Papanoa, Mexico earthquake of 18 April 2014 (M_w7.3)

Abstract

Papanoa earthquake broke the plate interface NW of the Guerrero seismic gap. In this region, previous large earthquakes occurred in 1943 (M_S7.4), 1979 (M_w7.4) and 1985 (M_w7.5). The earthquake was recorded in the near-source region by several accelerographs. Severe damage was reported in Papanoa (where PGA of ~ 0.9 g was recorded on one of the horizontal components at a soft site) and other nearby coastal towns. It was also felt strongly in Mexico City where the ground motions were comparable to those recorded during the 1979 and 1985 events. A careful analysis of the near-source data, including P-wave polarization, yields an epicenter at 17.375 ºN, 101.055 ºW, close to the coast, near the town of Papanoa. Effective duration of ground motion at near-source coastal stations to the NW of the epicenter is 10-15 s, while it is 20-35 s to the SE, demonstrating rupture directivity towards Zihuatanejo. Three (in some cases only two) bursts of high-frequency radiation are visible in the accelerograms. Near-field records show that the slip was small during the initial 2-3 s of rupture which, subsequently, cascaded in two or three larger subevents. Slip inversion using teleseismic waves, along with GPS data from a couple of near-source sites, reveals that the rupture mainly consisted of two subevents. The first one was centered close to the hypocenter and had a radius of ~ 15 km. The second subevent, roughly of the same dimension as the first, was centered ~ 25 km SSE of Zihuatanejo. Previous analysis of three slow slip events (SSEs) in the region (2001-2002; 2006; 2009-2010) had revealed that this region had a high inter-SSE coupling ratio (> 0.5) with a slip deficit about four times greater than in the adjacent NW Guerrero seismic gap (Radiguet et al., 2012). It seems that the large slip patch corresponding to the first subevent of the 2014 earthquake experienced a cumulative slip of ~ 20 cm during the SSEs, suggesting that seismic and SSE slip may share the same area of the interface. Alternatively, the SSE slip may have occurred over an area surrounding the large slip patch, a physically more plausible model. Epicenters of aftershocks (M ≥ 3.5), which occurred in the next 36 hours, define a rectangular area of ~ 40 km × 70 km, oriented ~ N75ºE; about half of this area lies onshore. This area encloses the inverted slip region. More than half of the aftershock area overlaps with that of the 1979 earthquake and a small fraction with that of the 21 September 1985 earthquake. As we only know the slip distribution of the 2014 earthquake, it is not known if the two large-slip patches had also slipped similarly during the previous earthquakes. The earthquake was followed by two moderately large events that occurred on 8 May (M_w6.5) and 10 May (M_w6.1). The epicenters of these events fall near Tecpan, within the NW Guerrero seismic gap (which extends from 100 ºW to 101 ºW), outside the aftershock area of the Papanoa earthquake. No large earthquake has occurred in this part of the gap, between Papanoa and Acapulco, since the events of 1899 (M_S7.5) and 1911 (M_S7.6). However, seismicity in the region (at M_w ≥ 5 level) appears normal. A few moderate earthquakes of unusually large duration and deficient high-frequency radiation have been identified near the trench. In contrast to the Papanoa – Zihuatanejo region, in this segment the inter-SSE coupling ratio from ~ 10 km inland towards sea is very low (< 0.2) and the slip deficit is about one-fourth that of the Papanoa- Zihuatanejo region (Radiguet et al., 2012). As a consequence, the expected recurrence period of large/great earthquakes may be relatively long, in agreement with the seismicity of the Papanoa-Acapulco region.

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