GEOLOGICAL NOTE
Darwin seismic gap closed by the 2010 Maule earthquake
Laguna sísmica de Darwin cerrada por el terremoto del Maule 2010
]]> Daniel Melnick1, Marcos Moreno2, Marco Cisternas3, Andrés Tassara4
1 Institut für Erd-und Umweltwissenschaften, UniversitatPotsdam, 14476 Potsdam, Germany. melnick@geo.uni-potsdam.de
2 GeoForschungsZentrum Potsdam, 14473 Potsdam, Germany. marcos@gfz-potsdam.de
3 Escuela de Ciencias del Mar, Pontificia Universidad Católica de Valparaíso, 1020 Valparaíso, Chile. marco.cisternas@ucv.cl
4Departamento de Ciencias de la Tierra, Universidad de Concepción, 160-C Concepción, Chile. andrestassara@udec.cl
ABSTRACT. The Maule earthquake (Mw 8.8) that affected south-central Chile on February 27, 2010 was preceded by the 1835 event documented by FitzRoy and Darwin. The relation between both events has been controversial. Fault slip in 2010 estimated by Lorito et al. (2011) is less than expected from 175 years of strain accumulation, leading them to conclude only limited overlap between the 2010 and 1835 events, and that a Mw 7.5-8 event could still strike the Concepción region. However, Lorito et al. 's model was based on displacements obtained from only 6 GPS stations and underpredicts observations from recent studies. Here we show that an alternative model based on 169 GPS displacements reproduces the data better, suggesting Lorito et al. 's main conclusion is not correct. Based on a slip deficit map, we suggest the seismic gap opened in 1835 was most likely closed in 2010.
Keywords: Maule earthquake, Coseismic slip distribution, Slip deficit, Seismic gap.
RESUMEN. El terremoto del Maule (Mw 8.8) que afectó el centro-sur de Chile el 27 de febrero del 2010 fue precedido por el evento de 1835 documentado por FitzRoy y Darwin. La relación entre ambos eventos ha sido controversial. Los desplazamientos para el 2010 estimados por Lorito et al. (2011) son menores a los esperados en 175 años de acumulación de acortamiento, observación que los llevó a concluir que los eventos del 2010 y 1835 tenían una similitud restringida y que, por ende, un evento de Mw 7.5 a 8 podría afectar la zona de Concepción en el futuro cercano. El modelo de Lorito et al. (2011) usó desplazamientos obtenidos de solo 6 estaciones de GPS y subestima observaciones de estudios recientes. Aquí mostramos que un modelo alternativo basado en 169 estaciones de GPS reproduce los datos mejor, y sugiere que la principal conclusión de Lorito et al. (2011) no es correcta. Basados en un mapa de déficit de deslizamiento sugerimos que la laguna sísmica abierta en 1835 fue muy probablemente cerrada el 2010.
]]> Palabras clave: Terremoto del Maule, Distribución de desplazamiento cosísmico, Déficit de desplazamiento, Laguna sísmica.1. Introduction: The 2010 Maule earthquake
The Maule earthquake of moment magnitude (Mw) 8.8 struck south-central Chile on February 27, 2010 causing a devastating tsunami. Its predecessor on February 20, 1835 was famously documented by FitzRoy and Darwin (Darwin, 1851; Fitzroy, 1839). This fame has concealed additional less considered reports from the scientists' view. These sources tell that shaking distribution (El Araucano, 1835; Guzmán, 1836; Lozier et al., 1835; El Mercurio de Valparaíso, 1835; Bonafous, 1835), tsunami inundation (Caldcleugh, 1836; Sutcliffe, 1839, 1841), and coastal uplift patterns (Dumont D'Urville, 1841; Caldcleugh, 1836; Guzmán, 1836) were rather similar in both events suggesting they ruptured an analogous segment of the plate boundary (Cisternas et al., 2010).
Before 2010, this region had been long recognized as a mature seismic gap (Barrientos, 1987; Campos et al., 2002), and surface velocities estimated from Global Positioning System (GPS) measurements suggested a high degree of coupling between the Nazca and South American plates (Ruegg et al., 2009; Moreno et al., 2010; Moreno et al., 2011; Métois et al., 2012). Theoretically, the average slip available for such a megathrust earthquake - the slip deficit - equals the plate convergence rate multiplied by the time since the last earthquake (175 years in this case) and the degree of plate coupling. This assumes that all the accumulated strain was released by the last earthquake (the 1835 event in this case) and neglects post-seismic deformation. Lorito et al. (2011) inverted geodetic and tsunami data to obtain plate-boundary slip during the Maule earthquake. Their modeled slip was lower than the theoretical slip deficit, particularly in a large area located immediately north of the city of Concepción, concluding that another damaging earthquake of Mw 7.5-8 could still strike this region (Lorito et al., 2011). This was further highlighted by Lay (2011) who discussed differences among existing slip models.
