 1a). At the same time, observations of bright material excavated from impact craters north of 30°N8,74 (Fig. 1a) have been interpreted as exposures of shallow ice. Moreover, in-situ measurements by Zhurong identified hydrated minerals and dust, suggesting recent groundwater rising or melting of subsurface ice at its landing site, which is located near the mid-latitudes of Utopia Planitia75 (Fig. 1a).

The brine reservoirs, however, are likely spatially localized rather than widespread for two main reasons. First, the narrow range of arrival time differences between P and S waves for seasonal marsquakes indicates similar epicentral distances39,41,55. Second, the large quality factor (~2500–6900) measured from the coda waves of seasonal marsquakes (Supplementary Fig. 12) suggest that the Martian crust along the propagation paths from the source to the InSight is generally very dry31. This measurement rules out the widespread underground water but consistent with localized reservoirs in the source region.

Taken together, these observations support the hypothesis that the brine source for seasonal marsquakes is localized and situated in the north of about 30° N. Regions with subsurface brine may have the potential to preserve organic materials and microbial life, offering critical insights into Mars habitability. Both the American Mars Sample Return mission and China’s Tianwen-3 mission are equipped with drilling systems capable of reaching depths of approximately 2–3 m76,77, which align with the range of near-surface brines inferred from this study. Consequently, these regions represent highly promising candidates for landing sites in the context of the Mars Sample Return mission.

Melting point of subsurface ice
Seasonal marsquakes emerge after Ls=25 (Fig. 2), at which point the surface temperature, according to the proposed model (Fig. 3), provides an upper bound on the melting point of subsurface ice. The midday (LMST 10:00–14:00) surface temperature at the onset (Ls=25) of seasonal marsquakes within the potential brine source regions (Fig. 1b) is approximately 250±13 K, as derived from the Mars Planetary Climate Model (see “Methods”). A melting point of at least 23 K lower than that of pure ice (273 K) strongly suggests the presence of brines. While temperature alone cannot fully determine brine composition, a melting point below 250±13 K provides a critical and independent constraint. Such constraints may be combined with other approaches, such as the seismo-electric method of ref. 78, to further characterize aquifers within the Martian crust.

Water volume required to induce seasonal marsquakes
Laboratory and field studies on Earth demonstrate that relatively small pore pressure perturbations (~0.01–0.1 MPa) can trigger earthquakes when faults are near failure49,79. Thus, seasonal marsquakes are likely to exhibit comparable sensitivity under near-failure conditions. Such pore pressure perturbations can be achieved during fluid injection49,79, for example, in wastewater disposal operations, where fluid is injected at moderate pressures, distinct from high-pressure hydraulic fracturing, into deep geological formations. Critically, the process of injecting wastewater into deep aquifers on Earth is analogous to our model where meltwater percolates to deeper faults on Mars. Well-documented research of wastewater disposal on Earth has quantified a systematic positive correlation between the volume of injected fluid and the maximum induced seismic magnitude80. Thus, by projecting the observed magnitudes of seasonal marsquakes onto this Earth-derived relationship (Supplementary Fig. 13), we estimated that a fluid volume of ~0.1 m3 per unit area (equivalent to 0.1 m water depth) would be sufficient to trigger seasonal marsquakes with maximum moment magnitudes (Mw) of ~3.0.

The estimate of the required water volume assumes that fault slip is primarily driven by elevated pore pressure. Meanwhile, our proposed model incorporates another complementary process: the same brines responsible for melting may also reduce the effective friction coefficient along fault planes. This dual mechanism (pore-pressure enhancement and friction reduction) would act synergistically to lower the threshold for fault activation. Consequently, if both processes operate in tandem, the meltwater volume required to trigger seasonal marsquakes could be even smaller.

To assess whether the estimated water volume could be generated under Martian conditions with latent heat included, we performed thermal conduction modeling. Our synthetic calculations show that, given a melting point of 240 K, seasonal surface temperature variations can bring the melting isotherm to a depth of ~2 m within the conductive layer under a porosity of 0.4 (Supplementary Fig. 14). This penetration depth yields a meltwater volume per unit area of ~0.7 m3 m-2 (Supplementary Text S3) that exceeds the threshold of 0.1 m3 m-2 required to induce marsquakes. Below this conductive layer, salinity-drive melting in the underlaying convective zone generates substantially more fluids, further supporting the physical mechanism proposed for induced seasonal marsquakes.

Annual variations in seismicity?
In addition to seasonal variation, annual temperature changes may also regulate ice-brine phase transitions, potentially driving yearly fluctuations in the occurrence rate of seasonal marsquakes. The total number of seasonal marsquakes detected between MY35 and MY36 exhibits a distinct annual variation, with the number recorded in MY36 nearly doubling that of MY35 (734 versus 417) (Fig. 2b). This trend holds true even when considering only events above the completeness magnitude (Supplementary Figs. 2 and 15), with MY36 still showing approximately 100 more seasonal marsquakes than MY35. In contrast, the number of tectonic marsquakes remains relatively stable between the two years (46 in MY35 versus 49 in MY36).

We investigated whether the increased seismic activity of seasonal marsquakes in MY36 is associated with annual temperature variations. Using Mars Planetary Climate Model (see “Methods”), we compared the average midday (LMST 10:00–14:00) surface temperatures between MY36 and MY35 during Ls=25 to Ls=200, the period when seasonal marsquakes occur (Fig. 2). Since nighttime temperatures on Mars are consistently low44 and do not affect brine-ice transitions, midday period is more relevant for this analysis.

Based on the proposed surface warming hypothesis (Fig. 3), seasonal marsquakes are concentrated in the northern hemisphere. The Mars Planetary Climate Model reveals that in several mid-latitude regions, including the north of the Utopia Planitia, mean surface temperatures during warm seasons (Ls=25 to Ls=200) in MY36 are higher than in MY35, with a maximum difference exceeding 4.0 K (Supplementary Figs. 16 and 17). This increase in temperature may have contributed to enhanced amount of molten brines, potentially driving the higher occurrence of seasonal marsquakes in MY36.

However, caution is needed when interpreting the correlation between the occurrence rates of seasonal marsquake and annual temperature variations. First, the two-year marsquake catalogue may not provide a statistically reliable basis for annual analysis. Second, the largest recorded quake, S1222a (Mw 4.7±0.2)81, occurred near the end of MY36, suggesting that stress state may have been higher in MY36 than in MY35. This could influence the occurrence of seasonal marsquakes with moderate magnitudes (Mw 1.2–3.0), though it likely did not affect larger tectonic marsquakes.

Near-surface brines cycle on Mars
To sustain the seasonal variation of seismicity, both the inflow and outflow of brines are essential. Without inflow at shallow depths, brine reservoirs on Mars would gradually deplete, preventing the occurrence of seasonal marsquakes. Similarly, without outflow at greater depths, brines could remain stagnant, potentially triggering seismic events year-round.

Water inflow may originate from Mars’ interior through non-eruptive degassing in volcanic regions82 or other processes. Numerical simulations suggest that present-day water in Martian interior can be released into the atmosphere via non-eruptive degassing processes and subsequently accumulate as ice in the shallow subsurface or on the surface82. Over time, a layer of dust can protect these ice deposits, likely allowing them to build up to several meters in thickness. This accumulation could provide a sufficient ice reservoir that, when melted, would reinfiltrate into the subsurface, completing a hydrological cycle on Mars. Alternatively, modern atmospheric water, in the form of frost or snow on Mars, may also contribute to the water inflow83.

