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TitleThe Mw 5.7 Pica earthquake: a crustal event in northern Chile with large ground accelerations and stress drop
AuthorHerrera, C; Cassidy, J FORCID logo; Dosso, S E; Dettmer, J; Rivera, E; Ruiz, S; Vasyura-Bathke, H
SourceSeismological Research Letters vol. 93, no. 2B, 2022 p. 1255-1256
Alt SeriesNatural Resources Canada, Contribution Series 20210550
PublisherSeismological Society of America
MeetingSeismological Society of America Technical Sessions; Bellevue; US; April 19-23, 2022
Mediadigital; on-line
File formatpdf
Lat/Long WENS -69.3833 -69.3667 -20.4667 -20.4833
Subjectsearthquakes; crustal movements
ProgramPublic Safety Geoscience Assessing Earthquake Geohazards
Released2022 04 19
AbstractImportant crustal earthquakes in Chile are less frequent compared with interplate and intraplate events. However, even moderate-magnitude onshore crustal events can pose a major hazard due to their closer proximity to population centers. We study the rupture properties of a Mw 5.7 crustal earthquake that occurred in 2008 near the town of Pica, under the Central Valley of northern Chile, which was well recorded with both strong motion and broadband instruments. This event generated maximum ground accelerations of up to 0.67 g in Pica, one of the largest recorded for this type of events. Overall, its observed ground motions are larger than expected by prediction models, particularly at short periods. The mainshock rupture exhibits a large stress drop of 250 MPa, inferred via S-wave spectrum modeling, which is likely a contributing factor to its large ground accelerations. Its hypocenter was located at 33 km depth in a cold and brittle region of the crust, with an inferred 9 km long by 8 km wide rupture via Bayesian inversion of waveforms. The fault plane is striking towards the NW and dipping towards the NE, as defined by the moment tensor solution and aftershock distribution. This geometry is similar to that of crustal earthquakes occurring under the neighboring Coastal Cordillera, including the large 2020 Mw 6.3 Loa River crustal earthquake, suggesting that the Pica earthquake is under the same stress field, and likely occurred in a buried branch of the margin-perpendicular reverse faults mapped in the Coastal Cordillera. These factors indicate the possibility of more similar events occurring in northern Chile, where our recurrence estimates show that Mw greater than or equal to 5.7 crustal events have occurred on average, every three years.
Summary(Plain Language Summary, not published)
In this presentation we examine one of the largest (and best-recorded) crustal earthquakes in northern Chile. We examine the type of rupture, the location and depth, aftershock patterns, and relationship with local geology. The ground motions show higher than expected shaking, and have implications for improving earthquake hazard models in northern Chile. Results of this study also have applications in northern Cascadia - which is also an active subduction zone with a similar subduction geometry and types of earthquakes.

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