SEAFLOOR GEODESY UNVEILS SEISMOGENESIS OF THE LONG-FEARED GUERRERO GAP
Noise could be overwhelming, particularly when it is non-harmonic and dominates the bandwidth of interest. This is the case at the seafloor when looking for transient tectonic deformations using ocean bottom tiltmeters and pressure gauges. Non-tidal oceanographic fluctuations of the water column can seriously obscure the information. This means that the stillness of the deep ocean and the extreme sensitivity of some geophysical instruments do not necessarily allow us to see what we look for; i.e., seeing everything may be too much. An alternative to alleviate this problem might come from instruments designed to look primarily at the object of study. That is, noise-insensitive devices that amplify potentially useful signals over a known bandwidth. Here we show that low-cost tiltmeters mounted on a steel tripod over highly compressible marine sediments act as ultra-long-period tilt mechanical amplifiers (ULP-TMA) that see slow tectonic deformations and are blind to most oceanographic noise. These devices, which operated continuously from November 2017 to March 2024 in the Guerrero Seismic Gap (GGAP) at depths between 1,000 and 5,200 m, enabled measuring for the first time worldwide seafloor rotations associated with Slow Slip Events (SSE) as follows.
On September 8, 2021, a Mw7.0 thrust earthquake occurred beneath Acapulco, Mexico, causing significant local damage and triggering the public early warning system in Mexico City 280 km north of the epicenter. Four months before the earthquake, a slow slip event was initiating to the east, in Oaxaca, and propagated to the Costa Chica of Guerrero over the deep segment of the plate interface. Dense GNSS data and unprecedented seafloor geodetic observations acquired aboard the UNAM R/V El Puma in seven oceanographic campaigns revealed that another SSE, this time shallow, initiated in April 2021 next to the oceanic trench of the GGAP, first observed in Mexico, and propagated downdip towards the earthquake hypocentral region during the five months prior to rupture. This SSE as well as the mainshock were recorded either by seafloor hydrostatic pressure sensors (vertical displacement) and/or collocated ULP-TMA (two tilt components), and imaged through the join inversions of GNSS, InSAR and seafloor geodetic data last acquired at sea in March 2024. The earthquake and its postseismic relaxation triggered a Mw7.3 long-term SSE deep in Guerrero that significantly increased the seismicity of the region. Between December 2022 and March 2023, during its late phase, the SSE climbed up the plate interface from 20 to 5 km depth reaching 35 km offshore just southwest of Acapulco. Then a new shallow short-term SSE (3-15 km depth) occurred offshore Acapulco starting around July 2022. These results indicate that the genesis of potentially devastating earthquakes in the GGAP is controlled by the sway of slow slip transients interacting from the trench to 50 km depth that eventually break locked asperities at seismogenic depths, phenomenology with important implications for the origin of large subduction earthquakes in the globe.