Sustaining the brine circulation also requires a continuous salt supply to maintain the necessary chemical gradients. Multiple processes likely contribute to the enrichment of salts in the shallow subsurface, such as the deposition of dust-born salts (e.g., perchlorates, sulfates, and chlorides)84. Additionally, relict evaporitic deposits from past aqueous activity, along with ongoing atmospheric-surface interactions, may serve as further supplementary sources85.

Brine outflow could occur through crustal faults. Shallow groundwater infiltrates deep regions, triggering seismic fault slip and rupture. This fault movement creates new pathways within the Martian crust, facilitating the downward migration of brines into deeper layers, a process commonly observed on Earth86. Therefore, seasonal marsquakes resulting from repeated cycles of stress redistribution and fluid infiltration can, in turn, activate previously isolated hydrological processes along seismic faults.

Methods
Calculation of the seismic b-value
We calculated the b-value using two different methods: least squares regression and maximum likelihood estimation87,88. The two methods, respectively, present seismic b-values of 2.52 and 2.42 for seasonal marsquakes, with an average of 2.47 (Supplementary Fig. 18a), and the maximum likelihood estimation gives one standard deviation of 0.10. This is in sharp contrast to the lower b-value (least squares regression: 1.07, maximum likelihood estimation: 1.31±0.18) for tectonic marsquakes (Supplementary Fig. 18b), findings that align closely with prior research58. In addition, a remarkable increase in the b-value of seasonal marsquakes is observed from MY35 to MY36, rising from 2.2 to 2.9 (Supplementary Fig. 18c). This increase may reflect the influence of surface temperature changes, where warming enhances the phase transition from ice to brine, thereby contributing to the higher number of small seasonal marsquakes observed in MY36 compared to MY35.

Laboratory experiments for salinity-driven convection
To illustrate the potential of the salinity-driven convection model in triggering seasonal marsquakes, we conducted a series of laboratory experiments on the infiltration of brine through ice-acrylic mixtures. By maintaining isothermal conditions between brine and ice, these experiments were designed to replicate brine percolation within the convective layer, where temperature variations are minimal.

In this framework, the ice-acrylic mixtures serve as analogues for Martian frozen soil in the convective layer, while the overlying brine represents meltwater derived from an ice-rich shallow surface. By systematically varying experimental temperature, acrylic content (to simulate the fraction of soil or rock fragments), and NaCl brine salinity, we investigated how these parameters influence brine percolation. This approach enabled us to evaluate the applicability of the salinity-driven convection model to plausible Martian settings.

Sample preparation followed a standard ice synthesis method89. Ice cubes made from ultrapure water were crushed at −25 to −35 °C, sieved at −20 to −25 °C, and sorted into particle sizes of 180–250 µm. Cylindrical acrylic particles (~3 mm in diameter, 5–7 mm in height) were then mixed uniformly with the sieved ice powder, packed into a transparent cylindrical acrylic mold (~5 cm in diameter), and sealed with metal plugs fitted with O-rings. An aluminum spacer with small holes was inserted above the mixture to allow fluid exchanged during evacuation. Because acrylic has low thermal conductivity, the assembly was equilibrated for one hour in an environmental chamber as the temperature increased from −30 to −3 °C. The mold was then connected to a vacuum line, air was removed while immersed in an ice-water bath, and degassed ultrapure water was introduced into the pores of the mixture. Freezing was induced from the bottom by placing the mold on a copper plate inside a freezer (−35 to −30 °C) and insulating it with a polyester foam sleeve. After overnight freezing, the mold contained the intact ice-acrylic sample, ready for experiments.

Brines were prepared by dissolving 15 or 20 wt.% NaCl in ultrapure water, with food coloring added to trace infiltration pathways. Prior to each run, frozen brine was crushed into powders and compacted onto the sample surface at −30 °C. The samples were then transferred to an environmental chamber maintained at −5 or −10 °C. Supplementary Table 1 summarizes the experimental conditions, including chamber temperature, acrylic content, and brine salinity. Throughout each experiment, a Sony ILCE-7M4 camera captured time-lapse images at one-minute intervals, and demonstration videos were later compiled.

The experiments consistently demonstrate that salinity-driven convection enhances the ability of brine to melt underlying ice (Supplementary Fig. 9). All runs were maintained for more than 45 h, ensuring that the percolation velocity stabilized. Results show that brine infiltration reliably induces melting of the underlying ice, with percolation depths varying according to experimental conditions. In addition, compositional convection is observed during the later stage of each experiment: salt enrichment promotes melting at greater depths, while re-precipitation of ice occurs in shallower layers.

Among the parameters tested, temperature exerts the strongest control. An increase of 5 °C in chamber temperature results in nearly a fourfold increase in percolation depth (Supplementary Fig. 9b, e). Acrylic content also plays a significant role, with up to a 3.5-fold increase in percolation depth between mixtures containing 0 and 55 vol.% acrylic (Supplementary Fig. 9c, d). The presence of acrylic, serving as a rock analogue, promotes melting along phase boundaries in the two-phase ice-rock system, though the difference between 21 and 55 vol.% acrylic is modest. By comparison, brine salinity exerts only a moderate influence: an increase of 5 wt.% in NaCl concentration produces a smaller enhancement (~1.3 times) in percolation depth (Supplementary Fig. 9a, b) than either temperature or acrylic content.

Most critically, once channelization occurs during percolation, both infiltration depth and rate increase by nearly an order of magnitude (Fig. 4). This nonlinear transition highlights the fundamental role of preferential flow paths in accelerating brine transport. Complete experimental videos illustrating these processes are provided in the Supplementary Materials.

Rupture size estimation for Mw 1.2–3.0 marsquakes
According to Hanks and Kanamori90, the relationship between seismic moment M0 and moment magnitude Mw is given by:
Mw = (log10 M0 − 9.05) / 1.5, (1)
where M0 is measured in N·m. Seasonal marsquakes with Mw from 1.2 to 3.0 then correspond to seismic moments of 7.08×1010 N·m to 3.55×1013 N·m. For seismic events with Mw ≤ 6 that do not saturate the seismogenic zone, a circular crack model is commonly assumed. The scalar seismic moment of such events can be estimated by:
M0 = 16/7 * μ * Δε * R3, (2)
where R is the radius, μ is the shear modulus, and Δε is the uniform strain drop, typically assumed to be 10−491. Adopting strain drops of 10−3 or 10−5 would not significantly affect the estimated magnitude but would lead to approximately a factor of 2.15 variation in the inferred rupture radius.

On Mars, the surface rocks are predominantly tholeiitic basalts, formed through extensive partial melting and exhibiting minimal weathering. These rocks typically have a shear modulus of 20–40 GPa92. In comparison, the shear modulus of fresh water ice and briny ice is approximately 3 GPa93, while frozen soil exhibits temperature-dependent variations in shear modulus, generally ranging from 6 to 15 GPa. For the purpose of this analysis, representative shear modulus values of 3, 10, and 30 GPa are adopted for ice, frozen soil, and Martian crustal rock, respectively.

