13th World Conference on Earthquake Engineering
Vancouver, B.C., Canada
August 1-6, 2004
Paper No. 1572

SOURCE CHARACTERIZATION OF MEXICAN SUBDUCTION
EARTHQUAKES FOR THE PREDICTION OF STRONG GOUND
MOTIONS
Jorge AGUIRRE1 and Kojiro Irikura2

SUMMARY

The empirical Green’s function method allows to simulate records of big earthquakes using
records of small ones. The great advantage of this method is that the information of the path andsite effects are included in the small records which allows to simplify the calculations in theabsence of nonlinear effects at the sites. However to carry out simulations of future earthquakes
it is necessary to construct source model consisting on the outer parameters (i.e. focal
mechanism, total area of rupture) and the inner ones (size, number and position of theroughness).
This study uses data of Mexican subduction earthquakes with the objective of proving thedifferent relationships presented previously, with the objective of validating them or in its caseto propose some other of regional validity.
Using records of the Guerrero Accelerograph Array from earthquakes occurring during the last
15 years we found that the relationships proposed by Dan et al. (2001) works appropiately for
the Mexican subduction earthquakes. We also found that the fault areas obtained with data of
this regional array, gave a good estimation of the size of the biggest asperity. These sizes arewell predicted by the relationship for subduction earthquakes given by Somerville et al. (2002).
INTRODUCTION

The evaluation of the seismic risk has become a topic of relevance especially in countries thathave great seismic activity. With that purpose many methods have been developed that focus on
the evaluation of the risk from different perspectives. One of these focuses is the called seismic
scenario that consists of carrying out the evaluation of the risk being based on simulations of1

Instituto de Ingeniería, Universidad Nacional Autónoma de México.

Ciudad Universitaria. Apartado postal 70-472, Coyoacán, 04510 México D.F
2Disaster Prevention Research Institute, Kyoto University.
Gokasho, Uji, Kyoto 611-0011, Japan..

seismic movements of well-known active faults that constitute a potential seismic source in thenear future. Inside this focus several methods are used for the simulation of the movements
generated by the potential seismic source. The methods based on simulating the propagation bymeans of finite elements or finite differences are commonly used for this purpose. Although thecomputation capacity no longer constitutes an obstacle for these techniques, the distribution of S
and P wave velocities in the different strata in one, two or three dimensions are not known
accurately enough as to carry out simulations to high frequencies. Another method that allows
the evaluation of seismic motions is the empirical Green’s function method. It has the great
advantage of being able to reach frequencies beyond 10 Hz in the simulations.
Recently, some seismologists have been carrying out several studies to understand therelationships between some inner and outer parameters in order to be able to make moreaccurate predictions of strong motions for future earthquakes. They have been studying thoseparameters that are responsible for the generation of strong motions especially. These
relationships in general, and for each region in particular, allows to carry out evaluation of theseismic hazard following a series of steps first proposed by Irikura (2002) like recipe to estimate
the strong movements of earthquakes taken place in active faults.
A lot of work has been done in the case of inland earthquakes. Somerville et al. (1999), using
results of many kinematic inversions obtained some relationships that synthesize the main
characteristics of the earthquakes. Especially they are focused in those that are the main strong
ground motions generators. It was found that the area of the asperities represented 22% of thetotal rupture area. Likewise they presented the relationships between the seismic moment andthe rupture area, the slip average, the combined area of asperities, the area of the biggest
asperity, among others. These relationships are of great utility for the prediction of seismic
movements limiting the values that can be considered in particular for the simulation of anactive fault.
The same relationships for subduction earthquakes with inverse focal mechanism have beenobtained, although they are not equally convincing. It is needed to continue collecting data forthe earthquakes of this type which occurred in subduction areas. Somerville et al. (2002) using
heterogeneous rupture slip models of big subduction earthquakes, studied the scale relationships
systematically with the seismic moment, and they compared them with those obtained for inland
earthquakes. They found that the main differences between the models of subduction slip andinland earthquakes were in the total rupture area. They concluded that the areas of the
subduction earthquakes were, on the average, two to three times larger that those of the inland
earthquakes having the same seismic moment. Consequently this produced smaller stress drops
for the subduccion earthquakes than the inland ones.
On the other hand, Dan et al. (2001), studied the quantity of effective stress and slip taken placein an asperity and in the rest of the rupture area based on models of rupture of variable slip andtheir relationship with the flat levels of acceleration spectra.
The present study uses data of some Mexican subduction earthquakes with the objective ofproving the different relationships presented previously, with the objective of validating them orin its case to propose some other one of regional validity.
DATA

