M7.1 Iniskin Earthquake Evolving Content

Realistic ground motion simulations

Computer simulation of the Iniskin earthquake. The 3D simulation demonstrates the complexity of the seismic wavefield that arises from realistic models of Earth structure. Note the striking effect due to the slow wave speeds within Cook Inlet sedimentary basin. The simulation was performed on the high-performance computing cluster at the University of Alaska Fairbanks, Research Computing Systems. Animations and full description on Carl Tape's research page: http://www.giseis.alaska.edu/input/carl/research/earthquakes/iniskin.html

Anchorage ground motions

The seismograms on this figure show 30 seconds of shaking at strong motion seismic stations across Anchorage. See the Alaska Dispatch News for a thorough discussion of the data and the history of these stations.

Fairbanks strong motion data

This map shows the earthquake as it was recorded by our eight strong motion stations in Fairbanks. 400 miles from the epicenter, the P-wave and S-wave arrivals were separated by about a minute. Many residents reported feeling both shocks distinctly. The differences in shaking from one neighborhood to the next are no surprise. Less shaking on the firm ground up on Chena Ridge (at the fire station), more shaking on the looser soils below.

Aftershocks, January 24-26

We have recorded about 135 aftershocks as of Tuesday, January 26. Our smallest automatic aftershock detections are at about magnitude 2, so additional analysis will identify more of the smaller events. The largest aftershock remains the magnitude 4.7 early Sunday morning. The automatic locations are quite good. A combination of Alaska Volcano Observatory, Alaska Earthquake Center, and Transportable Array stations provide comprehensive coverage of the earthquake source region.

Earthquake cross section through the Cook Inlet region

Here is a cross section through the Cook Inlet region. The circles are historical earthquakes. Note that this morning’s earthquake occurred inside the Pacific Plate. This is a frequent source of earthquakes in the region. Its depth is very important because it demonstrates that the earthquake has nothing to do with processes in the crust including known faults, volcanoes, bodies of water, sedimentary basins---or any other type of local geology. This earthquake occurred in what was once (millions of years ago) the crust under the Pacific Ocean. As the Pacific Plate is pulled slowly into the earth (a couple inches per year), the stretching creates earthquake. The location, fault orientation, and slip of the M7.1 earthquake align perfectly with this stretching action.

Aftershocks and Historic Seismicity

This map shows the January 24 magnitude 7.1 (furthest south of the red stars) and its early aftershocks in relation to historic seismicity in the Cook Inlet region.

Early Aftershocks

In the first few hours after this morning’s earthquake, we have had hundreds of aftershocks, though most are too small to be felt. This figure shows these aftershocks recorded in three locations. The largest aftershock (thus far) is a M4.7 at 5:29am. Aftershocks large enough to be felt should be anticipated in the days and potentially weeks to come. However, we anticipate these earthquakes to be smaller than the M7.1 mainshock. While any significant earthquake brings with it the slight chance of a similar, or larger, companion earthquake, we have no reason at all to expect this.

Aftershocks up to 5:00am.

Cartoon cross section

Nice cross section showing the setting for the earthquake. Posted to Facebook by Peter Haeussler. Original post and comments: https://www.facebook.com/photo.php?fbid=1037265803003506&set=gm.563282997170004&type=3&theater

ShakeMap Comparison: July 2015 Iliamna M6.4 vs. Jan 2016 Iniskin M7.1

The M7.1 Iniskin Earthquake occurred at about the same depth as and not very far south of last July's M6.4 Iliamna Earthquake, which was very strongly felt in Southcentral Alaska. Putting the ShakeMaps from the two events side by side demonstrates just how much stronger the M7.1 was. The M6.4 event tops out at intensity IV-V, while the M7.1 generated shaking of intensity VI over a wide area.

USGS Community Internet Intensity Map

This map shows the intensity of shaking by zip code as reported by residents to the USGS via their Did You Feel It page.

Screenshot of the real-time display

This is a screen shot of the earthquake detection system. The seismograms (left) show data from a handful of the closest seismic stations. The map (center) shows the stations, in blue, used to determine the initial location. Orange stations detected the earthquake, but are not used in the solution. The right side panel shows a brief snapshot of the earthquake activity through the morning. The "Southern Alaska" posts are automated detections of the various aftershocks.

Ground Motion Visualization

Between locating aftershocks and making some on-the-fly website additions, Matt Gardine put together this ground motion visualization. Each circle represents a seismic station. You can see them turn dark as the shaking from the earthquake spreads across the state.

Finite fault earthquake solution

Gavin Hayes at NEIC has produced a finite fault solution for the earthquake. The salient results are that the maximum slip on the fault is estimated at 2-3 meters and the rupture probably occurred over about 10-15 seconds. Shaking, of course, lasted much longer as these waves dispersed and reverberated around the Cook Inlet basin. The extent of the fault is estimated at a couple tens of kilometers.

Regional moment tensor solution

UAF graduate student Vipul Silwal ran this moment tensor solution using regional body waves. The orientation of the fault agrees well with solutions from other sources, reflecting the downward pull on the subducting plate.

Predicted static displacement

Green arrows represent the static (permanent) horizontal displacements *predicted* from the earthquake. This model used the fault rupture estimated by NEIC (linked above). The earthquake epicenter is shown by the red star. The largest static displacements are about 6 mm. Because the earthquake is deep, the displacements directly above the source are small. The pattern from a shallow earthquake would be very different. The green arrows are placed at locations where geodetic-grade GPS stations exist. At these locations it will be possible to compared the predictions to actual data. It may, however, take a few days before the GPS pattern is clear enough to distinguish. Figure courtesy of Jeff Freymueller.

Seismic waves across the lower 48

This is a snapshot from a video, produced by the IRIS Consortium (http://www.iris.edu) shows waves from the earthquake rippling across the EarthScope USArray seismic stations in the contiguous 48 states. Full explanation and related content available at http://ds.iris.edu/spud/gmv/11253227