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Strain accumulation along offshore faults of Puerto Rico and implications for tsunami hazard

PUERTO RICO SEA GRANT

FINAL REPORT

Pamela Jansma, PhD Professor Department of Geosciences

University of Arkansas

Fayetteville, AR 72701

Overview and introduction

The island of Puerto Rico is part of the deformation zone associated with east-west relative motion between the Caribbean and North American plates and thus has experienced destructive earthquakes throughout its history. Because most of the seismicity occurs offshore, the potential for tsunamogenic events is high. Indeed, the most recent large event in 1918 that originated in the Mona Rift offshore northwestern Puerto Rico was accompanied by a damaging tsunami of 4 to 6 m along the west side of the island (Mercado and McCann, 1998). Whether other offshore regions of Puerto Rico have the potential for tsunamogenic earthquakes is unclear. The number of active faults, their mechanical behavior, and their likelihood to accommodate dipslip motion (a necessary condition for tsunami generation) was unknown. Two end-members of mechanical behavior of faults are possible: 1) stable sliding, where opposite sides of the fault creep past each other and energy release is slow, producing little seismicity, or 2) stick-slip, where opposite sides of the fault are locked until a critical threshold is reached when the rocks can no longer absorb the accumulating slip, resulting in sudden motion of the two sides, rupture along the fault, and rapid energy release in the form of a devastating earthquake. Seismic risk is far greater when stick-slip behavior occurs. Determining the mechanical characteristics of fault zones traditionally has focused on calculating if the energy released from historical seismicity is enough to offset the amount of motion documented along active faults by geologic or geodetic methods. Problems include, but are not limited to, the large errors associated with dating offsets measured along faults and the potential incorrect location of historic earthquakes. The advent of the Global Positioning Sytem (GPS) made possible the direct measurement of relative plate motions and slip along faults, thereby, reducing much of the uncertainty associated with the evaluation of seismic risk. The primary objective of this two-year proposal, therefore, was to collect, process, and model GPS geodetic data that directly impact displacement rates across seismogenic structures in the northern offshore region of Puerto Rico. Although we cannot isolate the amount of displacement on offshore faults directly from the GPS geodetic data, we can test whether significant elastic strain accumulation occurs on any offshore structures.

Tectonic setting

The North American-Caribbean plate boundary is characterized by left-lateral motion along predominantly east-west striking faults (Figure 1). In the west, the structure is relatively simple, consisting of the Swan and Oriente transform faults, which define the EW-trending Cayman trough and bound the short (~100 km), NS-trending Mid-Cayman spreading center. In contrast, the eastern half of the boundary in Hispaniola, Puerto Rico and the Virgin Islands is a complex deformation zone approximately 250 km wide, whose northern and southern limits are defined by the Puerto Rico trench and the Muertos trough, respectively. Three proposed microplates lie within this diffuse boundary zone (Figure 1). From west to east, there are: 1) the Gonave (Mann et al., 1995); 2) the Hispaniola (Byrne et al., 1985); and 3) the Puerto Riconorthern Virgin Islands (PRVI) (Masson and Scanlon, 1991). Such a microplate model assumes that nearly all of the deformation associated with North America-Caribbean motion is concentrated along the faults that bound the three rigid blocks: the Oriente, Septentrional, Enriquillo-Plantain Garden, and Anegada faults, the Muertos trough and North Hispaniola deformed belt, and the Mona Rift faults northwest of Puerto Rico (Figure 1).

Seismicity within and around Puerto Rico and the Virgin Islands averages hundreds of earthquakes per year (Figure 2). Although most are small (< 4.0), several large events have occurred during historic time, including the 1916, 1918, and 1943 Mona Passage (Rift) earthquakes (Ms=7.2, 7.3, and 7.5 respectively), the 1867 Anegada earthquake (Ms=7.3), the 1787 Puerto Rico trench earthquake (M=7.5?) and the 1670 San German earthquake (M=6.5?) (Pacheco and Sykes, 1992). With most events concentrated offshore, current seismicity mimics the pattern of large, historic events (Figure 2), leading workers to propose a rigid Puerto Riconorthern Virgin Islands block (PRVI) in the northeastern corner of the Caribbean (Byrne et al., 1985; Masson and Scanlon, 1991). GPS geodetic results corroborate the overall rigidity of Puerto Rico, which is consistent with the concentration of seismic activity offshore (Jansma et al., 2000: Jansma and Mattioli, in press). Small displacements, on the order of a few millimeters per year, are permissible across the island of Puerto Rico within the error of the GPS-derived velocities. The most likely scenario is east-west extension of up to 5 mm/yr localized within the Mona Rift and on unidentified faults east of the San Juan metropolitan area (Jansma and Mattioli, in press). Most of the deformation, however, occurs along offshore faults north of the island.

