Tectonic patterns of the Panama Block deduced from seismicity, gravitational data and earthquake mechanisms: implications to the seismic hazard

Tectonophysics,

253

154 (1988) 253-267

Elsevier Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

Tectonic patterns of the Panama Block deduced from seismicity, gravitational data and earthquake mechanisms: implications to the seismic hazard A. VERGARA MUROZ Seismological Department,

Institute of Geophysics, Uppsala University, Box 12019, S-750 I2 Uppsala (Sweden)

(Received

October

27, 1987; revised version

accepted

March

28,1988)

Block deduced

from

seismicity,

Abstract Vergara

Muiioz,

earthquake

A., 1988. Tectonic mechanisms:

An integration information Panama beneath break

of seismicity

inland

Zone represents adds

degree

boundary

of the PB is the Panama-South

American Panama,

is coupled

with

plates. The southern and a broad

Cumulative

of tectonic

Deformed

major

America consists

Suture forces

boundary

with

The Caribbean-South America plate boundary is a broad zone of deformation characterized by a number of fault-bounded blocks engaged in a complex pattern of relative motions (Bell, 1972; Case, 1974). Some aspects of the seismicity of the Panama Block (PB), i.e., in the region between 7-12ON and 77-83” W have been studied by Vergara Mutioz (1987,1988,1989). Figure 1 shows locations of the most important features mentioned in this paper. The purpose of this paper is to establish new relationships between the Modified Mercalli (MM) intensity and the available instrumentally measured data. The regression formulas will permit B.V.

and

data and geological

which control

In the north,

the boundary

of the Isthmus

of Panama.

the shear motion the convergence

of a left-lateral Ridge

data

them. The subduction

of the block from the Panamanian

margin

Zone, in which

the Coiba

regimes

and Cocos plate and is undergoing

between

shelf

with

the

The eastern

the PB and the

of the Nazca

transform

and the Azuero

in zones with more complex

0 1988 Elsevier Science Publishers

other geophysical

to this area.

associated

of a combination

shear zone between

Index are greater

complexity

Introduction

0040-1951/88/$03.50

the Nazca

Belt along the Caribbean

compressive

boundary

left-lateral

Seismic Hazard

between

gravitational

154: 253-267.

Block (PB) and tectonic

with the Cocos Ridge at the western

a higher

plate lies on the Panama

plate

Tectonophysics,

data, focal mechanisms,

of the Panama

a clear boundary

Caribbean Nazca

gravitational

boundaries

the PB. Its interaction and

of the Panama

to the seismic hazard.

parameters,

is used to determine

Fracture

patterns

implications

and

South

fault, south of the Gulf of

Peninsula.

The values

of the

tectonics.

the evaluation of some other seisrnlcity parameters. These will complete the data set and a correlation with other geophysical parameters will be studied, e.g., gravity data as well as other geological information and focal mechanism solutions. Even though the Bouguer anomaly is not the best gravitational information to be correlated with dynamic tectonic processes, the agreement with the seismicity patterns of the region is encouraging. It is expected that by integrating the available information, reliable constraints on previous studies will merge and help to pinpoint seismically hazardous areas. Seismic hazard will be assessed using the Cumulative Seismic Hazard Index (Howell, 1974) which is based on MM intensities

85O

81’

79O

NICARAGUA

STRIKE-SLIP THRUST

FAULT

FAULT

NAZCA

Fig. 1. Schematic CF-Coiba Fault;

g~lo~~a~-tecto~c

Fault;

SF-SambG

map of the Panama

ST-Sam-Tonosi Fault;

JF-Jaqui:

Fault Zone; Fault;

Block showing

OF-Oria

UF-Unguia

Fault; Fault;

America

observed area.

during

a given period

within

the study

Intensity relationships

tectonic CD-Canal

UUF-

features

PLATE

and locations

Discontinuity;

of areas discussed

GF-Gatin

Urab& Fault (Silver, pers. commun.,

Fault;

in the text:

SB-San

Bias

1985); K’-Middle

Trench.

small compared to that of geometric spreading, and thus the epicentral intensity, lo, can be expressed as: Z0 = Z - a, + b, In A

(1)

Relations between MM intensity and other seismological parameters, e.g., the epicentral distance, instrumentally-measured magnitude and focal depth, for the PB will be given by Vergara Muiioz (1989). In this section we will develop regression formulae between intensity and other

where Z is the MM intensity at epicentral distance A, a1 is the source characteristics coefficient, and b, is a constant representing the geometric spreading. For all events with a maximum reported MM

seismic quantities which could then be used to interpret macroseismic data in terms of instrumental data, and, whenever necessary, vice versa.

