Seismic moment tensor catalog for southern Alaska

The files here are part of the PhD thesis work by Vipul Silwal. This supplement will be cited in a manuscript to be submitted. A seismic moment tensor catalog of 106 events was generated using body waves and surface waves. The best solution (M0) was obtained through a grid-search in the double-couple moment tensor space (M) using the "cut-and-paste" (CAP) approach of Zhu and Helmberger (1996). The waveform fits for the 21 events in the Part I catalog is shown in Figure A, with the best-fitting depth plots in Figure B. The uncertainty analysis for these Part I events is shown in Figure C. The waveform fits for the other 85 events (Part II) is shown in Figure D; for these events the best depth search and uncertainty analysis was not performed.This work was supported by the Alaska Earthquake Center and by a grant from the National Science Foundation (EAR 1251971)

Seismic moment tensors for six events in the Minto Flats fault zone, 2012-2016

Using seismic waveform data, we determine seismic moment tensor solutions for six events in the Minto Flats fault zone, 2012-2016.This project was supported by the National Science Foundation, Grant EAR-1352688

Seismic moment tensor catalog for Minto Flats fault zone (2000-2014)

We present moment tensor solutions for 15 earthquakes in the Minto Flats fault zone. Moment tensor solutions were generated using body waves and surface waves. The best solution was obtained through a grid-search in the double-couple moment tensor space using the "cut-and-paste" (CAP) approach of Zhu and Helmberger (1996). The files here are part of the PhD thesis work by Vipul Silwal. The materials were included as part of the supplement to Tape et al. (2015, BSSA).This work was supported by the Alaska Earthquake Center and by a grant from the National Science Foundation (EAR 1251971)

Seismic moment tensor catalog for crustal earthquakes in the Cook Inlet and Susitna region of southern Alaska

We determine moment tensor solutions for 53 earthquakes in southern Alaska, which include 9 in Beluga, 22 in Cook Inlet, and 22 in Susitna. The moment tensors were estimated using both the waveforms and the first-motion polarities. The best solution was obtained through a grid-search in the double-couple moment tensor space using the "cut-and-paste" (CAP) approach of Zhu and Helmberger (1996).This project was supported by USGS Earthquake Hazards Program (contract G15AP00052)

Hypocenter estimation for 14 earthquakes in south-central Alaska (1929-1975)

We provide results from an analysis of 14 historical earthquakes in the region of Cook Inlet and Susitna, south-central Alaska. Using global arrival times of P and S waves, we estimate probabilistic hypocenters using the code NonLinLoc. We provide the complete results, as well as a set of plots to help interpret the likelihood of each earthquake being within the crust, on the subduction interface, or within the subducting Pacific slab.V. Silwal and C. Tape were supported by USGS Earthquake Hazards Program (contract G15AP00052)

Step-response signals recorded during earthquakes in Alaska

We present waveform record sections of 18 earthquakes recorded the Minto Flats fault zone in central Alaska. These include the largest earthquakes to have occurred within the Minto Flats fault zone since the installation of the 13-station FLATS network in September 2015 (Tape and West, 2014). Several seismograms from these earthquakes exhibit a ``step-response signal'' that is a long-period, unwanted signal that does not reflect regional ground motion. We use the term ``anomalously high amplitudes'' to refer to amplitudes within a certain bandpass that exceed the amplitude of earthquake ground motion (within the same bandpass). We attribute anomalously high amplitudes to three possibilities: (1) step-response signal due to local tilt or other effect, (2) step-response signal due to defective sensor, (3) digitizer clipping, (4) high noise (especially before the earthquake). We find widespread occurrences of the step-response signal for earthquakes in the Minto Flats fault zone.This project was supported in part by the National Science Foundation under Grant No. EAR-1352668

Earthquake source mechanisms and three-dimensional wavefield simulations in Alaska