GPS data collection and processing

GPS measurements were first collected in the northeastern Caribbean in 1986 at six locations (Figure 1) and were re-occupied as part of CANAPE (CAribbean-North American Plate Experiment) in 1994. The network was densified during CANAPE and each subsequent year (Dixon et al., 1998; Jansma et al., 2000; Calais et al., 2002). Since 1994, measurements have been made on subsets of the entire network each year.

The GPS network in Puerto Rico and the Virgin Islands (Figure 3) consists of the original 1994 CANAPE locations (ISAB, PARG, and GORD) plus campaign sites MIRA (MiraderoMayagüez), ZSUA and ZSUB (San Juan), MAZC (Mayagüez airport), CIDE (UPRM), MONA (Mona island), DSCH (Desecheo island), ADJU (Adjuntas), ARC1 and ARC2 (Arecibo), CCM5 (Ponce), VEGA (Vega Alta), CAJA, (Caja de Muertos Island), FAJA ( Fajardo), LAJ1, LAJ2, and LAJ3 (Lajas Valley), SALN (Salinas), VIEQ (Vieques), and ANEG (Anegada, British Virgin Islands) and continuous sites GEOL in Mayagüez, FAJA in Fajardo, UPRR in Rio Piedras, and UPRH in Humacao operated by the Department of Geosciences, University of

F i g u r e 2 . F o c a l m e c h a n i s m s f o r d e p t h < 3 5 k m f o r e a s t e r n

H i s p a n i o l a , P u e r t o R i c o a n d V i r g i n I s l a n d s . S o u r c e s a r e t h e

H a r v a r d C M T c a t a l o g u e , t h e P u e r t o R i c o S e i s m i c N e t w o r k ,

D e n g a n d S y k e s ( 1 9 9 5 ) a n d M o l n a r a n d S y k e s ( 1 9 6 9 ) D o t s

a r e U S G S e p i c e n t e r s f o r e a r t h q u a k e s a b o v e d e p t h s o f 6 0 k m

w i t h m a g n i t u d e s > 3 5 k m f r o m 1 / 1 / 1 9 6 7 u n t i l 4 / 2 8 / 1 9 9 9

( U S G S ) A P : A n e g a d a P a s s a g e M R : M o n a R i f t M A R :

M a i n R i d g e M T : M u e r t o s T r o u g h N P R S F : N o r t h P u e r t o

R i c o S l o p e F a u l t P R T : P u e r t o R i c o T r e n c h S F :

Septentrional Fault SPRSF: South Puerto Rico Slope Fault YR: Yuma Rift. Modified from Jansma et al. (2000).

Puerto Rico trench

Figure 1. Map of northern Caribbean plate

Anegada passage; BP, Bahama Platform; C

EPGF, Enriquillo-Plantain Garden Fault; G

Platelet; KD, Kallinago Depression; LAT,

;

Figure 3. Current mixed-

m o d e G P S g e o d e t i c network in Puerto Rico

a n d t h e Vi rg i n I s l a n d s . W B = W h i t i n g B a s i n .