Vergara Muiioz (1989, table 4). The root-meansquare value (r.m.s) between the predicted and observed epicentral intensities is kO.67 (in MM intensity units) for the PB.

Epicentral

intensity of Z 2 IV, Z, was estimated using eqn (1) and the numerical results are summarized in

intensity Intensity-radius

Ma~mum intensities felt in Panama have been listed by Vergara Muiioz (1987). Close to the epicentre, the effect of exponential absorption is

relationship

of isoseismd

ZV

Because, for the past large events, the level of perceptibility lies out of the study area, the iso-

255

seismal IV is chosen as the most representative isoline in the narrow region of the Panamanian territory. The goal here is to evaluate numerical values of the coefficients (Yand j3 in the relation (Bath, 1980): I,=alog(A&,/h2+1)+@

(2)

where An, is the radius of a circular isoseismal IV and h is focal depth. In case when ~st~rne~tal h-values are missing, use is made of macroseismic observations through the formula of Vergara Mu”noz (1989): h, = Arv[I()0.%-4)

_ I]+*

(3)

The r.m.s. error between k,-values and instrumental h-values for 44 pairs of observations is + 16 km. For 67 pairs of observations (Vergara Muiioz, 1985) the linear least-squares approximation between I, and A,v/k gives: lo = 0.92 log( A;,,‘h2 + 1) + 5.3 (u = kO.81) for I, 2 IV Approximation

and I, results in:

log( A$‘h2 + 1) = 0.381, - 1.43 ((I = + 0.61) for I, 2 IV

(5)

The former equation can be rewritten as follows: log( A:v/h2 + 1) = (I0 - 3.8)/2.63

the relation between

M=0.141,+5.50

+0.34)

(o=

for VII I 1, I X

(9)

It is interesting that this formula provides results which differ from those obtained through eqn. (7) in Vergara Munoz (1989) only by 0.19 for I,, = VII, 0.17 for 1, = VIII, 0.13 for 1, = IX and 0.09 for 1, = X. Extrapolation towards a larger epicentral intensity is not allowed since in the calculations they range between VII and X. Regressions between M and log(A$v + h2) for 61 pairs of observations read: M = 0.44 log( A&, + h2) + 4.50 (u = kO.27)

00) and: log(A~v+~2)

= 1.18M-

3.25 (a=

rtO.44)

(11)

Finally, a recurrence of the form: (4)

between A,,/h

For 61 pairs of observations M and I, is:

for 1,2 IV

In N = a + b1

02)

where N is the number of earthquakes at a particular intensity, I, experienced per year, and a and b are empirically determinable constants, was calculated for the PB for a radius between 300 and 400 km. Figure 2 plots the results and gives the numerical values of the coefficients at 9”N, 79.5O W, i.e., Panama City. As shown by Brazee

(6)

Intensity-magnitude

I,>

relationship

The macroseismic magnitude for events with IV was calculated using the relation:

MA = ;

[e-(%-b,

*.a

‘n AIV-+IV) + 5.771

4

where all coefficients are given in tables 2 and 6 in Vergara Muiioz (1989). The r.m.s. values on MA for 42 pairs of observations (I = IV-MA) is +0.37. To homogenize the data set a regression formula between the macroseismic magnitude, MA, and the homogenized surface-wave magnitude, M (Vergara Muiioz, 1987), was determined. Linear least-squares approximation gives: M = 1 .05MA - 0.37 ((I = + 0.37)

0.05 lNTENSiT~(l

Fig. 2. Earthquake

recurrence

9ON, 79.5 o W, i.e., Panama (8)

OI greater)

curve for the years 1913-1986 City. Standard * 0.08.

deviation,

(I =

at

256

(1976)

since

the macroseismic

data

The following block characteristics are found to be practical in the applications to our region:

are interde-

pendent, the curve should not be used to project the probability of occurrences to intensities beyond the highest intensity curve

is in good

information prediction the recorded

observed.

agreement

and should of future

with

However,

within

(2) Owing

for the

the range

our

of

differences,

intensities.

lations

the block

latitude

successive,

partially

method

(Bath, 1982a, b) with

overlapping

blocks

and

of

the calcu-

block

positions

energy

by 0.25”

The

total

steps

in

number

of

is 173, of which

129

sums.

