Intraslab Mexican earthquakes of 27 April 2009 (Mw5.8) and 22 May 2009 (Mw5.6): a source and ground motion study

UNAM Seismology group

X. Pérez–Campos¹*, S. K. Singh¹, A. Iglesias¹, L. Alcántara², M. Ordaz² and D. Legrand¹ with contributions (in alphabetical order) by L. A. Aguilar², D. Almora², M. Ambriz², M. Ayala², C. Cárdenas¹, G. Castro², J. L. Cruz¹, R. Delgado², J. Estrada¹, S. I. Franco–Sánchez¹, M. A. Macías², I. Molina², F. Navarro¹, C. Pérez², J. Pérez¹, A. Quezada¹, L. Quintanar¹, L. E. Rodríguez¹, A. L. Ruiz², H. Sandoval², M. Torres², C. Valdés–González¹, R. Vázquez², J. M. Velasco², J. Velázquez², and M. Velázquez²

¹ Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, Del. Coyoacán, 04510, Mexico City, Mexico. *Corresponding author: xyoli@geofisica.unam.mx

² Instituto de Ingeniería, Universidad Nacional Autónoma de México, Ciudad Universitaria, Del. Coyoacán, 04510, Mexico City, Mexico.

Resumen

Dos tipos de sismos intraplaca en la placa de Cocos que subduce debajo de la placa Norte America ocurren en Guerrero, México, y áreas adyacentes: (A) inversos de gran echado y (B) de fallamiento normal. Los de tipo A se localizan a ~10–35 km de la costa, a una profundidad de ~35 km, y revelan compresión en la dirección del echado de la placa, probablemente causada por su desdoblamiento. Los de tipo B son ligeramente más profundos que los del tipo A cuando ocurren cerca de la costa, pero si ocurren más adentro del continente, donde la placa se vuelve horizontal, alcanzan profundidades de 40–50 km. Estos eventos revelan extensión en la placa subducida orientada en la dirección de su echado.

El análisis de los sismos del 27 de abril y del 22 de mayo de 2009 revela que se trata de eventos intraplaca en la placa de Cocos subducida del tipo A y B, respectivamente. Los espectros de fuente obtenidos a partir de datos locales y regionales dan una caída de esfuerzos de Brune, Δσ, de ~49 y 34 MPa, respectivamente, un poco mayores que la mediana de Δσ de 30 MPa reportada previamente para sismos intraplaca mexicanos. Nuestras estimaciones de energía radiada, E_R, son 3.55x10¹³ J y 2.29x10¹³ J, lo que arroja un cociente E_R/M₀ de 5.63x10^–5 y 6.54x10^–5, y un esfuerzo aparente, σ_a, de 3.9 MPa y 4.6 MPa, respectivamente (correspondientes al M₀ reportado en el catálogo Global de CMT), valores razonables para sismos intraplaca.

Las aceleraciones máximas del terreno (PGA), como una función de la distancia están en buen acuerdo con relaciones de atenuación previamente obtenidas para sismos intraplaca mexicanos. Los movimientos del terreno en el Valle de México generados por sismos intraplaca son más ricos en altas frecuencias en comparación con aquéllos de eventos interplaca, especialmente en la zona dura. Esto refleja tanto una naturaleza más energética de las fuentes intraplaca como una distancia más cercana a la fuente de muchos de estos sismos. Los resultados obtenidos en este estudio nos dan confianza en nuestro conocimiento de la naturaleza de las fuentes intraplaca y nuestra habilidad de estimar movimientos fuertes para eventos futuros.

Palabras clave: Sismos intraplaca, geometría de la placa, energía radiada, movimiento del terreno.

Abstract

Analysis of earthquakes of 27 April 2009 and 22 May 2009 reveals that they were intraslab events in the subducted Cocos plate of type A and B, respectively. The source spectra retrieved from local and regional data yield Brune stress drop, Δσ, of ~49 and 34 MPa, respectively, somewhat greater than the median Δσ of 30 MPa previously reported for intraslab Mexican earthquakes. Our estimates of the radiated energy, E_R, are 3.55x10¹³ J and 2.29x10¹³ J, which gives E_R/M₀ of 5.63x10^–5 and 6.54x10^–5, and apparent stress, σ_a, of 3.9 MPa and 4.6 MPa, respectively (corresponding to M₀ reported in Global CMT catalog), reasonable values for intraslab earthquakes.

