Generation of Sintered Fault Rock and Its Implications for Earthquake

Home , Fault gouge

ARTICLE https://doi.org/10.1038/s43247-020-0004-z OPEN Generation of sintered fault rock and its implications for earthquake energetics and fault healing ✉ Tetsuro Hirono 1 , Shunya Kaneki 2, Tsuyoshi Ishikawa 3, Jun Kameda4, Naoya Tonoike1,7, Akihiro Ito5 & Yuji Miyazaki6

1234567890():,; After an earthquake, faults can recover strength through fault healing, but the mechanisms responsible are not well understood. Seismic slip may induce sintering, a bonding process between solid particles in contact under high temperatures without melting, which could produce a fault rock with elevated strength and chemical stability. Here we present results from electron microscope analyses that show a typical sintered structure in a black disk- shaped rock from the Chelungpu fault, Taiwan. This structure is experimentally reproducible in simulated fault material, prepared from the local host-rock, by heating at 800–900 °C. Through thermal and kinetic analyses of experimental materials, we show that sintering is an exothermic process which can generate energy to enhance post-slip thermochemical reactions in the fault. We propose that sintering substantially contributes to earthquake energetics and fault healing and that its occurrence can be a useful indicator of past seismic slip.

1 Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan. 2 Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto 611-0011, Japan. 3 Kochi Institute for Core Sample Research, Japan Agency for Marine-Science and Technology (JAMSTEC), Nankoku, Kochi 783-8502, Japan. 4 Department of Natural History Sciences, Graduate School of Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan. 5 Analytical Instrument Facility, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan. 6 Research Center for Thermal and Entropic Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan. 7Present address: Nippon Light ✉ Metal Company, Ltd, Nagoya 460-0003, Japan. email: [email protected]

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echanisms that control the physicochemical properties (SEM and TEM, respectively, see “Methods”)aroundtheedges of fault materials during and after an earthquake are of individual ultrafine particles in the black disk (Fig. 1f–h). M 7,8 of particular importance, because mechanical and Such structures are typical in sintered materials ,although thermochemical processes crucially affect earthquake ener- they have previously been interpreted to result from frictional getics, the style and magnitude of fault slip, and fault healing1,2. melting5. Thus, to examine the sintering process in a fault and Field and laboratory investigations of fault rocks from exhumed its role in earthquake mechanism and energetics, we focused on faults and those recovered by drilling of deep fault zones have the black disk as a potential sintered fault rock (here named contributed to our understanding of faulting mechanisms and sinterite). related physicochemical processes3,4.AspartoftheTaiwan To examine whether sintering can occur as a result of earth- Chelungpu-fault Drilling Project,undertakenin2002underthe quake slip, when heating occurs for only several to several tens of auspices of the International Continental Scientific Drilling seconds, we carried out a series of heating experiments. A host- Program (ICDP), borehole rock samples of the Chelungpu rock sample nearby the Chelungpu fault was prepared to simu- fault, which slipped during the 1999 Chi-Chi earthquake, were lated fault material, and heated to temperatures from 600 to successfully recovered (Fig. 1a, b). The fault zones are devel- 1200 °C at 100 °C intervals, and to 850 °C for 30, 90, or 150 s in a oped within the early Pliocene Chinshui shale5.Adisk-shaped tube furnace apparatus (heating rate ~100 °C s−1) (Supplemen- black fault rock (black disk) was found at 1194-m depth within tary Fig. 1). Before heating, the sample was milled for 6 h one of the Chelungpu-fault zones6 (Fig. 1c).Thisrockis2-cm to produce ultrafine particles with diameters from several tens thick, stiffer, and more cohesive than the surrounding fault to several hundreds of nanometers (see “Methods”), and gouge, and its wet–bulk density is 2.6 g cm−3, higher than that the resulting powder was shaped into pellets with a density of of the gouge (2.2 g cm−3), but similar to that of the Chinshui 2.1 g cm−1. We used this sample preparation routine because not shale host rock (approximately 2.6 g cm−3)5.Italsoshowsshear only frictional heating but also comminution and densification foliation and striation (Fig. 1d, e). Bonding structures (referred commonly occur in the slip zone during an earthquake9. For to here as neck growths), which are partly amorphous, are comparison, we also prepared synthetic samples with varied observed by scanning and transmission electron microscopies mineral assemblages. Then, we performed comprehensive

a b Tohkoshan Fm. 120°E 122°E 120°30’ Toukoshan Fm. Alluvium 25°N TCDP Drill hole 0 (Hole B) 1 24°20’ Chelungpu FaultCholan Fm. Sanyi Fa Cholan Fm. 2 ult 3 (km) 22°N Chinshui Shale Keuichulin Fm. Changhua Fault 1 km c 120°10’ core top FG