VIB=Virgin Islands Basin

A r r o w s a r e G P S

v e l o c i t i e s r e l a t i v e t o t h e Caribbean for east-central

D o m i n i c a n R e p u b l i c

( D R ) , w e s t e r n P u e r t o

R i c o , a n d Vi rg i n G o r d a

R e d d o t s a r e c a m p a i g n sites and blue squares are continuous sites

Septentrional Fault

Puerto Rico Slope Fault North Puerto Rico Slope Fault Mona Rift Yuma Rift

Puerto Rico

Ridge

Muertos Trough

TURK

ROJO

Arkansas, and PUR3 in Aguadilla maintained by the U.S. Coast Guard. All of the sites in Puerto Rico and its islands (Mona, Desecheo, Vieques, Caja de Muertos) were re-occupied during 2003 and 2004. Occupations consist of a minimum of 8 hours of data collection each day for 3 days. Several improvements were made in our GPS data analysis methods using GPS-OASIS II. Our entire analysis engine has been migrated from a Sun Ultra60 running Solaris (2.8) to a Dell Precision 650 running RH Linux (WS 3.3). This transition allowed us to take advantage of the most recent release of GOA-II from JPL (v. 4). Processing speeds have improved by better than 1 order of magnitude: from ~120 s per station-day to <11 s per station-day for a 24 hr absolute point position estimate. In addition, we have included the effect of ocean loading in our Earth models within GOA-II on the calculated point positions because we expected the effect to be significant for most of our sites. Ocean loading coefficients for every PRVI observation site and all IGS sites analyzed by our lab have been obtained using the model of olfg/olmpp by Scherneck. In addition, we have calculated common mode corrections for all our observations to the September 2004 epoch. These corrections are done off-line and after the initial processing. The final results have yet to be incorporated into our regional analysis or modeling efforts.

GPS derived velocity field for PRVI

The current GPS-derived velocity field for PRVI is shown in Figure 4. Recent results are consistent with an independently translating PRVI whose motion is 2.6±2.0 mm/yr toward N82.5°W±34° (95%) with respect to the Caribbean and with east-west extension of several mm/yr from eastern Hispaniola to the eastern Virgin Islands. Extension increases westward with the most, 5±3 mm/yr, accommodated in the Mona rift. East-west extension of 3±2 mm/yr per year also is observed across the island of Puerto Rico. Although the loci of extension are not known, similarity of GPS-derived velocities among sites in eastern Puerto Rico suggest the active structures lie west of the San Juan metropolitan area. Re-activation of the Great Northern and Southern Puerto Rico fault zones as oblique normal faults with right-lateral slip is a possibility. East-west extension of 2±1 mm/yr also likely occurs offshore between eastern Puerto Rico and Virgin Gorda, but additional measurements are required to verify this result. The accommodation of this extension has implications for tsunami risk from earthquakes generated along dip-slip faults within the Virgin Islands and Whiting Basins. Indeed, a large (7<M<7 3/4) tsunamogenic earthquake occurred along the north wall of the Virgin Islands Basin in 1867 that caused extensive damage in St. Croix and St. Thomas (Reid and Taber, 1920).

GPS-derived velocities, however, are consistent with most of the displacement occurring offshore northern Puerto Rico. The predicted and GPS-derived velocities of the Caribbean with respect to North America for Puerto Rico are 19.4±1.2 mm/yr toward N79°E±3° (1 s ) and 16.9±1.1 mm/yr with an azimuth of N68E±3 (1s ), respectively, with the latter slightly slower and more northerly than that for Caribbean-North American plate motion as a whole. Thus nearly 85% of the relative motion between the Caribbean and North American plates is accommodated offshore northern Puerto Rico.

Quantitative modeling of elastic strain accumulation along the Puerto Rico Trench and South Puerto Rico Slope Fault

We have developed kinematic-coupling models to examine elastic strain accumulation and co-seismic displacements in obliquely convergent margins, such as along northern PRVI in the northeastern Caribbean. Our initial efforts used simple 2D elastic models based in the formulation of Savage (1982). Preliminary results from the quantitative modeling of locking on

offshore faults north of Puerto Rico and on the Caribbean-North American plate interface suggest that elastic strain accumulation along these structures is low (Figure 5).

While these simple models allow one to explore the control of slab dip angle, degree of kinematic coupling, and locking depth, they are not adequate to address the complex geometry and kinematics of the PRVI subduction zone and Lesser Antilles arc. We currently are exploring 3D elastic dislocation models for the geometry and plate kinematics of PRVI using the Department of Energy code DISL (Larsen, 1998), which is based on the formulations of Chinnery (1961), Savage and Hastie (1966), and Mansinha et al. (1971). This code allows one to include a more realistic geometry of a curved subduction interface, by calculating the displacement on a series of linked fault patches. It retains the constraint of determining the displacements for an elastic half-space, however. We have also obtained the license for GeoFEST, which is a 3D finite element code developed by NASA at JPL to investigate a wide variety of geophysical phenomena. GeoFEST will allow us to investigate the effects of viscoelastic and other crustal/m antle rheologies as w ell as slab texture on the observed surface displacement field. GeoFEST is designed to run in a parallel computing environment, and its successful operation depends on a number of subsidiary software modules.