This section has the purpose of validating the relationship proposed by Dan et al. (2001) among
the plane level of acceleration in high frequencies and the seismic moment. The relationshipproposed by Dan et al. (2001) isA[1dina − cm] = 2.46 x1017 xM 0 3
2
s

(1),

where A it is the flat level of the source acceleration spectrum and M 0 is the seismic
moment.
In order to find these spectra based on observed data we considered the stations Atoyac (ATYC),
Coyuca (COYC), Filo de Caballo (FIC2), Los Magueyes (MAGY), Ocotillo (OCLL), San
Marcos (SMR2), Súchil (SUCH), Tonalapa (TNLP), and Zihuatanejo (AZIH), that according to
Humphrey and Anderson (1994), present a flat site response in the band from 0.3 to 30 Hz. All
these stations belong to the Guerrero Accelerographic Array.
The selected earthquakes were recorded in some of the above mentioned stations, and all ofthem had reverse focal mechanism (reported by Harvard-CMT in the period from the January 1stof 1990 to December 31st of 1999). The epicenters were located between 15 and 20 degrees
latitude, and -107 and -95 degrees of longitude. Additionally, five very well-known earthquakes
September 19th of 1985 (09-19-85), February 8th of 1988, March 10th of 1989, April 25th of
1989, and May 2nd 1989; were included. In total 31 earthquakes were considered, 24 of themregistered in two or more stations. The epicentres of the considered earthquakes appear in the
Figure 1.

Figure 1. Epicentral localization of the 32 earthquakes used in this study.

FLAT LEVEL OF ACCELERATION SPECTRA

The acceleration records of the horizontal components were corrected as is indicated by thefollowing equation:
πfR

R Q( f )βA0 = A( f ) * 4πρβ
e
0.85

(2),

Q( f ) = 273 f 0.66

(3),

3

where

R is the hipocentral distance, β is the S wave velocity (here considered as 3.61 km/s like in
Humphrey and Anderson (1994) and Valdez et al. (1986) appropriate for the region), ρ is thedensity (2.83 g / cm 3 ), the constant 0.85 is used to consider the average S waves radiation
pattern and the partition of the energy in two horizontal components. The equation (3) is takenfrom Ordaz and Singh (1992). The epicentral distances have been calculated with thehipocentral localizations of the Instituto of Ingeniería (I. de I.) and of the Servicio Sismológico
National (SSN), except for the earthquakes of 09-19-85 and 10-09-95 whose hipocentral
localizations were taken of the Harvard CMT. Some examples of the spectra already corrected of
the 31 earthquakes appear in the Figures 2.

Figure 2. Spectra of acceleration corrected for the earthquakes of the 04-25-89 (left side) and 1024-93 (right side). The component EW is in the upper part and the NS in the bottom

To characterize the flat level of frequency in the acceleration spectra we proceeded as follows.
First, based on the graphs of the spectra for all the 31 earthquakes (as those in Figure 2), we
determined the band width for the frequencies inside which the flat levels would be estimated.
Taking into account that band width, the flat level was obtained for each spectrum averaging thespectral amplitudes logarithmically in that interval. For each event with records in more than one
station it was obtained the logarithmic average and the standard deviations of the flat levels.
This procedure is exemplified for the case of the earthquake of the 09-19-85 in the Figure 3. In
these graphs the horizontal lines show: the average flat level and their standard deviations in theband width where they were calculated. Table 1 shows the dates of the used earthquakes, thestations that recorded them, the seismic moment, the epicentral localizations, as well as the usedranges of frequency and the values of the flat levels. Note that the Harvard CMT's epicentral
localizations as well as the epicentral locations determined from the local array data (I. de I. andSSN) are also shown.

