Assessing the Seismic Hazard in Northwestern Peru

Abstract eng:
Northwestern Peru is a highly seismically active region situated above the downgoing Nazca plate within the South America subduction zone. Historically, a total of 28 earthquakes of moment magnitude (M) 7.0 and greater have occurred in the region, including the 1619 M 7.7 to 8 Trujillo earthquake. Several active crustal faults have been identified in the region but likely many more have yet to be discovered. Probabilistic seismic hazard analysis (PSHA) was performed to assess the hazard in the region as exemplified by the city of Cajamarca. The basic inputs into the PSHA include the seismic source model, ground motion prediction models, and a description of the geologic and geotechnical conditions beneath the site. The seismic source model includes 21 Quaternaryage (active) crustal faults, the South America subduction zone, including both the megathrust and intraslab (WadatiBenioff) zone, and an areal crustal source zone. As part of reconnaissance field investigations, several previously recognized crustal faults were characterized as being Quaternary active. The unrecognized La Quinua fault was identified as an active fault capable of generating a maximum earthquake of M 6.0 to 6.6. Its slip rate is estimated, however, to be relatively low at less than 0.1 mm/yr. Other local faults characterized include for example, the Carbon, Tapado, Conga, Perol, and Antena faults. A review of the surfacefaulting earthquakes in Peru indicates that fault displacements are significantly larger than expected given the mapped fault lengths and empirical relationships. Other significant regional faults include the Chaquilbamba and Bambamarca faults. The South America subduction zone megathrust is poorly understood in northwestern Peru. Whether it can generate large (M > 8.0) megathrust earthquakes is highly uncertain because few large historical earthquakes have been definitely identified as being associated with the megathrust. In this study, we estimate a maximum magnitude for megathrust earthquakes of M 8.0 ± 0.5. Its recurrence interval is estimated to be anywhere from 400 to 10,000 years, very uncertain given the lack of data. State-of-the-art ground motion prediction models were used in the PSHA for the subduction zone and crustal earthquakes. The latter included the recently developed Next Generation of Attenuation (NGA)-West2 models for actively tectonic regions such as Peru. For the subduction zone, we selected three state-of-the-art models. A generic soil Vs30 of 270 m/sec was used in the PSHA. The hazard in Cajamarca is relatively moderate with PGA values at the building code return periods of 475 and 2475 years of 0.31 and 0.55 g, respectively. The hazard is lower in this part of Peru compared to other regions in the country due to the uncertainty on whether the megathrust to generate very large earthquakes (M > 8) because of the flat subduction of the South America plate beneath northwestern Peru.

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