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Article |
1 Instituto de Geofisica
Universidad
Nacional Autónoma de México
Del. Coyoacán, C.P.
04510
Mexico, D.F.
(S.K.S., J.F.P.)
2 Departmento Geofisica y
Meteorología
Facultad de Ciencias Físicas
Universidad
Complutense de Madrid
Av. Ciudad Universitaria
28040 Madrid,
Spain
(D.G.)
3 Instituto de
Ingeniería
Universidad Nacional Autonóma de
México
Del. Coyoacán, C.P. 04510
Mexico,
D.F.
(A.I.)
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Abstract |
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 Top Abstract IntroductionDataAnalysisResultsDiscussion and ConclusionsAppendix |
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120f0.75 (0.1 
f 10 Hz), as compared with
Q(f) = 251 f0.58 for the average
path through subducted slab and continental lithosphere. Curiously, the
estimated Q of the mantle wedge at station SCIG, which is
located on the Yucatan Block, is about the same as the average Q in
southern Mexico. There are several possible explanations for the difference: (1)
very few recordings at SCIG, (2) a site effect at SCIG
masking the effect of low Q of the mantle wedge, and (3) relatively
high-mantle Q beneath the Yucatan Block. The third possibility is
supported by surface-wave tomography that reveals a thick, cold, mantle
lithosphere below the Yucatan Block and near absence of mantle lithosphere in
the backarc region of central Mexico. A higher density of seismographs along the
gulf coast is needed to resolve these issues. Our study predicts diminished
ground motions at the Laguna Verde nuclear power plant if the seismic waves pass
through the mantle wedge or traverse below the Popocatepetl or Orizaba
volcanoes. |
Introduction |
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TopAbstractIntroductionDataAnalysisResultsDiscussion and ConclusionsAppendix |
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20°) slab without unbending
has been proposed in central Oaxaca to explain the observed seismicity
(Singh et al., 1985;
Pardo and Suá rez, 1995).
The inslab seismicity in this segment extends to about 400 km from the trench
and reaches a depth of about 140 km. Thus, it is possible to delineate the
mantle wedge here.
Low Q in the mantle wedge was postulated and confirmed in the early
days of plate tectonics
(e.g., Oliver and Isacks, 1967;
Baranzagi et al., 1973).
In recent years, the Q of the mantle wedge below Japan has been mapped
in great detail (see, e.g.,
Takanami et al., 2000;
Tsumura et al., 2000),
thanks to the favorable location of seismic stations with respect to inslab
seismicity and a high density of these stations. Low Q of the mantle
wedge implies high attenuation of seismic waves reaching the backarc. An
impressive demonstration of this comes from the extensive recordings of recent
Japanese inslab earthquakes. A rather low Q for S waves
(60) in the mantle wedge is required to explain recorded seismic waves on
the western seaboard of Japan, across the volcanic front, as compared with
eastern Japan where the high-frequency seismic waves propagate through the
subducted slab and the continental lithosphere with much less attenuation (T.
Furumura, personal comm., 2004).
Mapping of Q and other physical properties of the mantle wedge in Mexico has been difficult because of the lack of suitably distributed seismic stations in the backarc region and because the ideal location for some of these seismographs would be in the Gulf of Mexico. Only three seismic stations are currently in operation along the gulf coast: LVIG, TUIG, and SCIG (Fig. 1). Each of these stations is equipped with a broadband seismograph and an accelerograph. TUIG is a highly noisy site located on thick gulf-coast sediments. For this reason, the seismograms from this site are of limited use. SCIG is located on limestone. LVIG, situated in the Laguna Verde nuclear power plant facility, is located on basaltic flows underlain by vulcanites that overlie a granitic inclusion. Logistical difficulties and instrumental failures, however, have resulted in few recordings at these stations, especially at SCIG.