Figure 3. Example of the determination of levels planes their averages and standard deviations
for the case of the earthquake of 09-19-85, the NS component is in top and EW component inbottom, the horizontal lines show: the level plane average and their logarithmic deviations
standard in the interval where they are calculated.

TABLE 1(ew)
DATE

1985/09/19
1988/02/08
1989/04/25
1989/05/02
1989/03/10
1990/01/13
1990/05/11
1990/05/31

Stations

ATYC COYC FIC2
SUCH
ATYC COYC MAGY
SUCH TNLPATYC COYC FIC2
OCLL SMR2
COYC FIC2 OCLL
SMR2ATYC MAGY SUCH
TNLP
FIC2 OCLL SMR2TNLP
ATYC FIC2 MAGYSUCH
ATYC AZIH FIC2MAGY SUCH

Moment[dyne-cm]

1.10e+28
7.37e+24
2.39e+26
1.91e+24
1.35e+24
1.00e+24
2.48e+24
7.49e+24

1991/01/14

AZIH FIC2 TNLP

1.89e+24

1991/04/01

TNLP

6.25e+24

1992/02/12

AZIH

7.03e+23

1992/03/31

ATYC FIC2 MAGY
TNLP

1.55e+24

1992/04/01

AZIH TNLP

7.33e+23

1992/06/07

TNLP

1.19e+24

1993/03/31

ATYC FIC2 MAGY
TNLP

1.95e+24

1993/05/15

COYC SMR2 TNLP

1.41e+25

1993/10/24

ATYC COYC FIC2
OCLL SMR2 TNLP

1.01e+26

1995/04/27

AZIH

7.29e+23

1995/09/14

ATYC COYC OCLL
TNLP

1.31e+27

1995/10/09

TNLP

1.15e+28

1996/02/25

TNLP

5.51e+26

1996/03/13

SMR2 TNLP

6.37e+23

1996/04/23

ATYC AZIH

2.42e+24

1996/07/15

ATYC AZIH OCLL
TNLP

9.95e+25

1996/07/18

ATYC AZIH TNLP

1.59e+24

1997/01/21

SMR2

2.09e+24

1998/05/09

ATYC AZIH COYC
SUCH

8.44e+23

1998/05/16

ATYC SUCH

6.85e+23

1998/07/05

ATYC COYC OCLL
SMR2 TNLP

1.07e+24

1998/07/11

ATYC SUCH TNLP

1.83e+24

1998/07/12

ATYC COYC OCLLSUCH TNLP

1.93e+24•

CMT Epicenter(IdeI SSN)

CMT Depth(IdeI SSN)

17.91,101.99(18.081,102.942)
17.03,100.35(17.494,101.157)
16.83,99.12(16.603,99.400)
16.82,99.35(16.637,99.513)
16.91,100.44(17.446,101.089)
16.33,99.67(16.820,99.629)
17.24,100.56(17.134,100.304)
16.77,100.12(17.106,100.893)
18.39,101.37(17.838,101.854)
16.70,97.68(16.044,98.387)
17.78,101.14(17.733,101.058)
16.99,100.65(17.233,101.302)
17.10,100.51(17.333,101.266)
16.40,98.28(16.222,98.870)
17.29,100.89(17.180,101.020)
16.45,97.92(16.430,98.740)
16.77,98.61(16.54,98.98)
18.04,101.66(17.880,101.64)
16.73,98.54(16.31,98.88)
19.34,104.80(18.74,104.67)
15.88,97.98(15.83,98.25)
16.93,98.86(16.52,99.08)
17.13,101.84(17.11,101.60)
17.50,101.12(17.45,101.16)
17.35,101.02(17.54,101.20)
16.49,97.99(16.44,98.15)
17.31,101.00(17.34,101.41)
17.56,101.54(17.25,101.35)
16.92,99.73(16.83,100.12)
17.28,101.17(17.25,101.54)
16.78,99.91(16.83,100.44)