2. Coseismic slip distribution and slip deficit after the 2010 earthquake
Because of the shallow dip of subduction faults, inversions for coseismic slip are mostly sensitive to horizontal displacements commonly obtained from Global Positioning System (GPS) stations in the near field (Okada, 1985). Lorito et al. 's model included only six GPS displacements, located mostly in the northern rupture sector (squares in Fig. 1a), and the adopted fault geometry was based on a single profile propagated along strike, discarding along-strike variations in dip and the complexity evident from a wealth of geophysical data (Tassara et al., 2006). Oversimplified fault geometries may introduce significant artifacts in the slip distribution (Moreno et al., 2009). Using near-field tsunami waveforms in the slip inversion requires detailed local bathymetry, data not included in Lorito et al. 's model. Their model failed to reproduce coastal uplift measurements in the southern part of the rupture (Farías et al., 2010), where residuals from radar interferometry (InSAR) were also high (Fig. 3c and 3d of Lorito et al., 2011), and it significantly misfits GPS displacements from other studies (Vigny et al., 2011; Moreno et al., 2012) (Fig. 1a). In addition, Lorito et al. 's model underpredicts their own GPS displacements by as much as 0.9 m at Constitución (Fig. 3b of Lorito et al., 2011). The main conclusions of Lorito et al. (2011): 'limited overlap with the seismic gap' and 'zones of very high coupling in the Darwin gap remain unbroken' are likely incorrect because: 1. their model underpredicts slip in the region where this coupled zone was located and 2. the 1835 rupture was a priori assumed to be much smaller than suggested by historical data (Cisternas et al., 2010).
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| FIG. 1. Modeled plate-boundary slip during the 2010 Maule earthquake and slip deficit of Darwin seismic gap. a. Slip from Lorito et al. (2011) with computed (grey arrows) and GPS (orange arrows) displacements (Vigny et al., 2011; Moreno et al., 2012). White squares show GPS sites used by Lorito et al. (2011). Inset shows histogram of residuals between measured and modeled displacements. SE-standard error. Note that Lorito et al. 's model underestimates GPS displacements between 36-37.5°S where they forecasted a Mw 7.5-8 earthquake; b. Alternative slip distribution (Moreno et al., 2012) with modeled and GPS displacements. Inset shows histogram of residuals; c. Slip deficit after the 2010 earthquake including plate coupling over 175 years at an heterogeneous rate (Moreno et al., 2010; Moreno et al., 2011), slip release by the 1960, 1928, and 1985 events (Moreno et al., 2009; Moreno et al., 2012), and the slip distribution of Moreno et al. (2012). Note that the deficit is negative or null over most of the rupture zone, suggesting the 2010 earthquake closed the gap opened in 1835. Extent of 1835 rupture inferred from a compilation of historical sources (see text). The Santa María splay fault system, which slipped during the Maule earthquake (Melnick et al., 2012b), may be associated with the positive slip deficit near Concepción; d. Slip deficit using the same constraints as in (c) but with the slip model of Lorito et al. Note the large positive region northwest of Concepción. The extent of the 1835 rupture assumed by Lorito et al. (2011) is shown. |
An alternative slip model (Fig. 1b) has been derived from 169 GPS vectors covering the entire rupture zone and the far field (Moreno et al., 2012), together with InSAR (Tong et al., 2010) and land-level change data (Melnick et al., 2012a). This new finite-element model integrates realistic fault and lithospheric geometries compiled from various geophysical data sets (Tassara and Echaurren, 2012), which include a seismic profile in the epicentral region of the Maule earthquake (Moscoso et al., 2011). Compared to Lorito et al., the new model reproduces the data with half the residuals standard error, resulting in more slip in the southern and less in the northern sectors.
]]> Furthermore, a slip deficit map including: 1960 earthquake slip that overlapped ~150 km of the 2010 rupture (Moreno et al., 2009); the 1928 and 1985 events; heterogeneous plate coupling (Moreno et al., 2010, 2011); and the new slip distribution (Moreno et al., 2012) shows null or negative values in most of the 2010 rupture with a small positive area (Fig. 1c). The slip deficit map obtained from this new slip model results in a much smaller area of positive deficit in comparison with the slip deficit map obtained using the same constraints but the slip distribution of Lorito et al. (Fig. 1d). The small area of positive deficit might be associated with a splay fault that slipped during the 2010 earthquake (Melnick et al., 2012b), not yet included in the model; in addition, deep afterslip occurred in this region immediately after the earthquake (Vigny et al., 2011), and might also partly account for the positive deficit.Thus, we propose that the 2010 event most likely closed Darwin seismic gap. Our re-analysis suggests the 34-38°S region is unlikely prone to the occurrence of another large plate-boundary earthquake in the near future. Potential earthquakes will rather occur on adjacent plate-boundary segments such as the Valdivia segment to the south (Moreno et al., 2011) or the Central Chile segment to the north. Additional sources of future earthquakes within the Maule earthquake rupture could be the outer rise and faults in the upper-plate.
3. Discussion
The tenuous conclusion of Lorito et al. (2011), which was based on a basic model fed with limited data led to a premature alarm, rapidly divulgated by the media to the already scared population (Diario Cooperativa, 2011; El Mercurio, 2011; The New York Times, 2011). Although after a great earthquake the seismic hazard in neighbor regions usually increases (e.g., Parsons and Velasco, 2011), the hazard assessment should be based on a thorough analysis of a comprehensive data set. On the other hand, two studies published before the Maule earthquake based on a wealth of data had shown that the area between Concepción and Constitución was highly locked and that it could produce a M>8.5 event anytime (Ruegg et al., 2009; Moreno, 2010). Such alarms should be taken seriously, although their validity may be hard to judge for non-specialists, emphasizing the need for better communication and development of a tight link between scientists and local authorities.
Acknowledgements
This work was supported by German Science Foundation (DFG, Deutsche Forschungsgemeinschaft) grants ME3157/2-1 to Melnick and MO2310/1-1 to Moreno, Chilean Science Foundation (Conicyt) Fondecyt 1110848 to Cisternas and 1101034 to Tassara, and the GeoForschungsZentrum Potsdam. We thank Christophe Vigny and Eduardo Contreras-Reyes for constructive reviews.
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Manuscript received: March 20, 2012; revised/accepted: May 31, 2012; available online: June 19, 2012.
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