References cited

Byrne, D. B., G. Suarez, and W. R. McCann, Muertos Trough subduction--Microplate tectonics in the northern Caribbean?, Nature, 317, 420-421, 1985.

Calais, E., Y. Mazabraud, B. Mercier de Lepinay, P. Mann, G. Mattioli and P. Jansma, 2002, Strain partitioning and fault slip rates in the Caribbean from GPS measurements, Geophy. Res. Lett., 29 (18), 1856, doi: 10.1029/2002G1015397.

Chinnery, M. A., The deformation of the ground around surface faults, Bull. Seis. Soc. Amer. 51, 355-372, 1961.

Dixon, T. H., Farina, F., DeMets, C., Jansma, P., Mann, P. and E. Calais, Caribbean-North American plate relative motion and strain partitioning across the northern Caribbean plate boundary zone from a decade of GPS observations, J. Geophys. Res., 103 , 15157-15182, 1998.

Jansma, P., G. Mattioli, A. Lopez, C. DeMets, T. Dixon, P. Mann and E. Calais, Neotectonics of Puerto Rico and the Virgin Islands, northeastern Caribbean from GPS geodesy, Tectonics , 19, 1021-1037.

Jansma, P. E. and G. S. Mattioli, in press, GPS results from Puerto Rico and the Virgin Islands: Constraints on tectonic setting and rates of active faulting, in, Mann, Paul and Carole Prentice, (eds.) Geological Society of America Special Publication

Mansinha, L., D. E. Smyllie, and D. L. Oprhal, The displacement fields of inclined faults, Bull. Seis. Soc. Amer., 61, 1433-1440, 1971.

Masson, D. G. and K. M. Scanlon, The neotectonic setting of Puerto Rico: Geol. Soc. Amer. Bull., 103, 144-154, 1991.

McCann, W. R., On the earthquake hazards of Puerto Rico and the Virgin Islands, Bull. Seismol. Soc. America, 75, 251-262, 1985.

McCann et al., Origin, neotectonics, and seismic hazard of the Anegada Passage, northeast Caribbean, Geol. Soc. Amer. Spec. Paper, in press.

Mercado, A., and W. McCann. 1998. Numerical simulation of the 1918 Puerto Rico tsunami. J. Natural Hazards, 18:57-76.

Pacheco, J. F. and L. R. Sykes, Seismic moment catalog of large shallow earthquakes, 1900 to 1989, Bull. Seism. Soc. Amer., 82, 1306-1349, 1992.

Reid, H. and S. Taber, 1920, The Virgin Islands earthquakes of 1867-1868: Bulletin of the Seismological Society of America, v. 10, p. 9-30.

Savage, J. C. A dislocation model of strain accumulation and release at a subduction zone, J. Geophys. Res., 88, 4984-4996, 1983.

Savage, J. C. and L. M. Hastie, Surface deformation associated with dip-skip faulting, J. Geophys. Res. 71, 4897-4904, 1966.

Refereed articles (in press):

Refereed articles (in preparation):

Jansma, P. E., G. S. Mattioli, and C. DeMets, Kinematic constraints on elastic strain accumulation in the northeastern Caribbbean: observations and models, Geophysical Research Letters, to be submitted spring 2005

Presentations at (inter)national meetings:

Jansma, P.E. and G.S. Mattioli, 2003, Diffuse extension across and active faulting within the Puerto Rico and northern Virgin Islands microplate: GPS geodetic results from 1994-2002 (INVITED), Seismo. Soc. America Annual Mtg., San Juan, PR.

Jansma, P. and G. Mattioli, 2002, GPS results from Puerto Rico and the Virgin Islands: constraints on tectonic setting and the rates of active faulting (INVITED), Eos Trans. AGU, (83) 19, Spring Meet. Suppl., T31A-05.

Student theses:

Shane Madson, MS, in progress, kinematics of PRVI and Lesser Antilles subduction

Henry Turner, PhD, in progress, quantitative modeling of oblique subduction in PRVI, Lesser Antilles and Central America

Students supported:

Caen Dowell, MS student, University of Arkansas, fieldwork in PRVI

Anita Stone, MS student, University of Arkansas, fieldwork in PRVI

Henry Turner, PhD student, University of Arkansas, GPS data processing and modeling