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- Title
GRACE and GRACE Follow‐On Gravity Observations of Intermediate‐Depth Earthquakes Contrasted With Those of Shallow Events.
- Authors
Han, Shin‐Chan; Sauber, Jeanne; Broerse, Taco; Pollitz, Fred; Okal, Emile; Jeon, Taehwan; Seo, Ki‐Weon; Stanaway, Richard
- Abstract
Earthquakes involve mass redistribution within the solid Earth and the ocean, and as a result, perturb the Earth's gravitational field. For most of the shallow (<60 km) earthquakes with Mw > 8.0, the GRACE satellite gravity measurements suggest considerable volumetric disturbance of rocks. At a spatial scale of hundreds of km, the effect of volumetric change exceeds gravity change by vertical deformation; for example, negative gravity anomalies associated with volumetric expansion are characteristic patterns after shallow thrust events. In this study, however, we report contrasting observations of gravity change from two intermediate‐depth (100–150 km) earthquakes of 2016 & 2017 Mw 8.0 (two combined) Papua New Guinea thrust faulting events and 2019 Mw 8.0 Peru normal faulting and highlight the importance of compressibility in earthquake deformation. The combined 2016/17 thrust events resulted in a positive gravity anomaly of 5–6 microGal around the epicenter, while the 2019 normal faulting produced a negative gravity anomaly of 3–4 microGal. Our modeling found that these gravity changes are manifestation of vertical deformation with limited volumetric change, distinct from gravity changes after the shallow earthquakes. The stronger resistance of rocks to volume change at intermediate‐depth results in largely incompressible deformation and thus in a gravity change dominated by vertical deformation. In addition, malleable rocks under high pressure and temperature at depth facilitated substantial afterslip and/or fast viscoelastic relaxation causing additional vertical deformation and gravity change equivalent to the coseismic change. For the Papua New Guinea events, this means that postseismic relaxation enhanced coseismic uplift and relative sea level decrease. Plain Language Summary: For the last two decades, changes in the global gravity field have been measured by the US‐German Gravity Recovery And Climate Experiment (GRACE) and GRACE Follow‐On satellites. The data quantify surface mass redistribution associated with water and ice mass changes as well as solid Earth mass changes such as earthquake deformation. From the GRACE data at a scale of several hundreds of km, we inferred that gravity changes after shallow (<60 km) earthquakes involve significant volumetric changes within the crust as well as vertical deformation. By contrast, in this study, we report gravity changes following two thrust faulting intermediate‐depth (100–150 km) earthquakes in Eastern Papua New Guinea in 2016 and 2017, combining to Mw 8.0, and a normal faulting one in 2019 Peru (Mw 8.0), which differ significantly from the signals following shallow events. Our numerical modeling of coseismic and postseismic processes suggests that the deformation associated with these intermediate‐depth earthquakes was largely incompressible due to higher resistance of rocks to volume change. Gravity changes observed by the satellites was due to vertical deformation, with little effect from volume change. This study demonstrates the contrasting examples of gravity changes depending on depth of ruptures (compressibility of rocks) and reiterates the importance of volume change in large‐scale gravity changes. Key Points: Gravity changes after the intermediate‐depth earthquakes are unexpectedly large and different from those of shallow eventsStronger resistance of rocks to volume change at intermediate‐depth results in highly incompressible deformation and gravity changesTwo Mw 8.0 intermediate‐depth earthquakes of 2016 & 2017 Papua New Guinea and 2019 Peru highlight compressibility in gravity change
- Subjects
PAPUA New Guinea; PERU; GRAVIMETRY; GRAVITY anomalies; GRAVITY; GRAVITATIONAL fields; EARTHQUAKES; ROCK mechanics; PALEOSEISMOLOGY
- Publication
Journal of Geophysical Research. Solid Earth, 2024, Vol 129, Issue 2, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2023JB028362