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.

In this paper, seismic activity characteristics in the East Sea area was analyzed by authors on the base of the unified earthquake catalog (1900-2017), including 131505 events with magnitude 3 ≤ Mw ≤ 8.4. The seismic intensity in the East Sea during the period 1900-2017 is characterized by the earthquake representative level Mw = 4.7. The strong earthquake activity in the East Sea area clearly shows the regularity in each stage. In the period from 1900 to 2017, the East Sea area has four periods of strong earthquake activity, each stage is nearly 30 years with particular characteristics. The distribution of the maximum earthquake quantities by years has a cyclicity in all four periods. In each stage there are 1-2 strong earthquakes with M_max ≥ 8.0. The strong earthquakes with M_max ≥ 7.5 have occurred by a repeatable rule of 3-5 years in all four stages. This allows the prediction of the maximum earthquake repeat cycle of M_max ≥ 7.5 in the study area is 3-5 years. 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The Muong La–Bac Yen–Cho Bo fault is one of the seismic faults in the northwest region of Vietnam. Neo-tectonic activities and exogenous processes have influenced the drainage system and topographic–geomorphologic features on both walls of the fault. The results of topographic analysis and geomorphological indices have confirmed the active tectonics of the fault during the Neo-tectonic period (Pliocene–Quaternary, about 5 million years). The valley floor width-to-height ratio (V_f) of less than 0.5 indicates the “rejuvenation” of the streams and the obvious influence of tectonic activities on the two walls of the studied fault. The Hypsometric curve (HC) in the study area has a straight–convex shape and the basins on the northeast wall have hypsometric integral index values ranging from 0.46 to 0.481, which are lower and more convex than those of the southwest wall. The Mountain-front sinuosity index (S_mf) from 1.92 to 3.28 along the foot slope of the Hoang Lien Son range (the segment from Than Uyen to Bac Yen and Phu Yen) and the highly variable stream-length gradient index (SL) value on the northeast wall signify the relative tectonic uplift on the northeast wall of the fault. The deformed geomorphological indications (steep cliffs, slip surfaces, etc.) in the field confirm the active tectonics of the Muong La–Bac Yen–Cho Bo fault during the Neo-tectonic period.