• 2020 Alaska Seismicity Summary

      Ruppert, Natalia A.; Gardine, Lea (2021-02)
      The Alaska Earthquake Center reported about 49,250 seismic events in Alaska and neighboring regions in 2020. The largest earthquake was a magnitude 7.8 event that occurred on July 22 in the Shumagin Islands region. It was followed by about 6,000 aftershocks including a magnitude 7.6 event on October 19. Other active spots include the 2018 M7.1 Anchorage, 2018 M6.4 Kaktovik, 2018 M7.9 Offshore Kodiak aftershock sequences, Purcell Mountains earthquake swarm, and Wright Glacier cluster northeast of Juneau.
    • AACSE earthquake catalog: January-August, 2019

      Ruppert, Natalia; Barcheck, Grace; Abers, Geoffrey (2021-05)
      The Alaska Amphibious Community Seismic Experiment (AACSE) comprised 75 ocean bottom seismometers and 30 land stations and covered about 650 km along the segment of the subduction zone that includes Kodiak Island, the Alaska Peninsula and the Shumagin Islands between May 2018 and September 2019. This unprecedented offshore dataset has the potential to support a greatly enhanced earthquake catalog by both increasing the number of detected earthquakes and improving the accuracy of their source parameters. We use all available regional and AACSE campaign seismic data to compile an enhanced earthquake catalog for the region between Kodiak and Shumagin Islands including Alaska Peninsula (51-59N, 148-163W). We apply the same processing and reporting standards to additional picks and events as the Alaska Earthquake Center currently use for compilation of the authoritative regional earthquake catalog. This release includes earthquake catalogs for the time period between January 01 and July 31, 2019. We include monthly CSS database tables (aecevent, arrival, assoc, event, netmag, origerr, origin) and quakeml files.
    • AACSE earthquake catalog: May-December, 2018

      Ruppert, Natalia A.; Barcheck, Grace; Abers, Geoffrey A. (2021-02)
      The Alaska Amphibious Community Seismic Experiment (AACSE) comprised 75 ocean bottom seismometers and 30 land stations and covered about 650 km along the segment of the subduction zone that includes Kodiak Island, the Alaska Peninsula and the Shumagin Islands between May, 2018 and September, 2019 (Barcheck et al., 2020). This unprecedented dataset has the potential to support a greatly enhanced earthquake catalog by both increasing the number of detected earthquakes and improving the accuracy of their source parameters. We use all available regional and AACSE campaign seismic data to compile an enhanced earthquake catalog for the region between Kodiak and Shumagin Islands including Alaska Peninsula (51-59N, 148-163W). We apply the same processing and reporting standards to additional picks and seismic events as the Alaska Earthquake Center currently use for compilation of the authoritative regional earthquake catalog. This release includes earthquake catalogs for the time period between May 12 and December 31, 2018 (3829 events total 1132 of which are newly detected). We include monthly CSS database tables and quakeml files. The data analysis is ongoing and more catalogs will be released in the near future.
    • Air Force Contract No. AF 19(604)-1089

      Geophysical Institute at the University of Alaska, 1954
      This report briefly describes the progress made at the Geophysical Institute of the University of Alaska during the past three months, in the study of arctic radio wave propagation under the Air Force Contract No. AF 19(604)-1089.
    • Alaska Earthquake Center Quarterly Technical Report April-June 2021

      Ruppert, Natalia (2021-08)
      This series of technical quarterly reports from the Alaska Earthquake Center (AEC) includes detailed summaries and updates on Alaska seismicity, the AEC seismic network and stations, field work, our social media presence, and lists publications and presentations by AEC staff. Multiple AEC staff members contributed to this report. It is issued in the following month after the completion of each quarter Q1: January-March, Q2: April-June, Q3: July-September, and Q4: October-December.
    • Alaska Earthquake Center Quarterly Technical Report January-March 2021