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Data |
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TopAbstractIntroductionDataAnalysisResultsDiscussion and ConclusionsAppendix |
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Two of the 16 events analyzed here are shallow, thrust- faulting, interplate earthquakes (1 January 2004 and 14 June 2004); all others are inslab earthquakes (10 normal-faulting and 4 steeply dipping thrust-faulting inslab events). Our analysis is based on recordings from "hard" rock sites.
The events have been divided in four groups, A to D, indicated in Figure 1. Figure 2 shows the distances at which each event was recorded at all stations in the Mexican seismological and accelerometric networks. Figure 3 illustrates the paths from the epicenters to the stations. The events of each group were projected on a vertical plane that extends from the trench to LVIG. These sections are shown in Figure 4.
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Group A comprises five relatively deep inslab earthquakes that occurred near the northern Oaxaca-Veracuz border and in the Isthmus of Tehuantepec. The only three recordings available at SCIG are from events in this group. The waves from these sources travel through the mantle wedge and the continental lithosphere before reaching LVIG (Figs. 1 and 4) and SCIG. Group B consists of four inslab events that occurred near the coast of central Oaxaca and one shallow, thrust-faulting interplate event (depth H = 20 km). It includes the 30 September 1999 (Mw 7.4, H = 47 km) normal-faulting earthquake (Singh et al., 2000; Hernandez et al., 2001). The wave path from group B events to LVIG is probably mostly confined to the continental lithosphere. Group C consists of two inslab events: the 15 June 1999 (Mw 6.9, H = 61 km) Tehuacan earthquake (Singh et al., 1999) and the 21 July 2000 (Mw 5.9, H = 50 km) Copalillo earthquake (Iglesia et al., 2002). Group D includes three inslab, normal-faulting events located below the Balsas basin and one shallow, thrust-faulting interplate event (H = 17 km).
From Figures 1 and 4 it is at once clear that the waves from group A and C events to LVIG spend a larger fraction of their total travel time in the mantle wedge than group B and D events. Indeed, the events of groups B and D may barely sample the mantle wedge.
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Analysis |
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TopAbstractIntroductionDataAnalysisResultsDiscussion and ConclusionsAppendix |
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The Q from inslab events was estimated by García et al. (2004) for paths shown in Figure 3 (top). The figure suggests that the wave paths to stations, other than to LVIG, did not travel through the mantle wedge. Only 5 of the 268 records used in the study of García et al. (2004) were from LVIG. We recomputed Q by excluding data from LVIG. As expected, this resulted in an insignificant change in Q (from Q(f) = 251f0.58 obtained in García et al. to Q(f) = 256f0.59). Thus, we conclude that the Q reported in García et al. (2004) was not significantly affected by paths through the mantle wedge. Figure 3 (bottom) shows paths from events included in the present study but not in García et al. (2004). The median spectra of these events were computed, excluding data from LVIG and SCIG, and are not likely to be much affected by paths through the wedge. The median spectra of 6 of the 16 events are from García et al. (2004). The data of the 10 remaining events were processed in the same fashion as in García et al. (2004).
We follow a well-established procedure to compute S(f) (see, e.g.,
Singh et al., 1982;
Ordaz and Singh, 1992;
García et al., 2004)
that we outline briefly here. S(f) is estimated from S-wave
recordings on the two horizontal components. The horizontal components of the
accelerations at each station were windowed. The window length was chosen such
that it included the main S-wave arrival and 85% of the total
energy. The signals were then Fourier transformed, smoothed by a 1/6 octave-band
filter, and 5% tapered. Let Ai(f, Ri) be
the Fourier acceleration spectrum at station i located at a hypocentral
distance of Ri. Then Ai(f,
Ri) may be written as
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| (1) |
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| (2) |
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| (3) |
0(f) is the moment-rate
spectrum. In the preceding equations, β = shear-wave velocity,
= density, Q(f) = quality factor,
R
is the average radiation pattern
(=0.55;
Boore and Boatwright, 1984),
F is the free-surface amplification (=2), P takes into
account the partitioning of energy in the two horizontal components
(=1/
2), and M0 is the seismic moment of the
earthquake. G(R) in
equation (1) is the geometrical
spreading term, which, for the inslab events, we take as G(R)
= R
(García et al., 2004).
