(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Soon after a large earthquake occurs, the USGS Hawaiian Volcano Observatory publicizes the preliminary location and magnitude. Have you ever wondered why the location and magnitude of the event sometimes change? Or why it may take awhile to provide the final “reviewed” earthquake report on our website (http://hvo.wr.usgs.gov/seismic/volcweb/earthquakes/)?

We hope to answer these and other questions by giving you an inside look at what happens at HVO after an earthquake.

When a large earthquake occurs, the HVO staff do what everyone should do—drop, cover, and hold on! As the shaking occurs, we can see the earthquake waves ring through the seismic network. The data are automatically collected and processed within seconds of the earthquake, and the first automatic earthquake location and magnitude are available about three minutes after the event. About a minute later, our automated earthquake location and magnitude are posted on the HVO website and transferred to the National Earthquake Information Center (NEIC).

Prior to the first release of earthquake information, the HVO seismic team assembles to wait for the results, after which they must follow up on several tasks, depending on the location of the earthquake and the size (magnitude) of the event.

By about the three-minute mark, our partners at the Pacific Tsunami Warning Center (PTWC) have also calculated a location and magnitude, which assist them in assessing the tsunami hazard from the earthquake. Because PTWC operates on a 24/7 schedule, the calculations can be quickly reviewed by a seismologist prior to public release. They are then plotted on our website if our automatic solution has not already been posted.

HVO’s automatic processing can be quite good, especially if an earthquake occurs in a densely instrumented area of the seismic network, such as the south flank of Kīlauea. The automated processing is not as good, however, in areas where seismometers are sparse, such as in off-shore locations. In any case, however, an analyst must review the event and refine the location and magnitude to correct any errors made by the computer when the event was first recorded. The result is a “reviewed solution,” which is always more precise than the initial automatic location and magnitude.

During regular business hours, when the HVO staff are on hand, a reviewed solution for a large earthquake can be completed in as little as 20 minutes. But after business hours and during weekends and holidays, the “duty seismologist” might take far longer to analyze an earthquake due to a slower internet connection at home or, especially, the need to find one if that person is not at home.

With more time for analysis following the initial quick assessment of an earthquake, more sophisticated (and time-consuming) calculations become possible. Since many stations around Hawai‘i send data only when they detect a large event, data arrival time may be several minutes. The additional information generated is then included on a “Shakemap,” a map of instrument-measured shaking. Using the additional data, we might also recalculate the location and magnitude of the earthquake and release an update. Each update in earthquake location or magnitude is reflected on the HVO and NEIC websites within minutes of being released and may result in a small shift in either location or magnitude or both.

Once the seismological calculations are completed, HVO issues a news release that includes information about what happened, how the event compares to other earthquakes in history, what caused it, and what people can expect going forward. We have found this process to be the best way to convey the correct information to the media, which, in turn, is disseminated to the public. Once the release goes public, we stand by to answer questions from the media. Then we wait for the next “big one.”