Preliminary slip model of the Feb 27, 2010 Mw 8.9 Maule, Chile Earthquake

Guangfu Shao, Xiangyu Li, Qiming Liu, Xu Zhao, Tomoko Yano, Chen Ji, UCSB

DATA Process

This is a 1-day UCSB preliminary solution of the 2010 Maule earthquake. We construct the fault plane using the USGS PDE epicenter (Lat.=-35.85^o; Lon.=-72.72^o) and the lower angle nodal plane inferred from the Global CMT project (http://www.globalcmt.org). The up-dip extension of the fault plane is constrained by the information of plate boundary [Bird, 2003]. After downloading the broadband waveforms from IRIS DMC, we have manually selected 35 body waves and 35 surface waves for this study. We have checked the alignments of body waves, particularly the SH waves, by performing multiple test inversions. This is, therefore, an update of the NEIC realtime solution obtained by Dr. Gavin Hayes (http://earthquake.usgs.gov/earthquakes/eqinthenews/2010/us2010tfan/finite_fault.php)

Results

1. Multiple double couple analysis

We first analyze this earthquake using long period surface waves. The complex rupture process was modeled as multiple point sources. A simulated annealing algorithm is used to invert for the strike, dip, rake, moment, centroid time, half-duration, and centroid location of each point source. We have gradually increased the number of point sources until there is no significant improvement in term of the waveform fits.

Our result reveals that this earthquake included two major asperities, separated by about 220 km. The first asperity located at the south of the epicenter with a deeper centroid depth of 30 km. It ruptured earlier with a centroid time of 58 s. The second asperity located at the north of the epicenter with a shallower centroid depth of 16 km. It ruptured later with a centroid time of 88 s. Two asperities have roughly same seismic moment and fault dip. The total seismic moment of entire rupture was 2.1x10²⁹ dyne.cm. However, their fault strikes are different by 15^o.

2. Finite fault analysis

We then conduct the finite fault inversion in the wavelet domain using both long period surface waves and teleseismic body waves. The fault plane has a strike of 17.5^o and dips 18.0^o to the east. We use a 1D crustal model interpolated from CRUST2.0 (Bassin et al., 2000). The preferred slip fault has a total seismic moment of 2.6x10²⁹ dyne.cm.

2.1 Cross-section of slip distribution

Figure 1.1 Cross-section of the inverted slip distribution. The black arrow denotes the fault strike direction and the red star shows the hypocenter location. For each subfault, the color denotes its average slip amplitude and a white arrow shows the average motion direction of the hanging wall relative to the footwall. Contours show the rupture initiation time in seconds.

1.2 Moment rate function

Figure 1.2 Inverted moment rate function.

2 Comparison of data and synthetic seismograms

Figure 2.1. Comparison of synthetic seismograms and teleseismic body waves. The data is shown in black and the synthetic seismograms are plotted in red. Both data and synthetic seismograms are aligned with the P or SH arrivals. The number at the end of each trace is the peak amplitude of the observation in micro-meter, which is used to normalize both data and synthetics. The number above the beginning of each trace is the source azimuth and below is the epicentral distance.

Figure 2.2. Comparison of synthetics and observed long period surface waves. The data is shown in black and the synthetic seismograms are plotted in red. Both data and synthetic seismograms are aligned on original time. The number at the end of each trace is the peak amplitude of the observation in millimeter. The number above the beginning of each trace is the source azimuth and below is the epicentral distance.

Figure 3. Surface projection of the slip distribution superimposed on ETOPO2. The black line indicates the major plate boundary [Bird, 2003]. The white dots are background seismicity from 1964 to 2004 (Relocated ISC catalog, Engdahl et al, 1998). The red dots are 1-day aftershocks (NEIC USGS). The focal mechanisms denote the locations of two major asperities. The red arrow indicates the plate convergence rate across the trench [Ruegg et al., 2009].

CJ's Comments:

Based on the inverted total seismic moment, the 2010 Mw 8.9 Maule, Chile earthquake is the fifth largest earthquake since 1900. However, the magnitude of the 1906 Ecuador earthquake was estimated using the size of its aftershock zone [Kanamori, 1977]. The uncertainty is high.

This earthquake ruptured the Concepcion-Consititucion gap in the Central Chile, which had been considered as a likely spot for a major subduction earthquake [e.g., Beck et al., 1998; Lopaz et al., 2002; Ruegg et al, 2009]. The southern and northern asperities mentioned above might rupture during the earthquakes in 1835, 1928, respectively. A recent geodetic study based on GPS data [Ruegg et al, 2009] showed that the subduction interface was fully locked and the surface projection of this locked zone extends about 50 km east of the coastal line. Therefore, both the >10 m peak slip and the downdip extension of the co-seismic slip are consistent with the result of recent interseismic strain accumulation [Ruegg et al, 2009].

Slip Distribution

SUBFAULT FORMAT

COULOMB INPUT FORMAT

CMTSOLUTION FORMAT

References

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.

Beck, S., Barrientos, S., Kausel, E., Reyes, M., 1998. Source characteristics of historic earthquakes along the central Chile

subduction zone. J. South Am. Earth Sci. 11, 115–129.

Bird, P. An updated digital model of plate boundaries, Geochemistry Geophysics Geosystems 4. 2003

Engdahl, E. R., R. van der Hilst, et al., Global teleseismic earthquake relocation with improved travel times and procedures for depth determination, Bull. Seism. Soc. of Am. 88(3): 722-743, 1998.

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.

Lopez et al, The 1835 seismic gap in South Central Chile, Phys. Earth Planet. Int. , 132, 177-195, 2002.

Ruegg et al., Interseismic strain accumulation measured by GPS in the seismic gap between Constitucion and Concepcion in Chile, Physics of the Earth and Planetary Interiors, Vol 175, 78-85. 2009.

Acknowledgement and Contact Information

This work is supported by National Earthquake Information Center (NEIC) of United States Geological Survey. This web page is built and maintained by Dr. C. Ji at UCSB.