Ground deformation associated with a seismic swarm near Aysen, southern Chile, measured by ALOS/PALSAR interferometry
(1) Kyoto University, Gokasho, 611-0011 Uji, Japan
In a region near Aysen, southern Chile, a seismic swarm started in January 2007. The activity intensified in the following months, generating several M5 earthquakes and a M6.2 earthquake. The M6.2 event occurred on 21 April 2007 presumably under the bed of a deep fijord. Some slope failures triggered by this earthquake generated tsunamis, killing several people. GPS measurements showed ground surface movements of about 10cm/month preceeding the M6.2 earthquake.
In order to measure the ground displacements in detail and to understand the mechanism of the seismic swarm, SAR interferometric analyses using ALOS/PALSAR images are performed. Using available data, two interferograms were computed for pairs of 15 Feb. – 2 Apr. 2007 and 2 Apr. – 3 Jul. 2007. The former maps the ground displacements of 46 days (before the M6.2 event), while the latter maps those of 92 days that include the M6.2 event. All the acquisitions have been done from ascending orbits with an off-nadir angle of 34.3 degrees, corresponding to an incidence angle of about 39 degrees at the target area.
The first interferogram detects ground displacements up to 10cm in the line-of-sight direction. The main deformed region has spatial dimensions of about 20km and is centered at the future M6.2 earthquake epicenter. Some areas show slant range increase, while others show slant range decrease. Some phase discontinuities are observed, suggesting fault reactivations caused by some external stress perturbation. Considering the presence of hot springs and volcanoes in the region, the faults may possibly have been reactivated by magmatic intrusions. The second interferogram contains a dense fringe pattern that presumably corresponds to coseismic displacements of the M6.2 earthquake, in addition to patterns similar to those observed in the first interferogram. The coseismic displacements seem to be consistent with a right-lateral fault slip with its strike directing north-south, although the signals are not fully explained by a simple dislocation source.
The two interferograms also show signals that are consistent with several centimeters of uplift in a wider area of more than 20km in diameter. While a solid conclusion requires a more thorough examination, this may be attributed to an inflation of magma reservoir that lie around 10km or deeper beneath the ground.
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,