Abstract: The Alaska Transportable Array deployment was completely installed as of Sept 2017, totaling 280 stations, with 194 new stations and 86 existing stations, 28 of those upgraded with new sensor emplacement. We briefly summarize the deployment of this seismic network, describe the added meteorological instruments and soil temperature profilers, and review the overwinter performance, current operation, and plans for demobilization set to begin in 2019. The Alaska Transportable Array is entering a period of routine data collection, though certain data collection functions as well as field activities are seasonally dependent. We describe how we have addressed these challenges with emerging technologies, and more technical details are provided by Bloomquist (SSA, 2018). Performance of the stations are generally high quality, with very low noise and high data return despite the challenging environment. As with previous TA deployments in Cascadia and the Central and Eastern US (CEUSN), efforts are ongoing to extend observations in Alaska through station adoption or cooperation with other agencies. Some of these agencies represent interests outside of seismological research yet take advantage of the micro-research station features of an operational Alaska TA station: primarily the enclosure, power, and data communications. We identify timelines for that collaborative planning so as not to interfere with NSF objectives and land management agency approvals.
Slidecast:
https://vimeo.com/278188032
Abstract: The structural style of a fault is generally thought to reflect its mechanical behavior. Stick slip behavior is associated with simple narrow fault zones that have a single high-strain core. These are commonly developed from quartzo-feldspathic protoliths and this structural style is thought to result from the positive feedback between comminution and transformation weakening (Chester et al., 1993). Stable sliding behavior is commonly associated with complex wide fault zones that have multiple anastomosing high-strain cores. These are commonly developed from phyllosilicate rich protoliths, and are thought to reflect strain hardening (Faulkner and Rutter, 2003). The MW 7.2 El Mayor-Cucapah earthquake of 2010 produced a cascading rupture that propagated through a complex network of intersecting faults that cut metamorphic and plutonic rocks exposed in northern Baja California. Coseismic slip of 1-6 m was accommodated by fault zones displaying the full spectrum of previously mentioned styles, which demonstrates that stick-slip mode is not restricted to a certain type of fault. As fault zone complexity and width increase, coseismic slip becomes more broadly distributed on a greater number of scarps that form wider arrays. Thus the slip of a single earthquake replicates many of the fabric elements of the long-lived fault. In addition to protolith, we find that orientation strongly affects fault zone complexity, which increases with decreasing dip. Projecting regional stress onto individual faults, shows that fault zone complexity increases systematically with resolved normal stress, which is known to increase gouge production in laboratory experiments (Yoshioka, 1986). The progressive rotation of faults is thus a previously unrecognized form of strain hardening. We also conclude that regardless of protolith, all faults should experience alternating cycles of stick slip, gouge production and weakening followed by creep, ductile attenuation of gouge and strengthening.
Slidecast:
https://vimeo.com/278182103
Abstract: Slow Slip Events (SSEs) have been discovered at the Southcentral Alaska Subduction Zone since GPS measurement started in Alaska in 1993. Ohta et al. [2006] identified a 3 year long-term SSE between 1998 and 2001 in Upper Cook Inlet. Wei et al. [2012] found another SSE between 2010 and 2011 in Lower Cook Inlet. Fu and Freymueller [2013] reported a large SSE in Upper Cook Inlet starting from the end of 2008. Li et al [2016] identified a slow slip event lasting at least 9 years occurred from 1995 to 2004 beneath Lower Cook Inlet. In addition to those long-term SSEs, we also discovered several short-term SSEs at the Southcentral Alaska Subduction zone, which lasted from weeks to months. For example, continuous GPS measurement clearly showed transient short-term SSEs in 2005, 2006 and 2007. In this study, we analyze the distributions of those long-term and short-term SSEs, and discuss their features of spatiotemporal variations. Our preliminary results indicate at upper Cook Inlet, both long-term and short-term SSEs occurred at the same location, which is downdip of the rupture of the 1964 M9.2 Prince William Sound earthquake. This part of plate interface behaves as a buffer region to transfer and release strain and stress with time in a complicated way.
