IMAGING METER-SCALE SURFACE CRACKS FROM SPACE: GOOGLE EARTH'S INSIGHT INTO FOREARC TECTONICS

Meter-scale open surface cracks in the Andean forearc provide insight into the structural reponse of the region to strong subduction zone earthquakes. The hyperarid climate of northern Chile and southern Peru preserves such minor structures, enabling their use as a long-term record of plate boundary seismicity. In our previous study of a subset of these cracks, we used high-resolution imagery from the IKONOS satellite to map more than 38,000 cracks over 500 km² in the Salar Grande region (21ºS) of northern Chile. The broad distribution, spatial correlation with regional faults, and consistency in strike indicate a genetic relationship between the cracks and subduction processes, most notably extension in the direction of plate convergence during strong interplate earthquakes. Given the extent of seismicity beneath the northern Chile-southern Peru forearc, we expected cracks to be concentrated in other localities throughout the region. However, due to the prohibitive costs of satellite imagery with resolution sufficient to map these features, comprehensive regional mapping efforts were not possible.

Now, the free access to 2.4 m-resolution images from the QuickBird satellite provided by Google Earth allows a spatially-complete sampling of these surface cracks. In this study, we use Google Earth to examine coastal regions of the Andean forearc from 16º to 25ºS and find numerous localized concentrations of surface cracks. The strikes of cracks vary between localities and correlate with the orientations of principal coseismic extension axes due to underthrusting earthquakes that rupture distinct segments of the margin. The magnitude of extension represented by the cracks exceeds that caused by a single earthquake, indicating that these features record permanent strain associated with multiple events afflicting each segment of the plate boundary; morphological characteristics offer further evidence for repeated episodes of opening. Despite their small scale, open cracks in the central Andean forearc provide important information about the surficial effects of plate boundary segmentation and seismicity.