Synopses of Master Papers

Bulletin of IISEE, 47, 13-18, 2013

SUBDUCTION GEOMETRY BENEATH THE
DOMINICAN REPUBLIC AND AFTERSHOCK
DISTRIBUTION OF THE 1946 NORTHEASTERNHISPANIOLA EARTHQUAKE BY HYPOCENTER
RELOCATION
Jottin M. Leonel Collado ∗MEE11603

Supervisor:

Nobuo HURUKAWA∗∗ABSTRACTThis research proposes a new model of slab geometry beneath the Dominican Republic (DR), whichfaces the most significant seismic risk in the region, based on the relocation of earthquakes dating from1964 to Aug. 2010, using the Modified Joint Hypocenter Determination (MJHD) method. Subduction
thrust events represent the higher seismic risk for Hispaniola and the Septentrional (SFZ), Camu, and
Enriquillo Plantain Garden (EPGFZ) faults show the seismic risk of strike slip earthquakes, where theSFZ represents the most imminent risk.
We found that the North American and Caribbean slabs converge beneath the central area ofDR, between 69.9˚W – 69.3˚W. In our model we explain that the deeper area (69.1˚W – 68.2˚W) insoutheastern Hispaniola belongs to the Caribbean slab as it evidences continuity of seismicity of
Caribbean slab and as the available focal mechanisms of the down–dip extension type, whileearthquakes deeper than 110 km belong to the subducting Atlantic slab in the previous model. The
Wadati – Benioff zone in the southeastern part of the island of Hispaniola reaches around 180 km in
depth.
We also found that the seismogenic slab on northwest of DR reaches until SFZ, whilenortheast extend beyond this limit. The Camu Fault was responsible for the 2003 Puerto Plata
Earthquake (Mw 6.4), with multiples gaps around these two faults.
We also relocated the 1946 Northern Hispaniola Earthquake (Mw 7.9) and almost two yearsof aftershocks related with this mainshock. The area of the aftershocks distribution for 1946 Northern
Hispaniola Earthquake was confirmed as being between 70.4˚W – 68.4˚W and 18.9˚N – 20.0˚N.
Keywords: MJHD, Relocation, Caribbean Slab, Hispaniola, Subduction.

1. INTRODUCTION
The island of Hispaniola is located in the North American–Caribbean plate boundary zone (NCPBZ)(Figure 1) and in its northeastern part begins the tectonic transition between the oblique subduction toeast-west pure strike-slip motion along the Cayman Trough to the south of Cuba.
Oblique convergence between the Caribbean and North American plates is partitioned
between thrusting along the North Hispaniola fault and strike slip faulting under the overriding plate
along the left lateral Septentrional and Enriquillo faults systems. In the vicinity of Hispaniola, theNorth American and Caribbean plates have a 250 km wide zone of deformation (Dolan and Bowman,2004). By GPS observation, Calais et al. (2002) estimated the relative velocity of NCPBZ at 6.3, 21.0,∗

National Geological Survey (SGN) of Dominican Republic.
Building Research Institute (BRI), Japan.

∗∗

8.0 and 9.2 mm/year (red arrows in Figure 1) (NUVEL-1A, blue arrows, 11 mm/year), although the
MORVEL model suggests, based on GPS observation, that the Caribbean–North American motion are
twice as fast as given by NUVEL-1A (DeMets et al., 2010).
Seismicity is related primarily to subduction around the DR and internal faults that have. In
the west part, the seismicity is shallow (less than 55 km), not very active, which is probably caused by
the lack of seismometers in the local network that can record small earthquakes. The Septentrional
Fault Zone (SFZ) has produced over 8 earthquakes with magnitudes (Mw) between 6.4 and 7.9 in thislocation in the last 100 years, being the last 2003 Puerto Plata Earthquake (Mw 6.4).
Our purpose is to clarify the seismicity of the Hispaniola (Areas 1 and 2 in Figure 1), using
the Modified Joint Hypocenter Determination (MJHD) method, developed by Hurukawa and Imoto(1990, 1992), which is used to relocate all hypocenters in an area simultaneously. We have interest in
proposing a new model or confirming the seismotectonic geometry beneath DR, find gap zones oflarge earthquakes, as the greater frequency of medium and large earthquakes in the last 100 years in
the Caribbean zone is concentrated close to the SFZ. We also find the aftershock distribution of the1946 Northeastern Hispaniola Earthquake (Mw 7.9) as the largest event in this zone in instrumental
time.
2. DATA

Figure 1. Active tectonic faults around Hispaniola, thesquares show the target Areas 1 and 2 in this study.
The red and blue arrows (calculated by NUVEL-1A)
indicate the relative velocity of DR (Caribbean Plate)relative to North American Plate (fixed). The active
faults are: CF, Camu fault, EPGFZ, Enriquillo
Plantain Garden Fault Zone, MT, Muertos Trough,
NHFZ, North Hispaniola Fault Zone, PRT, Puerto
Rico Trench, and SFZ, Septentrional Fault Zone.
NCPBZ, represent the North American Caribbean
Plate Boundary Zone. Country names in blue.
Sources: Bird (2003) for subduction zone, Bakun etal. (2012) for strike slip zone, Calais et al. (2002) for
GPS data.