21.3
(15)
47.8
(19)
15.0
(19)
47.9
(13)
46.8
(18)
34.2
(12)
15.0
(18)
26.0
(16)
67.8
(25)
39.8
(26)
33.9
(<5)
15.0
(11)
60.0
(18)
15.4
(5)
26.2
(8)
38.5
(20)
21.8
(19)
41.6
(61)
21.8
(22)
15.0
(5)
15.0
(3)
29.4
(18)
36.8
(17)
22.4
(20)
26.2
(20)
39.7
(18)
36.2
(18)
33.0
(14)
37.6
(5)
24.1
(5)
15.0
(4)

Frequency Range
for Flat

(ns)

cm
]
s2

cm
[dyne − 2 ]s[dyne −

0.63-7.59

1.37697e+27 *3.27458e+27

1.47918e+27 *3.45341e+27

2.29-15.14

9.98472e+25

9.89133e+25

0.52-8.71

1.0641e+26

1.08935e+26

1.00-10.00

3.01895e+25

2.94489e+25

1.90-15.85

2.54377e+25

2.55001e+25

2.75-12.02

2.39457e+25

2.40409e+25

1.15-9.12

1.49803e+25

1.51555e+25

0.91-8.32

4.51428e+25

4.31058e+25

1.66-9.12

5.61566e+25

5.53798e+25

1.20-10.00

0.74365e+26

0.78978e+26

0.91-22.91

0.17764e+26

0.12840e+26

1.15-13.18

3.49965e+25

3.81667e+25

2.51-10.00

2.52916e+25

2.46588e+25

0.76-7.60

0.25466e+26

0.24343e+26

0.72-18.20

2.21248e+25

2.10620e+25

1.00-8.71

8.80735e+25

1.07528e+26

0.69-10.00

1.18195e+26

1.261195e+26

1.00-10.00

0.24292e+26

0.23018e+26

0.51-8.32

1.96679e+26

2.17228e+26

0.275-1.74

0.12812e+28 *0.11331e+28

0.13424e+28 *0.11872e+28

0.07-1.99

0.66472e+26

0.64340e+26

1.20-12.00

2.98936e+25

2.70708e+25

0.66-13.18

1.90404e+25

1.78913e+25

1.00-10.00

1.15609e+26

1.16396e+26

1.10-12.02

3.34649e+25

2.97328e+25

1.00-8.32

0.25734e+26

0.36306e+26

2.00-13.80

2.69882e+25

2.73504e+25

1.00-7.94

1.39468e+25

1.55086e+25

1.90-13.18

1.56746e+25

1.43779e+25

0.72-11.50

2.81928e+25

3.03071e+25

0.9-8.3

1.19396e+25

1.13429e+25

These values were calculated using the distance calculated with Harvard CMT'S
epicentral localization.

Figure 4. Relationship between the flat level of acceleration spectrum and the seismic moment of
Mexican subduction earthquakes. The points represent individual events, the line is therelationship proposed by Dan et al. (2001), for subduction earthquakes. Here the results areshown for the component EW.

Figure 5. Relationship between the flat level acceleration spectrum and the seismic moment of
Mexican subduction earthquakes. The points represent individual events, the line is therelationship proposed by Dan et al. (2001), for subduction earthquakes. Here the results areshown for the component NS.

The averages of the flat levels of the acceleration spectra were obtained independently for eachcomponent. In the Figures 4 and 5 the graphs of the averages of these plane levels are shown
versus seismic moment as well as the standard deviation when it has been calculated for thecomponent EW and NS respectively. The straight line that appears in the Figures is calculatedwith the equation (1) proposed by Dan et al. (2001).
We can observe that the relationship proposed by Dan et al. (2001) predicts the flat levels of
acceleration appropriately for the Mexican subduction earthquakes. The Michoacán earthquakeof 09-19-85 is included in the earthquakes reported by Dan et al. (2001) with a flat level smaller
than the values obtained in this study for both components. The difference resides in the waythat Dan et al. (2001) use to determine it. They extrapolated the low frequency spectra based onheterogeneous slip displacements from kinematic inversions. They did not use any records as is
the case in the present study.