With the blocks specified as above, the operational procedure follows Bath (1982b) with the difference that our period covers the interval 1904-1986 and the normalization factor S = 1. Figure from:

tudes. In addition, the magnitude of 36 events for which macroseismic observations were at hand are calculated using eqns. (6) and (7) this being useful to complete the existing set of data. This is as complete and consistent as the available information permits, and it forms the basis for all calcula-

Fig. 3. Seismic energy

extent

no significant

is applied

several earlier papers of Vergara Mufioz (1987a, b, c), which also include an earthquake catalogue with homogenized surface and body-wave magni-

MT=

ENERGY

MAP

3 shows the “total

&(logCE-

magnitude”,

calculated

12.24)

(13)

where E is the seismic energy released by an individual earthquake (Vergara Mufioz, 1987). In Fig. 3, the discontinuous space distributions of the energy release are replaced by high gradients, i.e.,

study. SEISMIC

have

are shifted

longitude.

have non-zero

to map the seismic wave energy released within the PB. The seismicity of the PB is dealt with in

tions in the present

areas

(Bath, 1982b).

investigated The moving-block

square areas.

latitudinal

and hence will not influence

(3) The blocks

Seismic energy mapping

to practically

to the limited

region,

0.5O,which

block size is 0.5 o x

in our case corresponds

the historical

be a good index

activity

(1) The chosen

the

OF

THE

PANAMA

BLOCK

1904-96

map of the Panama Block for the period 1904-1986 with isolines for the total magnitude (0.5 o x 0.5 o ) of the block. All MT-values larger than 5.0 included.

(MT)

per unit

257

compressed mark

isomagnitude

the areas

energy different

release, tectonic

lines.

of large and

space

hence

These

gradients

variations

delineate

of the

zones

with

Comparing between

between

seismicity

and the gravita-

tional field

the distribution

is to some extent the gravitational foci usually

of earthquake

connected

with the character

field. The position

coincides

epicentres

with crustal

of

of earthquake fractures

and

with contacts of individual crustal blocks. These areas usually appear on gravitational anomaly maps in the form of highly characteristic gravitational discontinuities (Artemjev, 1963). Tectonic forces, generated by different sources, overcome the resistance of the isostatic forces and drive the areas out of equilibrium. After a period of tectonic activity, a period of quiescence dominates the tectonic regime; the crust regains a state of equilibrium after short stretches of geological time. Thus, we may assume that large deviations from crustal isostatic equilibrium in particular areas indicate enhanced tectonic activity. The derivatives of the gravitational field determine the dynamic forces acting in those areas. Free-air gravity anomaly data has been used in Panama, to study the uplift of the PB (Bowland, 1984) and regional stresses acting on the boundary between the Cocos Ridge (see Fig. 1 for locations) and

amounts

lowing

outstanding

of seismic

correlation

the Panama

Fracture

Zone

(PFZ)

(Bentley,

1974). Barday (1974) and Briceno-Guarupe (1978) used marine gravity data to contour the mantle depth in the Panama Basin region, south of PB, and the structure of the crust in the Gulf of Panama, respectively. Figure 4 shows a map of the Bouguer anomaly in the PB, compiled from all data in the study area (land and marine stations) made available by the Inter-American Geodetic Survey for the period prior to 1984, and data from land stations measured by our team at the National Geographical Institute of Panama during 1984-1985. The map distinctly shows that the steep gradients have no evident connection with such primary structures as the sea and the continental sectors.

in Figs. 3

a correspondence energy

of the Bouguer

release

anomaly.

emerges

The

in the fol-

regions:

(1) The PFZ between

It has been noted that in regions of high seismic activity

large

to identify

and steep gradients

patterns.

most Correlation

the shape of the gradients

and 4, it is possible

the Panama-Costa tinuing structural

82” and 83” W through

Rica border

northward,

following

highs and seamounts

Hess Escarpment

in the western

(Case and Holcombe,

region the

and con-

alignment

of

connecting

to the

Colombian

Basin

1980; Bowland,

1984).

