Forming a Mogi Doughnut in the Years Prior to and Immediately Before the 2014 M8.1 Iquique, Northern Chile, Earthquake. Issue 16 (21st August 2020)

• Record Type: Journal Article
• Authors: Schurr, B.; Moreno, M.; Tréhu, A. M.; Bedford, J.; Kummerow, J.; Li, S.; Oncken, O.
• Is Part Of: Geophysical research letters. Volume 47:Issue 16(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-08-21
• Subjects: subduction zone -- earthquake -- Chile -- Mogi Doughnut; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2020GL088351
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Asperities are patches where the fault surfaces stick until they break in earthquakes. Locating asperities and understanding their causes in subduction zones is challenging because they are generally located offshore. We use seismicity, interseismic and coseismic slip, and the residual gravity field to map the asperity responsible for the 2014 M 8.1 Iquique, Chile, earthquake. For several years prior to the mainshock, seismicity occurred exclusively downdip of the asperity. Two weeks before the mainshock, a series of foreshocks first broke the upper plate then the updip rim of the asperity. This seismicity formed a ring around the slip patch (asperity) that later ruptured in the mainshock. The asperity correlated both with high interseismic locking and a circular gravity low, suggesting that it is controlled by geologic structure. Most features of the spatiotemporal seismicity pattern can be explained by a mechanical model in which a single asperity is stressed by relative plate motion. Plain Language Summary: At the northern Chile subduction zone, where the oceanic Nazca plate glides several centimeters per year under the South American continent, the two plates got stuck at an irregularity. This irregularity got stressed for more than 100 years until it broke in 2014 in a magnitude 8.1 earthquake. We mapped small earthquakes in the years preceding the large earthquake. We found that small earthquakes formed a ring around the irregularity. First, a half circle formed along the lower side of the irregularity, probably, because the oceanic Nazca plate is pulled down by its own weight and is thus mainly stressing along the lower side. Then, 2 weeks before the largest earthquake, a series of smaller earthquakes closed the circle around the upper side of the irregularity. Finding such irregularities between tectonic plates is important to assess earthquake risk. We also found that the irregularity can be seen in data derived from Earth's gravity field, hinting that it is due to geologic structure. Key Points: Seismicity in the years prior to and immediately before the mainshock form two halves of a "Mogi Doughnut" surrounding the main slip patch Mechanical asperity model predicts stress increase where earthquakes are observed Slip region correlates with high interseismic locking and a circular gravity low, suggesting that the asperity is controlled by geologic structure