This form of G(R) implies dominance of body waves at all
distances. For the interplate events, we take G(R) =
R for R < 100 km and R1/2 for
R 
100 km
(Ordaz and Singh, 1992). For
inslab events we take β = 4.68 km/sec and
= 3.2 g/cm3, whereas for the interplate events the
corresponding values are taken as 3.5 km/sec and 2.8 g/cm3,
respectively. |
Results |
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TopAbstractIntroductionDataAnalysisResultsDiscussion and ConclusionsAppendix |
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For all five events of group A, SL(f) is consistently
lower than SM(f) (see left frames of
Fig. 5). A likely explanation
for this difference is a smaller-than-average Q along that fraction of
the path that traverses through the mantle wedge before reaching
LVIG. If we assume that the lower Q in the mantle wedge is
the correct explanation, then it is possible to establish a bound on its value.
The source spectra from LVIG for events A2, A3, and A5 can be made to
roughly coincide with the corresponding median spectra if Q along the
path to LVIG is taken as Q(f) =
120f0.75 (right frames of
Fig. 5). This provides an upper
bound for the Q in the mantle wedge for the three events. The true
Q(f) must be less because the waves to LVIG travel partly
through the subducted slab and continental lithosphere. Let us assume that half
of the total path to LVIG passes through the mantle wedge and the
other half through the subducted slab and continental lithosphere. It is
straightforward to infer that, in this case, the Q of the mantle wedge
is roughly given by Q 80f0.82.
Note that for SL(f) to match SM(f), the Q required for event A1 is much lower, Q(f) = 75f0.90, and for event A4 it is somewhat higher, Q(f) = 160f0.85. Local/regional and teleseismic recordings of event A1 show clear source directivity toward the southeast, away from LVIG. This may explain the lower spectral level at LVIG and, hence, the lower Q estimated for this as compared with the other three events. The cause for the relatively high Q for event A4 is not clear. We note that average S-wave radiation pattern of the group A events at LVIG is about 0.4, close to 0.55 taken in the calculations. Thus, a consistent bias in the radiation pattern cannot explain the observed lower spectral amplitudes at LVIG. Curiously, the source spectra of the three events retrieved from SCIG, SS(f), fall close to the median spectra SM(f). This suggests that the Q for paths to SCIG is close to the average Q or that there is a local site effect. We return to this issue in a later section.
Source spectra of events in group B are shown in Figure 6. SL(f) values of all events of this group fall within the curves defining ± one standard deviation from the median source spectra. Thus, the Q for paths from group B events to LVIG is roughly the same as for an average path, a result anticipated earlier based on the vertical section in Figure 4.
The wave paths to LVIG from the two events of group C are similar to those for events in group A (Figs. 1 and 4). Indeed, the SL(f) values as compared with the SM(f) values are similar to those for the group A (compare Figs. 5 and 7). Taking Q = 120f0.75 for the total paths to LVIG brings the SL(f) of the two events close to the SM(f) (see right frames in Fig. 7).
Group D events are farthest from LVIG (see Figs. 1, 2, 3, and 4). They range in depth from 20 to 64 km. Three of these events are inslab earthquakes (D1 to D3), and one of them is an interplate event (D4). In general, SL(f) is less than SM(f) for f >0.8 Hz and greater than SM(f) at lower frequencies (Fig. 8). At first glance this result is surprising because we anticipated group B and D events to show a similar trend. The wave paths from events D1, D2, and D3 to LVIG, however, pass below Popocatepetl, presently an active volcano (Fig. 1). The evidence suggests high attenuation of S waves crossing the volcano (Shapiro et al., 2000). Thus, SL(f) < SM(f) for f >0.8 Hz for these events may be a consequence of low Q below Popocatepetl. We note that event D4 shows the same trend as events D1 to D3 (Fig. 8), although its wave path does not cross the Popocatepetl volcano but the Orizaba volcano (Fig. 1). A likely explanation is that the Q below Orizaba is also low.