(2) The Azuero-Coiba region, between 80 o and 82“ W, extending to the north up to 8 o N. (3) The Panama -South America Suture Zone (PSZ), between 77 o and 79” W, with a NW-SE trend which crosses the whole of the PB and disappears at the Caribbean margin at about 9.5 o N. There is high seismicity at the PSZ (Vergara Munoz, 1988) which may correlate very well with this gradient. However, its continuation northwestward through the Panama City area up to the Caribbean Sea may be associated with the on- and offshore Sambu Basin (Lowrie et al., 1979; Mann et al., 1987), since this area has very low reported seismic activity. As proposed by Lowrie et al. (1979), a major tectonic discontinuity exists between 79 o and 80 o W along the Isthmus of Panama. Within the framework of plate tectonics, this discontinuity coincides with the predicted locus of the intersection between a marine fracture zone, marking a palaeoplate boundary, and the Isthmus of Panama (Van Andel et al., 1971; Lonsdale and Klitgord, 1978). The Bouguer anomaly gradient clearly follows this discontinuity across the Panama City area (Figs. 1 and 4). (4) The eastern Panama Deformed Belt (PDB), with a NW-SE trend along the deformation front (Bowland, 1984; Reed et al., 1985) from approximately 80’ W continuing to the eastern Colombian Basin. Focal mechanisms Many authors have published focal mechanisms for the region around the PB (e.g. Molnar and Sykes, 1969; Pennington, 1981; Adamek,

83O

Fig. 4. Simple Bouguer

7c

8”

anomaly

MAP

1984-1985

BLOCK

77O

Institute

of Panama.

data and land data prior to 1984 from the Inter-American Geographic

marme by the National

790

OF THE PANAMA

10 mGa1. The map includes during

interval,

measured

Block. Contour

81°

ANOMALY

Survey, and land stations

map of the Panama

BOUGUER

Geodetic

7O

259

TABLE

1

Earthquake No.

focal mechanism

solutions

Date Yr.

Mon

Day

Lat.

Long.

(“N)

(“W)

Source

*

m

M

Strike

Dip

Slip

Depth

Notes

(“)

(“)

(“)

(km)

**

Source

Type

1

1941

12

05

8.67

83.16

A

7.1

7.0

295

32

90

26

A

I

2

1962

07

26

7.49

82.75

V

7.4

7.2

180

75

202

33

RL

3

1962

09

18

7.56

82.40

V

6.8

6.8

358

80

180

33

M,F M

4

1963

06

26

7.08

82.35

V

6.1

6.7

358

79

180

34

M

RL

5

1963

07

09

8.50

83.00

V

6.1

5.8

295

52

270

31

M

N

6

1965

08

02

7.51

78.57

A

5.5

6.2

331

31

105

20

M

7

1965

12

15

7.59

82.24

A

6.6

6.6

14

70

156

26

1971

01

20

8.84

79.07

v

6.2

6.2

142

45

66

17

M,W P

RL

8 9

1974

07

13

7.80

77.52

A

7.2

7.0

174

29

70

12

P

ss

10

1975

01

25

7.16

77.71

A

6.6

6.9

337

12

77

40

P

11

1975

11

21

7.62

77.39

A

5.4

6.5

350

50

24

36

P

ss

12

1976

07

11

7.43

78.12

A

6.7

7.0

37

30

90

22

ss

13

1978

04

04

10.07

77.57

A

61

323

35

1978

04

04

10.00

77.89

V

6.4 _

40

14

5.9 _

P,W MC

296

56

74

11

MC

Th

RL

I

ss

15

1978

07

01

9.39

78.26

A

5.4

6.0

30

70

290

66

A

16

1979

07

01

8.36

82.91

A

6.3

6.4

345

59

287

35

A

N

17

1979

06

27

7.11

82.31

V

6.1

6.6

20

90

180

10

W

RL

18

1979

09

29

9.74

78.10

V

4.7

5.6

30

70

290

33

A

19

1983

04

07

8.01

82.79

A

5.4

6.2

311

22

113

35

Hl

Th

20

1983

05

09

8.26

82.95

V

6.1

6.3

332

31

142

39

Hl

Th

21

1983

10

12

8.07

82.73

A

5.7

5.5

296

24

92

20

H2

22

1983

11

26

7.34

82.43

A

5.3

6.3

357

72

183

10

H2

Th _

23

1985

01

05

10.24

79.92

A

6.8

6.5

301

48

128

35

H3

Th

24

1985

04

20

8.99

77.20

A

6.3

6.0

327

82

115

38

H4

DS

25

1986

08

07

7.43

81.24

V

6.1

5.8

356

67

330

8

HRV

ss

* Source of epicentral ** Explanation (1984); Moment

of notes:

data:

H3 = Dziewonski Tensor

A = Adamek

(1986); V = Vergara

Source for focal mechanisms:

solution

et al. (1985); from

P = Pennington

(1981);

SS = strike-slip;

Th = thrusting.