Shuichangliu Fault

Chelungpu Fault 20 km black disk 2 cm

Alluvium Terrace Deposits d black disk e black disk Toukoshan Fm. Cholan Fm. Changhua fault Chinshui Shale 23°40’ core top Miocene Rocks Oligocene Rocks foliation FG striation epicenter 120°50’ 100 µm 30 µm Shuangtung Fault

f gh

bonding by neck growth 1 µm 50 nm

Fig. 1 The Chelungpu fault and the black disk-shaped fault rock. a Geological map of central Taiwan5. The 1999 Chi-Chi earthquake initiated on the southern Chelungpu fault, which ruptured both upward and northward. The Taiwan Chelungpu-fault Drilling Project (TCDP), started in 2002, penetrated the fault and recovered core samples from two holes (Hole A, total depth 2003 m; Hole B, total depth 1353 m)5. b East–west cross section5. Fault rocks were recovered from three dominant fault zones, at depths of 1136, 1194, and 1243 m, in Hole B. c Core photo of a black disk-shaped fault rock, the surrounding fractured rock, and fault gouge from 1194-m depth in Hole B. Microstructures of the black disk observed under an optical microscope (d) and SEM (e, f). g Bright-ﬁeld TEM image of the neck between two ultraﬁne particles and h the electron diffraction pattern (within the white circle in g), indicating the imperfect crystalline structure. Arrows in f and areas outlined by dotted red curves in g indicate neck growths. FG fault gouge.

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a b Q Q Al Al halo Al 2500 Pl Q Q Q Al Al Q Al Kao/Chl Q Q host rock 1500 500 cps (unheated) black disk

black Halo area 500 disk unheated 600 800 1000 1200 600 °C Temperature (°C) 700 °C cdhost rock 700 °C (unheated) 90 s 800 °C Intensity 900 °C

1000 °C 1 µm 1 µm Sp Sp 1100 °C e 800 °C f 850 °C 90 s 90 s 1200 °C 5 152535455565 2θ (degree) illite/muscovite 1 µm 1 µm

g 900 °C h 1000 °C i 1100 °C j 1200 °C 90 s 90 s 90 s 90 s

1 µm 1 µm 1 µm 1 µm

850 °C k 90 s l m

50 nm

Fig. 2 Sintering at high temperature accompanied by crystallinity and microstructural changes. a X-ray diffraction patterns of samples before and after heating, and of the black disk from the Chelungpu fault. Broad, rounded peaks (halos) at ~20–30° 2θ are shown by orange and pink shading, where the latter represents the increase in the area of the halo compared with the unheated sample. b Change in the halo area (intensity × 2θ) with temperature. The horizontal blue bar indicates the initial size of the halo, which corresponds to the initial amount of minerals with imperfect crystal lattices. SEM images of samples (c) before and after heating for 90 s at d 700 °C, e 800 °C, f 850 °C, g 900 °C, h 1000 °C, i 1100 °C, and j 1200 °C. k Bright-field transmission electron micrograph of the neck between two ultrafine particles and (l) the electron diffraction pattern (within the white circle in k). m Diffraction pattern of an ultrafine particle within the blue circle in k. Red arrows in e, f, and g and area outlined by dotted red curves in k indicate representative examples of neck growth. Kao kaolinite, Chl chlorite, Q quartz, Pl plagioclase, Sp spinel, Al α-alumina. analyses and observations of the heated samples, and examined around 20–30° 2θ on the XRD spectra of samples heated to the sintering process in these samples. temperatures above 1000 °C, indicating the presence of imperfect crystal lattices11, and the area of the halo increased in samples heated to higher temperatures (Fig. 2b). The XRD spectrum of the Results black disk most closely resembles that of the sample heated to X-ray diffraction analysis. The starting material composition, 800 °C (Fig. 2a, b; Supplementary Fig. 2). analyzed by X-ray diffraction (XRD), consists mainly of quartz, feldspar, illite/muscovite, kaolinite, and chlorite, similar to the fault gouge surrounding the black disk10 (Fig. 2a, “Methods”). Electron microscopic observation. Although SEM images of the The intensities of the illite/muscovite peaks on the XRD spectra starting sample and of the samples heated to ≤700 °C show decreased when the sample was heated to temperatures above similar structures, the samples heated to 800, 850, and 900 °C for 800 °C and disappeared at 900 °C, and the kaolinite/chlorite peak 90 s show neck growths, mainly between ultrafine particles disappeared at 700 °C. Quantitatively, the illite/muscovite content (Fig. 2e–g). The samples heated above 1000 °C show highly became 0 wt.% above 900 °C, and the quartz and feldspar con- connected smooth structures together with thick bridges and tents were decreased at temperatures above 1000 °C (Supple- hollows where particles are no longer observed (Fig. 2h–j). A mentary Fig. 2). Wide, rounded peaks called halos evolved at bright-field TEM image clearly shows the presence of a neck