Amax at LVIG
These results predict lower-than-expected peak accelerations,
Amax, at LVIG for the events of groups A, C, and
D, and close to the expected values for group B events.
Figure 9 shows plot of
residuals (logarithm to the base 10 of the ratio of observed to expected
Amax) for events of each group. Here the expected
Amax has been computed for inslab and interplate events from
regression relations of
García et al. (2004)
and Ordaz et al. (1989),
respectively. As Figure 9
illustrates, however, the residuals at LVIG are negative for events
of groups A and C, and are close to zero for events in group B. The tendency for
group D events is less clear. Most data for group D have negative residuals. The
residuals at LVIG, however, are not more negative, as we expected,
but seem to follow the general tendency of the data.
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Less-than-expected Amax values at LVIG for group A and C events is a welcome result for the seismic hazard for Laguna Verde nuclear power plant. The critical earthquake in the design of Laguna Verde nuclear power plant, however, was a local Mw 6.6 crustal event. Q in the wedge has no impact on the expected ground motions from such an earthquake.
Low Q in the Mantle Wedge or an Extremely Competent, "Hard" LVIG Site?
The left frames in Figure 5
also include source spectra obtained from PNIG, a station located in
Pinotepa Nacional on the Pacific coast of Mexico (see
Fig. 1 for its location). We
note that the level of source spectra retrieved from LVIG
(Fig. 5) are lower than those
from PNIG, which, in turn, fall near minus one standard deviation
curves of the median spectra. Could the low levels of source spectra at
LVIG and PNIG be a consequence of both of these sites
being extremely hard, LVIG being harder than PNIG? If so,
then this could also explain negative residuals of Amax at
both LVIG and PNIG for group A and C events
(Fig. 9). In this scenario, the
Q of the mantle wedge would be about the same as the average Q
through the slab and the continental lithosphere. An examination of
Figure 6, however, does not
support this hypothesis. As seen in this figure, the source spectra from
LVIG for three of the group B events are now higher than that those
from PNIG. The simplest explanation, consistent with the
observations, is to assume that (1) the LVIG site is similar to the
typical hard site of the network, (2) PNIG is an extremely hard site,
and (3) the Q in the mantle wedge is relatively low. This would explain
the spectral curves of group B events in
Figure 6, with the exception of
B5. The wave paths from these events to LVIG involve little, if any,
propagation through the mantle wedge. For this group of events, the spectral
levels essentially reflect the site condition; higher and lower levels for
LVIG and PNIG, respectively, are a result of larger and
smaller amplification of seismic waves at these sites. For group A and C events,
the levels at LVIG are lower than those at PNIG, which, in
turn, is near minus one standard deviation curves of the median spectra. The
levels at PNIG are low because it is a very hard site with relatively
small amplification. The much lower level at LVIG is attributed to
the low Q in the mantle. The spectral curves for group D events can be
explained in much the same way as those for the other groups, except that the
lower levels of LVIG are now attributed to propagation below the
Popocatepetl or Orizaba volcanoes, regions of low Q.
Our assumption that PNIG is situated on an extremely hard site is
supported by geologic and geophysical data. PNIG is situated on
Xolapa Precambrian metamorphic complex. The P-wave velocity below the
station is high, about 6.2 km/sec, and the Bouguer gravity anomaly in the area
is anomalously positive, 60 mGal
(Valdes et al., 1986).
