Transcript Lecture 1: Class Objectives and Overview of Geoscience Case Studies

GPS geodesy: applications to geosciences
case overview
1)
2)
3)
4)
Puerto Rico-Virgin Islands microplate
Soufriere Hills volcano, Montserrat
Nicaragua forearc deformation
New Madrid Seismic Zone
CASE 1
GPS results from Puerto Rico and the Virgin Islands:
Constraints on microplate tectonics
overview
• general tectonics of Caribbean plate
• model for northeastern Caribbean
• Caribbean reference frame
• microplate tectonics in northeastern Caribbean
• deformation across Puerto Rico and the Virgin Islands
What is societal interest in the research? SEISMIC HAZARD!
Why is it important in Puerto Rico?
probability of various ground motions in specified time
USGS curves equate San Juan, Mayagüez with Seattle
population density of Puerto Rico: 1,124 per square mile
if US had same population density as Puerto Rico….
…population of US would be 3 billion
circum-Caribbean seismicity (defines plate)
Caribbean
epicenters < 30 km depth from USGS
Caribbean bathymetry
BR: Beata Ridge; HE: Hess Escarpment
NPDB: North Panama Deformed Belt;
SCDB: South Caribbean deformed belt
depth to slab
NOAM subducts
westward below
Caribbean along
Lesser Antilles
http://www.ig.utexas.edu/research/projects/venez_margin/venez_margin.htm
positions of the leading edge of Caribbean plate in the past…
i.e. where subduction was
http://www.ig.utexas.edu/research/projects/venez_margin/venez_margin.htm
bathymetry “around the corner”
note: Bahamas colliding with Hispaniola
North American-Caribbean plate boundary
MC: Mona Canyon
AP: Anegada Passage
MT: Muertos trough
proposed microplates of the northeastern Caribbean
from west to east:
Gonave
Hispaniola
Puerto Rico-Virgin Islands (PRVI)
(modified from Mann et al., 1995)
bathymetry offshore Puerto Rico and Virgin Islands
from: http://www.vangestel.com
block-faulting west of St. Croix
profiles across Puerto Rico
from: http://www.vangestel.com
models for Puerto Rico-Virgin Islands microplate
….Caribbean moves east relative to NOAM….
…left-lateral strike-slip along northern boundary
~40° CCW since Eocene;
~12° CCW since Miocene;
0 last 2 m.y.
• seismicity mostly offshore supports rigidity of PRVI
• thrust events along western MT and eastern PRT support rotation
• little seismicity along AP provides no constraint
• setting of western PRT isn’t clear
GPS geodesy in northeastern Caribbean
1986 (NASA):
6 sites
1994 (NSF):
6 original sites plus new sites in Hispaniola, Puerto Rico
and British Virgin Islands
1995 to present (NSF; NASA):
expansion of network throughout
• Hispaniola (Calais),
• Puerto Rico, Virgin Islands (Jansma, Mattioli)
occupations of subsets of sites done yearly since 1995
definition of Caribbean reference frame
…most of the islands sit on edges of the plate…
interior of plate has lots of ocean, not much land
…one spot in eastern Caribbean…Aves Island
sites for Caribbean reference frame (stars)
DeMets et al. (2000): AVES, SANA, ROJO, CRO1 plus transforms
…currently, we use this plus BARB (unpublished)
best fit NOAM-CARIB rate
~19 mm/yr
ENE azimuth
• twice as fast as NUVEL-1A
• azimuth ~N70°E at PR
problems with Caribbean reference frame:
1) too few sites
2) time series too short (noise significant) for key sites (AVES)
3) potential that Caribbean is TWO plates and/or deforming
potential improvements:
1) additional sites in Honduras, Nicaragua, Lesser Antilles
2) additional occupations (AVES for March--volunteers?)
too few sites…
Nicaragua
can use
BARB (already do)
FSDO
other sites in Lesser Antilles also may be on stable Caribbean…
…. too early to tell
only 2 occupations
Caribbean deformation? (affects reference frame)
two plates: western (SANA) and eastern (AVES) plates
little seismicity in interior says no
…some features suggest maybe
we know that NOAM and SOAM are converging…
“subduction” of Caribbean below Muertos Trough and South America
GPS network in northeastern Caribbean
potentially active faults on Puerto Rico
data from continuous GPS sites (ITRF00)
western Puerto Rico
eastern Puerto Rico
data from campaign GPS sites (ITRF00)
western Puerto Rico
eastern Virgin Islands
do separate microplates exist?…yes
clear difference between Hispaniola and Puerto Rico
…extension of ~5 mm/yr across Mona rift
• Hispaniola moves rapidly relative to Caribbean (slowly wrt NOAM)
• PRVI moves slowly relative to Caribbean (rapidly wrt CARIB)
does Puerto Rico rotate?…no
• component of convergence across western Puerto Rico trench
not compatible with rotation model…
• similar azimuths of velocities throughout Puerto Rico not
compatible with rotation about nearby vertical axis
does Puerto Rico escape to east? have active faults?
• island moves at ~85% of total CARIB plate rate wrt NOAM
• Virgin Gorda has no motion wrt CARIB (no tectonic escape)
• increasing velocities westward across PRVI (EW extension)
• velocities for western PR sites are similar (faults?)
CASE 2
The Dynamics of an Ongoing Andesitic Eruption: What we
have learned from Surface Deformation at Soufriere Hills,
Montserrat, BWI
eruption began in July 1995 as phreatic (steam)
…became phreato-magmatic in October 1995
…explosions began in September 1996
initial GPS occupations began in August 1995
• L1 only receivers early
• campaigns with dual frequency receivers began October 1995
• continuous GPS in summer 1996
data from August 1995-October 1996
note large error bar on circle (L1 measurement)
Tar River Valley Looking ENE
October 1995
English Crater
4 Ka Sector Collapse
GPS analysis strategy
• Campaign and continuous GPS geodesy done from 1995-2002
• Several CGPS sites destroyed over the years; current network is 6 sites
- Choke Ring Antennae with UNAVCO radomes and FreeWave RF
telemetry
• Data processed in absolute point positioning mode with final JPL
orbits, clocks, and earth orientation parameters - Time series in
ITRF97 and the referenced to fixed CAR (DeMets et al., 2000)
• GPS velocities are modeled assuming linear segments for each of three
periods
No Surface Flux
Explosions ->
Renewed
Dome
Growth
<-Dome Growth
Subsidence
Again! ->
Inflation ->
<- Subsidence