A high-resolution seismic catalog for the 2021 MS6.4/MW6.1 Yangbi earthquake sequence,Yunnan, China: Application of AI picker and matched filter

We present a high-resolution seismic catalog for the 2021 M_S6.4/M_W6.1 Yangbi sequence. The catalog has a time range of 2021-05-01 to 2021-05-28, and contains ~8,000 well located events. It captures the features of the whole foreshock sequence and the early aftershocks. We designed a detection strategy incorporating both an artificial intelligent (AI) picker and a matched filter algorithm. Here, we adopt a hybrid AI method incorporating convolutional and recurrent neural network (CNN & RNN) for event detection and phase picking respectively (i.e. CERP), a light-weight AI picker that can be trained with small volume of data. CERP is first trained with detections from a STA/LTA and Kurtosis-based method called PAL, and then construct a rather complete template set of ~4,000 events. Finally, the matched filter algorithm MESS augments the initial detections and measures differential travel times with cross-correlation, which finally results in precise relocation. This process gives 9,026 detections, among which 7,943 events can be well relocated. The catalog shows as expected power-law distribution of frequency magnitude and reveals detailed pattern of seismicity evolution. The main features are: (1) the foreshock sequence images simple fault geometry with consistent strike, but also show a variable event depth along strike; (2) the mainshock ruptures the same fault of the foreshock sequence and activate conjugate faults further to the southeast; (3) complex seismicity are developed in the post-seismic period, indicating complex triggering mechanisms. Thus, our catalog provides a reliable basis for further investigations, such as b-value studies, rupture process, and triggering relations.     

Geology of Pulikonda and Dancherla alkaline complexes,Andhra Pradesh

Geological studies on saturated to oversaturated and subsolvus aegirine-riebeckite syenite bodies of the Pulikonda alkaline complex and Dancherla alkaline complex were carried out. The REE distribution of the Dancherla syenite shows a high fractionation between LREE and HREE. The absence of Eu anomaly suggests source from garnet peridotite. The Pulikonda syenite shows moderate fractionation between LREE and HREE as reflected by enrichment of HREE and moderate enrichment of LREE. The negative Eu anomaly indicates role of plagioclase fractionation.Three distinct co-eval primary magmas i.e. mafic syenite-, felsic syenite- and alkali basalt magmas — all derived from low-degrees of partial melting of mantle differentiates and enriched metasomatised lower crust played a major role in the genesis and emplacement of the syenites into overlying crust along deep seated regional scale trans-lithospheric strike-slip faults and shear zones following immediately after late-Archaean calc-alkaline arc magmatism at different time-space episodes i.e. initially at craton margin and later on into the thickened interior of the Eastern Dharwar craton. The ductile sheared and folded Pulikonda alkaline complex was evolved dominantly from the magmas derived from partial melting of lower crust and minor juvenile magmas from mantle. Differentiation and fractionation by liquid immiscibility of mafic magma and commingling-mixing of intermediate and felsic magmas followed by fractionational crystallisation under extensional tectonics during waning stages of calc-alkaline arc magmatism nearer to the craton margin were attributed as the main processes for the genesis of Pulikonda syenite complex. Commingling and limited mixing of independent mantle derived mafic and felsic syenitic magmas and accompanying fractionation resulting into soda rich and potash rich syenite variants was tentatively deduced mechanism for the origin of Dancherla, Danduvaripalle, Reddypalle syenites and other bodies belonging to Dancherla alkaline complex at the craton interior. The Peddavaduguru syenite was formed by differentiation of alkali mafic magma (gabbro to diorite) and it’s simultaneous mingling with fractionated felsic syenitic magma under incipient rift. Vannedoddi and Yeguvapalli syenites were derived due to desilicification and accompanying alkali feldspar mestasomatism of younger potash rich granites along Guntakal-Gooty fault and along Singanamala shear zone respectively.