COMMUNICATIONS EARTH & ENVIRONMENT | (2020) 1:3 | https://doi.org/10.1038/s43247-020-0004-z | www.nature.com/commsenv 3 ARTICLE COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-020-0004-z between two ultrafine particles (Fig. 2k), and the electron dif- plagioclase, and K-feldspar with unmilled or 6-h-milled chlorite, fraction pattern of the neck indicates that it has a noncrystalline dehydroxylation of the latter occurred at a relatively lower tem- structure (Fig. 2l). Some of the ultrafine particles are amorphous perature (ca. 300–600 °C), indicating that the reaction was because the electron diffraction pattern indicates an imperfect crystalline structure (Fig. 2m); this characteristic is attributed to a a Host rock of distorted mineral structure. The amounts of crystalline quartz, Chelungpu fault dehydoxylation sintering melting feldspar, and illite/muscovite in the samples decreased after 6 h of 0.0 100 ) Remaining mass (%) 1 –1 99 milling (Supplementary Fig. 2). The neck structure and its crys- – –0.1 54.8 J g tallographic features, which were experimentally produced at 98 –0.2 TG 800–900 °C, are representative of sintered materials7,8 and also 97 resemble those observed in the black disk (Fig. 1f, g). –0.3 96 –0.4 DSC 95 Thermal analysis. To establish that sintering has occurred in the –0.5 94 – 93 temperature range of 800 900 °C, thermochemical examination is –0.6 92 required. A thermogravimetric curve, which depicts the change in Heat flux (mW mg the total mass of a sample during heating, is used to determine the –0.7 91 temperatures at which dehydration, dehydroxylation, and decom- –0.8 position of minerals occurs, and the amount of heat required to 0 100 200 300 400 500 600 700 800 900 1000 increase the sample temperature is used to determine not only the Temperature (°C) specific heat capacity but also the heats of reaction and transfor- mation. The simultaneous thermogravimetry–differential scanning b Simulated non-clay “ ” fi fault materials calorimetry ( Methods )pro le of the 6-h-milled host-rock sample 0.0 100 – shows endothermic peaks at around 100 700 °C together with a ) Remaining mass (%) 1 99

– –0.1 mass loss (Fig. 3a). It also exhibits a large endothermic peak without 98 any mass loss at ~920–1000 °C. The former peaks are mainly –0.2 97 attributed to dehydration of interlayer water and dehydroxylation of –0.3 clay minerals, whereas the latter peak corresponds to melting, 96 because a large amount of latent heat is required for melting of –0.4 95 silicate minerals12, and because our heated samples displayed a –0.5 94 93 glassy structure under SEM. In contrast, the exothermic peak of –0.6 −1 – 54.8 J g observed at ~800 920 °C is attributed to sintering, which Heat flux (mW mg 92 is an exothermic reaction because the associated decrease of surface –0.7 91 area releases surface energy7,8. The SEM images of the heated –0.8 samples in this temperature range in fact show neck growth, 0 100 200 300 400 500 600 700 800 900 1000 causing a decrease of the surface area. Melting of silicate minerals is Temperature (°C) also accompanied by a decrease of surface area, but its latent heat Q+Pl+Kspar Q+Pl+Kspar (unmilled) (6-h-milled ) requirement is much larger than the surface energy release due to c Simulated clayey this decrease7,8,12.Therefore,thethermochemicalproﬁle clearly fault materials indicates that sintering occurred at ~800–920 °C, and that melting 0.0 100 ) Remaining mass (%) started above 920 °C, and both results are supported by our crys- 1 99

