Deep intra-slab rupture and mechanism transition of the 2024 Mw 7.4 Calama earthquake

Abstract

While subduction zone hazard is dominated by the megathrust, intermediate-depth (70-300 km) earthquakes within the slab can likewise have catastrophic impacts. Their physics remains enigmatic, with suggested mechanisms including dehydration embrittlement and thermal runaway. Here, we investigate the 2024 Chile, M_w 7.4 intermediate-depth earthquake and compare the rupture extent with temperature conditions from thermo-mechanical models. We record regional geodetic co-seismic deformation and high-resolution seismicity associated with this type of event. Our analyses reveal a complex rupture spanning an exceptional depth range, with distinct asperities propagating deep into the subducting lithosphere. Comparison with thermal models shows that while the rupture initiated within the cold slab core, it extended well beyond the ~650 °C isotherm that typically delineates the boundary for efficient serpentine dehydration. We suggest that the rupture likely initiated with dehydration embrittlement within the cold core but then propagated into the warmer regions through shear thermal runaway. This implies a transition of mechanisms that facilitates large-scale rupture and activates typically aseismic, high-temperature slab regions. Our findings highlight the importance of considering interactions between rupture mechanisms as well as slab thermal and compositional settings to better understand the processes governing intermediate-depth earthquakes.

Fig. 1. Tectonic setting and seismicity in northern Chile.

a Map of seismicity in Northern Chile, based on the relocated catalog of Potin et al. . The largest historical events in the region, and the two used empirical Green’s function events, are marked with stars. The beachball shows the Global CMT solution (https://globalcmt.org) of the 2024 M_w7.4 Calama earthquake. b Distribution of seismic and geodetic stations used in this study. c Cross-section along the profile shown in (a), red circles are aftershocks relocated in this work. Black circles represent background seismicity within 25 km of the cross-section plane, and orange circles indicate the M_w7.4 mainshock and its aftershock sequence. Background seismic tomography is from Potin et al. .

Fig. 2. Rupture process of the 2024 Calama M_w 7.4 event.

a Subevent locations and focal mechanisms (red beach balls). The hypocenter is collocated with the first subevent E1. The aftershocks detected by Chile Seismic Network (CSN) are displayed by the gray dots in the background. The inset box shows the moment rate functions for all subevents. b Cross section along the profile in (a) showing the subevents propagated sub-vertically, consistent with the aftershock locations (gray dots), favoring the steep down-dip plane dipping eastward. The top black solid line indicates the slab surface from Slab2. Histogram shows the density of aftershocks as a function of depth. c Marginal probability distributions of subevent centroid depths. d–e Representative data (black) and synthetic (red) waveform fits for teleseismic P and SH waves (0.005–0.2 Hz), and regional strong motion (0.02–0.2 Hz) full waves. The numbers leading the traces are azimuths and distances.

Fig. 3. Determination of rupture extent and velocity using source time function deconvolution.

a Downward apparent source time functions (ASTFs) deconvolved from downgoing teleseismic P waves. The waveforms of the target event and EGFs are filtered between 0.02-0.2 Hz. STF durations are defined by the red shading which contains most of the energy. b Upward ASTFs deconvolved from upgoing regional SH waves. The waveforms of the target event and EGFs are filtered between 0.03–1 Hz. c Inversion for the rupture extent and velocity. Gray and orange squares indicate the apparent source durations as a function of the vertical slowness, before and after the horizontal rupture directivity correction, respectively. The orange line indicates a linear fit to the observed trend, corresponding to a rupture length of 78 km and rupture velocity of 4.2 km/s. The surrounding-colored area illustrates the 95% confidence limit of the slope, corresponding to the error range of the resolved rupture parameters. The black lines indicate the slopes for rupture lengths of 50, 70, and 100 km, respectively.

Fig. 4. Geodetic constraints on the fault geometry and slip distribution.

a Coseismic slip distribution and GNSS horizontal displacement fits for the sub-vertical fault plane. Blue and green arrows indicate observed and model-predicted displacements, respectively. b Coseismic slip distribution and GNSS vertical displacement fits for the rupture along the sub-vertical fault plane. Full fits to both vertical and horizontal component displacements for the two fault planes are shown in supplementary Figs. S17, S18.

Fig. 5. Thermal environment of the 2024 Calama earthquake rupture.

This figure compares the rupture extents and thermal environments for a the 2024 Chile M_w 7.4 earthquake with b–d other historical large intermediate-depth earthquakes. The color shows temperature derived from thermodynamic simulations along cross sections shown in Table S3. The dark red lines indicate the estimated rupture extent for the 2024 Chile M_w 7.4 (this study), 2005 Chile M_w 7.7, 2014 Aleutian M_w 7.9, and 1993 Japan M_w 7.6, respectively. The thin blue and red lines are the 650 °C and 1000 °C isotherms, respectively. Background gray circles indicate the seismicity from the NEIC catalog since 2000.

Fig. 6. Summary schematic of the intraslab rupture and mechanism transition for the M_w 7.4 event.

The earthquake ruptured seismic asperities of various scales (red polygons), and penetrated through the 650 °C boundary allowable for serpentine dehydration, which suggests transitioning to another mechanism, likely shear thermal runaway. Gray dots represent seismicity that delineates the upper and lower seismic zones.