The geometry and the rheological heterogeneity of a tectonic fault interface control both spatial features of seismicity and localization of large shallow earthquakes. The structural features of a fault interface are clearly highlighted in the characteristics of background seismicity. Here, we present the algorithm of Seismogenic Patches Detection (SPAD) to analyze background seismicity for revealing tectonic asperities. Tectonic asperities, predetermined by fault waviness, are associated with topologically dense clusters of background events, called seismogenic patches. In this study aimed at mapping tectonic asperities we have analyzed the seismicity of Kuril-Kamchatka subduction zone, which is one of the most active segments of the circum-Pacific subduction zone. The Kamchatka regional seismic catalogue includes events for the period from 1990 to 2024, and the magnitude of completeness Mc equals to 2.7. Basing on the nearest-neighbor method we divide seismicity into background and clustered modes. After that we apply fuzzy clustering for detecting spatial dense clusters of background mode, which are interpreted as seismogenic patches. Eighty one percent (18 of 22) of strong earthquakes M6-7 localize inside patches. Epicenters of major earthquakes M>7 (19 of 20) localize in the vicinity of patch contours. The characteristic size of seismogenic patches in the Kamchatka zone varies from 85 to 250 km, which corresponds to an earthquake source with a magnitude of 7.0-7.8. A rupture of a great earthquake M>8 covers several patches, the distance between them being less than half the length of a single patch.

The spectrum of slip modes on gouge-filled faults spans a continuum from fast ruptures to slow slip events. The nucleation of a certain slip mode is governed by the frictional heterogeneity of fault interface and the rheological fault stiffness. Though the pattern of mechanical parameter variation and dynamic stability loss during a seismic cycle is quite clear, it is important to have a unified seismic-acoustic signature of slow or fast slip event nucleation. We present laboratory acoustic emission (AE) experiments on a slider-model with a precise control of mechanical and AE parameters. A comprehensive analysis of AE activity points to the presence of two AE subpopulations. One of them manifests as pulses with harsh onsets. The second one exhibits a gradual amplitude rise and tremor-like signal. The second AE subpopulation shows a longer failure duration and increased energy dissipation. Regularities of changing the frequency-amplitude characteristics of AE subpopulations during a laboratory seismic cycle differ. The first AE subpopulation retains parameters of frequency-amplitude distribution, but the second one exhibits a pronounced cyclic recurrence of the b-value. The latter decreases before slip events and recovers after them. The detected features of AE subpopulations are common for the entire spectrum of slip modes. Findings reveal a coexistence of slow and fast modes at the same fault at the micro-scale and point to the unity of underlying physical mechanisms of different slip mode nucleation.

The spectrum of slip modes on gouge-filled faults spans a continuum from fast ruptures to slow slip events. The nucleation of a certain slip mode is governed by the frictional heterogeneity of fault interface and the rheological fault stiffness. There is a mounting evidence that a single fault can host multiple slip modes. We present laboratory acoustic emission (AE) experiments on a slider-model. The entire spectrum of fault slip modes with a precise control of mechanical and AE parameters was reproduced and the unity of underlying mechanisms of slip mode nucleation was detected. A comprehensive analysis of AE activity allows revealing coexistence of two distinct subpopulations of acoustic pulses (APs) emitted during a seismic cycle and accompanying nucleation of different fault slip modes. One of them manifests as APs with harsh onsets. The second one exhibits a gradual amplitude rise and a tremor-like waveform. The second AP subpopulation shows longer failure duration and increased energy dissipation. During a seismic cycle, the first AP subpopulation retains parameters of frequency-amplitude distribution, while the second one exhibits a pronounced cyclic recurrence of the b-value. The b-value of the second subpopulation decreases before slip events and recovers after them. The detected features of AE evolution are common for the entire spectrum of fault slip modes.