We present new geologic, tectonic geomorphic, and geochronologic data on the slip rate, timing, and size of past surface ruptures for the right-lateral Limón and Pedro Miguel faults in central Panamá. These faults are part of a system of conjugate faults that accommodate the internal deformation of Panamá resulting from the ongoing collision of Central and South America. There have been at least three surface ruptures on the Limón fault in the past 950-1400 years, with the most recent during the past 365 years. Displacement in this young event is at least 1.2 m (based on trenching) and may be 1.6-2 m (based on small channel offsets). A well-preserved 4.2 m offset suggests that the penultimate event also sustained significant displacement. The Holocene slip rate has averaged about 6 mm=yr, based on a 30-m offset terrace riser incised into a 5-ka abandoned channel. The Pedro Miguel fault has sustained three surface ruptures in the past 1600 years, the most recent being the 2 May 1621 earthquake that partially destroyed Panamá Viejo. At least 2.1 m of slip occurred in this event near the Canal, with geomorphic offsets suggesting 2.5-3 m. The historic Camino de Cruces is offset 2.8 m, indicating multimeter displacement over at least 20 km of fault length. Channel offsets of 100-400 m, together with a climate-induced incision model, suggest a Late Quaternary slip rate of about 5 mm=yr, which is consistent with the paleoseismic results. Comparison of the timing of surface ruptures between the Limón and Pedro Miguel faults suggests that large earthquakes may rupture both faults with 2-3 m of displacement for over 40 km, such as are likely in earthquakes in the M 7 range. Altogether, our observations indicate that the Limón and Pedro Miguel faults represent a significant seismic hazard to central Panamá and, specifically, to the Canal and Panamá City.

Probabilistic seismic hazard analysis (PSHA) is the state-of-the-art method to estimate ground motions exceeded by large, infrequent, and potentially damaging earthquakes; however, a fundamental problem is the lack of an accepted method for both quantitatively validating and refining the hazard estimates using empirical geological data. In this study, to reduce uncertainties in such hazard estimates, we present a new method that uses empirical data from precariously balanced rocks (PBRs) in coastal Central California. We calculate the probability of toppling of each PBR at defined ground-motion levels and determine the age at which the PBRs obtained their current fragile geometries using a novel implementation of cosmogenic 10 Be exposure dating. By eliminating the PSHA estimates inconsistent with at least a 5% probability of PBR survival, the mean ground-motion estimate corresponding to the hazard level of 10 −4 yr −1 (10,000 yr mean return period) is significantly reduced by 27%, and the range of estimated 5th-95th fractile ground motions is reduced by 49%. Such significant reductions in uncertainties make it possible to more reliably assess the safety and security of critical infrastructure in earthquake-prone regions worldwide. Plain Language Summary Rare earthquakes can be extremely destructive and costly, but existing hazard estimates for rare earthquake shaking are highly uncertain because observations are limited to historical records. This study utilizes unique prehistoric shaking constraints provided by precariously balanced rocks. At a site in Central California, we characterize the probability of toppling of such precariously balanced rocks and determine their age of formation. The precariously balanced rock constraints are used to directly eliminate estimates in the hazard model that are inconsistent with the preservation and antiquity of the rocks. These results dramatically improve the hazard model and significantly reduce uncertainties in the estimates. Our study demonstrates how constraints on seismic shaking for the geological past can be used to improve earthquake hazard estimates for the future.

[1] This paper reports a reconnaissance archeomagnetic study of mural paintings in various pre-Columbian sites in Mexico. The magnetic measurements of the pigments show that at least four murals (sites: Cacaxtla, Cholula and Templo Mayor) retain a remanent magnetization carried by a mixture of magnetite and minor hematite grains. In most specimens, a characteristic remanent magnetization is successfully isolated by alternating field demagnetization. The mean directions are reasonably well determined for each mural and within the range of secular variation during the last centuries. Studied Mesoamerican murals apparently retain the direction of the magnetic field at the time they were painted and therefore are an invaluable source of information concerning its secular variation. The archeomagnetic study of pre-Columbian mural paintings opens new alternatives to drawing a reliable reference master curve for the region and may largely contribute to the Mesoamerican absolute chronology.

<p>Probabilistic seismic hazard analysis (PSHA) models typically provide estimates of ground motions for return periods that exceed historical observations. It is therefore important to develop quantitative methods to evaluate and refine ground motion estimates for long return periods, especially in proximity to major earthquake sources where estimates can be very high. Here we provide empirical constraints over 10,000s years on ground motions from onshore and offshore seismic sources in central California using the distribution, age and fragility (probability of toppling given an intensity of ground shaking) of fragile geologic features.<br /><br />The fragility is estimated for seven precariously balanced rocks (PBRs) formed on uplifted marine terrace palaeo-sea stacks. The site is <10 km from the Hosgri fault, a major offshore fault considered part of the San Andreas fault system. PBR 3D models were constructed using photogrammetry and used to define normalized geometric measures that could be combined with empirical models to estimate the probability of toppling (i.e., fragility), over a range of vector ground motions (PGA and PGV/PGA). Using vector hazard and the fragility, the likelihood of survival was then computed. The PGA associated with a 50 percent chance of survival varies from ~0.4-1.3 g for the selected PBRs.</p> <p>We obtain fragility ages (time that each PBR achieved its current geometry) using Be-10 cosmogenic surface exposure dating. Extremely low Be-10 concentrations (~5000 at/g) in modern high-stand samples demonstrates minimal inheritance and reliability of chert age estimates. Additionally, the volume of colluvium surrounding the palaeo-sea stack outcrops, determined from LiDAR, combined with alluvial fan surface dating (using Be-10 and soil profile development indices) indicates low erosion rates (~2.5 mm/ky) and long-term stability. Exposure ages that bound the fragility age by approximating the removal of surrounding blocks range ~17-95 ky. The similar age distributions of block removal events around all of features suggests that all PBRs share a common evolution, and we interpret ~21 ka as the most defensible fragility age estimate of all seven PBRs, with negligible change to their fragility between that time and now. Despite the lack of constraints on the recurrence behaviour of the Hosgri Fault, the slip rate is such that the PBRs have almost certainly experienced multiple large-magnitude, near-field earthquakes, and therefore provide rare constraints on low frequency ground motions.<br /><br />Each estimate output from the PSHA model is evaluated against the ground-motion corresponding to the 95% probability of survival of the most fragile PBR over the 21 ka fragility age. The logic tree branches that produce estimates inconsistent with the survival of the PBR are removed from the PSHA model. From the consistent logic tree branches a new PSHA model is produced that has reduced mean ground-motion levels and reduced uncertainty between the estimates. At the 10<sup>-4</sup> hazard level, the mean ground motion estimate is reduced by ~30% and the range of estimated 5<sup>th</sup>-95<sup>th</sup> percentile ground motions is reduced by ~50%.</p>

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