      Ruppert, Natalia (2021-05)
      This is the first in a series of technical quarterly reports from the Alaska Earthquake Center (AEC). It includes detailed summaries and updates on Alaska seismicity, the AEC seismic network and stations, field work, our social media presence, and lists publications and presentations by AEC staff. Multiple AEC staff members contributed to this report. It is issued in the following month after the completion of each quarter Q1: January-March, Q2: April-June, Q3: July-September, and Q4: October-December.
    • Alaska Earthquake Center: A 2020 Perspective

      Grassi, Beth; West, Michael; Gardine, Lea (2021-03)
      The Alaska Earthquake Center is not historically in the habit of producing annual reports. We are in a dynamic time, however. Societally-significant earthquakes and multiple tsunami concerns over the past few years have brought more attention to what we do. At the same time, we are experiencing significant growth in several areas. Our goal in distributing this summary is to communicate the breadth of our activities and the diversity of our stakeholders, helping us become even more effective at meeting the earthquake and tsunami science needs of Alaska and the nation.
    • Alaska Earthquakes Poster

      Gardine, Lea; West, Michael; Grassi, Beth (2020-10)
      Alaska is one of the most seismically active places in the world. This poster connects the geographic distribution of earthquakes from the Alaska Earthquake Center catalog with the core concepts that drive Alaska seismicity. Rupture patches, how plate tectonics forms faults throughout Alaska, and how the angle of the sinking Pacific Plate affects earthquake distribution and creates volcanoes are some of the key concepts represented.
    • Analysis of regional seismograms and 3D synthetic seismograms for the 2016-01-24 Mw 7.1 Iniskin earthquake in southern Alaska

      Tape, Carl (2016-11-03)
      I perform two analyses to identify cases of seismogram clipping or other problems (e.g., data gaps) for the 2016-01-24 Mw 7.1 Iniskin, Alaska, earthquake. The first analysis is a comparison of synthetic and observed seismograms: three-component, displacement seismograms filtered between periods 4-80 s. The subset of 141 stations is limited to an oblique rectangular region that is 1200 km x 600 km (Figures 1 and 2) and used in a seismic wavefield simulation with a three-dimensional seismic velocity model. I identify 60 out of 141 stations that are suspected of clipping or other problems. Of the 81 good stations, only 8 are within 250 km of the Iniskin epicenter, and all 8 stations are outside of Cook Inlet basin, which strongly amplifies ground motion (both in data and in synthetics). The second, much simpler, analysis is to identify clipping based on the maximum counts on the waveforms. The max-counts approach reveals general agreeement with the classification based on long-period data and synthetics. The analysis suggests that (1) some recorded waveforms that exceed clipping levels may still be usable for some modeling purposes, and (2) some recorded waveforms that appear to be suitable for modeling purposes should probably be discarded due to clipping at high frequencies. The identification of suspected stations, along with the waveform comparisons, may help network operators assess the possibility of unexpected performance during the Mw 7.1 slab earthquake.
    • Archival search for felt reports for the Alaska earthquake of August 27, 1904

      Tape, Carl (2016-06-03)
      This report documents a search of primary sources from 1904 to identify felt reports of the 1904-08-27 earthquake in central Alaska. The objective is to use the felt reports to get a better idea for where the earthquake occurred.
    • Arctic Propagation Studies at Tropospheric and Ionospheric Modes of Propagation: Final Report