Using these shear moduli, the estimated rupture radius for seasonal marsquakes with Mw ~1.2 to 3.0 range from 47 to 373 m in ice, 31–249 m in frozen soil, 22–173 m in crustal rock. The corresponding rupture areas, determined using:
A = πR2, (3)
are approximately 6.9×103 to 4.4×105 m2 for ice, 3.0×103 to 1.9×105 m2 for frozen soil, 1.5×103 to 9.4×104 m2 for crustal rock (Supplementary Table 2).

Calculation of the average midday surface temperatures
The surface temperatures of Mars were obtained from the widely used Mars Planetary Climate Model (Mars PCM), developed at the Dynamic Meteorology Laboratory (LMD), which is extensively utilized within the planetary science community. Mars PCM is a general circulation model, which consists of a dynamical core that uses a finite difference method to solve primitive equations and a physical core considering a series of comprehensive processes, such as radiative transfer94, the dust cycle95, the water cycle, and energy and material transfer in the planetary boundary layer (PBL) region.

The temperature variations of Mars, including seasonal and interannual changes, are closely linked to the distribution characteristics of Martian dust. The vertical distribution of dust particles in the Mars PCM is derived using a semi-interactive method. In this study, we employed daily column-integrated dust scenarios based on comprehensive reconstruction dust maps, which provide the 9.3 μm absorption column dust optical depth for multiple Martian years (from MY24 to MY36). These maps are described in detail by Montabone et al.96. By using these semi-interactive dust scenarios and the corresponding input data, we were able to achieve atmospheric and surface temperatures that closely align with actual observations.

In addition, the surface temperature of the Martian is influenced by surface thermodynamic processes dominated by thermal inertia and surface-soil thermal conduction, as well as phase transition processes of surface CO₂ and water, and boundary layer mixing. On a diurnal timescale, it is also affected by the local topography, including slope and shadow effects. In the simulations we conducted, all of these physical processes were enabled to ensure that the Mars PCM simulation results closely resemble actual conditions.