For this study, earthquakes from the catalogsof the International Seismological Summary(ISS), International Seismological Centre(ISC) and United States Geological Survey(USGS) were relocated, for earthquakes
occurring between 1946 and 1948, 1964 and2008 and 2009 and Aug. 2010, respectively.
The hypocenters were located in an areabounded by 17˚N to 21.5˚N and 72˚W to68˚W, as shown in Figure 1.
1) For observation of temporal change andthe geometry structure for the subduction
beneath the DR, the period from 1964
until August 2010 was investigated, as
this is the total span of the database used
(more than 2,000 earthquakes).
2) For aftershock distribution of the 1946
Northeastern Hispaniola earthquake (Mw
7.9) and relative hypocenter location,
data from less than two years after themain event was used. The reason is thatsignificant foreshocks for this event were
not observed (68 events).
All hypocenters registered in these areas wereused without distinction between magnitudes.
Phase data of P wave arrival times were used.

3. METHODOLOGY
MJHD method, developed by Hurukawa and Imoto (1990, 1992) was chosen as it provides morestability than other methods such as the Joint Hypocenter Determination (JHD) method, from Douglas(1967) and Freedman (1967) who had generalized the Geiger’s method (Geiger, 1912) to calculate thehypocenters of a group of earthquakes and station corrections simultaneously. Station correction

parameter removes the effect of lateral heterogeneity on the earth and reflects a travel time difference
between the assumed velocity structure and the actual one.
When the media is overly heterogeneous and station coverage is poor, the method explained
above is unstable and unreliable because of the significant trade-off between station corrections andfocal depths of earthquakes. For this reason Hurukawa and Imoto (1990, 1992) developed the MJHD
method for local earthquakes and Hurukawa (1995) for teleseismic earthquakes, which is an
improvement of the JHD method using constraints that station correction is independent of both the
distance and azimuth from the center of the region in question to the station. Although these constrain
sacrifice absolute hypocenter, this makes the JHD method stable.

4. RESULTS AND DISCUSSION
4.1 Subduction Geometry beneath the Dominican Republic
To understand the geometry of Atlantic and Caribbean slabs beneath the DR we take hypocenter
distribution in Area 1, from 1964 to Aug. 2010. We were able to relocate 276 earthquakes precisely
with the following conditions. Each earthquake was recorded at least by 20 stations and each station
registered at least 20 earthquakes. We used phase data with travel time residuals (O-C) ≤ 1.5 sec. The
stations are shown in Figure 2.
Figure 3 shows the relocatedepicenter distribution and cross sections. Cross–sections A–B and
C–D are with orientation of S18˚W
to N18˚E and W18˚N to E18˚S,
respectively,
which
are
perpendicular and parallel to thesubduction structures around DR
and the SFZ. During the process of
the relocation we observed that all
hypocenters fit to a depth less than
180 km, when in the beginning
Figure 2. The 422 worldwide stations used for relocation in this some earthquakes had depths up to
study are shown. The green star shows the studied area.
300 km, even if the earthquakes
poorly cover the trend to look deep
but when the coverage was good and the teleseismic stations were used these offered more stability as
fixing simultaneously for all stations that registered these events. So, we can conclude that the
Wadati–Benioff zone is around 180 km on southeastern Hispaniola.
The Muertos Trough makes contact with EPGFZ, after that, it becomes the Peralta Belt.
Intermediate and deeper events are absent and no high seismicity was registered. On northeastern
Hispaniola where the 1946 North Hispaniola earthquake (Mw 7.9) occurred, seismicity is very low
probably as the energy release by the event and currently energy can be concentrated for the next large
earthquake in the zone.
In the cross section A-B, the right side represents the Atlantic slab and shows two very welldelimitated concentrations, the shallow one is related to the Camu fault, which caused the 2003 Puerto
Plata Earthquake (Mw 6.4) and extends to a depth of around 30 km. The second concentration isrelated to the SFZ and extends approximately 60 km, it is clear to see that this fault has a gap of 20 km
at the shallow part. The thickness of seismic slab in left side of the cross section A-B is 25 km at the
deepest part (Caribbean Plate).
We divide the area into four sub-regions; each sub-region shows the global CMT solutions
available in this area (Figure 4).