PDE,

W = Wolters

H4 = Dziewonski

August

1986; M = Molnar

The Panama Fracture Zone (1981), Adamek and others, the

(1986); Hl = Dziewonski

et al. (1986);

(1986). Type of mechanism:

1986; Wolters, 1986; Adamek et al., 1987). In this paper only those located in the PB as described before will be considered. Table 1 lists 25 faultplane solutions in the convention of Aki and Richards (1980), and refers to the original sources of publication in the last column. The focal mechanisms compiled for this study are displayed in map form in Fig. 5. The most important seismotectonic provinces will be discussed here in clockwise succession.

As reported by Penmngton (1986) Vergara Munoz (1988)

Muiloz (1987).

A = Adamek

and

F = Fujita

Sykes (1969);

I = inverse;

and

et al. (1983); H2 = Dziewonski Kanamori

MC = McCaffrey

DS = dip-slip;

(1981); (pers.

N = normal;

et al.

HRV = Centroid commun.,

1986);

RL = right-lateral;

distribution of earthquakes near 82.5 o W and north of 7O N clearly defines the northern part of the PFZ as a seismically active province. The focal mechanisms of events 2, 3,4,7,17 and 23 demonstrate N-S strike-slip as expected. Events 5 and 16 show normal faulting, the latter with a small strike-slip component. Focal mechanisms of events 19, 20 and 22 demonstrate thrust faulting. These are aftershocks of the destructive 4 April 1983, Costa Rica Thrust earthquake (Adamek, 1986). Event 1 also has a thrusting mechanism, but this earthquake has been relocated by Kelleher et al. (1973) in the Costa Rica region. It should be pointed out that seismotectonic patterns of the activity around the Burica peninsula (Fig. 1) are

FOCAL

hemisphere

of all available

Shaded projection,

quadrants

I

I

I Fig. 5. Compilation

790

810

83O

explanations),

MECHANISMS

earthquake represent

focal mechanism

compr~sionai

equal area plots. The numbers

solutions

wave arrivals, refer to Table displayed

affected both by the PFZ (normal faulting) and by the collision of Ihe Cocos Ridge with the southern Central American arc and its subduction beneath the continental crust (thrust faults); both normal and thrusting mechanisms are typical for trench and near-trench en~ronments (Pennington, 1981). This area marks the end of the Middle America Trench (Fig. 1). Lowrie et al. (1979) propose along E-W spreading centres westward migration of the PFZ the Coiba Trough (CT), the (MT), the Balboa Trough (BT) Trough (PT).

770

,

L

that N-S rifting accompanied the along four faults: My~and Trough and the Panama

from various

sources for the Panama

and open quadrants 1. Lines A-D

1

represent

are locations

Block (see Table 1 for

dilatational

arrivals

on lower

of the 35 km wide cross sections

in Fig. 6.

Figure 6 shows 35 km wide cross sections of seismicity in the PFZ using data from Vergara Mufioz (1987a), excluding all events with normal focal depths (e.g., 33N). The larger events (I,> IV) have depths controlled with macroseismic determinations. Possible errors in location and depth are discussed in Vergara Munoz (1988). Cross section A, along the northern segment of CT, shows rather shallow seismicity dipping close to the coastline. Section B shows the interaction of the MT with the continental crust of the PB. Section C along the PT indicates the effects of the interaction between PFZ and PB or the effects of the subduction of the Cocos Ridge beneath south-

ff v

A

t

70

8'c I_

90

. 40 -

.

60 1 60 -

Fig 6. Cross sections of the seismicity of the Panama Fracture Zone (for location see Fig. 5). Dots are hypocentres measured instrumentally and open triangles are events with macroseismic focal depth determinations. The position of the Bark volcano as projected onto section C is shown at the top with a small arrow pointing upward. The arrow in the bottom of section A shows the intersection with the proposed broad shear zone between Coiba Ridge and Azuero Peninsula. CL on top of each section indicates the location of the coastline. Numbered events refer to individual focal mechanism solutions (Table 1, Fig. 5).

ern Central America. The effects of the Cocos-Nazca-Caribbean (PB-Costa Rica) junction are demonstrated in cross section D, with most of epicentres close to the coastline and a few of them occurring below the arc up to the Caribbean margin of northwestern Panama having increasing focal depth. The Panama Defo~e~