The horizontal resolution of the simulations is 5.625 × 3.75 degrees with 73 sigma-pressure levels covering the altitude range from near surface to ~250 km. We conducted multiple Mars PCM simulations for different Martian years (MY30 to MY36) using the corresponding reconstruction dust maps and interpolated the surface temperature data from the simulation results to local mean solar time (LMST). Subsequently, we applied average kernel processing to the data near midday (LMST 10:00–14:00), obtaining surface temperature values for different Martian sols around local noon.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Please see attached", "reviewer_2": "REVIEW COMMENTS: Evidence for Near-Surface Liquid Water on Mars from Seasonal Marsquakes by Shi et al.\nThis manuscript reports a novel and potentially high-impact finding: the detection of near-surface brines on Mars inferred\nfrom seasonal variations in marsquake seismicity. The innovative aspect lies in using InSight seismic data not only for\nstructural studies but also as a proxy for active hydrological processes—specifically, ice-to-brine phase transitions that may\nregulate seismic activity. This approach offers a creative bridge between planetary seismology and hydrology, opening a\nrelatively unexplored pathway for inferring present-day brine behavior.\nThe proposed mechanism—seasonal warming leading to brine melting, pore-pressure increase on pre-stressed faults, and\nresulting seasonal marsquakes—is conceptually sound and aligns with observations of clustering, shallow focal depths, and\nhigh b-values. The estimated brine melting point (<250 ± 13 K) is an important constraint for modeling the Martian brine\ncycle and has implications for habitability assessments.\nHowever, the model rests on several key assumptions that require stronger quantitative support:\n1. Completeness and bias of the seismic catalog – The absence of fall–winter detections could reflect true quiescence, poor\ndetectability, or both. Current interpretation leans toward quiescence without fully demonstrating it. This gap should be\ntreated as censored data, not as absence.\no Explicitly state that wind noise during these seasons saturates the sensor and raises the nighttime detection threshold.\no Test detectability by applying matched filtering (spring templates to fall–winter data) and report whether subthreshold\nrepeats are found.\no Explore techniques such as polarization filtering, coda normalization, or array beamforming to extend detection into noisy\nperiods.\no Provide a contingency explanation if the true rate in fall–winter matches that of spring–summer.\n2. Feasibility of the brine-triggered pore-pressure model – While the conceptual chain is plausible, it needs order-of-\nmagnitude estimates to demonstrate feasibility.\no Calculate the pore-pressure increase required to trigger slip under plausible Martian stress and friction conditions.\no Estimate the meltwater volume necessary to achieve this pressure change for the inferred rupture areas.\no Compare seasonal melt production estimates to these requirements.\no State the likely depths of the seasonal marsquakes and assess whether the magnitudes (Mw 1.2–3.0) are compatible with\nthe proposed mechanism.\no Evaluate whether brine could reach these depths within the what timescale (weeks–months) along faults. Could they have\na time lag between temperature and quakes?\n3. Alternative mechanisms – While the paper notes that thermal fracturing and CO₂ sublimation cannot fully explain the\nobservations, the argument would be stronger with a quantitative comparison showing why these processes cannot\nreproduce the observed abrupt onset, clustering, and b-value patterns.\nMinor language typos: “bines” -> “brine”\nYou missed one great paper on this topic: Roth N., Zhu T. and Y. Gao, (2024). Characterizing Liquid Water in Deep Martian\nAquifers: A Seismo-Electric Approach, Journal of Geophysical Research – Planets, 129, e2024JE008292.\nOverall assessment:\nThe manuscript is scientifically significant, offering new observational evidence and a plausible physical mechanism for\npresent-day liquid water in Mars’ near-surface environment. With strengthened treatment of catalog completeness,\nquantitative feasibility estimates, and systematic elimination of competing hypotheses, this work could make a substantial\ncontribution to our understanding of Mars’ hydrological processes and their link to ongoing seismicity.", "reviewer_3": "Review of Evidence for Near-Surface Liquid Water on Mars from Seasonal Marsquakes\nby Li et al.\nThis paper by Li et al. presents an intriguing hypothesis that seasonal Marsquakes may be triggered\nby subsurface ice melt during warmer periods, which increases pore pressure and induces seismicity.\nConceptually, the idea is compelling, and the paper makes a convincing case for a time lag or\nthreshold between diurnal temperature increases and the onset of seismic activity. The paper is very\nwell written, and the figures and captions are great. However, I remain skeptical about the overall\nefficiency of this mechanism under Martian conditions. As such, my primary comments below focus\nmainly on the thermal aspects of the authors' argument. I am sure other reviewers will comment on\nthe overall seismic analyses presented in the paper.\nMajor Comment 1: A central claim of the manuscript is that seasonal Marsquakes may be triggered\nby the melting of briny ice in the shallow subsurface, where eutectic temperatures (~250 ± 13 K) can\noccasionally be reached. While the hypothesis is conceptually interesting, a key parameter that\nremains underexplored is the volume of meltwater that can realistically be generated under Martian\nconditions.\nIt is reasonable to assume that some fraction of pore-filling ice could undergo partial melting;\nhowever, for such melting to reach substantial depth and induce seismicity, the generated water must\nbe of sufficient volume to meaningfully increase pore pressure. What is the volume necessary? This\nis neither addressed nor discussed from terrestrial perspective either. Can thin film layers of melt\nwater be sufficient for such a proposed hypothesis? A more quantitative treatment of this issue would\ngreatly strengthen the paper’s central argument. Specifically, the authors could estimate the potential\nvolume of meltwater produced as a function of Ls (or on a diurnal basis), and compare this with\nthresholds for pressure-driven seismic triggering.\nAlthough the data in Extended Data Fig. 10 and related sections are suggestive, they stop short of\ndemonstrating that the thermal energy available is sufficient to produce the necessary melt.\nIncorporating latent heat of fusion in the thermal budget would be a crucial step toward evaluating\nwhether the proposed mechanism is energetically feasible. I performed a simple back-of-the-\nenvelope calculation (1D thermal model) to estimate the daily meltwater that can be produced if you\nassume a sufficiently thick layer of ice underneath a thin regolith (0.1 to 0.2 m). The temperature\nvalues are for 40N 90E from MCD for Ls185 and assumes thermal diffusivity values similar to what\nthe authors use. Even when I assume a melting temperature of 230 K, the thickness of liquid water\ngenerated on a daily basis is relatively low. I encourage the authors to carry out a similar analysis or\ninclude a discussion to assess whether the hypothesized melting is quantitatively sufficient to explain\nthe observed seismic activity.\nMajor Comment 2: A further point of concern regarding the proposed brine melting mechanism is\nthe plausibility of downward migration of meltwater. Specifically, it remains unclear how liquid water\ngenerated in the shallow subsurface—above an underlying layer of pore-filling ice—could traverse to\ngreater depths where the convective brine layers may exist, as suggested by the authors. While the\ncited reference (https://www.science.org/doi/10.1126/sciadv.adp2808) discusses the potential for\nsuch migration, the schematics presented in the current manuscript imply that meltwater would need\nto travel downward over hundreds of meters potentially to km depths.