– –0.1 tallographic analysis and microscopic observation. 98 –0.2 97 –0.3 Evaluation of the effect of multicomponent mineral assem- 96 blages. Because rock samples in fault zones have multiple com- –0.4 95 ponents, including comminuted quartz, feldspar, and clay –0.5 94 minerals, we also considered the effects of the eutectic system and 93 of mineral distortion (“Methods”). In a comparison of unmilled –0.6

Heat flux (mW mg 92 and 6-h-milled mixtures of quartz, plagioclase, and K-feldspar, –0.7 91 neither mixture showed an exothermic peak at temperatures up –0.8 to 1000 °C nor any neck growth at 850 °C (Fig. 3b; Supplementary 0 100 200 300 400 500 600 700 800 900 1000 Fig. 3a, b). Therefore, in this condition, sintering did not occur Temperature (°C) even when amorphous particles were included. In comparisons of Q+Pl+Kspar Q+Pl+Kspar Q+Pl+Kspar 6-h-milled mixtures of quartz, plagioclase, and K-feldspar to +illite/muscovite +chlorite +illite/muscovite (6-h-milled) (6-h-milled) +chlorite+kaolinite which clay minerals (illite, chlorite, smectite, or kaolinite, or all of +smectite them) were added, only those mixtures that included chlorite Q+Pl+Kspar Q+Pl+Kspar (6-h-milled) +smectite +kaolinite showed exothermic peaks and neck growth (Fig. 3c; Supple- (6-h-milled) (6-h-milled) mentary Fig. 3c–g). This result is consistent with a previous thermochemical result that sintering started at 800 °C in slate Fig. 3 Simultaneous thermogravimetry (TG)—differential scanning composed of mica, quartz, feldspar, and chlorite, from Berja, calorimetry (DSC) results. TG–DSC proﬁles of a a host-rock sample Almería, Spain13. Thus, sintering can occur in a simulated fault collected near the Chelungpu fault, b simulated non-clay fault materials material that includes chlorite. Because at ~800–900 °C, chlorite composed of quartz, plagioclase and K-feldspar, and c simulated clayey decomposes and is neocrystallized into new minerals such as fault materials composed of non-clay minerals (quartz, plagioclase, and K- olivine, spinel, and enstatite14; in our experiments, sintering may feldspar) and one or all of illite/muscovite, chlorite, kaolinite, and smectite. be driven by the diffusion of atoms from the decomposed chlorite Dotted and solid curves show the TG and DSC data, respectively. Q quartz, matrix. In comparisons of 6-h-milled mixtures of quartz, Pl plagioclase, Kspar K-feldspar.

