Caribbean OBS Fault Research Survey. Iniciative (COFRESI)

Transcription

1 Caribbean OBS Fault Research Survey Iniciative (COFRESI) May 6, 2013 PROPOSAL Submitted to: National Science Fundation Submitted by: Department of Geology, University of Puerto Rico at Mayagüez Proposal Duration: 16 months Proposed Amount: $ 411,009. Starting Date: January 1, 2014 Principal Investigator: Fernando Martínez Torres, Bs. Graduate Student

2 1 Proposal Statement PROJECT DESCRIPTION I propose to deploy five Ocean Bottom Seismometer (OBS) at the Northeastern of Puerto Rico-Virgin Islands in order to record earthquake hypocenters from a swarm of earthquakes that occurred in this region every year. These seismic eventes will be relocate using a double difference earthquake location algorithm and the data will be use to create a tridimensional wave velocity model of this region. The creation of such a model is intended to better understand the location and geometry of the North American subducting slab under the Caribbean plate. Visualizing the geometry of the subducting slab is critical in helping understand the aspects of earthquake rupture processes in this region and is a key component for constraining calculations such as inference of subduction zone mechanics, interpretation of seismic structure and dynamic modeling (Anderson et. al., 2007). 2 Objective Statement The purpose of my research is to improve the current wave velocity model creating a tridimensional model that takes into account the location of the slab, for having a better location of earthquakes close to the Puerto Rico Trench. The goal of this research is to: (1) Relocate a one-year period of events using joint data from the Puerto Rico Seismic Network (PRSN) land stations and Ocean Bottom Seismometers, and (2) Produce a tomographic image of the forearc that will be valuable for location operations at the PRSN. This exercise is expected to test my hypothesis of quantifying the PRSN bias towards offshore event locations. If our relocated events show a depth dependency on slab interface, then I can be certain that our velocity model is precise, and therefore ready to implement in PRSN daily location operations. 1

3 3 Introduction/Background The Puerto Rico Seismic Network (PRSN) has recorded seismic swarms data at the northeastern region of Puerto Rico up to the present, since it began operating in Scientist believe that these seismic swarms are caused by a tear in the North American slab due to the increases of the plate boundary arc curvature (Brink, 2005). This hypothesis has not been yet proven due to sparse and inaccurate data sets. Puerto Rico-Virgin Islands (PRVI) microplate is part of the northeastern island arc of the Caribbean. The placement of seismometers has been inhibited as a result of poor land exposure, limiting and thus preventing us to install seismometer at the north area where the seismic swarms are taking place, having an azimuthal gap of 270º. The closest seismic station at the North of the seismic swarms may be in the Bahamas or Florida, where stations are 1,500 kilometers away, making unable to record the type of low magnitude events characteristic of these swarms. At such distant stations, swarm earthquakes from the northeastern PRVI region may not be detected, on account of these seismic swarms are not capable of releasing enough energy, hence instruments may collect incomplete or no data from them. This serve as evidence that such region is complex, at least in the seismological sense. Therefore, it is vital to have instruments at the north of these seismic swarms to be able to identify active faults where the swarms are occurring, constrain their depth and estimate faults geometries. Another limiting factor for offshore earthquake locations at PRSN is the existing velocity model, were this one-dimensional velocity model assumes that all layers in the trench are horizontal, limiting the ability to locate hypocenters. (Figure 3). This limitation we currently face created a bias at the time of locate earthquakes, obtaining ambiguous hypocenter location. Therefore, 2

4 we may not know if the published hypocenters are correct. These seismic hypocenter locations are essential to understand and locate the rupture processes of earthquakes. This results in the inability to explain the nature of seismic swarms. In order to circumvent this problem the United States Geological Survey (USGS) deployed five OBSes at the North of the Virgin Islands for a period of six months in 2007 (Figure 1)(López et. al., 2009). With the OBS data we are able to decreased the azimuthal gap from the PRSN land station from 270 to 70, improving hypocenter relocations (Vieten, 2012). The data recorded by the OBSes was merged with the data recorded by the land PRSN stations in order to produce a complete P and S phase arrival dataset(lópez et. al., 2009). To process the recorded data by the OBS, it was use the programs of HypoDD and TomoDD in order to constrain a three-dimensional velocity model of the Northeastern of the Caribbean. This contribute in decrease the existing bias in order to located earthquakes hypocenters and the data were processed. Due to the sparse dataset, we was not able to have a accurate image of the slab. To have an accurate image of the slab, more data are needed. 3.1 Tectonic Setting The Northeastern Caribbean is a complex region to study, at least in the seismological sense, given that as an island arc, there is always a limited location where to place seismic stations. The North America-Caribbean plate boundary zone (NBCPBZ, Mann et al., 2002), a 3200 km long buffer zone, extends from northern Central America to the Greater Antilles (Grindlay et. al., 2005). The northeastern Caribbean plate boundary zone can be subdivided into three microplates: the Gonave, the Hispaniola and the Puerto Rico-Virgin 3