\nGiven the top-down nature of the proposed melting process, it seems likely that any liquid produced\nnear the surface would encounter a colder, potentially impermeable ice-rich layer beneath it. Under\nsuch conditions, it is difficult to envision a viable mechanism for downward percolation of thin brine\nfilms through this barrier over substantial depth. The authors should clarify how this vertical\ntransport is physically feasible under Martian thermal conditions, or alternatively, discuss whether\nlocalized fractures, heterogeneities, or other transient features might facilitate such migration.\nWithout this clarification, the connection between shallow melting and deeper subsurface processes\nremains, in my opinion, very speculative.\nMajor Comment 3: The manuscript refers to a \"convective layer\" at depth, but it is not clear what\ntype of convection is being invoked—thermal or salinity-driven. If the convection is thermally driven,\nthe authors need to clarify what temperature gradient is driving it, especially given that surface\ntemperature variations are significantly attenuated at such depths. This point is supported by the data\nin Extended Data Fig. 10, which shows minimal seasonal thermal variation at depth.\nAlternatively, if the authors are suggesting salinity-induced (compositional) convection, further\nexplanation is needed regarding the source of heterogeneity in salinity. What is the origin of the\nrequired spatiotemporal variations in composition, and how is the density stratification maintained\nor destabilized?\nMoreover, the concept of this deep convective layer undergoing freezing or melting in response to\nseasonal forcing is difficult to reconcile with the thermal structure presented in the paper. If this layer\nis located well below the diurnally or seasonally influenced conductive zone, it should remain\nthermally stable on seasonal timescales. The manuscript would benefit from a clearer articulation of\nthe physical mechanisms driving convection at depth, and a justification for why such a layer would\nbe sensitive to surface or near-surface thermal forcing.\nMajor Comment 4: The melting of ‘Frozen faults/structures’ at substantial depth is not as easy as the\nauthors make it out to be in this manuscript. If there is already ice in the fault (as seems to be based\non their model schematic), then it will likely take longer to melt this ice to increase the pore pressure.\nThis would again require a robust thermal model.\nMinor Comments:\nTitle of the paper: I do not believe the current title accurately reflects the nature of the work\npresented. The authors put forward a hypothesis that may be consistent with the observed seasonality\nof Marsquakes, but the results fall short of providing definitive evidence. As such, describing the\nfindings as “evidence” does not sound right to me.\nLine 22: Is detection really the accurate word here?\nLine 53: Even more comprehensive refute is given in this paper:\nhttps://www.sciencedirect.com/science/article/abs/pii/S0019103523003494\nLine 24: I think this line should instead say something akin to “The absence of seasonal marsquakes\nduring colder periods and their abrupt resurgence in warmer seasons can be explained by ice-to-brine phase transitions.” Basically, I just find the definitiveness of the statement to be a bit too\noptimistic.\nFigure 1. Is the black line instet in the globe in Figure 1 accurate? It might be projection issue but\nthe inset location does not seem to match what is shown in Fig.1(a).\nLine 85: Both boxes are transparent. I would suggest referring to the box by its color.\nLine 281-282: “suggests the presence of near-surface liquid brines.” It does not. The meter-scale\nextent of the thermally influence conductive zone suggests just exactly that. It does not suggest or\nimplicate the presence of near-surface liquid brines.\nLine 296: Or alternatively, based on the thermal conditions one assumes, brines driven downwards\nby gravity through pore-spaces can lose heat along the way and re-freeze. One needs specific set of\nthermal conditions for the proposed scenario to be viable.\nLine 297: I am confused by this line. Are the authors stating that the brines produced in the shallow\nsubsurface need to reach hundreds to thousands of meter depth within day(s)? Assuming a purely\ngravity-driven flow, my back of the calculation suggests a modest speed of 10-4 m/s for saturated flow.\nMaybe hundred is conceivable but anything deeper than that would require extra momentum.\nThis point above may be also relevant to Line 229 where the authors state that the arrival time\ndifference between P and S waves to be consistently around 249+/- 24 S. What does the plot of the\nP-S wave time difference as a function of season look like? If liquid layer is present at certain times\nand not others, should we not expect to see potential change in the P-S arrival time as a function of\nseason?\nFigure 3. Are the authors stating that the convective layer is relatively pure water or a brine layer? In\neither way, the diffusion of the incoming salty layer needs to be accounted. See The Darcy Problem.\nS1: Capitalize W in “we analyzed”\nS1: Citation needed in support of this “The conclusion that tidal forces are not the primary driver\nof marsquakes is further supported by other studies employing different methodologies .”\nS1: missing t in “usually requires housands of pascals”"}, "author_response": "Manuscript Reference Number: NCOMMS-25-31203-T\nJournal: Nature Communications\nTitle: Near-Surface Liquid Water on Mars inferred from Seasonal Marsquakes\nAuthors: Jing Shi, Jiaqi Li, Caoanla Li, Haoran Meng, Cong Sun, Chao Qi, Lu Pan,\nSiteng Fan, Zhenliang Tian, Tao Wang, Lian Xue, Xie Hu, Ling Chen\nWe sincerely thank the reviewers for their constructive and insightful comments. We\nalso appreciate the recognition of the novelty of our findings. In preparing the revision,\nwe considered all comments and addressed each in detail. Beyond the requested\nclarifications, we also carried out a laboratory simulation to further validate the\nplausibility of our proposed mechanism.\nBelow we summarize the major improvements:\nTo address Reviewer #1’s request for thermal modeling, we calculated the thermal\nprofile under Martian conditions, explicitly considering the effect of latent heat. This\nanalysis constrains the depth range that can be thermally affected and directly addresses\nthe “speculative” aspect of our model in which seasonal marsquakes are triggered by\nice melt.\nIn response to Reviewer #2’s concern regarding seismic catalogue completeness, we\nperformed additional template-matching tests with varying signal-to-noise ratios and\ncross-correlation thresholds. These tests verify the robustness of seasonal marsquake\ndetection and provide a more rigorous assessment of the completeness of the seismic\ncatalogue.\nTo address the shared concern of Reviewers #1 and #2 regarding pore-pressure changes\nand meltwater production, we conducted estimations and calculations to evaluate three\nkey aspects: (1) the magnitude of pore-pressure changes required to trigger seasonal\nmarsquakes, (2) the associated meltwater volumes, and (3) the plausibility of these\nprocesses under Martian conditions.\nTo further validate the feasibility of our model, though not requested by the reviewers,\nwe conducted controlled laboratory experiments simulating brine-driven melting of\nunderlying ice. The results demonstrate that once shallow brines form, they can\nprogressively melt the ice beneath. Importantly, when channelization develops,\n1/73\npercolation rates can increase by orders of magnitude, supporting the plausibility of\nsignificant downward migration within seasonal timescales.\nWe included new experimental results in the revised manuscript, added a new figure\n(Figure 4) illustrating channel formation, updated Figure 1a with the latest water-ice\nconsistency map and craters containing water ice, and provided full laboratory\nexperiment videos as supplementary materials.\nWe have also revised the manuscript to address all other minor comments and\nsuggestions by clarifying key assumptions, refining figure presentation, and improving\nwording for clarity.\nWe believe these revisions substantially strengthen the manuscript and provide more\nrobust support for the plausibility of our proposed mechanism.