Locating Tremor Using Least-Squares Interferometric Source Location Imaging Method.

In volcanic monitoring, accurate source locations of volcanic tremors are helpful to understand the volcanic activity and further predict an eruption. The waveform-based interferometric source location imaging method is a conventional and effective tool for locating tremors, which usually have complex signals that lack clear phases and onsets. However, the imaged source locations are degraded by the blurring effect caused mainly by the limited source-receiver geometry, showing as low resolution and high uncertainty. We introduce the Hessian matrix, which governs the blurring effect, into the interferometric source location imaging method. The proposed Hessian-based least-squares interferometric source location imaging method consists of three steps: 1) construct Hessian matrix of source location; 2) obtain an initial source location image by summating backprojections of the correlograms; and 3) update the source location image using a Hessian-based least-squares inversion. We assess the robustness of the proposed method using synthetic data with the consideration of different source locations and velocity errors. Compared with the conventional method, the proposed method can reduce the blurring effect of Hessian matrix and provide high-resolution and low-uncertainty source locations, no matter whether the source is inside or outside the seismic network. We apply the proposed method to a real tremor that occurred at Katla volcano. The method recovers the tremor location within 1 km from its most likely source zone.