Peak ground accelerations (PGA) as a function of distance are in fairly good agreement with previously–derived attenuation relations for Mexican intraslab earthquakes. Ground motions in the Valley of Mexico from intraslab earthquakes are enriched at high frequency as compared to those from interplate events, especially in the hill zone. This reflects both a more energetic nature of the intraslab sources and, relatively, closer location of many of these earthquakes. The results obtained in this study give us some confidence in our knowledge of the nature of intraslab sources and our ability to estimate ground motions from future events.

Key words: Intraplate earthquakes, slab geometry, radiated energy, ground motion.

Introduction

Study of intraslab earthquakes in the subducting Cocos plate below Mexico is important in understanding the dynamics of the subduction as well as in the seismic hazard estimation. The intraslab earthquakes are often normal–faulting events involving down–dip extension. Some of these earthquakes have caused great damage in the Mexican altiplano. The earthquake of 15 January 1931 (M_w 7.8, H = 40 km) caused destruction to the City of Oaxaca (Singh et al., 1985); the earthquakes of 28 August 1973 (M_w 7.0; H = 82 km) and 24 October 1980 (M_w 7.0; H = 65 km) resulted in deaths and damage in the states of Veracruz, Puebla, and Oaxaca (Singh and Wyss, 1976; Lomnitz, 1982; Yamamoto et al., 1984; Nava et al., 1985). Tehuacán earthquake of 15 June 1999 (M_w 7.0, H = 60 km) caused damage in the State of Puebla, especially to colonial structures in and near the city of Puebla (Singh et al., 1999). The great earthquake of 19 June 1858, which caused severe damage to inland towns in the state of Michoacán, including its capital city of Morelia, and to Mexico City, may also have been an intraslab, normal–faulting event (Singh et al., 1996).

The epicenters of the earthquakes mentioned above were in the altiplano. However, normal–faulting, intraslab earthquakes also occur near the coast. Earthquakes of 11 January 1997 (M_w 7.1; H = 35 km) and 30 September 1999 (M_w 7.4, H = 40 km) are two such examples. The 1997 earthquake occurred near Caleta de Campos below the rupture area of the 19 September 1985 Michoacán earthquake (Santoyo et al., 2005). The 1999 earthquake was located near the coast of Oaxaca and caused damage to towns along the coast and between the coast and the City of Oaxaca (Singh et al., 2000).

The geometry of the subducted Cocos slab below Guerrero is somewhat unusual. Several studies based on seismicity and focal mechanisms confirm the subduction of Cocos plate below Guerrero at a shallow angle, reaching a depth of 25 km at a distance of 65 km from the trench. The plate begins to unbend at this distance and becomes horizontal at a distance of ~120 km at a depth of ~40 km (Suárez et al., 1990; Singh and Pardo, 1993; Pardo and Suárez, 1995; Pacheco and Singh, 2010). This is supported by recent observations on the geometry of the subducted Cocos plate imaged from receiver function analysis (Pérez–Campos et al., 2008). Pacheco and Singh (2010) report that the unbending of the slab results in steeply–dipping, thrust earthquakes at a depth of about 35 km. These intraslab events, whose focal mechanisms show down–dip compression, occur about 10 km inland from the coast. Atoyac earthquake of 13 April 2007 (M 5.9, H = 37.5 km) was this type of event (Singh et al., 2007). Thus, intraslab earthquakes below Guerrero are not only of normal–faulting, down–dip extensional type, but also of down–dip compressional type.

Preliminary location of the earthquakes of 27 April and 22 May 2009 and their focal mechanisms obtained from centroid moment tensor inversion suggest that these were intraslab earthquakes. Here, we undertake a more detailed source and ground–motion study of these earthquakes.