5 Island (PRVI) (Jansma and Mattioli, 2005). The PRVI region is located at a zone where oblique subduction between the North America and Caribbean plates take place (Figure 2) (Doser et. al., 2005). The PRVI microplate is surrounded by different faults features: at the east is the Anegada Passage, in the south is the Muertos Trough, in the west the Mona Passage and in the north the Puerto Rico Trench. Similar to other island arcs, PRVI is difficult to study seismologically and thus, tectonically, due to insufficient data collection from their surrounding oceanic crust (Brink, 2005). One explanation for this is the fact that the movement of the Caribbean plate is relative slow compared to other plates (Jansma et. al., 2000). Global position systems studies show that PRVI moves together with the Caribbean plate, towards the E-NE at a rate of ± 3 mm/yr (Mann et. al., 2002). 3.2 Previous Work In 2011, for my senior thesis I relocated 548 seismic events recorded by the OBS es in 2007 using hypodd and tomodd (Figure 4). This study show hypocenter relocations parallel to the Puerto Rico trench in an area of 150 Km 2. With the preliminary results we are able to image preliminarily the top 100 km of P-wave velocity structure in the swarm area suggesting preliminarily location, dimensions and orientation of the North America slab under Puerto Rico (Figure 5). The tomography shows a heretegenous material but due to the sparse dataset, we was not able to have a accurate image of the slab. A study in 2012 where only a second swarm (173 seismic events) was relocated using HypoDD (Vieten, 2012). Vieten uses two velocities models for doing the relocations: (1) PRSN model and (2) IASP91 model. Both models dosent represent the true velocity structure of the region but with the OBS the azimuthal gaps is reduce, having more accurate relocations of the hypocenters 4

6 than the original locations. Also due to the lack of vertical ray paths, the hypocenters relocation depths having high residual of 5 to 10 Km. 4 Methods Waldhauser and Ellsworth (2000) agree that is necessary to have knowledge of the precise spatial offset between earthquake hypocenters to study tectonic processes, earthquake recurrence, and earthquake interactions. To have better locations of these seismic events, I will relocate the hypocenters using data collected by the OBSes through a computer program called HypoDD. After acquiring results with HypoDD, I will use them in another computer program, TomoDD, to produce a tomographic image of the region. 4.1 HypoDD HypoDD is a FORTRAN computer program package developed by Waldhauser (2001) that uses a double-difference (DD) algorithm for relocating earthquakes hypocenters, which takes advantage of the fact that the hypocentral separation between two earthquakes is small. Therefore, the ray paths from the source to the station of this two earthquakes will travel almost the same materials having similar wave velocity. This algorithm is especially useful for regions where there are swarms present, because the distance between events are in the order of a few hundred meters (Waldhauser, 2001). As a result the program uses the location of larger events in the swarms as reference to relocate weaker events, clustering all the events along faults planes. To relocate the earthquake hypocenters with HypoDD, two essential steps are need. First, I will derive the travel time difference for two earthquakes by analyzing the catalog phase data (Waldhauser, 2001). Essentially, this step is 5

7 to minimize redundancy in the data set so that the catalog is converted to a phase data file using ph2dt (Waldhauser, 2001). The second step consists in using the differential travel time data to determine double-difference hypocenter locations (Waldhauser, 2001). During this process it is essential that all events have connecting vectors among them to avoid numerical instabilities in the code (Waldhauser, 2001). After processing the data in HypoDD the results will be plotted to visualize the difference in hypocenters. 4.2 TomoDD TomoDD, developed by Zhang and Thurber (2003) also employs the double difference technique to jointly determine event locations and velocity structure by using both absolute and differential travel time data from body waves. This program stems from the algorithm of HypoDD and thus it also relocates the earthquake hypocenters while it reduces systematic errors by choosing and using relative arrival times with minimal residuals to produce and improve a velocity model (Zhang and Thurber, 2003). This results in a three-dimensional velocity model of the trench, having a better representation of the geometry of the North America plate subducting under the Caribbean plate. 5 Time Line Term Spring 2014 Spring 2015 Fall 2015 Spring 2016 Plan OBS Deployment OBS Recovery Data Analysis Conclusions and resuts presentation 6