Slidecast:
https://vimeo.com/278061159
Abstract: We study temporal changes of seismic velocities associated with the June 2016 Mw 5.2 Borrego Springs earthquake that occurred at 12 km depth in the San Jacinto fault zone. This is done with 9 component Green’s function estimates from daily cross correlation functions of ambient noise between stations of two spatially dense linear arrays. The two arrays cross the fault surface trace and are located at Dry Wash (DW) and Jackass Flat (JF) about 2 km northwest and southeast of the epicenter, respectively. The DW and JF arrays have 9 and 12 stations, respectively, with instrument spacing 25-100 m. We first rotate the cross correlations into a system of fault perpendicular, along strike, and vertical directions. For each component, a reference waveform is obtained from stacking correlations from one year before the earthquake. We estimate relative velocity change (dv/v) from arrival time changes in the daily correlation coda waveforms compared to the reference stack. The obtained dv/v time series exhibit variations associated with seasonal velocity changes, linear trends, and the Borrego Springs event. The earthquake signal is characterized by an abrupt velocity drop at the time of the event that is followed by a gradual recovery. This behavior is consistently observed for different components in various frequency bands at both arrays obtained with a time- and frequency-domain analysis methods. After removal of the seasonal and linear components, the velocity drop observed with the vertical-vertical correlation data is around ~5%, 0.2%, and 0.1% at DW array, and ~0.6%, 0.2%, and 0.2% at JF array, for 0.1 – 0.5 Hz, 0.5 – 2 Hz, and 0.8 – 4 Hz, respectively. The larger changes for lower frequencies imply variations not limited to the very shallow material. The larger changes at the DW array may be associated with directivity of the Borrego Springs earthquake.
Slidecast:
https://vimeo.com/278061196
Abstract: High-latitude telemetry is limited by communication network infrastructure, both terrestrial and space based. Few high-latitude regions of the Earth have cabled Internet or mains power, and satellite coverage is often constrained to polar orbiting constellations. These limitations are further complicated by extreme environmental conditions, months without solar charging, and costly logistics. In 2006, the IRIS PASSCAL Instrument Center began developing communications and power solutions for deployment of portable seismic stations in Antarctica. Over the last decade, improvements in power systems, station enclosures, and Iridium based telemetry has enabled real-time seismic data from some of the most remote, high-latitude seismic stations. However, telemetry in these environments require evaluating trades between sample rate, power, and latency, and these decisions directly impact logistics costs. Here we discuss these trades by highlighting several successful station telemetry configurations.
Slidecast:
https://vimeo.com/278055833
Abstract: Following the 2012 Mw 7.6 Nicoya, Costa Rica earthquake, campaign and continuous GPS were used to collect 5 years of postseismic surface deformations. Linear yearly velocities show reversal from uniform seaward to uniform landward motion. This transition from afterslip to interseismic motion is expected. We also observe, unexpectedly, an intervening episode of purely northwestward trench-parallel motion, interpreted as forearc sliver translation. This forearc event occurs during the relocking phase, and involves trench-parallel velocities in excess of the background sliver transport rate. During this episode there is effectively no trench-normal component of velocity. We compare to non-forearc velocities, and contemporaneous seismicity. Inversion shows that afterslip is followed by relocking which propagates toward the center of the seismogenic zone. This relocking regresses during the sliver translation event, and resumes after it is finished. Together these observations suggest that during relocking, temporarily weakened oblique faults can generate unanticipated trench parallel motions as part of the postseismic repertoire. The downgoing slab exerts an oblique landward force on the forearc. Due to several years of unclamping on the arc fault system, the backstop fault has greatly diminished shear strength. This results in the purely strike-parallel motion of the forearc observed herein, with no strike-normal stress accumulation. This finding has important implications for active tectonics. Firstly, forearc-bounding faults may be unclamped by postseismic processes, allowing for creeping. This may affect activity at arc volcanic systems. Secondly, what has been regarded as constant, long-term sliver motion may be accommodated episodically when bounding faults are weakened. Finally, it appears that relocking involves unstable megathrust frictional conditions. Tenuous coupling on the fault interface during relocking can be interrupted by large-scale forearc sliding.
Slidecast:
https://vimeo.com/278060906