4 COMMUNICATIONS EARTH & ENVIRONMENT | (2020) 1:3 | https://doi.org/10.1038/s43247-020-0004-z | www.nature.com/commsenv COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-020-0004-z ARTICLE enhanced mechanochemically, but thermal decomposition and time, we applied a temperature–time profile estimated based on phase transition of the chlorite and sintering occurred at similar the chemical kinetics of the thermal decomposition of carbonate temperatures (Supplementary Fig. 4). These findings indicate minerals15, which compared with the surrounding host rock that, in sedimentary rocks, chlorite can function as an auxiliary showed, had nearly disappeared from the black disk. We assumed sintering agent. We observed no glassy structures at 850 °C in any a 6-s slip time, the same as that of the 1999 Chi-Chi earthquake16 of these mixtures (Supplementary Fig. 3), so melting associated (“Methods”). The simulation results showed that X/D hardly with eutectic system effects did not occur at ≤850 °C in these increased during coseismic slip, and that sintering occurred mineral assemblages. mainly during the post-seismic phase (Fig. 4b). At 1500 s, X/D was 0.30, approximately the same as the X/D value of 0.36 ± 0.07 in the black disk from the Chelungpu fault. Chemical kinetic analysis. To understand when sintering can occur during seismic slip, we conducted a chemical kinetic analysis of neck growth. We focused on sintered spherical or ellip- Discussion soidal particles ~300 nm in size on SEM images because neck Our experimental results revealed that sintering occurs at growth is dependent on particle size7,8. We measured the major ~800–920 °C in simulated gouges of the Chelungpu fault, and the long and short axes of the particles and calculated their diameter microstructural and crystallographic features of the heated sam- (D) by assuming them to approximate circles, and we also mea- ples coincide with those observed in the black disk. In addition, sured the interparticle neck width (X). The ratio of neck width to the black disk lacked the major and trace- element signatures that particle diameter (X/D) increased with heating time t, and would be produced by melting of sediment-hosted fault zone showed a good correlation with the first-order reaction time, rocks17 (Supplementary Methods and Supplementary Figs. 5 and expressed by X/D = 1 − exp(−kt), where k is the reaction rate 6). Therefore, the black disk should be identified as sinterite constant (Fig. 4a). Then, using the Arrhenius equation, which rather than as pseudotachylyte, a solidified friction-induced melt relates k to temperature, we determined the activation energy of that has been considered as earthquake fossil because it records the sintering reaction to be 60.0 ± 9.5 kJ mol−1 and the pre- past activity on a fault18,19. The sinterite from the Chelungpu exponential factor to be 5.3 ± 2.7 s−1. Because the temperature on fault might have experienced 800–900 °C during a seismic event. a fault during an earthquake is not constant but changes with Total energy released from a fault during an earthquake is generally partitioned into energy radiated as seismic waves, frictional heat, surface fracture energy (which creates a new rupture 20 a 1.0 surface), and energy for endothermic chemical reactions . X/D =1–exp(–2.21 x 10–2 t) However, during the post-seismic phase, the exothermic sintering reaction may serve as an energy source for thermochemical 0.8 reactions. In the case of the black disk, we calculated the energy

X/D 900 °C released by sintering to be 1.03 MJ m−2 (=54.8 J g−1 exotherm × 2.6 g cm−3 density × 0.36 reacted fraction × 2-cm thickness), 0.6 850 °C which is almost the same as the amount of surface fracture energy X/D =1–exp(–1.13 x 10–2 t) used (0.65 MJ m−2)21 during the 1999 Chi-Chi earthquake. Thus, 0.4 neck the exothermic energy associated with sintering has the potential grain a to enhance post-seismic healing processes, such as pressure- grain X 800 °C D b solution welding22 and mineral precipitation23. In addition, sin- 0.2

Neck size ratio, –3 tered materials generally have high bulk density, stiffness, X/D =1–exp(–7.40 x 10 t) D = (a b)0.5 300 nm strength, and chemical stability7,8, e.g., sintered kaolinitic clay, a 0 possible sinterite analog, has a high ﬂexural strength of ~30 MPa, 0 50 100 150 200 whereas the initial kaolinitic powder, an analog of fault gouge, has ﬂ 24 Time, t (s) a exural strength of close to zero . Changes in the mechanical properties of fault rock caused by sintering may affect the post- coseismic seismic fault healing. Friction-induced melting of fault rocks, that b 1000 1.0 is, formation of pseudotachylyte, may also contribute to fault post-seismic healing25,26, but pseudotachylyte formation consumes a large 800 0.8 Neck size ratio 12 °C) amount of latent heat . Thus, sintering may be a central reaction in fault healing, one that not only promotes other healing 600 0.6 mechanisms, but also contributes directly to strength recovery7,8,24. 400 black disk 0.4 At present, reports of sintered fault rocks are scarce. However, Currewitz and Karson27 interpreted pseudotachylyte from nor- 200 0.2 mal faults developed in gneiss in East Greenland that exhibit

