Looking at the M3.9 Fall City Earthquake with PNSN.org Tools
A magnitude 3.9 earthquake occurred near Fall City, WA, in the early morning of Aug. 8, with several small aftershocks recorded in the hours after. Felt by thousands of people, the shaking was mostly constrained between Highway 2 to the north, I-90 to the south, and east to west between 405 and the foothills of the Cascades.
There are many ways that seismic activity can be explored on our website. This post is intended to show them all in one place. The following images are available via our interactive Recent Earthquake and Event pages.
Recent Earthquakes Map
Regional view showing the locations of earthquakes in the Puget Sound. The primary way earthquakes are viewed on our website, the interactive Recent Earthquakes Map portrays each earthquake as a color-coded dot. Red dots indicate events in the past 2 hours, orange dots indicate events between 2 to 48 hours old, and yellow dots indicate events between 2 to 14 days old. For this event you can see a string of aftershocks that appear to form a line.
This zoomed in view shows the main shock highlighted. Clicking on the dots on the Recent Earthquakes Map will pop up info about each event and the event page can be accessed through this pop up as well. The spread seen in this image is likely artificial and caused by the geometry of the stations that were used to locate the events. See a future blog post for a more detailed explanation.
ShakeMap
Using instrument recordings, did-you-feel-it reports, an attenuation model, and basic characteristics of the shallow subsurface, ShakeMap produces an interpolated image of the intensity of shaking. ShakeMap uses color to represent the Modified Mercalli Intensity Scale (MMI), with a range starting from “not felt” and going up to “extreme shaking”. You can also select contours of other ground motion intensity measures, like peak ground acceleration in %g or peak ground velocity in cm/s.
Did You Feel It
Found under the Event page’s Maps tab, Did You Feel It? (DYFI) collects information from people who felt an earthquake and creates maps that show what people experienced and the extent of damage felt. We strongly encourage people to fill out DYFI reports as they provide us with vital insights about events.
Waveforms
Found under the Waveforms tab on the Event page, waveforms from the earthquake show the arrival of seismic waves at nearby stations, with each wave color distinguishing the different types of seismic sensors. The closest station to the earthquake — in this case TLW1 at the Tolt Reservoir — is listed first, and arrival at further locations are listed in ascending order.
Plots
Plots can be made by using the Analyze tool on both the Recent Earthquakes page the Custom Search page of the PNSN earthquake catalog. By drawing an A-B line and stretching it over the desired area, several types of plots can be generated.
Magnitude v. Time shows the main shock M3.9 and subsequent aftershocks in the immediate moments afterwards as well as a few throughout the following 48 hours. For large crustal earthquakes it would be expected that medium-sized aftershocks give way to smaller and smaller aftershocks as time moves on.
Cumulative Count shows the number of earthquakes related to time. The steep increase in number of events after the initial M3.9 shows the rapid set of aftershocks that decreases in frequency over time. This is typical of most aftershock sequences.
Depth v. Time shows the different depths at which the events occur over time. In this case, the depth of these events is well constrained and shows very little variation over time.
Conclusions
All of these tools and images are available via PNSN.org. We’ve listed them here all in one place to show just how many cool ways there are to view an individual earthquake. The M3.9 felt event this week near Fall City is a little event that has helped us show off our website but it should also serve as a reminder to us that we live in earthquake country. All of us should take steps to prepare for a larger earthquake. Practicing Drop, Cover, and Hold On (DCHO) as well as packing an emergency kit for your home, workplace, car and/or classroom will make a big difference when our next big earthquake rolls around.