\nIn the following, we present point-by-point responses with reviewers’ comments shown\nin black, our responses in blue, and the corresponding revisions in the main text\nhighlighted in red for clarity. To improve readability, each issue is organized as a Q\n(question) followed by our A (answer). Figures in our responses are labeled as “Figure\nR” to distinguish them from figures in the main manuscript. In addition, we highlighted\nthe major changes in red within the revised main text. After the main text, provided a\nseparate version with tracked changes for reference.\n2/73\nResponse to comments from Reviewer #1\nThis paper by Li et al. presents an intriguing hypothesis that seasonal Marsquakes may\nbe triggered by subsurface ice melt during warmer periods, which increases pore\npressure and induces seismicity. Conceptually, the idea is compelling, and the paper\nmakes a convincing case for a time lag or threshold between diurnal temperature\nincreases and the onset of seismic activity. The paper is very well written, and the\nfigures and captions are great. However, I remain skeptical about the overall\nefficiency of this mechanism under Martian conditions. As such, my primary comments below\nfocus mainly on the thermal aspects of the authors' argument. I am sure other reviewers\nwill comment on the overall seismic analyses presented in the paper.\nReply:\nWe sincerely thank you for taking the time to review our work, for your thoughtful\nevaluation, and for your kind words about the manuscript’s novelty, writing, figures,\nand captions. We greatly appreciate your clear summary of our study and your\nrecognition of the conceptual appeal of our hypothesis that seasonal marsquakes may\nbe triggered by subsurface ice melt during warmer periods.\nYour concern regarding the overall efficiency of this mechanism under Martian\nconditions is entirely reasonable and raises important points for further discussion.\nBased on our analyses and additional laboratory experiments, we found that the\nproposed process is indeed feasible under Martian conditions. In addition, to address\nthe thermal aspects you highlighted, we carried out thermal modeling that explicitly\nconsiders latent heat effects and constrains the depth range influenced by seasonal\ntemperature variations.\nThese results, together with our experimental demonstrations of salinity-driven melting\nand channelized infiltration, indicate that the efficiency of the mechanism is\nsufficient to plausibly generate seasonal seismicity.\nBelow, we provide detailed responses to each of your specific comments, along with\nadditional analyses and experimental tests.\n1 - Major Comment 1: A central claim of the manuscript is that seasonal Marsquakes\nmay be triggered by the melting of briny ice in the shallow subsurface, where eutectic\ntemperatures (~250 ± 13 K) can occasionally be reached. While the hypothesis is\nconceptually interesting, a key parameter that remains underexplored is the volume of\nmeltwater that can realistically be generated under Martian conditions.\nIt is reasonable to assume that some fraction of pore-filling ice could undergo partial\nmelting; however, for such melting to reach substantial depth and induce seismicity,\nthe generated water must be of sufficient volume to meaningfully increase pore pressure.\nWhat is the volume necessary? This is neither addressed nor discussed from terrestrial\nperspective either. Can thin film layers of melt water be sufficient for such a proposed\nhypothesis? A more quantitative treatment of this issue would greatly strengthen the\n3/73\npaper’s central argument. Specifically, the authors could estimate the potential\nvolume of meltwater produced as a function of Ls (or on a diurnal basis), and compare this with\nthresholds for pressure-driven seismic triggering.\nAlthough the data in Extended Data Fig. 10 and related sections are suggestive, they\nstop short of demonstrating that the thermal energy available is sufficient to produce\nthe necessary melt. Incorporating latent heat of fusion in the thermal budget would be\na crucial step toward evaluating whether the proposed mechanism is energetically\nfeasible. I performed a simple back-of-the-envelope calculation (1D thermal model) to\nestimate the daily meltwater that can be produced if you assume a sufficiently thick\nlayer of ice underneath a thin regolith (0.1 to 0.2 m). The temperature values are for\n40N 90E from MCD for Ls185 and assumes thermal diffusivity values similar to what\nthe authors use. Even when I assume a melting temperature of 230 K, the thickness of\nliquid water generated on a daily basis is relatively low. I encourage the authors to carry\nout a similar analysis or include a discussion to assess whether the hypothesized melting\nis quantitatively sufficient to explain the observed seismic activity.\nReply:\nWe thank you for this insightful and constructive comment. Based on your suggestions,\nwe distilled the key issues into the following guiding questions:\nQ1: What level of pore-pressure change is required to trigger seasonal marsquakes?\nQ2: How much meltwater is necessary to induce seasonal seismicity?\nQ3: When the latent heat is included, what is the depth of the thermally affected layer?\nQ4: Can the required amounts of meltwater realistically be generated under Martian\nconditions?\n4/73\nWe fully agree that assessing the realistic volume of brine meltwater, and whether it is\nsufficient to perturb pore pressure and trigger seasonal marsquakes, is a crucial issue\nfor evaluating the plausibility of our hypothesis. We also note that, in Mars studies,\nlarge uncertainties remain in key parameters such as permeability, making it\nchallenging to precisely estimate pore pressure changes and the required water content.\nNevertheless, following your suggestion, we carried out relevant estimates by using\nEarth as an analogue, in addition to performing further calculations and a literature\nreview, which we summarize below.\nQ1: What level of pore-pressure change is required to trigger marsquakes?\nA1: Regarding the magnitude of pore pressure perturbations required to induce\nseismicity, terrestrial studies of induced seismicity have shown that coulomb failure\nstress changes on the order of ~0.01 to 0.1 MPa are sufficient to trigger earthquakes\nwhen faults are near failure (Stein, 1999; Keranen et al., 2014; Deng et al., 2016). Such\nperturbations may arise from variations in shear stress or effective normal stress, and\npore-pressure changes of comparable magnitude could likewise meet this threshold. We\ntherefore considered it reasonable to assume that pore-pressure changes within a similar\nrange (i.e., ~0.01 to 0.1 MPa) would also be enough to induce seasonal marsquakes\nunder near-failure conditions on Mars. We added corresponding discussion and\nclarification in the main text (Lines 430 – 432):\n“Laboratory and field studies on Earth demonstrate that relatively small pore pressure\nperturbations (~0.01 to ~0.1 MPa) can trigger earthquakes when faults are near failure\n(Stein, 1999; Keranen et al., 2014; Deng et al., 2016). Thus, seasonal marsquakes are\nlikely to exhibit comparable sensitivity under near-failure conditions.”.\nQ2: How much meltwater is necessary to induce seasonal seismicity?\nA2: We estimated the amount of meltwater required per unit area on Mars by analogy\nwith Earth, because estimating the total meltwater volume would require knowledge of\nthe full extent of seismogenic regions involved in seasonal seismicity, which remains\nuncertain. Therefore, we converted the problem into estimating the meltwater volume\nper unit area. On Earth, the relationship between injected fluid volume and the\nmaximum induced earthquake magnitude has been well established under conditions\nwhere seismicity is triggered (McGarr, 2014). Specifically, during wastewater injection,\nfluids are introduced into deep aquifers under low pressure, a scenario analogous to our\nmeltwater model. Observations show that the induced magnitude increases with\nincreasing injected volume (McGarr, 2014) (Figure R1a).\n5/73\nTable R1 (Supplementary Table. 