DO PRESSURE AND TILT MEASUREMENTS ON THE OCEAN FLOOR REVEAL SUBDUCTION TECTONICS IN THE GUERRERO SEISMIC GAP?
Ocean bottom pressure and tilt data are essential for studying tectonic deformation associated with large earthquakes and slow slip events (SSEs) in subduction zones. The Mexico-Japan offshore experiment, conducted off the Guerrero seismic gap in Mexico, provided over five years of OBP and tilt measurements in the subduction zone. Analysing and modelling time series and bathymetry profiles for the deepest OBP-07 forebulge site enables us to estimate the contribution of Cocos plate motion to the overall OBP record. The results show that only around 3% of the large, steady linear trend observed in the OBP record at OBP-07 can be attributed to Cocos plate motion. The remainder of the trend is likely to be related to local subsidence of the fractured outer bulge of the subduction plate, as well as to instrumental drift of the pressure sensor. The modelled tilt rate at the OBP-07 site is 3–4 μrad/yr, whereas a tiltmeter sensor yields a similar trend on the mrad/yr scale. This discrepancy is likely caused by tilt amplification due to the nonlinear compaction of unconsolidated pelagic sediments and turbidites, and the local rotation of oceanic crustal blocks may also play a role. Detailed processing of other OBP records and inland GPS time series reveals that long-term SSE in 2018 and 2019 has had no noticeable effect on bottom pressure at the continental shelf. However, comparing tectonic tremors recorded by ocean bottom seismometers (OBS) with the OBP records reveals apparent independent offshore SSE in the second half of 2018.