Source parameters

The earthquakes were well recorded at local and regional distances by broadband and accelerographic networks of Instituto de Geofisica, UNAM and accelerographic networks of Instituto de Ingeniería, UNAM. We use these data, augmenting them with a few recordings from stations operated by CENAPRED, to study the characteristics of the two sources and the ground motions.

Locations of the two earthquakes, computed from local and regional data, are listed in Table 1 and their epicenters are plotted in Fig 1a. The locations of the events in the section shown in Fig. 1b at once reveal that they were intraslab earthquakes.

(a) Regional moment tensor inversion

A regional moment tensor (RMT) inversion scheme has been implemented by the Mexican National Seismogical Service (SSN) which routinely and automatically estimates moment tensor of moderate and large Mexican earthquakes using broadband seismograms (http://laxdoru.igeofcu.unam.mx/cmt). Here we briefly summarize the procedure.

An e–mail report by the SSN triggers the automatic computation of RMT for M ≥ 5.0 earthquakes. The system chooses appropriate broadband stations according to magnitude reported as indicated in Table 2. Records from stations relatively close to the epicenter are discarded to avoid finite fault effects, and stations far from the epicenter are discarded to ensure good signal to noise ratio. Records of selected stations are integrated to obtain displacements and are band–pass filtered using frequencies that depend on magnitude (see Table 2). This procedure tries to use low frequencies (where displacement spectrum is flat), avoiding high frequencies at which the structure is poorly known. Selected seismograms are inverted using a time domain scheme implemented by Dreger (2003). Computation time is dramatically reduced using a set of stored Green's functions, which are computed at nodes of a grid of distances vs. depths and using an average S–wave velocity model. For Mexico, the SSN uses a model reported by Campillo et al. (1996).

Figures 2 and 3 illustrate the fit between observed and synthetic waveforms for the two earthquakes. These figures also provide a summary of the results. The focal mechanism of 27 April earthquake corresponds to steeply–dipping thrust faulting with downdip compression, while 22 May earthquake is a typical normal–faulting earthquake with downdip extension. The origin of 27 April earthquake may be interpreted as a consequence of unbending of the slab. Such unbending would give rise to compressional stress in the upper part of the subducted slab. The tensional stress corresponding to the 22 May earthquake may be attributed to negative buoyancy of the slab.

We inverted near–source displacement records of the 27 April earthquake at POZU, VNTA, and OCLL (Fig. 1a), obtained by integration of the accelerograms, for focal mechanism and seismic moment. The inversion assumes that the events may be approximated by a point–source shear dislocation in an infinite space. Synthetic seismograms include near– and intermediate–field contributions (Singh et al., 2000). Theory predicts simple unipolar P and S displacement pulses. The effect of free surface is approximately taken into account by multiplying the infinite–space synthetics by two. This approximation is acceptable if the epicentral distance, A, is smaller than the depth, H. The fit between the observed and synthetic seismograms, shown in Fig. 4, is good. The focal mechanism and the moment are listed in Table 1. The source parameters obtained from near–source recordings and RMT inversion, and those reported in the Global Centroid Moment Tensor (CMT) catalog are reasonably similar.

The near–source displacement records of the 22 May earthquake are complex, and P and S pulses are not unipolar. For this reason, the simple inversion scheme, mentioned above, could not be used for this event.

(c) Source spectra

We estimated source displacement and acceleration spectra, M₀ (f ) and f ²M₀( f ), of the two earthquakes from the analysis of the S–wave group recorded at hard sites. The Fourier acceleration spectral amplitude of the intense part of the ground motion at a station, under far–field, point–source approximation, may be written as

In equations above, M₀ (f) is the moment rate spectrum so that M₀(f)M₀ as f0, R = hypocentral distance, R_θΦ = average radiation pattern (0.55), F = free surface amplification (2.0), P takes into account the partitioning of energy in the two horizontal components , β = shear–wave velocity at the source, ρ = density in the focal region, and Q( f) = quality factor which includes both anelastic absorption and scattering. The appropriate geometrical spreading term, G(R), for inslab Mexican earthquakes is R ^–1 and the corresponding Q( f) is 251f^0.58 (García et al., 2004). As discussed in Pacheco and Singh (2010), the intraslab earthquakes below Guerrero occur in the lower oceanic crust or upper most part of the oceanic mantle. Assuming the source region of the earthquakes to be in the upper mantle, we take β = 4.68 km/s and ρ= 3.2 kg/m³. Taking logarithms of Equation (1) we obtain.

