Detecting the boundaries of geologic structures is one of the main tasks in interpreting gravity anomalies. Many methods based on the derivatives of gravity anomalies have been introduced to map the source boundaries. The drawbacks of traditional methods are that the estimated boundaries are divergent or false boundaries appear in the output map. Here, we use the exponential transform of the tilt angle of the horizontal gradient to improve the edge detection results. The robustness of the presented method is illustrated using synthetic data and real examples from the Voisey’s Bay Ni-Cu-Co deposit (Canada) and the Tuan Giao (Vietnam). The findings show that the presented technique can produce more precise and clear boundaries.

We have developed maps of the predominant period of the ground and average S-wave velocity to depths of 10, 20 and 30 m (VS10, 20 and 30) as the fundamental data for seismic hazard assessment for Hue City, Central Vietnam. We conducted microtremor single-station measurements at 461 sites and microtremor array measurements at 157 sites in and around the city (i.e., within an area of about 15 km x 15 km). The peak period of horizontal-to-vertical spectral ratios (HVSR) obtained from three-component microtremor waveforms at a single station were regarded as a S-wave predominant period of the ground. Rayleigh-wave phase velocity at wavelengths of 13, 25 and 40 m obtained from vertical-component array measurements were regarded as VS10, 20 and 30, respectively. It revealed that the predominant periods range from 0.03 to 1.98 s (i.e., 0.5 to 33 Hz) in Hue City, which show a tendency to be longer in the northern part of the city than in the southern part. The analysis results of the average S-wave velocity indicate that soft sediments are very thick in the northern part of the city, where VS10, 20 and 30 ranges from 149 to 189 m/s. These analysis results are consistent with an existing geological map and quantify the geoenvironmental conditions in Hue City, indicating the effectiveness of the microtremor measurements for a seismic assessment.

Following the reservoir impoundment, the seismic activity in the Lai Chau reservoir area and its vicinity has increased significantly. The changes in frequency of local earthquakes depends on: i) The type of rocks below the reservoir; ii) The slip tendency of faults in the reservoir area; iii) The Coulomb stress change on faults in the reservoir area; and iv) The history of the reservoir filling and spillage. The results show that the granite massif of the Phu Si Lung complex is located below the upstream area of ​​the reservoir at a depth of about 3,000-4,000 m; when water percolates down through this type of rocks, it will cause some fractures in the rock to become saturated with water, making stress in the rock reach the failure threshold. Under the impact of the modern regional tectonic stress field, the faults connected to the reservoir (F-1, F-2, F-3, F-4) tend to slip at moderate to high rates, and the Coulomb stress change is also clearly shown on these faults. These factors, together with the annual fluctuations in the reservoir water level between the rainy and dry seasons, have mutual influences, triggering the earthquakes to occur in the Lai Chau reservoir area and its vicinity in the period 6/2015 - 6/2024.