Temperature ( interlocking particles as sintered ultracataclasite, and were the ﬁrst to propose a role of sintering in fault healing. On the mirror- 0 6 0 0 50 100 500 1000 1500 like surfaces of carbonate-bearing faults associated with the Dead Time (s) Sea transform, coatings were observed on tightly packed nano- particles28; these coatings, together with structural traces of neck Fig. 4 Neck growth kinetics during heating and simulation of neck growth growth between nanoparticles, were experimentally reproduced during an earthquake. a Changes in the neck-size ratio (X/D) with heating by using a rotary shear apparatus29. Similar structures have been duration. b Simulation of neck growth during coseismic and post-seismic observed in synthetically heated carbonate minerals at >600 °C phases based on the temperature–time proﬁle (red curve) for the 1999 Chi- (ref. 30) and also in experimentally sheared calcite gouge when the Chi earthquake. Error bars indicate standard deviations from the mean temperature reached ca. 700 °C (ref. 31). Formation of connected values. The purple curve shows the evolution of the neck-size ratio. nanoparticles might be controlled by thermally activated

COMMUNICATIONS EARTH & ENVIRONMENT | (2020) 1:3 | https://doi.org/10.1038/s43247-020-0004-z | www.nature.com/commsenv 5 ARTICLE COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-020-0004-z dislocation and decomposition of carbonate minerals30. Densifi- samples before and after heating were determined by the reference-intensity ratio cation and a decrease in the specific surface area of experimen- method42,43. tally sheared quartz gouge and sheared clayey gouge, collected from the Nojima fault, have also been attributed to sintering Simultaneous thermogravimetry–differential scanning calorimetry. A Netzsch fl 32,33 STA 449 C Jupiter balance was used; the resolutions of TG, DSC, and temperature induced by ash heating at particle contacts . In addition, were 1 µg, 1.25 µW, and 0.01 °C, respectively. An approximately 30-mg powder −1 connected hematite polygonal nanocrystals are observed on a sample was placed in a covered Pt90Rh10 crucible and heated at a rate of 20 °C min fault mirror that cuts Fe ore, reflecting sintering and interlocking from room temperature to 1000 °C under a flow of argon gas (50 mL min−1). of crystals across the slip surface during and after seismic slip34. Moreover, sinter hardening of materials by subjecting fine pow- Evaluation of the effect of multicomponent mineral assemblages.Asan ders to heat and pressure is a frequently applied industrial pro- additional heating experiment to evaluate the eutectic system, standard mineral cess35. The temperature range, process, and mechanism of samples of quartz, plagioclase, K-feldspar, illite, chlorite, kaolinite, and smectite were obtained from quartz sand (Wako Pure Chemical Industries, Japan), peg- sintering might vary greatly, depending on the mineral or rock matite (Shinyashiki, Fukushima, Japan), and the Clay Mineral Society (IMt-2, illite, type, the sintering agent, and environmental conditions, such as Silver Hill Montana, USA; CCa-2, chlorite, Flagstaff Hill, USA; KGa-1b, kaolinite, stress and fluid content. Warren County Georgia, USA; SWy-2, Na-rich montmorillonite, Crook County Some previously reported cataclasites/ultracataclasites, espe- Wyoming, USA). The minerals were mixed so that their mass ratios were the same as in the host-rock sample, and then heating experiments were performed as cially within plate-subduction faults with a chlorite component, follows: (1) the sample mixtures were milled for 6 h, (2) the resulting particles were may actually be sinterites. In the Nankai Trough, a shear- shaped into pellets, (3) the pellets were inserted into a quartz tube under vacuum localized dark fault gouge retrieved from the megasplay fault that condition, and (4) the pelleted samples were heated to 850 °C for 90 s in a tube branches from the plate interface included ~10 wt.% chlorite36 furnace apparatus. and showed an exothermic peak when heated to 750 °C (Sup- plementary Fig. 7). Therefore, this gouge has the potential to Reconstruction of the earthquake’s temperature–time profile. The disk-shaped black fault rock had a low inorganic carbon content (mainly carbonaceous minerals become sinterite during seismic slip. In sediment-hosted fault such as calcite)5, which was attributed to the thermal decomposition of calcite zones with low permeability, fault strength may often be dra- during the earthquake5,15. The amounts of carbonaceous minerals in and around matically decreased by friction heating-induced pressurization of the fault zone at 1194-m depth were measured by coulometric titration with a CO2 interstitial fluid during an earthquake (i.e., thermal pressuriza- coulometer (UIC Inc., Coulometrics model CM5012)44. The amount of carbonate 37 minerals, mainly calcite, in the disk-shaped black fault rock, was 0.05 wt.%, tion ), thereby preventing the fault from attaining the high compared with 0.43 wt.