Geodetic Exploration of the Kinematics and Fault Slip Rates of the Southeastern and Western Caribbean Plate Boundaries and the Caldera Dynamics of the Sierra Negra Volcano.
Open Access
- Author:
- Higgins, Machel
- Graduate Program:
- Geosciences (PHD)
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 17, 2021
- Committee Members:
- Charles Ammon, Major Field Member
Peter La Femina, Chair & Dissertation Advisor
Douglas Miller, Outside Unit & Field Member
Derek Elsworth, Major Field Member
Mark Patzkowsky, Program Head/Chair - Keywords:
- Geodesy
Fault Slip Rate
Strain Partitioning
GPS
InSAR
Trapdoor faulting - Abstract:
- Determining plate boundary kinematics and fault slip rates is critical to gaining an understanding of the seismogenic nature of the faults. The kinematics of plate boundaries vary widely and are a reflection of plate boundaries history, current tectonic regime, geology, and crustal properties. Geodesy is a useful tool for probing the kinematics of plate boundaries, active fault identification, slip rates, and stress accumulation. This dissertation presents research on the faulting and kinematics of the western and southern Caribbean plate boundaries. Specifically, this research focuses on the 900 km long Caribbean-South American transform plate boundary and the Caribbean-Cocos plate convergent margin in Nicaragua. This dissertation is composed of four main chapters: (1) the interseismic strain accumulation and slip partitioning at the CA-SA transform boundary, (2) exploring bookshelf faulting in Nicaragua using upper-plate earthquakes, (3) modeling bookshelf faulting in Nicaragua using geodetic data, and (4) investigating the faulting of intra-caldera earthquakes during the 2018 eruption of the Sierra Negra volcano. We combined Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data to characterize the interseismic behavior (i.e., locked or creeping), and strain partitioning for the faults along the Caribbean – South American transform plate boundary. Interseismic strain is distributed mainly on three faults, the San Sebastian, El Pilar, and Central Range faults, but partitioning occurs across multiple faults in the west (San Sebastian and La Victoria faults) and east (Sub-Tobago Terrane, Central Range, and South Coast faults). In northern Venezuela, slip is partitioned on the San Sebastian (16.4 ± 1.7 mm/yr) and La Victoria (4.3 ± 0.9 mm/yr) faults. In north-eastern Venezuela, the El Pilar fault accommodates slip at a rate of 18.6 ± 1.8 mm/yr. In Trinidad and Tobago, slip is partitioned between the Sub-Tobago Terrane (3.0 ± 0.1 mm/yr), Central Range (14.5 ± 2.0 mm/yr), and South Coast (3.0 ± 0.1 mm/yr) faults. The La Victoria, San Sebastian, the western El Pilar segment, and Sub-Tobago Terrane faults are locked to depths of 16 ± 4 km, 7 ± 5 km, 6 ± 2 km, and 8 ± 1 km, respectively. The eastern segment of the El Pilar, the Central Range, and the South Coast faults all creep. Our new InSAR results indicate that the entire Central Range Fault is creeping. The locked western segment of this transform plate boundary is capable of producing a Mw 8 earthquake, which is a significant finding regarding seismic hazard and risk. Oblique convergence and strong mechanical coupling along subduction zones result in strain partitioning and the development of translating forearc terranes. Translation of the fore-arc relative to the over-riding plate is typically accommodated by strike-slip fault systems; for example, the Great Sumatran Fault, Indonesia. The Central American Fore-Arc (CAFA) is a northwestward translating (8 mm/yr to 14 mm/yr) fore-arc sliver, the result of oblique Cocos - Caribbean convergence, and Cocos Ridge collision. However, the CAFA in Nicaragua does not have the expected trench-parallel, strike-slip fault system to accommodate its relative motion with the Caribbean Plate. It has been proposed that CAFA-Caribbean dextral shear is accommodated by clockwise rotating tectonic blocks (bookshelf faulting), where faulting is characterized by NE-trending left-lateral faulting. Using Global Positioning System data, and a Bayesian inversion approach, the kinematics and geometries of three moderate-magnitude upper-plate earthquakes in Nicaragua were determined. The April 10th, 2014 Mw 6.1, September 15th, 2016 Mw 5.7, and September 28th, 2016 Mw 5.5 earthquakes were investigated. It was found that the April 10th, 2014 earthquake occurred on a NW-SE (313°) striking fault with right-lateral coseismic slip. This is the first well-documented historical earthquake with this geometry and kinematics. The September 15th and September 28th, 2016 earthquakes were located on faults with strikes of N55° & N22°, respectively, with left-lateral and dip-slip coseismic slip. Coulomb failure stress analysis suggests that the 2016 earthquakes were promoted by the 2014 earthquake and that the September 28th, 2016 earthquake was triggered by the September 15th, 2016 earthquake. It was also found that the April 15th earthquake occurred in the vicinity of the Momotombo volcano and would have dilated the volcano magmatic system, allowing magma ascent and the subsequent December 1st, 2015 eruption. The determination of the geometry and kinematics of these upper-plate earthquakes provides support for CAFA-Caribbean dextral shear via bookshelf faulting and important implications for seismic hazard estimates. In Nicaragua, the CAFA-CA relative motion is accommodated primarily by bookshelf faulting. This project models Global Position System (GPS) data to explore bookshelf faulting. First, elastic and hetero-elastic models were investigated for arc-parallel fault systems. A two-fault elastic dislocation model fits the geodetic data but does not honor the real-world fault configuration. Second, the boundary element method (BEM) was used to investigate the arc-normal and arc-oblique faults of bookshelf faulting. BEM models show that an array of arc-normal faults, with lengths of 15 km to 20 km and spaced by 5 km or less, fits the geodetic signature seen across-strike the CAFA. These faults also have an average slip rate of 3 mm/yr. The CAFA-CA interface produces destructive shallow Mw 5.5 earthquakes. These earthquakes would have a reoccurrence interval of ~50 years. The Sierra Negra volcano is the largest of the basaltic shield volcanos on the western volcanic province of the Galapagos Islands, Ecuador. Prior to the June 26th, 2018 Sierra Negra eruption there was 6.5 m of inflation of the caldera. A Mw 5.3 earthquake occurred on the intra-caldera trapdoor fault ~8 hours before the eruption began. This project determines the coseismic faulting and kinematics of the earthquakes that occurred during the eruption. These are the Mw 5.3 June 26th, Ml 4.9 July 5th, and Ml 4.8 July 22nd earthquakes. It was found that these earthquakes occurred on the southern limb of the trapdoor fault system. It was also found that the June 26th had coseismic slip of 1.8 m right-lateral slip and 4.5 m of dip-slip (reverse motion). While the July 5th earthquake had coseismic slip of 0.4 m left-lateral and -3.2 m of dip-slip (normal faulting) and the July 22nd earthquake coseismic slip was pure dip-slip of -1.2 m (normal faulting). These earthquakes are events of rapid inflation and subsidence of the caldera floor. Over the course of the eruption net uplift of the caldera floor, with a maximum of 1.5 m, was observed. The net coseismic reverse dip-slip over the three earthquakes was of 0.2 m, which is sufficient enough to produce the observed net uplift over the eruption, leading to building the resurgent caldera. Similar earthquakes over many eruptive cycles are responsible for caldera resurgence and some of the features within the caldera.