Belt

The six available focal mechanisms along the PDB (see Fig. 7) show that complex faulting occurs here. Event 23 is located at the western limit of the seismic activity in the area where Bowland (1984) reported a change of structural style along the deformation front, and represents thrusting. Event 14 also demonstrates thrust faulting. Event 24 has a mainly dip-slip mechanism with a small component of thrusting. Although the relative location of this event with respect to the wedgeshaped structure of the PDB is poorly constrained, its focal depth of 38 km suggests that the event

occurs in the lower part of the crust (Adamek et al., 1987). The remaining events (13, 15 and 18) have focal mechanisms with nodal planes striking appro~mately parallel to the convergence direction of the PB and the Caribbean plate, and are normal-faulting earthquakes. The location and depths of these events suggest that they occurred within the underriding Caribbean plate and not within the Pancake lithosphere. A possible explanation is that the PB would provide the loading stress which causes the underriding lithosphere plate to fail, in the same way as vertical load or an oceanic plateau may perturb the local stress field while the oceanic lithosphere deforms in response, as described by Lambeck and Nakiboglu (1981) and Bergman et al. (1984). The Panama-South

America

Suture Zone

This area is characterized by scattered and diffuse seismicity (for location see Fig. 7). The focal mechanisms of events 6, 9, 10, 11 and 12 show

262

CUMULATIVE

Fig. 7. Cumulative

that

seismic hazard

there is no single

SEISMIC

HAZARD

INDEX

index map for the period 1913-1986.

boundary

compression is being acco~odated faults striking NW-SE. Polygonal

and

that

E-W

along many fracturing has

been described from structural analysis using radar imagery by MacDonald (1969), Vicksne et al. (1969), Wing and Dellwig (1970), Wing (1971a, b) and Wing and MacDonald (1973), and from geological studies by Terry (1956), and is consistent with the focal mechanism solutions.

DURING

1913

Contours

- 1996

in units of Modified

Event 8, which occurred close to Panama City, is the single known earthquake in the neighbourhood of the palaeoplate boundary of Van Andel et al. (1971) and Vitali et al. (1985) (see Fig. 1). It is

intensity

of a series of NW-striking left-lateral faults between the Coiba Ridge and Azuero Peninsula. Three of these faults are seismically active (Fig. 1): Coiba, Son&-Tonosi and Oria (Mann et al., 1987; Vergara Muiioz, 1989).

Cumulative !3eismic Hazard Index (CSHI) To complete the seismicity portant to include an evaluation

The Canal Discontinuity

Mercalli

analysis, it is imof the effects that

seismic energy release would produce at different epicentral distances. Since a numerical model for

slip components On the other hand, it could represent the northwestern extension of the JaquC River Fault (Fig. 1) of Mann and Burke (1984). However, the seismicity of this subprovince during the

the decrease of intensity with distance is proposed by Vergara Munoz (1989, eqn. (5) and table 3), it is possible to estimate the CSHZ following the method of Howell (1974). For VII I: I, 5 X, eqn. (9) gives &#-values which agree with those obtained with the formula proposed by Gutenberg and Richter (1956). Hence, for the PB the CSHZ is defined in units of log 1014 erg as

present century speculation.

CS’HI = loge

a thrust-fault

event with a small left-lateral

does

not

give

support

strike-

to this

10’“’ ‘2’

($4)

N

The Azuero region

The single focal mechanism published to date is number 25 in Table 1. The event demonstrates Ieft-lateral strike-slip faulting. This mechanism suggests that the Coiba Ridge is colliding with the PB. This process may be responsible for the origin

For a single earthquake, the CSHI at any observation point is thus numerically equal to the MM intensity of the site. As reported by Calgagnile and Panza (1976), the CSHI is very sensitive to the largest values of the intensity of each point, and hence by considering VII I I0 I X we reduce the effect of incom-

263

pleteness

of the

smaller

reported

data

to

shocks.

CSHI

The

was

calculated occurring

with parameters Munoz

(1987)

184-1986. is shown

at 0.5O

for 1913-1983 and

A composite

during (1986)

the PB. The 74-year

respect

to historical

study

area.

lines

and island

period

areas in the

to be representative

with

at different

though

CSHI

tectonics,

e.g., as expected

around

the

PB.

