The south central Chilean margin is one of the most seismically active subduction zones on Earth, generating some of the largest earthquakes on the planet, including the largest ever recorded in 1960 near Valdivia, Chile (Mw 9.5). Using the 15 km streamer and 6600 in3 tuned seismic airgun array aboard the R/V Marcus G. Langseth, multichannel seismic (MCS) data were collected over the forearc and trench from ~30∘𝑆 to 44∘𝑆 in 2017 as a part of the Crustal Examination from Valdivia to Illapel to Characterize Huge Earthquakes (CEVICHE) experiment.
Seismic reflections originating in the lower crust and from the crust-mantle boundary were observed intermittently along the north-south transect beneath the forearc from 35.3∘𝑆 to 41.4∘𝑆 and 43.6∘𝑆 to 44∘𝑆. Deep reflection package pairs at ~6 s and 8 s two-way travel time (TWTT) were observed from ~36.2∘𝑆 to 39.3∘𝑆 and are interpreted to be the top and bottom of the roughly 7 km thick subducting Nazca plate crust, dating to the late Paleogene. Triple reflection packages were observed from 41∘𝑆 to 41.4∘𝑆 and 43.3∘𝑆 to 44∘𝑆 where the subducting plate dates to the Neogene.
The change in spectral amplitudes along the north-south transect were analyzed to characterize the effects of background noise along the line in order to evaluate whether the presence or absence of reflections from the lower crust could be attributed to changes in the background noise level rather than geologic changes in lower crustal reflectivity. Most of the energy in the signal reflected back from the lower crust and upper mantle lies within the 3-10 Hz frequency band, followed by the 10-20 Hz band.
Wide offset fan shot data were recorded onshore using short period seismometers placed in east-west transects at 36.4∘𝑆, 38.2∘𝑆, and 40.5∘𝑆. One dimensional forward models were generated for the far segments north and south of each transect, constraining the velocity of the deep crust, the depth of the Moho discontinuity between oceanic crust and upper mantle, and velocity of the upper most mantle. 2D tomography based on first arrivals recorded on the 15 km long streamer constrained upper crustal velocities over the forearc to ~5 km depth at 37.1∘𝑆 to 38.2∘𝑆 (MC09) and 40.7∘𝑆 to 41.5∘𝑆 (MC20B). The 2D velocity models were combined with the 1D models to convert MCS09 and MCS20B from TWTT to depth. The depths of the reflection package pairs in MCS09 agree with the Moho depth in the combined velocity model, placing the top and bottom of the subducting crust at approximately 15 km and 23 km respectively. Reflections in MCS20B are correlated to the depths of a high velocity zone, plate boundary, and Moho in the velocity model, showing the possibility of a high velocity structure (7.5 km/s) centered at 41.1∘𝑆, overlying the subduction channel (5.5 km/s) and subducting Nazca plate (6.3 km/s).
Comparing the location of deep reflections with seismic activity in the region from 1980 to present shows a correlation between strong deep reflections and recent seismicity in south central Chile. Clear deep reflections are observed in the seismically active 2010 Maule rupture area and Arauco Peninsula, as well as the 2016 Puerto Quellon rupture area, with weak or no reflections observed in the less active 1960 Valdivia rupture region. Increased permeability after large earthquakes allow the redistribution of fluids through new fractures along the plate interface, creating high acoustic impedance contrasts that may explain increased reflectivity in these regions. Frequent seismicity can prevent these reflective regions from recovering. The Valdivia region may have mostly healed since the Mw 9.5 event in 1960, where low reflectivity and low seismicity may be an indication of closing fractures, reduced porosity, and fault strengthening.