Sub-region 1 shows
the continuation of theseismogenic slab of the
Caribbean SSW to NNE, in
the B side between 60 km
and 110 km where only twoevents appears. These eventsare located in the border ofthe sub-region, so in thecenter of the cross section
the full gap still continues inthis range of depth.
CMT solutions show
low angle thrust along theplate boundary shallower
than 50 km and down-dip
extension deeper than 70 km.
ThisindicatesthecontinuationoftheFigure 3. Hypocenter distribution of relocated events, 1964–2010. The Caribbean slab and thedotted line represents the original area of the hypocenter distribution. evolution of the gradual
CF, Camu Fault, NHF, North Hispaniola Fault. 1, 2, 3 and 4 indicate change of the slab direction,
four sub regions, of which cross sections are shown in Figure 4. The in this side the seismogenic
size of each symbol is proportional to the magnitude of the earthquake, slab shows a line of eventsas shown in the figure legend. Bars represent the standard errors of withfocal
mechanisms
hypocenter.
opposite to the main slab.
We interpreted this result asinternal events in the slab due to the bending of theslab in this range of depth.
Sub-region 2 shows the Atlantic slab
continuation NNE to SSW, the convergence area isshown in this sub region, the contact area extends
from the west around 60 km eastward of about 110
km in depth. Where the Atlantic slab is above the
Caribbean slab and it is clear the gap on the left
side in the shallow part with a depth of around 30km; on the right are two lines of events, on the left
is the SFZ and the right is a Northeast Hispaniola
Slope Fault Zone (NEHSFZ) possibly continuationof the Camu Fault.
In sub-region 3 we can observe the
Camu Fault and SFZ well demarcated on the rightside, in this side the gap in the shallow zone of the
SFZ is notorious, and the left side shows a large
gap zone that includes the sub regions 2 and 3.
Here, it is clearer how the contact zone of theAtlantic slab in the NCPBZ starts to change from
Figure 4. Cross sections defined in Figure 3. A subduction to strike slip westward, both plates are
and B represent the Caribbean and Atlantic shallow. The CMT solutions show the sameslabs sides, respectively. Global CMT solutions behavior as that of sub-region 2 and both slabs start
are also shown. Opposite triangles are the limits to look shallower. It is obvious the separationof the trench. S, Septentrional Fault; C, Camu between slabs and look clear the Camu Fault andFault. ?, Northeast Hispaniola Slope Fault Zone. SFZ concentrations.

Sub region 4 includes the hypocenter of 2003 Puerto Plata Earthquake (Mw 6.4); this is thelast of the series of moderate to great events propagating westward from 1946 to the present. Since
these earthquakes do not cross the SFZ, we cannot interpret if the subduction continues more deepthan 60 km in this zone. Only shallow events are recorded in this area (less than 50 km).
4.2 New Model of the Caribbean and Atlantic slabs Beneath DR
For a clear comprehension of this geometry, Figure 5 shows the conceptual drawing of seismogenic
slabs between both plates relating the 4 sub-regions. Space between slabs represents the gaps observedin this study which are absent of seismicity. The Caribbean slab had a perpendicular extension relative
to the contact area, indicating that the eastern part becomes deeper, opposite to the western part that is
kept shallow. The important features of this schematic figure are that the SE part of Hispaniola extends
around 180 km in depth and belongs to the Caribbean Plate. The converge area between Caribbean and
Atlantic slabs is located beneath the central part of DR, between 69.9˚W – 69.3˚W, up to 110 km in
depth. Other feature is that in the NW part of DR, the subduction from the Atlantic slab does not crossthe SFZ, possibly for the transition between subduction to strike slip of this slab in this area.
Although the intermediate part of theslab cannot be clear with the interpretationof the bending of the seismogenic slab, as
the double seismic layers or continuationfrom Atlantic slab with a gap in the center
part on sub-region 1, the sub-region 2 canclear this point with two focal mechanisms
available in the contact area when the Taxis, down-dip extension is conclusive.
4.3 1946 Hispaniola Earthquake
Sixty eight events in Area 2 (Figure 1) weretaken from ISS; the locations of these events
are adjusted for the epicenter area of the
Figure 5. Schematic figure of geometry structure of the three largest events in this period, the main
subduction zone beneath DR. This construction is made event of Aug. 4, 1946 (Mw 7.9), strong
from the seismicity observed during the period under aftershock of Aug. 8, 1946 (Mw 7.3) andstudy. Note that the Caribbean slab does not move April 21, 1948 (Mw 7.0). In this relocation
below the Atlantic slab (North American Plate).
we prefer high accuracy instead of thenumber of events, so, we used minimum
number of stations (MNST), 10, and minimum number of events (MNEQ), 10. Total number ofstations used was 50. Travel times residuals (O-C) ≤ 2 sec were used for all cases. Then, we relocated28 events accurately using all available readings.
Most aftershocks are less than 60 km in depth; this is precisely the depth value corresponding
to SFZ. The aftershock area of our relocation is almost the same as Russo and Villaseñor (1995).
Because of the long time of the aftershocks period, could be that near events, but not related
to the main shock, happened, these events are included in the list and possibly related to thebackground seismicity.
The deeper area has less than 110 km and matches with the contact area explained by ourmodel. Limit of 110 km is consistent with the contact area of sub-region 2 of this study, where mostaftershocks are concentrated.
After the sequence of 1946-1953 on NE Hispaniola, events with M > 6 did not occur,
indicating a concentration of stress in this zone and following the Coulomb stress modeling for thissequence by Dolan and Bowman (2004), our model is consistent.