Discussion

for

covered

records

Even

reports

map of the iso-CSHI

in Fig. 7 for the land study is believed

intervals

CSHI values are larger in areas with

mechanisms

1913-1986,

taken from Vergara

PDE

present the

respect

complex

using 61 earthquakes

within

with

places in values

are

neither corrected for incompleteness nor grouped in tectonic provinces, as Howell (1974) did, it is believed that our map will be useful for future subdivision of the region for seismic zoning purposes. Because CSHI units are the same as those of the MM intensity (Howell, 1974), the iso-CSHI lines contour areas in which the size of the largest earthquakes is likely to be equal. Since intensity VIII corresponds to the threshold of structural damage to well-built buildings, the areas within iso-CSHI 2 VIII will need special building code provisions to ensure that all buildings will withstand earthquake forces. Because intensity VII is the threshold of structural damage to poorly built buildings, special care is required in areas with CSHI values between VII and VIII. For the remaining area with CSHI values smaller than VII the danger of earthquakes, especially those due to the effects of local geology, needs to be taken into account in building codes and in disaster planning. Iso-CSHI lines show a variable degree of correlation with the isomagnitude lines. This could be due to the fact that, for the seismic energy mapping, all events with M = 5.0 and larger were entered into the calculations, while for the CSHZ evaluation only 61 events fullfiled the requirements of eqn (14). The higher values correspond to the PSZ, the PFZ and the eastern PDB. The iso-CSHI contours agree rather well with steep gradients of the Bouguer anomaly which are considered to delineate areas where dynamic forces are present. An interesting aspect is that the zone with the lowest CSHI value is contoured by the 50 mGa1 isoline (Fig. 4). With respect to focal

In the preceding sections, a description of the seismicity, gravitational data and earthquake focal mechanism sented. here

solutions

The by

integrating

geophysical

for the PB have been

tectonic

patterns these

and geological

pre-

will be discussed

data

with

information.

additional Figure

8

shows the main tectonic features around the PB which are relevant to a better understanding of their interaction at a regional level.

Cocos-Nazca-Caribbean

(PB-Costa Rica) junction

The PFZ splays into four faults north of 6 o N, three of which (CT, MT and PT) are active. The MT and PT are presently being subducted beneath the lithosphere of the PB. The subduction of the Cocos Ridge produces both normal and thrusting events where the Middle America Trench peters out. The normal faulting events (5 and 16) are related to the interaction with the PT. This process extends northward along the Panama-Costa Rica border region and marks the western boundary of the PB. Hence, the driving forces of seismic activity in this area are a combination of the interaction of a fracture zone (PI-MT) with the subduction Ridge). consists

of a buoyant

feature

(the Cocos

South of the continental slope, the PFZ of at least three bowl-shaped bathymetric

lows or depressions. Single-channel profiles suggest, according to Adamek (1986) that the deeper wedge-shaped feature is a graben-like structure. Oblique convergence has been reported by Moore et al. (1985), where the PFZ is being subducted under the continental slope. Wrench fault structures dominate the slope, and the convergence zone is defined by landward-dipping thrusts without associated folds. A steep gradient of the Bouguer anomaly values with a N-S trend east of 83” W along the Panama-Costa Rica border region is considered to be an indication of active processes within the continental crust. The same

264

compressive

effect upon the isolines

is observed

in

the seismic energy map.

trending is defined by the toe of a wedge-shaped structure. In contrast, along the NE-SW trending segment of the fold, the deformation front is

Northern Panama Block

defined

by the first antiform

show both landward The PDB is a thrust tation

belt with a uniform

of folds and faults.

formation NE-SW

front

changes

The trend from

near 81” 25’ (Bowland,

the deformation

of the de-

almost

E-W

to

vergence

between

(Reed

the style of

front

of the PDB and the struc-

ture of the Colombian

Basin crust (Fig. 8), indi-

cates that the basin

1984). The E-W

85’

and seaward

et al., 1985). The relationship

orien-

of the belt. The folds

crustal

structure

controls

the

80’

15’

Fig. 8. Major tectonic this study.

Depth

features

around

&he Panama

Block, with data from Pennington

(1981), Mann and Burke (1984), and results of

Fracture Zone; PDB-Panama Deformed Belt; contours in metres. PFZ-Panama America Suture Zone. The arrows indicate the predominant direction of motion.

PSZ-Panama-South

265

northward

motion

which influences

of the PB and the deformation

fold belt and probably

produces

tion

Vitali

(Bowland,

Panamanian relatively more

crust thin

easily

Mono

northwestern

tures,

thick

which

formation

the

Panama.

and is pinned

1985).

The

to overthrust

the

Rise (Fig.

thicker

small

the rota-

crust

plateau

structures it impinges

in thrust

relative

beneath the North Andean 8) and is slipping eastward

to the PB along

boundary

(Jordan,

Azuero

1975)

a left-lateral which

transform

extends

to the

region.