Thesis (Ph.D.) University of Alaska Fairbanks, 2018This thesis presents: (1) a set of earthquake source mechanism catalogs for Alaska and (2) a threedimensional seismic velocity model of Alaska. The improved earthquake sources are used within the velocity model for generating synthetic seismograms, which are then compared with recorded seismograms to assess the quality of the velocity model. An earthquake source mechanism can be modeled as a moment tensor, which is a 3 × 3 symmetric matrix. We estimate the moment tensor for earthquakes by comparing observed waveforms (body waves and surface waves) with synthetic waveforms computed in a layered model. The improved moment tensor solutions are obtained by utilizing both the body waves and surface waves at as many broadband stations as possible. Further improvement in the inversion technique is obtained by (1) implementation of L1 norm in waveform misfit function and (2) inclusion of first-motion polarity misfit in the misfit function. We also demonstrate a probabilistic approach for quantifying the uncertainty in a moment tensor solution. Moment tensors can be used for understanding the tectonics of a region. In the Cook Inlet and Susitna region, west of Anchorage, we determined moment tensor solutions for small-tointermediate magnitude (M ≥ 2.5) crustal earthquakes. Analyzing these small earthquakes required us to modify the misfit function to include first-motion polarity measurements, in addition to waveform differences. The study was complemented with the probabilistic hypocenter estimation of large historical earthquakes (Mw ≥ 5.8) to assess their likelihood of origin as crustal, intraslab, or subduction interface. The predominance of thrust faulting mechanisms for crustal earthquakes indicate a compressive regime within the crust of south-central Alaska. Wavefield simulations are performed in three regions of Alaska: the southern Alaska region of subduction, the eastern Alaska region with the accreting Yakutat microplate, and the interior Alaska region containing predominantly strike-slip faulting, including the Minto Flats fault zone. Our three-dimensional seismic velocity model of Alaska is an interpolated body-wave arrival time model from a previous study, embedded with major sedimentary basins (Cook Inlet, Susitna, Nenana), and with a minimum shear wave velocity threshold of 1000 m/s. Our comparisons between data and synthetics quantify the misfit that arises from different parts of each model. Furtherwork is needed to comprehensively document the regions within each model that give rise to the observed misfit. This would be a step toward performing an iterative adjoint tomographic inversion in Alaska.National Science Foundation, Alaska Earthquake Center, U.S. Geological Survey Earthquake Hazards Program, Air Force Research Laborator

Enhanced crustal and intermediate seismicity in the Hindu Kush-Pamir region revealed by attentive deep learning model

The Hindu Kush-Pamir region (HKPR) is characterized by complex ongoing deformation, unique slab geometry, and intermediate seismic activity. The availability of extensive seismological data in recent decades has prompted the use of deep learning algorithms to extract valuable insights. In this study, we present a fully automated approach for augmenting earthquake catalogue within the HKPR. Our method leverages an attention mechanism-based deep learning architecture to simultaneously detect events, perform phase picking, and estimate magnitudes. We applied this model to a ten-month dataset (January 2013–October 2013) from 83 stations in the region. Utilizing a robust criterion to evaluate the model's probabilities, we associated phases at different stations and pinpointed earthquake locations in the region. Our results demonstrate a significant enhancement, revealing nearly four and a half times more earthquakes than previously documented in the International Seismological Center (ISC) catalogue. A notable portion of these newly detected events falls within the category of very low-magnitude earthquakes (<3), which were absent in the ISC catalogue. Notably, our spatiotemporal analysis reveals a concentration of crustal seismicity along poorly mapped neotectonic north and northeast-oriented faults in the western Pamir, as well as the Vakhsh Thrust System and the Darvaz Karakul Fault. These findings underscore potential sources of future seismic hazards. Furthermore, our expanded earthquake catalogue facilitates a deeper understanding of the interplay between crustal and intermediate seismic activity in the HKPR, shedding light on the deformation and active faulting resulting from Eurasian-Indian plate interactions

Author Correction: Earthquake nucleation and fault slip complexity in the lower crust of central Alaska

International audienceIn the version of this Article originally published, the `Data availability' section contained an incorrect DOI for data from the FLATS (XV) seismic network (10.7914/SN/ZE_2015); the correct DOI is: 10.7914/SN/XV_2014. This has now been corrected in the online versions

Earthquake nucleation and fault slip complexity in the lower crust of central Alaska

International audienceEarthquakes start under conditions that are largely unknown. In laboratory analogue experiments and continuum models, earthquakes transition from slow-slipping, growing nucleation to fast-slipping rupture. In nature, earthquakes generally start abruptly, with no evidence for a nucleation process. Here we report evidence from a strike-slip fault zone in central Alaska of extended earthquake nucleation and of very-low-frequency earthquakes (VLFEs), a phenomenon previously reported only in subduction zone environments. In 2016, a VLFE transitioned into an earthquake of magnitude 3.7 and was preceded by a 12-hour-long accelerating foreshock sequence. Benefiting from 12 seismic stations deployed within 30 km of the epicentre, we identify coincident radiation of distinct high-frequency and low-frequency waves during 22 s of nucleation. The power-law temporal growth of the nucleation signal is quantitatively predicted by a model in which high-frequency waves are radiated from the vicinity of an expanding slow slip front. The observations reveal the continuity and complexity of slip processes near the bottom of the seismogenic zone of a strike-slip fault system in central Alaska