INTERNAL TECTONIC STRUCTURE

OF THE CENTRAL AMERICA WADATI-BENIOFF ZONE

BASED ON ANALYSIS OF AFTERSHOCK SEQUENCES

Aleš Špičák, Václav Hanuš, Jiří Vaněk

Institute of Geophysics,

Academy of Sciences of the Czech Republic,

Prague

MARGINS, June 18 – 22, 2007, Heredia, Costa Rica

MOTIVATION

- to contribute to the understanding of the internal tectonic

structure of subducting slab of the Cocos plate by defining

fault zones that rupture during strong earthquakes

OUTLINE

1) region of interest

2) global seismological data – their benefits for understanding structure, recent dynamics and tectonic history of convergent plate margins

3) method of hypocentral data analysis

4) aftershock sequences – selection, data analysis, determination of geometrical parameters and tectonic regime

5) conclusions - tectonic interpretation of aftershock sequence parameters

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

16

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

94° 82°90° 85° [W]

[N]

94° 82°90° 85° [W]

[N]

Cocos Plate

Nicaragua

CostaRica

Guatemala

El Salvador

HondurasMexico

Panama

Guatemala Basin

smoothsegment

FischerRidge seamount

segmentCocosRidge

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

GLOBAL SEISMOLOGICAL DATA

- 44 years ( since 1964) of continuous observation and standard data processing (since 1976 accompanied by focal mechanism determination of stronger events)

- homogeneous coverage of the whole Earth from mb ~ 4.0

- precision of hypocentral determination is sufficient to contribute to understanding the structure and dynamics of convergent plate margins

- enable comparative studies among individual convergent margins

- with global seismological data, both SUBFAC and SEIZE problems(and some others) can be addressed

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

ISC hypocentral determinations 1964 – 2005

relocated by Engdahl et al. (1998) m < 4

4 < m < 4.54.5 < m < 5

5 < m < 6

m > 6

DATA 1 (EHB data)

fault plane solutions of

Harvard Centroid Moment Tensor Solutions (1977 – 2005)

B 1 2 1 2 8 6 EDATA 2 (HCMTS data)

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

Global seismological datareflect SUBFAC operation

(SE Asia – Krakatau vers. Merapi volc.)

0

100

200

300

dep

th [

km]

0 100 200 300 400 500[km]

Krakatauvolcano

depth[km] SSW NNE

[km]

0

100

200

300

400

500

600

700

dep

th [

km]

0 100 200 300 400 500 600[km]

depth[km]

Merapivolcano

S N

[km]

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

16

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

Global seismological data reflect SUBFAC operation

(Costa Rica – volcán Irazú)

ŠPIČÁK, A., et al. (2005), Source region of volcanism and seismicity pattern beneath Central American volcanoes. N. Jahrbuch fuer Geol. Palaeont, 236 (1/2), 149-172.

1

5

10

15

20

24

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

10 aftershock sequences in the Wadati-Benioff zonebeneath Central America between 1964 - 2005:

aftershock sequence no. depth mW

of events range [km]

1970 Chiapas 58 6-87 7.41983 Osa 27 6-96 7.41985 10 25-67 5.61992 Nicaragua 136 4-76 6.21993 48 16-73 7.21996 18 24-89 6.51998 34 4-96 6.51999 Quepos 10 9-46 6.92000 Nicoya 18 5-49 6.42001 El Salvador 61 37-88 7.7

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

transformation of coordinates

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

slab dip = 29°slab strike = 294°

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

slab dip = 41°slab strike = 303°

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

10 aftershock sequences in the Wadati-Benioff zonebeneath Central America between 1964 - 2005:

aftershock sequence no. depth mW

of events range [km]

1970 Chiapas 58 6-87 7.41983 Osa 27 6-96 7.41985 10 25-67 5.61992 Nicaragua 136 4-76 6.21993 48 16-73 7.21996 18 24-89 6.51998 34 4-96 6.51999 Quepos 10 9-46 6.92000 Nicoya 18 5-49 6.42001 El Salvador 61 37-88 7.7

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

position of aftershock sequencesalong the plate margin(epicentral map)

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

sq2000 (Nicoya earthquake)

(a) standard epicentral map

step 1:transformationof coordinates

(b) projection to the slab surface

step 2:approximation of fociby a (fault) plane(d) position of approximating planes

(fault planes) in the slab

(c) vertical section in the slab

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

sq1983 (Osa earthquake)

(a) standard epicentral map

step 1:transformationof coordinates

(b) projection to the slab surface

step 2:approximation of fociby a (fault) plane

(d) position of approximating planes(fault planes) in the slab (c) vertical section in the slab

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION

CONCLUSIONS – tectonic interpretation

- distribution of foci in aftershock sequences is quasiplanar; the geometry of the respective planes – fracture zones (FZ) - can be determined !

- all mainshocks and almost all events with available CMT (95 %) are of normal character in the coordinate system anchored to the subducting slab

- a majority of fault planes offer nodal planes parallel to the trench

- occurrence of events off the main hypocentral clusters („outliers“) point to seismogenic rupturing along two or more faults in fracture zones

- geometrical parameters of fracture zones probably reflect the interaction of the subducting slab with the continental wedge of the Caribbean Plate in the hanging wall:

- areas with smooth sea floor morphology – slab-parallel FZ- seamounts and ridges on the sea floor – steeply inclined FZ oriented

quasiperpendicular to the trench

Špičák, A., Hanuš, V., and Vaněk J. (2007), J. Geophys. Res., in press.

1) REGION OF INTEREST

2) GLOBAL SEISMOLOGICAL DATA

3) METHOD 4) AFTERSHOCK SEQUENCES

5) TECTONIC INTERPRETATION