Southern

Panama

Block

offshore

northeastward, on these

faulting

is rapidly subducting Block (Figs. 7 and

8)

of the

The PB crust is being forced

crustal where

results

et al.,

west of Mono

the much

and

style within a clockwise

is expected

crust

than

Rise

between

1984;

is the factor

struc-

at the de-

front. The fold belt has grown by defor-

The southern Azuero region, (1987,

1988).

compared between

boundary of the PB lies on the as described in Vergara Muiioz Here,

the seismicity

with the PFZ 79”

and

is rather

low

and the PSZ, especially

81” W. A possible

explanation

ming and incorporating autochthonous sediments in the Colombian Basin (Lu and McMillen, 1982; Bowland, 1984; Vitali et al., 1985). According to

could be that the left-lateral transform fault affects a zone of weak coupling between the crust of the Gulf of Panama and the crust of the Panama

Reed et al. (1985), deformation of thick layers along the footwall ramps has forced material out of the backlimb of folds along antithetic thrust faults. Landward vergence is the dominant polarity of the toe structure, suggesting very low basal shear stresses associated with initial imbrication. The seismicity of the PDB indicates that northeastward convergence between PB and the Caribbean plate is responsible for the seismic ac-

Basin (Fig. 8). If so, the 20 January 1904 event (Mb = 7.1 in Vergara Muiioz, 1987) should be investigated to determine its relationship with the above-mentioned boundary. The spatial distribution of epicentres clearly shows an alignment of events with a NW-SE trend between the Azuero

tivity here. The gravity data does not cover the whole PDB; however, steeper gradients in the northwestern and northeastern areas coincide with a higher seismic activity (Figs. 3 and 4). The Nazca-South America-Caribbean ombian Basin) junction

(PB-Col-

The PSZ represents the coupling zone of the shear motion between the Caribbean and Nazca plates and major compressive forces associated with the convergence of the Nazca and South American plates. All the associated seismically active faults appear to be dominantly of the thrust type. The earthquakes in this area occur southeast of the PDB, and to the north of the Colombia-Ecuador Trench (see Figs. 7 and 8). The PDB and South Caribbean join to form a S-pointing cusp which is separated from the northern termination of the Colombia-Ecuador Trench by the PSZ. Thus, the PSZ represents the Nazca-South America-Caribbean (PB-Colombian Basin) triple junction. Nazca oceanic crust

Peninsula and Coiba Ridge, with practically no events south of it. Event 25 (Table 1 and Fig. 5) exhibits a mechanism which characterizes shear zones. If this is accepted as a characteristic example of the faulting style for this area, then one conclusion is that the Coiba Ridge is converging with a northeast direction, causing the formation of a 100 km broad shear zone between the Sona-Tonosi Fault Zone and the Coiba Fault (Fig. 1, with locations from Case and Holcombe, 1980). As described recently by Mann et al. (1987), these faults are predominantly of the left-lateral strike-slip type with a NW-SE trend. The distribution of aftershocks of event 25 seems to verify our hypothesis. As reported by Moore et al. (1985) migrated seismic reflection profiles image a welldefined zone of folds and thrusts at the toe of the slope east of the Coiba Fracture Zone (or CT). Thus, the Coiba Ridge (another buoyant feature) is colliding with the PB, and the process produces shear stresses with characteristic left-lateral strike-slip faults which appear landward of the convergence zone, cross the Azuero Peninsula diagonally and end at the intersection with the CT (see Fig. 6 for location of this intersection). The Bouguer anomaly shows a slight gradient along

266

the proposed left-lateral boundary and it extends inland of the region of wrench faults.

Adamek,

S.H., Frohhch,

croplate

tectonics

Aki.

Our main conclusions may be formulated as follows: (1) New intensity relations~ps have led to some improvements of our earlier results, shedding new light on the seismicity of the PB. (2) The moving-block method applied to the study area to map the seismic energy provides a suitable and informative dynamic parameter which graphically shows an aseismic block surrounded by seismically active zones. (3) Integration of known focal mechanisms with other geophysical and geological information provides a better understanding of the tectonic regimes around the PB. (4) The iso-CSHI map gives a realistic picture of areas which have suffered major shaking during the 74-year period studied. The highest values of the CSHI are obtained in the vicinity of two triple junctions: Cocos-CaribbeanNazca and Nazca -South America-Caribbean. (5) As a consequence of the tectonic processes described above, the PB is believed to be undergoing a clockwise rotation with a principally northeast convergence. A diffuse internal deformation may be expected within the PB.

of the Panama

K. and Theory

Richards,

and

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