6 in the Supplementary Materials). Induced\nearthquakes: injected wastewater volumes and maximum induced magnitudes\n(modified from McGarr, 2014). From left to right, the table lists the site location,\nmaximum induced seismic magnitude, injected wastewater volume, area of the\nseismogenic region, and the corresponding reference.\nMagnitude Water Seismogenic\nSite Reference\n(Mw) volume (m3) areas (m2)\nEastern Bavaria, (Zoback & Harjes,\n1.4 200 8*105\nGermany 1997)\nSoultz, France 2.9 3.98*104 1.2*107 (Majer et al., 2007)\nDallas-Fort Worth (C. Frohlich et al.,\n3.3 2.82*105 5.6*105\nAirport, TX 2011)\nBasel, Switzerland 3.4 1.15*104 \\ (Majer et al., 2007)\nAshtabula, OH,\n3.6 6.17*104 7.5*105 (Seeber, 2004)\nJuly, 1987\nCooper Basin, (Baisch et al.,\n3.7 2.00*104 3*106\nAustralia 2006)\nAshtabula, OH,\n3.9 3.40*105 5*106 (Seeber, 2004)\nJanuary 2001\nYoungstown, OH 4.0 8.34*104 6*105 (Kim, 2013)\nParadox Valley,\n4.3 3.29*106 1.8*107 (Ake, 2005)\nCO\nRaton Basin, CO, (Meremonte et al.,\n4.4 4.26*105 1.4*107\nSeptember 2001 2002)\nGuy, AR 4.7 6.29*105 2.8*107 (Horton, 2012)\n(Nicholson et al.,\nPainesville, OH 4.8 1.19*106 1.2*107\n1988)\n(Herrmann et al.,\nDenver, CO 4.85 6.25*105 9.6*107\n1981)\n(Cliff Frohlich et\nTimpson, TX 4.8 9.91*105 3.6*107\nal., 2014)\nRaton Basin, CO, (Rubinstein et al.,\n5.3 7.84*106 2.5*107\nAugust 2011 2014)\n(Katie M. Keranen\nPrague, OK 5.7 1.20*107 3.6*107\net al., 2013)\n6/73\nFigure R1 (Supplementary Fig. 21 in the Supplementary Materials). Induced\nearthquakes from wastewater injection: magnitude-injected volume relationship.\na, Relationship between maximum induced seismic magnitude and injected fluid\nvolume. Scatter points and the solid line are adapted from McGarr, (2014), with the\nsolid line representing the upper bound. The x-axis shows the injected wastewater\nvolume, and the y-axis shows the seismic magnitude. b, Estimated relationship between\ninjected volume per unit area and maximum induced seismic magnitude. The x-axis\nrepresents injected volume per unit area, and the y-axis shows seismic magnitude. The\norange shaded box highlights the magnitude range of seasonal marsquakes. The red\ndashed lines indicate the estimated per-unit-area meltwater volume achievable on Mars.\nUsing previously compiled data on injected fluid volumes and induced seismic\nmagnitudes on Earth (McGarr, 2014), we further compiled the corresponding\nseismogenic areas (Table R1), thereby deriving the relationship between the injected\nvolume per unit area and the maximum induced seismic magnitude. As shown in Figure\nR1b, aside from a single outlier, the required injected volume to trigger earthquakes of\nMw ≤ 5 is generally below 0.1 m3/m2. For an injected volume of 0.3 m3/m2, earthquakes\nof Mw ≥ 5 can be triggered.\nThis per-unit-area estimate allows us to translate the Earth-based relationship into\nconstraints for Martian seasonal seismicity, providing a physically plausible range for\nthe amount of meltwater needed to activate faults. Given that most seasonal marsquakes\nhave magnitudes below Mw 3.0 (Lognonné et al., 2023), we inferred that a meltwater\nvolume of approximately 0.1 m3/m2 would be sufficient to trigger such events.\nIn the revised manuscript, we included an estimate of the required water volume in the\nmain text (Lines 438 – 444):\n“Well-documented researches of wastewater disposal on Earth have quantified a\nsystematic positive correlation between the volume of injected fluid and the maximum\ninduced seismic magnitude (McGarr, 2014). Thus, by projecting the observed\nmagnitudes of seasonal marsquakes onto this Earth-derived relationship\n(Supplementary Fig. 21), we estimated that a fluid volume of ~ 0.1 m3 per unit area\n7/73\n(equivalent to 0.1 m water depth) would be sufficient to trigger seasonal marsquakes\nwith maximum moment magnitudes (Mw) of ~3.0.”.\nWe also provided the methodological details in the Supplementary Text. S3.\nQ3: When the latent heat of fusion is included, what is the depth of the thermally\naffected layer?\nA3: You kindly provided a simple calculation of melt depths under different melting-\npoint assumptions. For comparison, we first reproduced your results on diurnal\ntemperature variations, then extended the calculations to seasonal temperature profiles\nunder heat conduction without considering latent heat, and finally computed the profiles\nincluding latent heat effects.\nDiurnal Case: The depth of penetration of diurnal temperature variations is shallow\n(~0.1 m). We retrieved the same temperature profiles (Figure R2a) from the Mars\nClimate Database (MCD) and solved the heat conduction equation to obtain thermal\nprofiles at the same location (40N, 90E) and time (Ls=185). The melting point of ~250\n± 13 K represents an upper bound, because this temperature coincides with Ls=25,\nwhen seasonal marsquakes begin to emerge (Fig. 1b in the main text). Therefore,\nmelting must already occur before Ls=25. Assuming a melting point of 240 K, we\nderived the maximum hourly melt depth corresponding to the depth at which the\ntemperature reaches 240 K. This daily calculation yields a depth of 0.08 m (Figure R2b,\nc), consistent with your estimate and with previous studies (Zhang et al., 2023). It is\nimportant to note, however, that this estimate only accounts for diurnal variations.\nFigure R2. Maximum melt depths induced by diurnal temperature variations. a,\nInput temperature profiles retrieved from the MCD. b, Time evolution of the estimated\nmaximum melt depth, assuming a melting point of 240 K. c, Temperature profiles\ncomputed from the heat conduction equation with a thermal diffusivity of 7.0 × 10-7\nm2/s. The x-axis denotes time and the y-axis denotes depth.\n8/73\nSeasonal case (without latent heat): When latent heat is neglected, the seasonal\nthermal wave penetrates to greater depths within the subsurface. From the MCD, we\nextracted the average midday (LMST 10:00 – 14:00) surface temperatures over one\nMartian year at 40°N, 90°E. As noted in the main text (Lines 129 – 134), daytime\nwarming is more likely responsible for melting. Neglecting latent heat, our calculated\nthermal profiles based on thermal diffusivities of 7.0 × 10-7 and 5.1 × 10-8 m2/s indicate\nthat temperatures can reach the melting point (240 K) down to depths of roughly 4.0 m\n(Figure R3b) and 1.5 m (Figure R3c), respectively.\nFigure R3 (Supplementary Fig. 22 in the Supplementary Materials). Penetration\ndepth of seasonal temperature variations without accounting for latent heat. a,\nInput temperature profiles derived from the Mars Climate Database (MCD), showing\naverage midday (LMST 10:00 – 14:00) temperatures at 40°N, 90°E. b, Simulated\ntemperature field for a thermal diffusivity of 7.0 × 10-7 m2/s. The x-axis represents solar\nlongitude, and the y-axis represents depth. The red solid line marks the 240 K isotherm.\nc, Same as (b), but for a thermal diffusivity of 5.1 × 10-8 m2/s.\n9/73\nThe inferred penetration depths associated with diurnal (~0.1 m) and annual (~4.0 m)\ntemperature variations are consistent with theoretical predictions using the equation\n(Equation 1) of thermal skin depth (Mellon et al., 2000; Hayne et al., 2017):\n!∙#\n𝑍 = # , (1)\n$\nwhere 𝑍 denotes the depth of thermal influence, 𝛼 is the thermal diffusivity, and 𝑇\nis the period. Employing the value of 𝛼 = 7.0 ×10−7 m2/s, the thermal skin depth is\nestimated to be ~0.14 m for the diurnal cycle (T = 1 sol) and ~4.0 m for the annual cycle\n(T = 668 sols).\nSeasonal case (with latent heat): When latent heat is taken into account, the melting\ndepth decreases but remains on the meter scale. We incorporated latent heat via\nadjusting the apparent heat capacity associated with phase change. Specifically, the\none-dimensional heat conduction equation describes heat flow in regolith is:\n%# % %#\n𝜌𝐶 = (𝜆 *, (2)\n%& %' %'\nwhere 𝑇 is temperature, 𝑡 is time, 𝑥 is depth, 𝜌 is regolith density, 𝐶 is specific\nheat capacity, and 𝜆 is thermal conductivity. Assuming constant thermal properties,\nthe Equation (2) is simplified using thermal diffusivity 𝛼 = 𝜆/(𝜌𝐶):\n%# %!#\n= 𝛼 . (3)\n%& %'!\nTo account for the latent heat absorbed or released during freezing and thawing, the\nspecific heat capacity 𝐶 is replaced by an apparent heat capacity 𝐶 , which is a\n())\nfunction of temperature:\n𝐶 , 𝑖𝑓 𝑇 < 𝑇 −∆𝑇 (𝐹𝑟𝑜𝑧𝑒𝑛)\n* +\n𝐶 (𝑇) = 0 , \" -,# + /$ , 𝑖𝑓 𝑇 −∆𝑇 ≤ 𝑇 ≤ 𝑇 +∆𝑇 (𝑃ℎ𝑎𝑠𝑒 𝑐ℎ𝑎𝑛𝑔𝑒),\n()) + +\n. .