The records of local earthquakes at PNIG are also very simple
(Singh et al., 1997;
Pacheco and Singh, 1998),
suggesting a simple crustal structure and the absence of a significant weathered
superficial layer. As mentioned earlier, LVIG is located on a
3-million-year-old basaltic flow, underlain by vulcanites that overly a granitic
inclusion. It seems reasonable to expect that the site effect at LVIG
be similar to an average site of the networks.
We estimated site effects at LVIG and PNIG from the spectral ratio of the horizontal-to-vertical component (H/V) of S waves. The results, illustrated in Figure 10, suggest little, if any, site effect at PNIG and a somewhat larger one at LVIG than at PNIG. Roughly, then, Figure 10 supports a low Q of the mantle wedge.
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Q of the Mantle Wedge along Paths to Yucatan Block
As mentioned earlier, the spectra of the three events of group A recorded at
SCIG (Fig. 5) do not
reveal low Q along this path, which partly traverses through the mantle
wedge (Fig. 1). SCIG
is located on the Yucatan block. It is possible that the low mantle-wedge
Q effect is masked by a site effect at SCIG. In fact, the
H/V plot at SCIG
(Fig. 10) does indicate a
relatively large site effect. It is also possible that both the site effect at
SCIG and the absence of a low Q mantle wedge along this path
are responsible for the spectral level at SCIG.
Recent results of surface-wave tomography of Mexico and Central America reveal low and high shear-wave velocities in the backarc region northwest and southeast of the Tehuantepec ridge, respectively (N. Shapiro and M. Ritzwoller, personal comm., 2002). These authors propose near absence of mantle lithosphere below the Mexican backarc and the existence of substantial lithosphere below the Yucatan block. For group A events, this model predicts diminished spectral levels at LVIG and average or higher levels at SCIG. Thus, low Q for paths to LVIG is supported by this model.
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Discussion and Conclusions |
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TopAbstractIntroductionDataAnalysisResultsDiscussion and ConclusionsAppendix |
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f 10 Hz); a realistic estimation may be Q(f) =
80f0.82. These Q values are smaller than
Q(f) = 251f0.58, the average Q for
Mexican inslab earthquakes with wave paths mostly confined to the subducted slab
and the continental lithosphere
(García et al., 2004). The three events of group A recorded at SCIG (Fig. 5) do not reveal low Q along this path, although it partly traverses through the mantle wedge (Fig. 1). SCIG is located on the Yucatan block. We consider the data from SCIG as inconclusive however, because of the small number of recordings and possible contamination from the local site effect. Only more extensive deployment of seismographs along the coast of Yucatan Block can resolve this important issue.
The waves from events of groups B and D to LVIG mostly travel through the continental lithosphere. Hence, we expect the Q for paths to LVIG to be similar to the Q of the average path. Indeed, the results for group B agree with this expectation. This is not true, however, for group D events. The waves from these events travel below the active Popocatepetl or Orizaba volcano before reaching LVIG and hence suffer high attenuation. This results in diminished amplitudes at LVIG. Low Q below Popocatepetl has been reported in a previous study (Shapiro et al., 2000). Our study suggests that the Q may be low below the Orizaba volcano also.
We have offered a qualitative explanation of the observations which are based on a small data set. A detailed tomography of the Q of the mantle wedge will require a much larger data set and generation of synthetic seismograms through appropriate 2D and 3D structures of the Mexican subduction zone (see, e.g., Furumura and Singh, 2002).
Our study suggests reduced seismic hazard at the Laguna Verde nuclear power plant from earthquakes with wave paths through the low-Q mantle wedge or the active volcanoes of Popocatepetl and Orizaba. The same must be true for other sites along the coast of Gulf of Mexico. The critical design earthquake for the nuclear plant, however, was a local, crustal earthquake. Seismic hazard from such an event remains the same.
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Appendix |
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TopAbstractIntroductionDataAnalysisResultsDiscussion and ConclusionsAppendix |
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Manuscript received January 3, 2005
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