      Owren, Leif; Bates, H. F.; Hunsucker, R. D.; Pope, J. H.; Stark, R. A. (Geophysical Institute at the University of Alaska, 1959-10)
      Two types of direct scatter from the F region are identified on the records from the oblique incidence sweep-frequency sounder located at College, Alaska. One type of echo appears to come from randomly distributed, field-aligned irregularities in the ionosphere and the other from discrete patches of irregularities. The former is essentially a nighttime phenomenon, while the latter occurs mostly during the day. From these direct scatter modes we can obtain an estimate on the horizontal and the vertical extents of the irregularities. Analysis of the data for the past year has shown that the randomly distributed irregularities commonly occur in regions having horizontal extents of more than 1000 km. The discrete irregularities appear to extend throughout most of the lower half of the F layer. The sequence of events near sunrise and sunset on a magnetically quiet winter day indicates that solar radiation eliminates the random irregularities and accentuates the discrete irregularities. Certain phenomena frequently recorded on high latitude ionograms such as Spread F and triple splitting are probably manifestations of backscatter from ionospheric irregularities. The occurrence of Z-traces in College ionograms is studied statistically and it is concluded that the majority, if not all, of the Z-traces are produced by backscatter of the radiation obliquely incident in the direction of the magnetic zenith. Fixed frequency oblique incidence soundings on frequencies of 12, 18 and 30 mc/s made at College, Alaska show both direct backscatter from the E and F layers and F layer propagated backscatter from the ground. The 12 mc/s soundings made during 1956 have been re-scaled under this contract to extract the available information concerning direct backscatter echoes at ranges below 1000 km. The direct backscatter echo from the F layer (IF echo) has a large diurnal maximum at approximately 1800 AST and a smaller maximum at 0400 AST. IF echoes are observed at ranges from 500 to 1000 km, usually occurring at approximately one-half the range of the 2F echo. The azimuth distribution of the IF echo has a maximum centered on magnetic north. Direct backscatter from the E layer (IE echo) occurs in the range interval of 200 to 800 km with a maximum between 300 and 500 km. The azimuth distribution maximum is centered on magnetic north and the diurnal distribution shows maxima from 0000 to 0200 AST and 0300 to 0400 AST. F layer propagated backscatter from the ground (2F echo) is investigated using both the 12 mc/s 1956 soundings and soundings on 12, 18 and 30 mc/s obtained during 1958. Histograms showing the diurnal distribution of 2F echo occurrence on 12 mc/s for 1956 and 1958 are essentially the same, and illustrate solar effects on the F layer. The behaviour of the regular 2F echo on 12, 18 and 30 mc/s for a typical day in December 1958 is illustrated by a series of PPI photographs. The results obtained during an experimental investigation of the drift motions of auroral ionization are summarized, and certain properties of the luminous aurora established by photo-electric measurements reviewed. Some preliminary observations of solar radio emission at 65 mc/s are reported. A technique of estimating the electron densities of the outer ionosphere by the use of nose whistlers is described. The method involves the numerical integration of the whistler dispersion equation after first assuming a model for the distribution in density. This technique is applied to several whistlers which occurred on 19 March 1959 resulting in estimates of electron densities between four and five earth's radii. The temporal variations in the occurrence of chorus during the IGY at College and Kotzebue, Alaska are studied. The results of an investigation of the effect of latitude on the diurnal maximum of chorus indicate that it is desirable to use a latitude based on the location of the eccentric dipole rather than the usual geomagnetic latitude for the study of chorus. The mathematical theory of longitudinally propagated whistlers in a magnetic dipole field is developed. The usual method for deriving electron density distributions in the exosphere from nose whistler observations by means of assumed distribution functions is criticized and shown to be ambiguous and subjective. A systematic method which avoids subjective assumptions is described. The whistler propagation problem is reduced to an integral equation and a first order principal value solution is obtained by using an approximate form of the equation. Higher order solutions may then be derived by an indicated iterative procedure. Five short-term transpolar transmission tests conducted jointly by the Geophysical Institute and the Norwegian Defence Research Establishment during 1956-59 are described briefly. Some preliminary results of a transarctic propagation study on 12, 18 and 30 mc/s made by the Geophysical Institute in cooperation with the Kiruna Geophysical Observatory, Sweden, are reported. Simultaneous backscatter soundings of the polar region from College, Alaska and recordings c£ the forward propagated signal at Kiruna, Sweden are used to deduce the propagation conditions and modes. The 12 mc/s and 18 mc/s pulse transmissions from College were received at Kiruna over 80% of the time during the month of December 1958. Groundscatter echoes from the polar regions indicated that a three-hop mode occurred 52% of the time on 12 mc/s and 49% of the time on 18 mc/s. Similarly, a two-hop mode occurred 9% of the time on 12 mc/s and 127. of the time on 18 mc/s. A signal was recorded at Kiruna 197. of the time without any corresponding groundscatter being observed from College. This could indicate propagation by a one-hop high ray (Pedersen) mode or by a lateral mode.
    • Arctic Radio Wave Propagation

