gps geodesy: applications to geosciences case overview 1)puerto rico-virgin islands microplate...

GPS geodesy: applications to geosciences

case overview

1) Puerto Rico-Virgin Islands microplate2) Soufriere Hills volcano, Montserrat3) Nicaragua forearc deformation4) New Madrid Seismic Zone

GPS results from Puerto Rico and the Virgin Islands:Constraints on microplate tectonics

CASE 1

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?

USGS curves equate San Juan, Mayagüez with Seattle

probability of various ground motions in specified time

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 EscarpmentNPDB: North Panama Deformed Belt; SCDB: South Caribbean deformed belt

http://www.ig.utexas.edu/research/projects/venez_margin/venez_margin.htm

depth to slab

NOAM subductswestward belowCaribbean alongLesser Antilles

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

from west to east:GonaveHispaniola Puerto Rico-Virgin Islands (PRVI)

proposed microplates of the northeastern Caribbean

(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

…most of the islands sit on edges of the plate…

definition of Caribbean reference frame

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

• twice as fast as NUVEL-1A• azimuth ~N70°E at PR

~19 mm/yrENE azimuth

problems with Caribbean reference frame:

1) too few sites2) 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 Antilles2) 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)

“subduction” of Caribbean below Muertos Trough and South America

two plates: western (SANA) and eastern (AVES) plateslittle seismicity in interior says no…some features suggest maybe

we know that NOAM and SOAM are converging…

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

• 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?)

does Puerto Rico escape to east? have active faults?

The Dynamics of an Ongoing Andesitic Eruption: What we have learned from Surface Deformation at Soufriere Hills,

Montserrat, BWI

CASE 2

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 ENEOctober 1995

English Crater4 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

<-Dome Growth

Explosions ->

<- Subsidence

No Surface Flux

Inflation ->

RenewedDome

Growth

SubsidenceAgain! ->