∆#\n𝐶 , 𝑖𝑓 𝑇 > 𝑇 +∆𝑇 (𝑈𝑛𝑓𝑟𝑜𝑧𝑒𝑛)\n1 +\nwhere 𝐶 and 𝐶 are the volumetric heat capacities for frozen and unfrozen regolith,\n* 1\nrespectively, 𝑇 is the melting point, ∆𝑇 is the small temperature interval of phase\n+ \nchange, and the volumetric latent heat of fusion:\n𝐿 = 𝜌 𝑤𝐿, (5)\n2 3\nwith 𝜌 being the density of water, 𝑤 the mass ice (or water) content of the regolith,\n3\nand 𝐿 the latent heat of fusion for water. For a porous and ice-saturated regolith with\nporosity 𝜙, given the density (𝜌 ), specific heat capacity (𝐶 ), and thermal conductivity\n4 4\n(𝜆 ) of rock matrix, the mass ice/water content (𝑤), the volumetric heat capacities and\n4\neffective thermal conductivities for frozen (𝐶 , 𝜆 ) and unfrozen regolith (𝐶 , 𝜆 ) can\n* * 1 1\nbe expressed as:\n10/73\n5 6\n𝑤 ≈ %\n⎧\n5%6-5& (896)\n⎪ ⎪ 𝐶 = (1−𝜙)𝐶 +𝜙𝐶\n* 4 ;\n𝐶 = (1−𝜙)𝐶 +𝜙𝐶 , (6)\n1 4 3\n⎨\n896 6\n𝜆 = 𝜆 𝜆\n⎪ ⎪ * 4 ;\n896 6\n⎩ 𝜆 = 𝜆 𝜆\n1 4 3\nwhere 𝐶 and 𝐶 are the specific heat capacities, and 𝜆 and 𝜆 the thermal\n; 3 ; 3\nconductivities of ice and water, respectively.\nThen, the continuous governing equation is discretized for a numerical solution using\nthe notation 𝑇(𝑥,𝑡) ≈ 𝑇< , where 𝑥 = 𝑖∆𝑥 and 𝑡 = 𝑖∆𝑡 . The Forward-Time\n; \nCentered-Space explicit scheme for Equation (3) is:\n#\n%\n'()9#\n%\n' \n= \n𝛼<#\n%\n' \n() \n9.#\n%\n'-#\n%\n' \n*). (7)\n∆& ; (∆')!\nNote that the thermal diffusivity 𝛼< = 𝜆(𝑇<)/𝐶 (𝑇<) depends on the temperature\n; ; ()) ; \nat node 𝑖 at time step 𝑛. Rearranging the above gives the formula to calculate the\ntemperature at the next time step:\n𝑇<-8 = 𝑇< + ! % '∆& (𝑇< −2𝑇< +𝑇< ). (8)\n; ; (∆')! ;-8 ; ;98\nIn the calculation, we input the initial surface temperature 𝑇(𝑥,0), and at a sufficient\ndepth (𝐻), the temperature is assumed to be constant 𝑇(𝐻,𝑡) = 220 𝐾. Meanwhile,\nFor the explicit scheme to be numerically stable, the following condition must be met:\n!+,-∆&\n≤ 0.5, (9)\n(∆')!\nwhere 𝛼 is the maximum expected thermal diffusivity during the simulation.\n+(' \nNext, we performed heat conduction simulations accounting for latent heat, using\nthermal parameters from previous studies (Table R2). The thermal properties of the\nsolid matrix were based on those of basalt (Shadi et al., 2023), the most commonly\nobserved rock type in the Martian uppermost crust (Golombek et al., 2017; Lognonné\net al., 2020), and the thermal parameters for ice and water were taken from Ojha et al.,\n(2020). We calculated thermal profiles for porosities of 0.2 and 0.4, which are typical\nvalues in the Martian uppermost crust (Rajšić et al., 2021).\n11/73\nTable R2 (Supplementary Table. 7 in the Supplementary Materials). Thermal\nparameters used in the simulations.\nVariable Description Value Units\n𝝓 Porosity 0.2, 0.4 -\n𝝆 Density of water 1000 𝑘𝑔⁄𝑚>\n𝒘\n𝝆 Density of ice 917 𝑘𝑔⁄𝑚>\n𝒊\n𝝆 Density of rock matrix 2900 𝑘𝑔⁄𝑚>\n𝒓\n𝑪 Specific heat of ice 2000 𝐽⁄(𝑘𝑔∙𝐾)\n𝒊\n𝑪 Specific heat of water 3985 𝐽⁄(𝑘𝑔∙𝐾)\n𝒘\nSpecific heat of rock\n𝑪 800 𝐽⁄(𝑘𝑔∙𝐾)\n𝒓 matrix\nThermal conductivity\n𝝀 2 𝑊⁄(𝑚∙𝐾)\n𝒊 of ice\nThermal conductivity\n𝝀 0.6 𝑊⁄(𝑚∙𝐾)\n𝒘 of water\nThermal conductivity\n𝝀 2.5 𝑊⁄(𝑚∙𝐾)\n𝒓 of rock matrix\n∆𝒕 Time step 60 𝑠\n∆𝒙 Space grid 0.1 𝑚\n𝑻 Melting point 240 𝐾\n𝒎\nTemperature interval\n∆𝑻 0.2 𝐾\nof phase change\nLatent heat of fusion\n𝑳 3.34*105 𝐽/𝑘𝑔\nfor water\n12/73\nFigure R4 (Extended Data Fig. 9 in the main text). Thermal profiles accounting\nfor latent heat. a, Input temperature profile, same as in Figure R3a. b, Calculated\nthermal profile for a porosity of 0.2. The frozen (𝛼 ) and unfrozen (𝛼 ) thermal\n* 1\ndiffusivities are 6.4 × 10-7 and 3.7 × 10-7 m2/s, respectively. c, Calculated thermal profile\nfor a porosity of 0.4. 𝛼 and 𝛼 are 6.5 × 10-7 and 2.5 × 10-7 m2/s, respectively.\n* 1\nThermal parameters used in the calculations are summarized in Table R2.\nBoth results show that even when latent heat is considered, the seasonal temperature\nvariations can still penetrate to meter-scale depths. When the porosity is 0.2, the frozen\n(𝛼 ) and unfrozen (𝛼 ) thermal diffusivities are 6.4 × 10-7 and 3.7 × 10-7 m2/s,\n* 1\nrespectively. Under these conditions, the 240 K isotherm reaches a depth of about 3 m\n(Figure R4b). When the porosity increases to 0.4, 𝛼 and 𝛼 become 6.5 × 10-7 and\n* 1\n2.5 × 10-7 m2/s, respectively, and the 240 K isotherm extends to about 2 m (Figure R4c).\nThe shallower penetration with increasing porosity is consistent with the theoretical\nexpectation that higher porosity leads to more meltwater formation and greater latent\nheat release, which limits the depth of thermal influence. This agreement further\nvalidates our thermal modeling. Therefore, even accounting for latent heat, seasonal\nthermal penetration can still approach ~2 m.\n13/73\nQ4: Can the required amounts of meltwater realistically be generated under Martian\nconditions?\nA4: Even when latent heat is taken into account, the seasonal melting still provides\nsufficient meltwater per unit area to meet the required amount. As shown in Figure R4,\nthe thermal wave can reach depths of ~3 m for a porosity of 0.2 and ~2 m for a porosity\nof 0.4. In these two cases, the estimated ice/water mass fractions (𝑤) are approximately\n7 and 17 wt.%, and the bulk densities of rocks (𝜌 ) are about 2500 and 2100 kg/m3,\nB\nrespectively. Then, the meltwater volume per unit area (𝜂) is calculated using the\nequation:\n𝜂 = \nC∙5.∙3\n, (10)\nD∙5/\nwhere 𝜌 is the density of water (or melt), 𝑉 is the volume affected by seasonal\n3\ntemperature variations, and 𝑆 is the area. Consequently, the resulting meltwater\nvolumes per unit area are approximately 0.525 m3/m2 for a porosity of 0.2 and 0.714\nm3/m2 for a porosity of 0.4, respectively. Both values substantially exceed the typical\nfluid amount (~0.1 m3/m2) (Figure R1b) required to trigger quakes up to Mw 3.\nIt should be noted, however, that the estimate above does not account for additional\nmelting that could occur if brines induce further melting of underlying ice within the\nconvective layer.\nIn summary, our estimations and calculations indicate that the hypothesized mechanism\nis energetically feasible. However, we emphasize that substantial uncertainties remain\nin many of the controlling parameters, particularly given the limited constraints\navailable for present-day Mars. Consequently, the estimated pore-pressure changes and\nthe required water volumes are guided by terrestrial analogs.\nIn response to this valuable comment, we added a discussion (Lines 452 – 461) in the\nmain text comparing the required meltwater volume per unit area to trigger marsquakes\nwith the estimated meltwater volume that can be generated under Martian conditions:\n“To assess whether the estimated water volume could be generated under Martian\nconditions with latent heat included, we performed thermal conduction modeling. Our\nsynthetic calculations show that, given a melting point of 240 K, seasonal surface\ntemperature variations can bring the melting isotherm to a depth of ~2 m within the\nconductive layer under a porosity of 0.4 (Extended Data Fig. 9). This penetration depth\nyields a meltwater volume per unit area of ~0.7 m3/m2 (see Supplementary Materials)\nthat exceeds the threshold of 0.1 m3/m2 required to induce marsquakes. Below this\nconductive layer, salinity-drive melting in the underlaying convective zone generates\nsubstantially more fluids, further supporting the physical mechanism proposed for\ninduced seasonal marsquakes.”.\nWe incorporated a latent-heat-inclusive thermal conduction scheme into the\nSupplementary Text. S3 and made the corresponding code publicly available to ensure\nreproducibility (https://doi.org/10.5281/zenodo.17341566).\n14/73\nReferences:\nAke, J. (2005). Deep-Injection and Closely Monitored Induced Seismicity at Paradox\nValley, Colorado. Bulletin of the Seismological Society of America, 95(2), 664–\n683. https://doi.org/10.1785/0120040072\nBaisch, S., Weidler, R., Voros, R., Wyborn, D., & De Graaf, L. (2006). Induced\nSeismicity during the Stimulation of a Geothermal HFR Reservoir in the Cooper\nBasin, Australia. Bulletin of the Seismological Society of America, 96(6), 2242–\n2256. https://doi.org/10.1785/0120050255\nDeng, K., Liu, Y., & Harrington, R. M. (2016). 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