      Owren, Leif; Little, C. Gordon (Geophysical Institute at the University of Alaska, 1958-03)
      The object of this investigation is to obtain additional information concerning the effects of aurora on high frequency radio signals which is essential to a complete understanding of new modes of propagation that have tactical and strategic applications.
    • Assessment of station metadata in Alaska based on analysis of Love waves from the 2012-04-11 Mw 8.6 offshore Sumatra earthquake

      Tape, Carl (2012-07-05)
      This report is part of a detailed investigation of a Mw 3.9 earthquake near Nenana, Alaska, that was triggered by Love waves from a Mw 8.6 offshore Sumatra earthquake. Results from that study appeared in Tape et al. (2013). We analyze all BH and HH channel waveforms that are available at the Alaska Earthquake Center. This report has three objectives: (1) to provide information that may help improve station metadata at Alaska stations; (2) to provide a snapshot of station performance in Alaska at one particular time (11-April-2012); (3) to provide details and figures on part of the waveform processing used in Tape et al. (2013).
    • Auroral Index for College, Alaska Derived from All-Sky camera Photographs, September 1957- December 1958

      Tryon, Helen M. (Geophysical Institute at the University of Alaska, 1959-11)
    • Auroral zone absorption of radio waves transmitted via the ionosphere

      Owren, Leif; Leinbach, Harold; Nichols, B.; Stark, R.; Smith, Carol (Geophysical Institute at the University of Alaska, 1956)
      TASK A: TRANSMISSION OF HIGH FREQUENCY RADIO WAVES VIA THE ARCTIC IONOSPHERE The experimental data collected from June, 1949, through October, 1955, under "Experiment Aurora" are summarized in tables and diagrams, and the results discussed. The monthly percentage of signal in-time is tabulated for all frequencies and paths» and depicted in diagrams which allow a comparison of the values for East-West and South-North propagation at each frequency. The average monthly percentage of signal in-time for the duration of the 6-year experiment is tabulated for each frequency and path. The seasonal variation in signal in-tim e over short and long paths is shown in diagrams. The relationship found between ionospheric absorption, as measured with a vertical incidence sounder, and signal outtime is summarized. The average diurnal variation in the hourly median signal strength during the different seasons of the year 1954-55 is given for all frequencies on both short and long paths in the East-West as well as the South-North direction. The diurnal variation in signal strength on the 4 me short paths and the 12 me long paths is compared for a year of high solar activity (1949-50) and a year of low solar activity (1954-55). The discussion of the data reveals that a statistically significant difference in signal in-time for the East-West and South-North paths exists only for the 12 me short paths. The larger percentage of signal in-time found in the East-West direction is believed to be due to a preferential orientation of sporadic ionization along parallels to the auroral zone. A study of the critical frequencies observed for the E and F -layers shows that the difference in daytime variation of median signal strength between the years 1949-50 and 1954-55 may be explained in terms of the normal changes in F -layer ionization and D -layer absorption in course of a sunspot cycle. The results indicate that in Alaska there will generally be F2 propagation during daytime of 4 me signals over 350 km paths throughout the solar cycle. Regular daytime F2 propagation of 12 me signals over 1100 km paths may be expected in years of reasonably high solar activity only. TASK B: PULSE TECHNIQUES. BACK-SCATTER AT 12 MC A 12 me radar has been constructed and operated using A -scope and PPI displays. Experimental results obtained during several months of continuous operation are reviewed and discussed. Both direct backscatter and ground back-scatter echoes, as well as possible combinations of these modes, have been observed. The echoes are classified in two groups according to their fading rates, those fading rapidly being associated with aurora. Figures show the diurnal, range and range-azimuth distribution of the observed auroral echoes as well as some special types of echoes recorded. The direct back-scatter echoes at 12 me associated with aurora show characteristics consistent with those observed at YHF when allowance is made for the frequency difference. At 12 me the fading rate is proportionally less than at higher frequencies; and aspect sensitivity, although weaker, still exists. The diurnal variation is similar to that found at VHF. Several types of echoes not observed at VHF are mentioned. TASK B: VISUAL OBSERVATIONS OF THE AURORA Analysis is made of the visual auroral data obtained at five stations in Alaska during the observing period of 1954-55. Graphs giving the percentage occurrence of aurora at each station as a function of latitude and time of day are presented. Graphs showing the variation of auroral occurrence with geomagnetic latitude as a function of magnetic K index are also given. The conclusions drawn from the 1954-55 data are substantially the same as those based on the 1953-54 data discussed in an earlier report.
    • Auroral zone absorption of radio waves transmitted via the ionosphere

