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: 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: A new component of the public GeoGateway mapping system allows determination and display of instances and networks of fault slip from radar interferometer observations of the earth’s surface. This component has added value through integration with a system that supports optional layered display of GPS stations, velocities, and patterns; radar interferometric images and displacement profiles; catalog seismicity; known fault systems; and user-supplied KMZ objects. Fault slip from creep, seismic triggering, aftershocks and general stress change is imaged and verified to better than 1 cm in desert environments and about 4 cm in moderately wooded and frequent-visit agricultural areas. The surface fracture characterization method has an initial version based on filling of bad data through interpolation and blurring, followed by the Canny edge detection algorithm from computer vision. This is used for highly stable (coherent) environments with largely continuous and packed unwrapped image products. A refinement begins with a raw, complex-valued interferogram and uses conjugation to find phase differences up to a full cycle, a method that works when image unwrapping fails. Illustration of the ulitity of the refined surface fracture imaging with other GeoGateway layers centeres on the Brawley Swarm deformation of 2012 and the South Napa earthquake slip of 2014.
Slidecast:
https://vimeo.com/278069475
Abstract: The EarthScope Plate Boundary Observatory (PBO) GNSS network, funded by the NSF and operated by UNAVCO, in Alaska is comprised of 140 permanent GPS and GNSS stations. Of the Alaska portion of the network, 87 stations sit within the subduction zone footprint, including the volcano clusters on Akutan, Augustine, and Unimak. UNAVCO operates 18 real-time GNSS stations in Alaska, the majority of which sit above the subduction zone and as new communications avenues open up, will be adding more. The 1-sps real-time data are available from UNAVCO in BINEX and RTCM formats with Precise Point Position (PPP) solutions generated for each site using the NMEA format. The size and remoteness of Alaska requires the use of multiple modes of communications, depending on the localities. Satellite (VSAT and BGAN) modems are sometimes the only viable means of telemetry, especially along the Aleutian Arc, and tend to be more costly, slower and use more power resources compared to other systems. As cellular service expands in the state, cell modems provide an easy and economical solution where coverage is available, but suffer from infrequent outages that are beyond our operational control. Radio networks are robust and economical when well designed, but depend on the telemetry solution at the central uplink site. Many of the villages are now receiving 3G and faster service via cell modem, and WISPs. In collaboration with other agencies UNAVCO operates 4 integrated GPS/seismic stations with the Alaska Earthquake Center, 9 with the Alaska Volcano Observatory, and 13 with the TA. These stations typically operate on shared telemetry with all parties sharing maintenance responsibilities. The Jan 23 2018 Mw7.9 Gulf of Alaska earthquake provided a great test case for the network as a whole. 14 of the 18 RT GNSS stations captured the earthquake. Given mid winter and the higher likelihood for stations going down, ~85% of the stations in Alaska returned high rate data from a post earthquake response and more recovery may be possible as some of the communications are restored
Slidecast:
https://vimeo.com/277561879
Abstract: The giant Mw 9.2 1964 Alaska earthquake was generated by > 25 m slip on the eastern Aleutian Megathrust that resulted in regional coastal uplift including the Copper River Delta (CRD) that extends 67 km eastward from Cordova along the Gulf of Alaska coast. Coseismic uplift of 2.2 ± 0.2 m in the Alaganic Slough estuary area of the CRD brought a 12 km wide intertidal zone of mud flats above the highest tide level (HTL) resulting in conversion of mud surfaces to fresh water peat marsh and forest. The base of the peaty soils records the 1964 uplift relative to HTL. Five power auger holes cored to 15 m depth, archive 8 pre-1964 peat layers ~0.02–35 cm thick separated by intertidal mud layers ~30–250 cm thick. Each peat/mud couplet defines a complete earthquake cycle with the base of the peat closest to the “event horizon”. Multiple high-precision 14C samples from peat event horizons were used to date and correlate between core holes and to calculate event return intervals (RI’s). Our data indicate: (1) The 8 pre-1964 mean event horizons range from ~780 (E-1) to ~6560 (E-8) calendar years before present (cybp); (2) The Mw 9.2 1964 Alaska earthquake (E-0), with coseismic uplift of 2.2± 0.2 m, followed a recurrence interval (RI) of ~780 cybp; (3) The 7 pre-1964 RI’s range from ~455–1300 cybp; (4) Assuming uplift and magnitude scale roughly with RI, uplift per event is ~1.2–2.9 m (average ~1.5 m); and magnitudes of the 8 events for which RI’s are known range from ~Mw 9–9.3; (5) No additional peat beds that reflect coseismic uplift events were found within the Alaganic Slough area stratigraphy; (6) Energy release in this segment of the Aleutian Megathrust is primarily during giant Mw > 9 seismic events; (7) Minimum mean RI is ~480 years and Mw ~9.05; and (8) Maximum mean RI is ~1125 years and Mw ~9.3.
Slidecast:
https://vimeo.com/278185802
Abstract: We excavated two trenches across topographic scarps on the Puerto Rico southern coastal plain: one across the west-striking Salinas fault and one across the northwest-striking Great Southern Puerto Rico fault zone (GSPRFZ). For the Salinas fault, topographic scarps are primarily south-facing and extend >22 km along hills several kilometers inland from the coast between Río Nigua and the city of Ponce. In contrast, topographic scarps along the southeast 8 km of the GSPRFZ face northeast and southwest. Based on stratigraphic relationships dated by optically stimulated luminescence and radiocarbon analyses, we interpret two surface-rupturing earthquakes on the Salinas fault in the past 20,000 years, and likely in the past 7400 to 10,400 years. Based on similarities in stratigraphy and soil development, but no numerical ages, we interpret that two surface-rupturing earthquakes on the southeast GSPRFZ also may have occurred in the past 20,000 years. The Salinas fault roughly aligns with the South Lajas fault that extends from the west coast of Puerto Rico into the hills west of Ponce. The last two surface-rupturing earthquakes on the South Lajas fault occurred in the past about 7000 to 7500 years. These results suggest the possibility that the southern coastal plain of Puerto Rico is traversed by a series of west-striking faults (e.g., Salinas, Lajas), which may or may not be continuous and kinematically linked, that these faults are distinct from the northwest-striking GSPRFZ, and that the eastern end of the Salinas fault approaches the GSPRFZ obliquely near the southeastern corner of the island. Although there is incomplete knowledge of faults on the southern coastal plain, reasonable scenarios for the rupture lengths, senses of slip, and timing of large earthquakes can be developed for probabilistic seismic hazard analyses of engineered facilities (e.g., bridges, dams, buildings).
Slidecast:
https://vimeo.com/278185653