The source displacement and acceleration spectra of the two earthquakes are shown in Figure 5. We interpret these spectra within the framework of a ω²–source model and obtain an estimation of the seismic moment (M₀) and corner frequency (f_c). The stress drop (Δσ) is computed using the Brune model (Brune, 1970). The source spectra in Figure 5 can be fitted by M₀ and Δσ of 7.8x10¹⁷ N–m and 49 MPa (f_c = 0.915 Hz), and 4.3x10¹⁷ N–m and 34 MPa (f = 0.99 Hz), respectively. We note that these stress drops are slightly greater than the median Δσ of 30 MPa reported by García et al. (2004) for inslab Mexican earthquakes. The earthquake of 27 April 2009 is remarkably similar to the Atoyac earthquake of 13 April 2007 which was located ~90 km to the west and had about the same magnitude, depth, focal mechanism, and Brune stress drop (M_w 5.9; H = 37.5 km; Φ = 284⁰, λ = 73⁰, I = 91⁰; Δσ = 47 MPa; Singh et al., 2007) as the 2009 earthquake.

We emphasize that the source spectra and the estimated Brune stress drops above are contaminated by site effect. Although the sites are nominally "hard, weathering of near–surface rocks would give rise to amplification of seismic waves. These stress drops, however, are useful in estimating ground motions during future earthquakes using random vibration theory.

(d) Radiated seismic energy

Following Singh and Ordaz (1994), radiated seismic energy, E_R, can be written as:

where V_j(f) is the velocity spectrum of j–th component of the S–wave group. All other parameters are defined earlier. We used only hard–site recordings in the computation. Following Pérez–Campos et al. (2003), we applied a site effect correction that includes the combined effect of frequency–dependent site amplification and site attenuation (Boore, 1996; Boore and Joyner, 1997),

where A₀(f) is the amplification factor, and was estimated from the generic rock model given by Boore and Joyner (1997), and k is the attenuation parameter. Values of k were obtained from Castro et al. (1990) and Humphrey and Anderson (1992), with an average value of k = 0.036 s. E_R for the two events is plotted as a function of R in Fig. 6. The median E_R for the 27 April earthquake is 3.55x10¹³ J with 67% of observations lying within a factor of 2.7 of this value. For the 22 May earthquake the median E_R is 2.29x10¹³ J with 67% of observations lying within a factor of 2.3 of this value. This yields E_R/M₀ of 5.63x10^–5 and 6.54x10^–5 (corresponding to M₀ reported in Global CMT catalog) for the earthquakes of 27 April and 22 May, respectively.

With µ = 7.0x10⁴ MPa, the apparent stress, σ_a = µE_R/ M₀, for the two earthquakes is 3.9 MPa and 4.6 MPa, respectively. These values of σ_a are similar to those found by Choy and Kirby (2004) for normal–faulting subduction earthquakes from a worldwide database. These authors report that the highest σ_a for inslab events occurred at 35–70 km depth, in proximity to zones of intense deformation such as sharp bends in the slab. The apparent stresses of these events were up to 5 MPa, significantly higher than for the interplate thrust earthquakes. Inslab earthquakes in Mexico are also significantly more energetic than interplate events (García et al., 2004).

For a Brune ω²–source model, it can be shown that σ_a= µE_R/M₀ = 0.23Δσ, where Δσ is the Brune stress drop (see e.g., Singh and Ordaz, 1994). From Δσ reported above, the expected ct for the two earthquakes are 11.3 MPa and 7.8 MPa, respectively, 2.9 and 1.7 times greater than the computed σ_a . The discrepancy, in part, arises from ignoring site effect in the estimation of Δσ.

