Seismic Analysis of The 30 July 1972 Sitka Earthquake Aftershocks, Stress Drop of Aftershocks in Cook Strait, New Zealand Following The 2013 Cook Strait and Lake Grassmere and 2016 Kaikoura Earthquakes, and 2d Seismic Tomography in The Brady Hot Springs Geothermal Site, Nevada

Juan Alejandro Ochoa Chavez, University of Texas at El Paso

Abstract

The 1972 Mw 7.6 Sitka earthquake is the largest earthquake to have occurred along the Alaska portion of the Queen Charlotte fault (QCF) in historic time. The QCF system forms the plate boundary between the Pacific and North American plates within southeastern Alaska and has accumulated enough slip since 1972 to produce a comparable sized event in the near future. Thus, it is important to better understand the controls on the rupture process of the 1972 mainshock and its aftershocks. Following the mainshock the U.S. Geological Survey installed a network of 11 portable seismographs that recorded over 200 aftershocks of the sequence. These locations were never published and the digital phase data were misplaced; however, we were able to scan paper-based copies of the data, convert the data to digital form and successfully relocate 87 of the aftershocks. The relocations show two clusters of aftershocks along the QCF, one to the north of the main shock and the other one to the south. More aftershocks were located within the northern cluster and some of the events appear to line up on structures at an angle to the QCF. These off-fault lineations are similar to aftershock patterns observed in the 2013 Mw = 7.5 Craig, Alaska earthquake. Recent and historical seismicity indicates that these clusters remain active through time and that there is a small gap between 56.8° and 56.9° N. Gravity and magnetic anomalies suggest that the structure of the Pacific plate plays an important role in the segmentation of the QCF rupture zone. I calculate stress drops for 26 events from three sequences of earthquakes in New Zealand: Cook Strait, Lake Grassmere and Kaikoura and evaluate differences within the sequences and also differences between them. The three sequences are in the over-riding plate of the subduction zone in the northern South Island. I use an empirical Green’s function (EGF) approach, and require the EGF earthquakes to be highly correlated to their respective main shocks. Since this approach also requires that events used as EGF are close to the main shocks, a hierarchical clustering algorithm was used to relocate all the earthquakes. I restrict my analysis to the better-constrained corner frequency measurements. Relocations show that the Kaikoura earthquake activated structures that were not activated during the 2013 sequences. Other structures show activity during both sequences. Most of the events are distributed along the previously unmapped transform fault. There could be a stress drop variation with time, but more measurements are needed to support this idea. Variation with other parameters like magnitude, latitude, depth or focal mechanism was not found. The Brady geothermal system is located 80 km northeast of Reno in the western part of the Basin and Range Province in Nevada. It is one of the three independent geothermal systems of the Hot Springs Mountains. The goal of this study was to use data from the PoroTomo project to generate 2-D subsurface images of the Brady system to constrain the general structure and composition of the area up to 200-300 m depth. First arrivals were manually picked for the sources and stations along four different seismic lines: two of them subparallel and the other two perpendicular to the strike of the main Brady fault. Ray coverage plots show the maximum area that can be resolved for each seismic line. Checkerboard tests were used to determine the areas with better resolution. They also indicate that the best spacing between the velocity nodes for this geometry is 150 m horizontally and 30 m vertically. Starting from a 1-D velocity model, initial 2-D velocity grids were generated to perform an inversion with the first arrival travel times. Parallel lines included more data, so more lateral variations resulted from the inversion. Results broadly agree with previous seismic studies in the Brady area. Evidence of thermal contraction of the rocks in different parts of the Brady area had been observed in previous studies, but no correlation was found between these rocks and the velocities estimated in this study.

Subject Area

Geophysics

Recommended Citation

Ochoa Chavez, Juan Alejandro, "Seismic Analysis of The 30 July 1972 Sitka Earthquake Aftershocks, Stress Drop of Aftershocks in Cook Strait, New Zealand Following The 2013 Cook Strait and Lake Grassmere and 2016 Kaikoura Earthquakes, and 2d Seismic Tomography in The Brady Hot Springs Geothermal Site, Nevada" (2019). ETD Collection for University of Texas, El Paso. AAI13883343.
https://digitalcommons.utep.edu/dissertations/AAI13883343

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