| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
|
| Initial Amendment Date: | February 8, 2013 |
| Latest Amendment Date: | February 8, 2013 |
| Award Number: | 1246850 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Eva Zanzerkia
EAR Division Of Earth Sciences GEO Directorate for Geosciences |
| Start Date: | April 1, 2013 |
| End Date: | March 31, 2014 (Estimated) |
| Total Intended Award Amount: | $115,468.00 |
| Total Awarded Amount to Date: | $115,468.00 |
| Funds Obligated to Date: |
|
| History of Investigator: |
|
| Recipient Sponsored Research Office: |
1608 4TH ST STE 201 BERKELEY CA US 94710-1749 (510)643-3891 |
| Sponsor Congressional District: |
|
| Primary Place of Performance: |
307 McCone Hall Berkeley CA US 94720-4767 |
| Primary Place of
Performance Congressional District: |
|
| Unique Entity Identifier (UEI): |
|
| Parent UEI: |
|
| NSF Program(s): | Geophysics |
| Primary Program Source: |
|
| Program Reference Code(s): | |
| Program Element Code(s): |
|
| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
The devastating March 11, 2011 Tohoku-Oki earthquake and its tsunami caused more than 15,000 fatalities and severe damage in NE Japan. The densest geodetic and seismic networks in Japan made this earthquake the best-recorded event ever. Seafloor geodetic stations recorded over 10s of m horizontal displacements during the earthquake. High-resolution seismic bathymetry data indicate that the rupture reached the trench. Model inversion studies indicate a slip distribution of up to ~50 m over a remarkable compact area of 400 km along strike and 200 km wide. Coseismic and postseismic crustal deformation recorded at unprecedented high spatial and temporal resolution and precision allow the researchers to further their understanding of the rheological structure of the Earth and the subduction zone processes. In particular, the high quality geodetic data provide a unique opportunity to constrain the three-dimensional (3D) rheology of the upper mantle and lower crust and the evolution of transient slip on the megathrust. The scope of the project will be broad enough such that results will be applicable to other subduction margins (Sumatra, Chile, Alaska and Cascadia) where deformation is currently at various stages of the subduction earthquake cycle.
This project will examine the postseismic deformation of the 2011 Tohoku-Oki earthquake. In previous investigations, it has been a challenge to separate the contributions of afterslip on the megathrust from viscoelastic relaxation of the earthquake-induced stresses in the upper mantle. Effects of the complex 3D rheology of convergent margins on subduction zone earthquake deformation have yet to be better understood. In the proposed research, we seek to understand: (a) What is the distribution and evolution of the afterslip on the megathrust following a giant earthquake? (b) How does the 3D rheology of the Earth control the postseismic crustal deformation? (c) What are the rheological properties of the oceanic and continental upper mantle? In this work the researchers will benefit from access to the wealth of Global Positioning System (GPS) data recorded by the Japan national network as well as regional stations across East Asia. They will also constrain afterslip using repeating earthquakes. A 3D finite element code will incorporate complex subduction slab geometry and advanced mantle rheology in the Earth.
PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
In this project we have studied the postseismic deformation of the 2011 Mw9.0 Tohoku-Oki earthquake. Afterslip of the fault and transient viscoelastic relaxation of the earthquake-induced stresses in the upper mantle are believed to be two primary processes that control the very rapidly-decaying postseismic deformation after a great earthquake, such as the recent great earthquakes in NE Japan, Chile and Sumatra. Both afterslip of the fault and viscoelastic relaxation of the mantle can have similar contributions to the surface deformation. The major challenge is how to quantify and distinguish the effects of these two processes. In this project, we have successfully used a three-dimensional (3D) spherical viscoelastic finite element model to simulate the two-year postseismic deformation of the 2011 Tohoku earthquake. Main achievements during this project include:
(1) We obtained daily-solution timeseries at more than 1200 continuous GPS stations in Japan. The GPS-measured displacements include contributions from interseismic coupling, non-tectonic seasonal variation, and postseismic processes. We subtracted the effects of the non-earthquake processes from the GPS timeseries and focus on the postseismic displacements due to earthquake-related processes that are examined in this project.
(2) Based on the previously published geometry of the subducting slab, seismicity data, and location of the arc, we compiled a 3D spherical finite element mesh.
(3) We use a narrow weak zone attached to the fault to model the stress-driven, time-dependent afterslip. Some earthquakes with small magnitudes (e.g., less than M5) and similar seismic wave forms repeatedly occur at the same location of the fault and are generally shallower than 50 km. These earthquakes are commonly called “repeating earthquakes”. Slip evolution of the fault can be derived at the location of the repeating earthquakes. We used published fault slip data derived from repeating earthquakes to constrain the viscosity of the shallow weak zone. The steady-state viscosity of the shallow weak zone is determined to be ~10^17 Pa s.
(4) We construct hundreds of test models to obtain the best-fit rheological parameters of the finite element model of NE Japan. Assuming that the transient viscosity is one order of magnitude lower than that of steady-state viscosity, test models determine the steady-state viscosities of the mantle wedge, oceanic mantle, and deep weak zone to be at orders of 10^19 Pa s, 10^20 Pa s, and 10^18 Pa s, respectively. Our best-fit model well reproduces the two-year GPS observations, in particular, the subsidence of marine GPS stations and uplift along the eastern coast.
(5) We also construct test models to study the rheology heterogeneity in NE Japan. (a) A test model that includes the subduction Philippine Sea plate (PHS) predicts less than ~10 cm surface motion above the subduction plate. (b) Fluids due to dehydration of the subduction slab migrate into the upper plate and weaken the rocks beneath the arc. Two years after the earthquake, a test model including a weak sub-arc zone produces up to ~20 cm horizontal displacements offshore, up to ~15 cm uplift at the seaward edge of the arc and up to ~20 cm subsidence to the west.
(6) We carried out comprehensive tests on the poroelastic response (PE) to the 2011 earthquake. A PE model with flow in response to coseismic pressure changes down to 6 km and 16 km in the continental and oceanic crust, respectively, predicts 0-6 cm uplift on land, up to ~20 cm uplift in the peak rupture area and up to ~15 cm subsidence elsewhere offshore.
Last Modified: 06/12/2014
Modified by: Roland Burgmann