RELATIONSHIP BETWEEN THE AREA OF THE BIGGEST ASPERITY AND THE SEISMIC
MOMENT FOR MEXICAN EARTHQUAKES.

To examine the relationship among the area of the larger asperity and the seismic moment lets
consider the results shown by Humphrey and Anderson (1994). They obtained the source
spectrum of 82 earthquakes after removing the site and path effects. We selected 8 earthquakes
from those, with reverse focal mechanism. We took the radius of the source that they obtainedusing the average corner frequency and the relationship by Brune (1970):
rc =

2.34β
2πf c

(4).

With the radii we calculate the equivalent area and then ploted it against the seismic moment, inFigure 6. In the same Figure the relationship proposed by Somerville et al. (2002) between theseismic Moment and the area of the large asperity for subduction earthquakes is plotted. That
relationship is
2/3
A1 (km 2 ) = C 4 x10 −16 xM 0
(5),
where M 0 is the seismic moment in dyne-cm, and C4 for subduction earthquakes takes thevalue of 8.87. In Figure 6 it can be observed that the predicted area based on the relationship fits
appropriately the area that is considered as the source area in Humphrey and Anderson.
Considering that the data used by Humphrey and Anderson are acceleration records it isprobable that indeed the estimates that they carried out are on the area of the largest asperity
instead of the total area of the source.
Using our data set, we also estimated the seismic moment and corner frequencies for all the 31
earthquakes from the displacement spectra (after integrating the corrected acceleration spectra).
In Figure 7, we ploted the estimated areas versus the seismic moment, resulting from our cornerfrequencies in the equation 4.

Figure 6. Relationship between the area of the large asperity and the seismic moment. The points
represent the area of the source reported by Humphrey and Anderson (1994). The line represents
the relationship reported by Somerville et al. (2002) for subduction earthquakes.

Figure 7. Relationship between the area of the large asperity and the seismic moment. The points
represent the area of the source obtained in the present study. The line represents the relationshipreported by Somerville et al. (2002) for subduction earthquakes.
The three graphs, in Figures 6 and 7, show the same tendency. For one side this allows us tovalidate the relationship of Somerville et al. (2002) for the Mexican subduction earthquakes,
especially for those that have moments larger than 2x10 24 dyne-cm, for those that therelationship seems to work better. From the Figure 6, the area of the largest asperity seems to beoverestimated by the Somerville et al. (2002) relationship for seismic moments smaller than
2x10 24 dyne-cm. On the other hand if we consider the seismic moments obtained with theGuerrero Accelerographic Array, like in Humphrey and Anderson, we can determine the area ofthe largest asperity that frequently is the responsible for great percentage of the generation ofstrong motions. For example the earthquake of 4-25-89 whose seismic moment using data ofacceleration is M 0 = 8.71x10 25 dyne-cm and it would have an area of 180.59 km 2 using theradius and moment reported by Humphrey and Anderson (1994). Using the relationship ofSomerville et al. (2002) for that moment the area of the largest asperity is of 174.28 km 2 . That
earthquake has been also simulated using empirical Green’s function method and using a square
fault area of 110.25.

STRESS DROP

Once the largest asperity has been determined, it is possible to estimate the stress drop of thesame asperity using the flat level of acceleration spectrum given by the next equation:
A0 ≈

Aasp
0.98

≈ 4π

S asp

π

∆σ asp β 2(6),where Aasp is the flat level of aceleration spectra due only to the asperity (98% of A0 ), S asp is
the asperity area and ∆σ asp is the stress drop of the asperity. Taking the areas determined withthe radi reported by Humphrey y Anderson (1994), considering β = 3.61 Km/s, and the flatlavels from the Table 1, we obtained the stress drop reported in Table 2.