Ground motions

(a) Peak Ground Acceleration (PGA) as a function of distance

Inasmuch as the Brune stress drops of the two earthquakes are about the same as the median stress drop of Mexican intraslab earthquakes, we expect the observed PGA to follow the attenuation relation obtained by García et al. (2005) for this type of earthquakes. The plot of PGA versus hypocentral distance R is shown in Fig. 7. In general, the observed PGA values follow the attenuation relation but there are some differences. For example, PGA for the vertical component of the 22 May earthquake is consistently lower than the expected values, especially for the broadband stations farther from the epicenter. This observation is also true for the horizontal component. For the 27 April event, the PGA values at close distances are consistently higher than those expected from García et al. (2005), no matter the type of site. There are some soft sites at ~300 km that show larger values of PGA than those expected. These correspond to sites within Mexico City.

In spite of the differences pointed out above, Fig. 7 gives us confidence in our ability to predict ground motion from future intraslab earthquakes using the available attenuation relations.

(a) Ground motion in the Valley of Mexico: interplate versus intraslab earthquakes

Felt reports of intraslab earthquakes in the Valley of Mexico differ from those of interplate earthquakes. For events of the same magnitude, the ground motion from the former type of earthquakes felt more intense than from the latter type. This may be due to: (1) Location of intraslab events which, often, occur closer to the valley than the interplate events. The closest, well–located such earthquake has a hypocentral distance of 145 km (Iglesias et al., 2002), whereas interplate earthquake can only occur at R > 250 km. For the 22 May 2009 earthquake R to the city is ~160 km. (2) Depth of the intraslab earthquakes, which occur in the lower oceanic crust or the upper most mantle of the subducted slab, may give rise to significantly different wavefield in the valley than the interplate earthquakes. This is supported by theoretical computations (Furumura and Singh, 2002). García et al. (2004) show that intraslab earthquakes in Mexico are significantly more energetic than interplate events.

The larger spectral amplitudes in the hill zone at high frequencies (f > 1 Hz) during intraslab earthquakes may explain why the ground motion in this zone is felt more intensely during intraslab earthquakes as compared to interplate events. As shown in earlier studies (Singh et al., 1996; Iglesias et al., 2002), the relatively enriched high–frequency radiation in the hill zone renders small buildings in this zone vulnerable to strong intraslab earthquakes.

Discussion and conclusions

The intraplate earthquakes of 27 April and 22 May 2009 are examples of the two types of events that occur in the subducted slab below Guerrero: (A) steeply–dipping thrust events with down–dip compression and (B) normal–faulting earthquakes with down–dip tension. The former type seems to occur over a small distance range of ~15 to 40 km, inland from the coast, at a depth of about 35 km. Type (B) is observed over a larger distance range, ~15 to 175 km inland from the coast. The depth of this type of events is also ~35 km near the coast but increases to 40–50 km in the part of the slab where is horizontal. Thus, near the coast the two types of earthquakes occur in close proximity of each other. Within the uncertainty of the location, it is quite possible that type (B) occurs immediately below the type (A). Unbending of the slab provides an explanation for the occurrence of type (A) earthquakes, while negative buoyancy of slab may be the cause of the extensional stress observed over the longer distance range.

Intraslab earthquakes are more energetic at high frequencies than interplate events. However, there seems no discernible difference between the two types of intraslab events below Guerrero: the source spectra of both types can be explained by a stress drop of 30 to 50 MPa. The normalized energy, E_R/M₀, and apparent stress of the two earthquakes are ~6x10^–4 and 4 MPa, respectively. These values are similar to those reported by Choy and Kirby (2004) for world–wide data base of normal–faulting intraslab earthquakes.

Observed PGA values of the two earthquakes are reasonably well fitted by a previously obtained attenuation relation (García et al., 2005). More energetic nature of the intraslab sources and their, often, closer location to the Valley of Mexico, produces ground motions in the hill–zone which are enriched in high frequencies. This may be the reason why felt reports of intraslab events suggest, relatively, intense motions. As shown in some previous studies (Singh et al., 1996; Iglesias et al., 2002), this is also the reason why smaller buildings in the hill zone may be vulnerable during large intraslab earthquakes.

Finally, the results of the analysis of the two earthquakes give us confidence in our knowledge of source characteristics of Mexican intraslab earthquakes and our ability to estimate ground motions from future such earthquakes.

Acknowledgements

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