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Source Parameter Analysis Indicates Both Hydrous Phase Breakdown and Thermal Shear Runaway Drive Unusual Subduction Seismicity in Central Colombia

Description: 

Unusual earthquakes occur 60–200 km beneath Colombia where the young, warm Nazca slab subducts. These earthquakes in the Cauca cluster, between 3.5°-5.5° N and 77.0°-75.3° W, occur within and up to 50 km above the Nazca plate. The earthquakes above the plate represent an instance of long-lived supraslab seismicity. To understand how rupture varies with different pressure-temperature regimes of the subduction process, we estimate the stress drop and energy efficiency for 472 earthquakes (magnitudes 1-4) in the slab and supraslab regions using P and S wave data from the Servicio Geológico Colombiano. We use a clustering approach, forming 10 Nazca slab and 3 supraslab subgroups, to reduce the tradeoff between source properties and along-path effects. Within a subgroup, each earthquake has an independent corner frequency but a common average along-path effect to each station. We jointly invert for these two parameters using simulated annealing. Initial results suggest that the supraslab region may have lower average stress-drops (0.2–12.7 MPa) and less spread in radiation efficiency (0.03–0.65) than the Nazca slab (stress drops of 0.2–77.4 MPa; radiation efficiencies of 0.01–1). Large errors limit resolution, so we test different models assumptions to check the robustness of observed trends.

High stress drop and low radiation efficiency suggest thermal shear runaway due to increased slip and increased heat generation, whereas low stress drop and moderate-to-high radiation efficiency indicate the breakdown of hydrous phases due to efficient slip across a hydrated rupture. While earthquakes in both regions have similar average efficiencies and stress-drops, the higher stress-drop and efficiency averages in the Nazca slab suggest that both thermal shear runaway and hydrous phase breakdown cause earthquakes in this region. In the supraslab region lower stress drops and efficiencies may indicate that most of these earthquakes are caused by the breakdown of hydrous phases.

Session: Earthquake Source Parameters: Theory, Observations and Interpretations [Poster]

Type: Poster

Date: 4/18/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: Pablo Aravena

Student Presenter: Yes

Invited Presentation: 

Authors