TABLE 2
DATE

Moment[dyne-cm]

Area[ cm 2 ]1988/02/08
1989/04/25
1989/05/02

7.37e+24
2.39e+26
1.91e+24

353e+08
18059e+08
581e+08A0 (ew)[dyne −cm]
s2

9.98472e+25
1.0641e+26
3.01895e+25∆σ asp (ew)∆σ asp (ns)

cm
]
s2

[ bar ][ bar ]

9.89133e+25
1.08935e+26
2.94489e+25

587
87
138

581
90
135

A0 (ns)[dyne −

Using the moments found by Humphrey and Anderson, we obtained the results shown in Table
3.
TABLE 3
MomentDATE(Humphrey&
Anderson)

1988/02/08
1989/04/25
1989/05/02[dyne-cm]
1.41e+24
8.71e+25
8.41e+23

Area

A0 (ew)

A0 (ns)∆σ asp (ew)∆σ asp (ns)(Humphrey&Anderson)[ cm 2 ]cm[dyne − 2 ]scm[dyne − 2 ]s[ bar ][ bar ]

353e+08
18059e+08
581e+08

9.98472e+25
1.0641e+26
3.01895e+25

9.89133e+25
1.08935e+26
2.94489e+25

587
87
138

581
90
135

Now, if we consider the areas given by the relation in Somerville et al. (2002) for subduction
earthquakes (equation 5), we obtained the results shown in Table 4.
TABLE 4
MomentDATE(Humphrey&
Anderson)

1988/02/08
1989/04/25
1989/05/02[dyne-cm]
1.41e+24
8.71e+25
8.41e+23

A0 (ew)

Area(Somerville et al (2002))[ cm 2 ]1117e+08
17428e+08
821e+08[dyne −cm]
s2

9.98472e+25
1.0641e+26
3.01895e+25∆σ asp (ew)∆σ asp (ns)

cm
]
s2

[ bar ][ bar ]

9.89133e+25
1.08935e+26
2.94489e+25

330
89
116

327
91
113

A0 (ns)[dyne −

If we consider the areas given by the relationship in Somerville et al. (2002) for subduction
earthquakes (equation 5) and the flat level of acceleration spectra given by the relationship in
Dan et al. (2001) in equation 1, we always get an stress drop of ∆σ asp = 91 bars. Using datafrom the Guerrero Acelerographic Array Ordaz et al. (1995) found stress drop of 150 and 200
bars for the 1998/04/25 and 1998/05/02 earthquakes respectively. In Humphrey and Anderson(1994) the stress drop reported for the three earthquakes are 525, 89, and155 (for 1988/02/08,
1989/04/25, and 1989/05/02 earthquakes respectively).
THE FLAT LEVEL OF ACCELERATION SPECTRA FOR TOKACHI-OKI, JAPAN
EARTHQUAKE OF SEPTEMBER 26, 2004 AND AFTERSHOCKS

On September 26 2003, the Tokachi-oki earthquake (Mw = 7.9) occurred off Tokachi,
Hokkaido Island, Norhern Japan. Two people were missing and more than 800 people were
injured. In this region the Pacific plate subducts toward N60 o W beneath the Hokkaido region(Yagi, 2004). In this fault zone large interplate earthquakes occurred in 1952 (Ms 8.2), 1958
(Ms 8.1), 1969 ( Ms 7.8), and 1975 (Ms 7.4).
We used the records of 17 events recorded in the 29 stations. The epicenter locations of theevents used in this study were plotted in Figure 8. The moment magnitude of the earthquakes
ranging form Mw = 4.7 to Mw = 7.9, been the largest one the magnitude of the mainshock of theTokachi-oki earthquake. Most of the events are aftershocks of Tokachi-oki earthquake.
We corrected the data using the corrections of the equation (2) using the following value for thequality factor
Q( f ) = 83.3 f 1.0(7)

obtained by Yamamoto et al. (1995). That value is similar to the one obtained by Morikawa andSasatani (2000) that is Q( f ) = 120.9 f 1.0 . Even though the stations are in borehole at depths

more than 100 m and can be regarded as rock sites; we believe that the destructive interferenceof the reflected waves can affect the flat level of acceleration spectra. Then we obtained andcorrected the site effect.

