02/27/2010 (Mw 8.8), Chile
Anthony Sladen, Caltech
Location of Epicenter
Amount of Slip on Fault
View in Google Earth (requires Google Earth)
Colors show the amount of slip on different sections of the fault zone. Two views are shown (either view can be de-selected on the Google Earth sidebar):
- Circles show a 3-D map of the fault. (The fault map has been raised above the Earth's surface. Click on a circle to get the depth of that fault segment.)
- Image shows the projection of the fault onto the Earth's surface.
Chile is located at the boundary between the Nazca plate and the South American plate. These plates are converging at 8 cm/year.
The rectangle in the figure shows the location and length of the rupture zone. The red
star indicates the epicenter. The rupture was bilateral, i.e., it spread away from the epicenter in both the north and south directions, taking two minutes to cover a total of 600 km (370 miles). This magnitude 8.8 earthquake was 500 times stronger than the recent January 12 Haiti earthquake.
The colors show the slip amplitude (i.e., the amount by which the land on one side of the fault moved with respect to the land on the other side). The motion was mainly thrust-fault (land on the Nazca plate subducting under land on the South American plate - see animation).
The rupture originated about 35 km (22 miles) below the surface. In some places, the slippage along the fault was as large as 9 meters (30 feet), shown by red-shaded areas. The coast of Chile sprang west, into the ocean, in some places by as much as 6 m (19 feet).
DATA Process and Inversion
We used the GSN broadband data downloaded from the IRIS DMC. We analyzed 24 teleseismic
P waveforms selected based upon data quality and azimuthal
distribution. Waveforms are first converted to displacement by removing the instrument
response and then used to constrain the slip history based on a finite fault
inverse algorithm (Ji et al, 2002). The epicenter location is based on the USGS estimate (Lon.=-72.719
° Lat.=-35.846 ° depth=35 km). The focal mechanism is taken from the GCMT solution (strike=18 °, dip=18 °) and the 1D
velocity model is extracted from the CRUST2.0 global tomography model (Bassin et al., 2000).
The solution is made of one major slip patch stretched about 20 km west and updip from the epicenter. The slip is mostly left-lateral, with a significant component of thrust motion.
Cross-section of slip distribution
Figure 1: The colors show the slip amplitude and white arrows indicate the direction of motion of the hanging wall
relative to the footwall. Contours correspond to the propagation of the rupture front, and the red
star indicates the hypocenter location. The big black arrow gives the orientation of the fault plane.
Comparison of data and synthetic seismograms
Figure 2: The Data are shown in black and the synthetic seismograms are plotted
in red. Data are aligned on the theoretical P-wave arrival (IASPEI earth model).
The number at the end of each trace is the peak amplitude of the observation
in micro-meter. The number above the beginning of each trace is the source azimuth
and below it is the epicentral distance.
Map view of the slip distribution
Figure 3: Surface projection of the fault plane slip distribution. The red star represents the epicenter of this event.
The white dots are the aftershocks located by USGS during the day following the event.
Map view of the surface deformation
Figure 3: Surface deformation predicted from the slip model. The vertical component of displacement is given by the color scale, and the horizontal motion by the arrows.
Ji, C., D.J. Wald, and D.V. Helmberger, Source description of the 1999 Hector
Mine, California earthquake; Part I: Wavelet domain inversion theory and resolution
analysis, Bull. Seism. Soc. Am., Vol 92, No. 4. pp. 1192-1207, 2002.
Bassin, C., Laske, G. and Masters, G., The Current Limits of Resolution for
Surface Wave Tomography in North America, EOS Trans AGU, 81, F897, 2000.
GCMT project: http://www.globalcmt.org/
USGS National Earthquake Information Center: http://neic.usgs.gov
Global Seismographic Network (GSN) is a cooperative scientific facility operated jointly by the Incorporated Research Institutions for Seismology (IRIS), the United States Geological Survey (USGS), and the National Science Foundation (NSF).