Magnitude 7.9 Offshore Kodiak Evolving Content Page

ShakeMap

A ShakeMap, which is a visual representation of peak shaking produced by the earthquake;.

Historical Seismicity

Red circles are preliminary aftershocks with historical background seismicity. Yellow are M>5, and blck dots are M>2.5.

Ground Motion Visualization

In 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. When you compare this to the visualization from the Iniskin earthquake two years ago, you can see the (temporary) dramatic increase in station coverage due to the Transportable Array!

Visualization from the January 23, 2018 earthquake

Visualization from the January 24, 2016 Iniskin earthquake

Seafloor faulting

This graphic combines background seismic data (the orange dots), the earthquakes that happened on Jan 23 (the red dots), the known faults in the area (the red lines), old fractures in oceanic crust (the black lines) with a magnetic map of the ocean floor (the black and white stripes. You can also see the coastline of mainland Alaska along the top, hidden underneath all the earthquakes. This graphic is helping to decipher which fracture zones the M7.9 earthquake likely ruptured. The earthquake might have happened on several different faults that were all in close proximity to each other.

Setting

Here's a picture of Monday night's magnitude 7.9 earthquake and the first few aftershocks (the red dots). Many more aftershocks are recorded than shown here.

The ground has been cut away to show where the subducting plate is - that's where most earthquake activity of this size usually happens. The town of Kodiak is 183 miles from the epicenter.

Earthquake and Tsunami Timeline

This timeline shows events in the first few hours following the M7.9 Offshore Kodiak earthquake on January 23, 2018. The red bar displays the rupture duration (i.e. how long the earthquake itself lasted, 52 seconds) compared with when earthquake energy (P- and/or S-waves reached certain communities (red boxes). The S-wave is responsible for the strongest shaking felt during the earthquake. The blue bar displays the timespan of the the tsunami warning (2 hours 37 minutes). The blue boxes note when alert messages were sent by the National Tsunami Warning Center and when the maximum tsunami wave heights reached certain communities (obtained from tsunami bulletins on the NTWC website). The maximum wave is not necessarily the first wave to arrive, as tsunami waves often continue for long periods of time.

Aftershocks, analyst reviewed and Network Match Filtered

When an earthquake occurs, the features of the seismic waves that are recorded at seismometers depend on the earthquake location and faulting mechanism. These recorded seismic waves can be considered a “fingerprint” of that specific earthquake. Much like fingerprint matching, the recorded waveforms of a known earthquake are used as a “template” to search for other times where a very similar earthquake may have occurred, but was too small or buried in seismic noise for analysts to detect. This procedure is called “Network Matched Filtering,” and has seen widespread use in building more complete catalogs of seismic activity.

This first plot investigates the seismic sequence of the M7.9 offshore Kodiak earthquake and it’s aftershocks. First, matched filtering is used to search for any evidence of foreshock activity. While many earthquakes do have foreshock sequences, many more do not. There was no seismic activity detected in the week before the mainshock, so it does not appear there were any foreshocks to this earthquake.

Next, matched filtering is used to populate the seismic catalog of the aftershock sequence. Starting with 128 known “template” earthquakes, an additional ~400 earthquakes are uncovered in the first two days of the sequence alone. As more aftershocks are cataloged, the potential to find even more earthquakes during these two days will increase, and the analysis will expand to include more up-to-date results as the coming days pass. Normally, aftershock sequences decay somewhat smoothly over time, and any variation in the decay of the earthquake rate can signify that something funky is going on, like propagating afterslip. In the catalog version of the seismicity rate, some variations are observed (where the red line shows some kinks). However, this should not be interpreted too carefully, as these seismic data are still being processed. In contrast, the matched filter enhanced catalog shows a much smoother decay of the seismicity rate. This is likely because the matched filter technique is finding many of the events missed by the automated processing system.

Here is a map showing analyst reviewed locations (red circles) as well as the Network Matched Filter aftershocks (yellow circles). Also plotted are known faults and seafloor fractures.

Anchorage Strong Motion Map

Here is a map showing 90 seconds of S-waves recorded on the Anchorage strong motion network following the M7.9 Offshore Kodiak Earthquake on January 23. The S-waves (shown here) contributed to the strongest shaking felt in the region. Anchorage is approximately 330 miles away from the epicenter.

Largest magnitude Alaska earthquake per year since 1964

These graphs show the magnitude of the largest Alaska earthquake each year since 1964 and zoomed in on the last 20 years. Each year since 1999 there has been at least a magnitude 6.4 somewhere across the state. Since 1964, only one year had a maximum earthquake of magnitude less than 6 (1984). To make recent comparisons, the Offshore Kodiak M7.9 Earthquake was 15 times stronger (in terms of energy released) than the M7.1 Iniskin Earthquake from 2016. It would take more than six Iniskin sized earthquakes to make one Offshore Kodiak earthquake.

Moment Tensor Inversions

(Top left) Distribution of stations used for moment tensor inversion. (Right) Moment tensor solution of offshore Kodiak event using surface waves (75-250 seconds) at regional distances (< 700 km). In black are the recorded data, and synthetics computed using 1D velocity model (Beaudoin et al 1992) are shown in red. Waveform fits are shown only for a subset of stations. For more information on header and labels please see https://scholarworks.alaska.edu/handle/11122/6025. (Bottom left) The depth is estimated by running inversion at multiple depths and finding the solution that gives the least misfit between the recorded seismogram and the synthetics.

Preliminary Coulomb Stress Change on the Aleutian Megathrust

When an earthquake occurs, the motion of the fault blocks pushes and pulls the rock volume surrounding the area. This changes the stress field, which can impact other nearby faults by either pushing them closer to failure (in increased stress regions), or push them farther from failure (in decreased stress regions). Coulomb modeling is one tool that researchers use to determine how a specific earthquake can impact nearby faults. Since this earthquake occurred near the Aleutian Megathrust, the objective here is to find out how the megathrust may have been impacted by this earthquake.

Here, are preliminary Coulomb modeling results for how the M7.9 offshore Kodiak earthquake may have affected the state of stress on the megathrust. The beachball and multi-colored line show the faulting mechanism and the presumed fault plane, respectively. For the beachball, the sense of motion is displayed in the black and white quadrants (the motion along each side of the fault is from the white quadrant and towards the black quadrant). The Coulomb modeling presented here shows that the megathrust near Kodiak is experiencing slightly increased stresses as a result of this earthquake, but the megathrust just east of Kodiak is experiencing slightly decreased stresses as a result of this earthquake. Based on this coulomb stress modeling, it is possible that the regions of the megathrust shaded in warm colors may see heightened earthquake activity as a result of the M7.9 offshore Kodiak earthquake, and the regions shaded in cool colors may see decreased earthquake activity. However, the magnitude of this stress change is small, on the order of 0.1 bar. While some studies have suggested that a 0.1 bar stress increase may be enough to trigger seismicity, the effects will likely be small, if noticeable at all.

Seafloor artistic rendering

This is an artistic rendering of the seafloor where the Offshore Kodiak  M7.9 occurred. You can see that the aftershocks (the red dots on the ocean floor) are spread across several big old cracks, along which the earthquake is believed to have ruptured. 

IMPORTANT NOTE: the depth of the ocean is exaggerated to show the aftershocks better. Kodiak Island is about 66 times longer than the water depth shown here.