% of carbonate minerals in the surrounding host rocks15; temperatures required for sintering and melting. However, recent therefore, the fraction of carbonate minerals lost by thermal decomposition from successive discoveries of pseudotachylytes in exhumed accre- the black disk was 0.88. tionary complexes38–40 imply that the occurrence of sinterites, The degree of thermal decomposition of calcite α (0 ≤ α ≤ 1, where α = 1 means which can be generated at lower temperatures than pseudo- total decomposition), is expressed as dα 2 tachylytes, may not be so rare. ¼ kð1 À αÞ3 ; Our discovery of the role of sintering in faults has important dt implications not only for estimating earthquake energetics, but where t is the reaction time, and k is the reaction rate35. The relationship between the reaction rate and temperature is expressed by the Arrhenius equation also for evaluating fault healing and earthquake cycles. Moreover, E we expect the presence of sinterite in a fault zone to become a k ¼ Aexp À a ; new useful indicator of rapid slip associated with past seismic RT events. where A is a constant (pre-exponential term), Ea is the activation energy necessary for a reaction to occur, R is the gas constant (8.31447 JK−1 mol−1), and T is temperature (K). By solving simultaneously the equations for chemical kinetics, Methods frictional heating and heat conduction, and one-dimensional diffusion under the Grinding procedure. A laboratory planetary mill (Pulverisette 6, Fritsch, Germany) constraint provided by the reacted fraction (i.e., the mass fraction of decomposed was used to grind samples for 6 h at 600 rpm. For each milling, a 5-g air-dried calcite, α = 0.88), the temperature–time profile (and also shear stress) could be sample was placed in a 45-cm3 sintered alumina pot with eight alumina balls uniquely determined15. (10-mm diameter). Grinding was suspended for 5 min after 20 min of milling to The temperature–time profile recorded in the disk-shaped black fault rock was prevent the sample temperature from rising; thus, the temperature at the end of reconstructed, and shear stress during the earthquake was determined to be 15 = 7 −1 = 1.31 MPa (ref. ), by applying the kinetic parameters A 2.68 × 10 s and Ea each experiment, measured with a needle-probe thermometer, was only slightly − higher than room temperature (~40 °C). 187 kJ mol 1 (ref. 45) for the thermal decomposition reaction, and along with other parameters during the earthquake [in situ initial temperature, 46.5 °C (ref. 46); total displacement, 8.3 m (ref. 21); slip time, 6 s (ref. 16); specific heat capacity, − − − Electron microscopy. Submicrometer-scale structures of the natural and experi- 300 J kg 1 K 1 (ref. 47); density, 2200 kg m 3 (ref. 48); thermal diffusivity, 1.0 × − − ment samples before and after heating were examined under an SEM (JSM-7600F, 10 6 m2 s 1 (ref. 47)] (Fig. 4b). JEOL, Japan) operated at an acceleration voltage of 15 kV, and a transmission electron microscope (JSM-2100, JEOL, Japan) operated at 200 kV. However, it has fi Data availability been technically dif cult to identify neck structures in natural fault rocks, probably – because of the use of resin in the sample preparation: when our heated host-rock Data of TEM, XRD, TG DSC, and chemical kinetic analyses in this paper are available at sample at 850 °C and 90 s was fixed by using an epoxy resin, no neck structure was https://doi.org/10.5281/zenodo.3903901. identifiable under either optical or electron microscopes (Supplementary Fig. 8). Received: 17 February 2020; Accepted: 19 June 2020; X-ray diffraction spectroscopy. X-ray diffraction patterns of the samples before and after heating were obtained by using a Spectris PANalytical X’Pert PRO MPD spectrometer with monochromatized CuKα radiation operated at 45 kV and 40 mA, with a step width of 0.004° (Δ2θ), 0.25° divergence and antiscattering slits, and a high-speed semiconductor array detector. The samples were blended with α- alumina (20 wt.%) as an internal standard and mounted on XRD glass holders by References the side-load method to minimize any preferred alignment of phyllosilicates. The 1. Niemeijer, A. et al. Inferring earthquake physics and chemistry using mineral assemblage of the host-rock sample, collected from the Chinshui Shale at an integrated field and laboratory approach. J. Struct. Geol. 39,2–36 1104.76-m depth in Hole A (40 m southeast of Hole B)5, was quantitatively (2012). determined to be composed of quartz, plagioclase, K-feldspar, illite, chlorite, 2. Rice, J. R. Heating and weakening of faults during earthquake slip. J. Geophys. kaolinite and smectite (40.0, 11.5, 6.9, 21.1, 13.1, 5.0, and 2.4 wt.%, respectively10) by using the XRD RockJock program41, which are similar to the fault gouge sur- Res. 111, B05311 (2006). rounding the black disk (43.4, 11.3, 7.7, 19.0, 10.8, 2.2, and 5.6 wt.%, respec- 3. Scholz, C. H. The Mechanics of Earthquakes and Faulting (Cambridge tively10). The mineral amounts of quartz, feldspar, and illite/muscovite in the University Press, 2002).