      Andersen, Soren; Leonard, Robert S. (Geophysical Institute at the University of Alaska, 1955)
      A discussion of the design for a new antenna system for the transmitter stations is presented together with the measurements and power computation made on the old and new antennas. In the 12 mc back-scatter program at College, the technique used to measure the amplitude of each individual echo and reanalysis of the range distribution previously reported are discussed. Revisions in the techniques of observation of visual auroras and the methods of recording the data for analysis are described in detail.
    • Auroral zone absorption of radio waves transmitted via the ionosphere

      Owren, Leif; Stark, Robert (Geophysical Institute at the University of Alaska, 1955)
      Signal intensity operations have stopped and the preliminary reduction of all field intensity records completed . The 12 me back-scatter equipment is described and operating conditions stated. Tentative interpretations of ob served echoes in terms of possible reflection mechanisms are given.
    • Brief Discriptions of Auroral Displays Over Alaska During 1957-58

      Davis, T. Neil (Geophysical Institute at the University of Alaska, 1960-01)
    • Catalogue of Huet auroral spectra 1957-1959

      Romick, Gerald J. (Geophysical Institute of the University of Alaska, 1961-03)
      The zenith auroral spectra at College, Alaska, obtained during the 1957-1959 observing seasons, has been assembled in catalogue form. The prime purpose of this catalogue is to present the auroral activity in a manner which can be used by others in the interpretation of aurorally associated phenomena. Prom the general appearance of the spectra and other factors, a table of daily index numbers (1 -9) is given for two observing periods. Although these numbers should not be used in themselves as correlation data they are valuable as representative indices. This point is indicated by the clear appearance of the spring maximum in activity and a general yearly decline in activity towards the minimum of the sunspot cycle.
    • Catalogue of IGY All-Sky Camera Data for Alaskan Stations

      Young, M. J. (Geophysical Institute at the University of Alaska, 1959-07-31)
      The earlier orbits and ephemerides for the Soviet satellites were not sufficiently accurate to be very useful in making observations in Alaska. Extrapolations from our own observations gave better predictions. This merely pointed out the fact that rough observations of meridian transits at high latitudes will give better values of the inclination of the orbit than precision observations at low latitudes. Hence, it was decided to observe visually the meridian transits estimating the altitude by noting the position with respect to the stars or using crude alidade measurements. The times of the earlier observations were observed on a watch or clock and the clock correction obtained from WWV. Later the times were determined with the aid of stop watches, taking time intervals from WWV signals. This rather meager program of optical observations of the Soviet satellites was undertaken to give supplementary data for use of the radio observations, and particularly to assist in the prediction of position of the satellite so that the 61-foot radar of Stanford Research Institute could be set accurately enough to observe it (the beam width at the half-power points is about 3°). This report contains primarily the visual observations made at the Geophysical Institute by various members of the staff, and a series of observations by Olaf Halverson at Nome, Alaska. In addition there is a short discussion of the geometry of the trajectory, the illumination of a circumpolar satellite, and a note on the evaluation of Brouwer's moment factors.