Quaternary seismo‐tectonic activity of the Polochic Fault, Guatemala

Abstract

[1] The Polochic-Motagua fault system is part of the sinistral transform boundary between the North American and Caribbean plates in Guatemala and the associated seismic activity poses a threat to $70% of the country's population. The aim of this study is to constrain the Late Quaternary activity of the Polochic fault by determining the active structure geometry and quantifying recent displacement rates as well as paleo-seismic events. Slip rates have been estimated from offsets of Quaternary volcanic markers and alluvial fan using in situ cosmogenic 36 Cl exposure dating. Holocene left-lateral slip rate and Mid-Pleistocene vertical slip rate have been estimated to 4.8 AE 2.3 mm/y and 0.3 AE 0.06 mm/y, respectively, on the central part of the Polochic fault. The horizontal slip rate is within the range of longer-term geological slip rates and short-term GPS-based estimates. In addition, the non-negligible vertical motion participates in the uplift of the block north of the fault and seems to be a manifestation of the regional, far-field stress regime. We excavated the first trench for paleo-seismological study on the Polochic fault in which we distinguish four large paleo-seismic events since 17 ky during which the Polochic fault ruptured the ground surface.

Geomorphic analysis of transient landscapes in the Sierra Madre de Chiapas and Maya Mountains (northern Central America): implications for the North American–Caribbean–Cocos plate boundary

“…These faults are seismically active ( Fig. 3a; White, 1984;Singh et al, 1984;Ambraseys and Adams, 1996;Guzmán-Speziale, 2010) and Quaternary displacements are documented (e.g., Burkart, 1978Burkart, , 1983Authemayou et al, 2012). described a relict middle Miocene planation surface which formed at low elevation and covered most of the Polochic-Motagua sliver (the so-called "Mayan paleosurface").…”

Guatemala paleoseismicity: from Late Classic Maya collapse to recent fault creep

“…Destruction of the topsoil prevented determining whether the trace disrupts the ground surface. However, observations conducted at the site before the opening of the trenches failed to detect any topographic discontinuity24, suggesting that the fault zone used to be sealed by the topsoil. 14 C dating indicates that sedimentation has been fairly uniform over the past 5–7 centuries throughout the floodplain, at a rate of 2–4 mm/y (Additional information, Table S1-2).…”

“…Indeed, the earthquake related to turbidite B occurred before 1557 CE, whereas displacements at Agua Blanca continued after 1680 ± 15 CE. Using the current <5 mm/y interseismic fault velocity17, the time-averaged fault velocity of 2.9 ± 0.4 over the past 7 ky32, and the time-average fault velocity of 1.3 mm/y over the past 84 ky24, we find that the locked zone of the fault has accumulated between 0.6 m and 2.2 m of potential slip since deposition of seismite B . If such potential slip were released during a single rupture event, it would generate a major earthquake.…”

“…The Polochic fault currently produces micro-seismicity to a depth of 10 km and M4.5–5.5 earthquakes between 5 and 10 km23, suggesting the existence of a locked zone at depth. There is also geological evidence for past episodic, possibly seismogenic deformation at the surface24. We therefore consider unlikely that fault creep also occurs at depth.…”

“…Over the 20 th century however, the instrumental record of M < 5.5 earthquakes does not display a severe deficit212223. Direct geological observations of displacements across the plate boundary are thus far restricted to observations in 10–17 ky-old alluvium along the Polochic fault24. Field evidence for more recent motion is lacking on both faults.…”