8 6 Figures Figure 1: Figure showing events recorded by the PRSN from April through September Light yellow color of events represent shallow depths, while darker colors are deeper. Stars represent GPS stations, triangles are PRSN land station and inverted triangles represent the 5 deployed oceans bottom seismometers (OBS) in 2007 (López et. al., 2009). Inset at the right top corner show a histogram of events as a function of time. Two peaks in the histogram represent the two swarms recorded during the six month period. 7

9 Figure 2: Plate tectonic setting of the Caribbean, with emphasis on the Northeastern area. White arrows show GPS data while solid lines trace main fault systems. Oblique subduction along the Northeastern Caribbean while frontal subduction occurs at the Lesser Antilles arc (Ali et. al., 2008). Figure 3: PRNS s current velocity model. 8

10 Figure 4: Relocations of all events were performed using first HypoDD (top) and then with TomoDD (below). The blue epicenters are the original locations after merge and initial 1-D relocation. Left figures are map views, while on the right are profiles with events within the rectangular box. Red epicenters are event relocations. Black lines joining red and blue locations indicate relocation tendency. Due to a lack of strong event pair links many events relocated to the surface indicating a lack of convergence in the process. Overall, HypoDD tend to bring events closer to the surface, while TomoDD is able to pull together closer events within the cluster. 9

11 Figure 5: TomoDD velocity model results. Cross-sections shown are contiguous north-south slices through the swarm region. The 0 km in the x axis is the center of the cluster and the y axis is depth in km. Colors represent P-wave velocity following the color scale bar on the right. Most of all profiles do not show a significant result except for the last three figures on the right column, where an apparent slab can be seen subducting towards the south. Black dots are relocated events. 10

12 References Ali, S., Freed, A., Calais, E., Manaker, D., and McCann2, W.R., 2008, Coulomb stress evolution in Northeastern Caribbean over the past 250 years due to coseismic, postseismic and interseismic deformation: Geophysis Journal, v. 174, p Anderson, M., Alvarado, P., Zandt, G., and Beck, S., 2007, Geometry and brittle deformation of the subducting Nazca Plate, Central Chile and Argentina: Geophysics Journal, v. 171, p Brink, U., 2005, Vertical Motions of the Puerto Rico Trench and Puerto Rico and their cause: Journal of Geophysical Research, v. 110, p Doser, D.I., Rodríguez, C., and Flores, C., 2005, Historical earthquakes of the Puerto Rico-Virgin Islands region ( ): Geological Society of America, Special Paper, v. 385, p Grindlay, N., Hearne, M., and Mann, P., 2005, High Risk of Tsunami in the Northern Caribbean: EOS, Transactions, American Geophysical Union, v. 86, p Jansma, P. and Mattioli, G., 2005, GPS results from Puerto Rico and the Virgin Islands: Constraints on tectonic setting and rates of active faulting: Geological Society of America Special Paper, v. 385, p Jansma, P., Mattioli, G., López, A., DeMets, C., Dixon, T., Mann, P., and Calais, E., 2000, Neotectonics of Puerto Rico and the Virgin Islands, northeastern Caribbean, from GPS geodesy: Tectonics, v. 19, p López, A., Pulliam, J., Brink, U.T., Mintz, H., and Hillebrandt-Andrade, V., 2009, Deployment of Ocean Bottom Seismometers South of the Puerto Rico 11

13 Trench Yields new Insights into the Behavior of Seismic Swarms: AGU 2009 Fall Meeting Poster Presentation. Mann, P., Calais, E., Ruegg, J., DeMets, C., Jansma, P., and Mattioli, G., 2002, Oblique collision in the northeastern Caribbean from GPS meassurments and geological observations: Tectonics, v. 21, p Vieten, R.M., 2012, Relocations of a Seismic Swarm in the Northeastern Caribbean Using Ocean Bottom Seismometers [Master s thesis]: Gottfried Wilhelm Leibniz University Hannover, 97 p. Waldhauser, F., 2001, HypoDD: A computer program to compute doubledifference earthquake locations: U.S. Geological. Survey Open-File Report, p Waldhauser, F. and Ellsworth, W., 2000, A double-difference earthquake location algorithm: Seismological Society America, v , p Zhang, H. and Thurber, C., 2003, Deployment of Ocean Bottom Seismometers South of the Puerto Rico Trench Yields new Insights into the Behavior of Seismic Swarms: Seismological Society of America, v. 93, p