Figure 8. Location of the KiK-net stations (left site) and epicentral localization of theearthquakes used in this study for the Tokachi-oki earthquake, Hokkaido, Japan.

Figure 9. Relationship between the flat level acceleration spectrum and the seismic moment for Tokachioki earthquake (green dot), after shocks, and other subduction earthquakes occurred in 2003. The points
represent individual events, the line is the relationship proposed by Dan et al. (2001), for subduction
earthquakes. Here the results are shown for the component EW.

This earthquake was recorded in many stations of the KiK-net. The KiK-net had instruments atborehole and surface sites. We selected borehole acceleration records from 29 KiK-net stations
for this study. The stations are located at depths more than 100 m and their locations are shownin Figure 8.
The averages of the flat levels of the acceleration spectra are plotted in the Figures 9 and 10
versus seismic moment as well as the standard deviation for the component EW and NS. The
straight line that appears in the figures is calculated with the equation (1) proposed by Dan et al.
(2001).

Figure 10. Relationship between the flat level acceleration spectrum and the seismic moment for Tokachioki earthquake (green dot), after shocks, and other subduction earthquakes occurred in 2003. The points
represent individual events, the line is the relationship proposed by Dan et al. (2001), for subduction
earthquakes. Here the results are shown for the component NS.

DISCUSION AND CONCLUSIONSIn the present study, acceleration records of Mexican subduction earthquakes were used tovalidating previously obtained scaling relationships and to propose some other relationships forregional validity.
Using records of the Guerrero Accelerograph Array from earthquakes occurring during the last
15 years we found that the relationships proposed by Dan et al. (2001) for estimating the flatlevel of acceleration spectra works appropriately for the Mexican subduction earthquakes. The
largest earthquake that we consider (09-19-85), shows a higher level than the value that Dan etal. (2001) relationship predicts. We also found that the fault areas obtained with data of thisregional array, gave a good estimation of the size of the biggest asperity. These sizes were well
predicted by the relationship for subduction earthquakes given by Somerville et al. (2002).

Also we used a set of 19 earthquakes recorded in 29 borehole stations that occurred during 2003
close to Hokaido Island in the north of Japan. After correcting the data for path and site effectthe high frequency level of acceleration spectra was determined. The results showed analignment parallel to the Dan et al. (2001) relationship for those earthquakes with a moment
smaller than 1x10 26 dyne-cm but systematically within larger values. There are two earthquakesthat have large acceleration level that departs substantially from the general tendency. One ofthem, the May 5, 2003 Miyagiken-oki earthquake exhibits the largest acceleration level. The flatlevel of acceleration spectra of this earthquake has been obtained by Satoh (2004). She
concluded that this earthquake has a higher level than the Dan et al. (2001) relationship. So thisearthquake is a special case. The other earthquake that showed large flat acceleration spectra is
the July 27, 2003 earthquake occurring in the northern part of the Japan Sea with a depth ofabout 500 km. These kinds of earthquakes exhibit an anomalous intensity pattern. Furumura(2003) suggested that for these deep earthquakes the waves were traveling as guided waves
through the plate producing larger intensities in places far from the epicenter and smaller
intensities in places closer to the epicenter. Then it is possible that this mechanism produces thehigh flat level of acceleration that we obtained. So these two earthquakes should be discussed
separately. Finally, the acceleration spectra for the 2003 Tokachi-oki earthquake obtained in thisstudy shows a higher flat level than the expected based on the Dan et al. (2001) relationship.
ACKNOLEDGMENTS
This work has been done during the visits of the first author to DPRI, Kyoto University in 2003 and 2004.
The Mexican waveform data used was obtained from the “Base Nacional de Datos de Sismos Fuertes”.
The Japanese waveform data used was provided by the KiK-net, which belong to the National ResearchInstitute for Earth Science and Disaster Prevention (NIED). Most of the graphs shown in this article weredone using Generic Mapping Tools (Wessel and Smith, 1998). Part of data processing was done using
SAC2000 (Goldstein et al., 1999).

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