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Earth We thank T. Kondo and G. Uramoto Ito for their technical support during our heating Planet. Sci. Lett. 206, 161–172 (2003). experiments and scanning electron microscope observations. We also thank S.-R. Song, 24. Martin-Marquez, J., Rincon, J. M. & Romero, M. Effect of firing temperature K.-F. Ma, and all other scientists and operation staff on the Taiwan Chelungpu-fault on sintering of porcelain stoneware tiles. Ceram. Inter 34, 1867–1873 (2008). Drilling Project and ICDP. This work was supported by JSPS KAKENHI Grant Number 25. Mitchell, T. M., Toy, V., Di Toro, G., Renner, J. & Sibson, R. H. Fault welding 19K04039. by pseudotachylyte formation. Geology 44, 1059–1062 (2016). 26. Proctor, B. & Lockner, D. A. Pseudotachylyte increases the post-slip strength Author contributions of faults. Geology 44, 1003–1006 (2016). T.H. proposed the investigation of the relation between sintering and earthquakes. T.H., 27. Curewitz, D. & Karson, J. A. Ultracataclasis sintering and frictional melting in S.K., N.T., and A.I. conducted the heating experiments, X-ray diffraction analysis, and pseudotachylytes from East Greenland. Jour. Struct. Geol 21, 1693–1713 microscopic observations, and T.H. and S.K. performed thermochemical and kinetic (1999). analyses. T.I. performed geochemical evaluation and modeling of the fault rock sample. 28. Siman-Tov, S., Aharonov, E., Sagy, A. & Emmanuel, S. Nanograins form J.K. and Y.M. contributed to discussion about mineralogy and thermochemistry asso- carbonate fault mirrors. Geology 41, 703–706 (2013). ciated with sintering process. All authors wrote the paper. 29. Siman-Tov, S., Aharonov, E., Boneh, Y. & Reches, Z. Fault mirrors along carbonate faults: formation and destruction during shear experiments. Earth Planet. Sci. Lett. 430, 367–376 (2015). Competing interests 30. Pluymakers, A. & Røyne, A. Nanograin formation and reaction-induced The authors declare no competing interests. fracturing due to decarbonation: implications for the microstructures of fault – mirrors. Earth Planet. Sci. Lett. 476,59 68 (2017). Additional information 31. Pozzi, G. et al. A new interpretation for the nature and significance of mirror- Supplementary information is available for this paper at https://doi.org/10.1038/s43247- like surfaces in experimental carbonate-hosted seismic faults. Geology 46, 583–586 (2018). 020-0004-z. 32. Sawai, M., Shimamoto, T. & Togo, T. Reduction in BET surface area of Correspondence Nojima fault gouge with seismic slip and its implication for the fracture energy and requests for materials should be addressed to T.H. of earthquakes. J. Struct. Geol. 38, 117–138 (2012). Peer review information 33. Togo, T. & Shimamoto, T. Energy partition for grain crushing in quartz gouge Primary handling editor: Joe Aslin. during subseismic to seismic fault motion: an experimental study. J. Struct. Reprints and permission information Geol. 38, 139–155 (2012). is available at http://www.nature.com/reprints 34. Ault, A. K., Jensen, J. L., McDermott, R. G., Shen, F.-A. & Van Devener, B. R. Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in Nanoscale evidence for temperature-induced transient rheology and fi postseismic fault healing. Geology 48, 1203–1207 (2019). published maps and institutional af liations.

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