5. CONCLUSIONSIn order to identify the slab geometry beneath DR, we relocated earthquakes during 1964 and Aug.
2010 by using the Modified Joint Hypocenter Determination (MJHD) method. We found that the
Wadati – Benioff zone in the southeastern part of Hispaniola reached to around 180 km in depth.
We proposed a new model of subducting slabs beneath DR with three important features.
Firstly, we found that the North American and Caribbean slabs converge beneath the central part ofDR, where a contact area of around 110 km in depth. Secondly, the deeper area in southeastern
Hispaniola belongs to the Caribbean slab and extends around 180 km in depth, although the previous
model by Dolan and Wald (1998) regarded that deeper earthquakes occurred inside the North
American slab. Our model is consistent with both, seismicity and focal mechanisms data available forthis area. Seismic behavior around DR certainly changed after the 1946 Hispaniola Earthquake andindicated the concentrations of stress in this zone.
Thirdly, northwest of DR slab reaches until SFZ, while northeastern of DR extend beyond
this limit. The Camu fault was responsible for the 2003 Puerto Plata Earthquake, and therefore, poses acontinuing risk to Hispaniola. In shallow areas the SFZ has important gaps that represent the mostimminent risk to Hispaniola. Further studies are necessary such as tomographic model in order toconfirm our model.
6. RECOMMENDATIONThe seismogenic geometry beneath DR is a new model that we were proposing, so, we recommendthat further study should be conducted to enrich this result and continue shedding light on the
Caribbean tectonics. Examples of these studies are seismic tomography, focal mechanism
determination for small earthquakes, etc.
ACKNOWLEDGEMENT
I would like to express my sincere gratitude to Dr. T. Yokoi and Dr. B. Shibazaki for their invaluablesuggestion and interest of this research.

REFERENCES
Bakun, W. H., Flores C. H. and Ten Brink U. S., 2012, Bull Seismol. Soc. Am., 102, 18–30.
Bird, P., 2003, Geochem. Geophys. Geosyst., 4(3), dio:10.1029/2001GC000252.
Calais, E., Mazabraud Y., De Lepinay B. M. and Mann P., 2002, Geophys. Res. Lett., 29, 18, 1856.
DeMets, C., Gordon R. G. and Argus D. F., 2010, Geophys. J. Int., 181, 1-80.
Dolan, J. F. and Wald D. J., 1998, Special paper 326, Geol. Soc. Am., Boulder, Colorado.
Dolan, J. F. and Bowman D. D., 2004, Seismol. Res. Lett., 75, 587–597.
Douglas, A., 1967, Nature, 215, 47–48.
Freedman, H. W., 1967, Bull.Seismol. Soc. Am. 57, 545–561.
Geiger, L., 1912, Bull. St. Louis Univ., 8, 60-71.
Hurukawa, N., 1995, Geophys. Res. Lett., 22, 3195 – 3162.
Hurukawa, N. and Imoto M. 1992, Geophys. J. Int., 109, 639–652.
Hurukawa, N. and Imoto M. 1990, J. Seismol. Soc. Jpn, 43, 413–429 (Japanese, abstract in English).
Russo, R. M., and Villaseñor A., 1995, J. Geophys. Res., 100, 6265–6280.
Website: Global Centroid Moment Tensor Solution. http//www.globalcmt.org