Volcano Watch: Aftershock forecast lets you know what to expect after a big earthquake: Big Island Now

Volcano Watch is a weekly article and activity update written by USGS Hawaiian Volcano Observatory scientists and affiliates. This week’s article was written by Andrew Michael, a research geophysicist with the USGS Seismology Center.

After devastating earthquakes, response and recovery occur under the threat of aftershocks. USGS aftershock forecasts can help you understand what might happen, promote public safety and understand that exposure to aftershocks is normal.

Many earthquakes are unpredictable but we do know that earthquakes are followed by additional earthquakes called aftershocks. For example, in the 10 days following the magnitude 5 earthquake near Pahala, Hawaii, on October 14, 2022, there were aftershocks of magnitude 6 on the Richter scale 3 or greater, more than 100 of the magnitude 2 and larger aftershocks within 6.2 miles (10 km). While most earthquake aftershocks are smaller than the main shock, they can be devastating or deadly, and feeling many small earthquakes can cause emotional distress. But 5% of earthquakes are followed by a larger earthquake, in which case previous earthquakes are referred to as foreboding tremors and the new largest earthquake becomes the main shock.

Plot showing the magnitude and time of earthquakes in the October 2022 Bahala sequence. The major 5.0 magnitude tremors are shown in blue, and the 6.2 mi (10 km) aftershocks from this event are shown in yellow to red with color based on their time of occurrence. USGS plot.

To help people deal with aftershocks, the USGS releases aftershock forecasts for the following day, week, month, and year that provide:

The expected number of aftershocks that can be felt (magnitude 3, 4 or greater)

– Possibility of aftershocks large enough to cause potential damage (Strength 5 or greater)

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The probability of future moderate earthquakes (magnitude 6) to major earthquakes (magnitude 7).

This forecast is released automatically after earthquakes of magnitude 5 and greater in the United States and its territories. The first forecast is released 20 minutes after the main shock has occurred and is updated 74 again during the first year. Forecasts are updated regularly because the rate of aftershocks changes over time, and generally decreases, although sometimes it increases temporarily after larger aftershocks. Therefore, forecasts are updated to keep pace with the changing rate of aftershocks. Updates also include information about the behavior of each series of aftershocks.

In addition to providing basic information about the earthquake and its aftershocks, the USGS reminds everyone to “fall, cover, and wait” during an earthquake. And remember, if you feel a strong earthquake while on the beach or in a low-lying area, you must immediately move to higher ground – a tsunami can reach the shore before the siren alerts you to the potential danger.

Forecasts can be found on the web at https://earthquake.usgs.gov by searching for the “Event Page” of the main shock. For example, the event page for the October 14 event near Pāhala is at https://earthquake.usgs.gov/earthquakes/eventpage/hv72000488/ and the aftershock forecast can be found by clicking on the “card” titled “Post-Shock Forecast” . The event page also provides estimates for shaking, earthquake damage, and other technical information, and you can contribute your experience during the event. Recent major earthquakes will be found in the Large Earthquakes List on the homepage, Older earthquakes can be found in Important Earthquake Archives, and all events can be found using the search tool at https://doi.org/10.5066/F7MS3QZH.

Aftershock forecasts combine three well-researched statistical characteristics of the aftershock sequence: larger mainshocks produce more aftershocks, smaller aftershocks are more common than larger aftershocks, and the rate of aftershocks decreases approximately in proportion to the time since the main shock. Preliminary predictions use parameters for that region or similar geological regions around the world. For example, in Hawaii, preliminary predictions use observations from similar oceanic volcanoes around the world. These parameters are then updated when we observe the specific behavior of each earthquake sequence.

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You may also have noticed that the most common word (it was in bold italics!) in the description when we automatically issue aftershock forecasts. The exception to the rule is earthquakes in Hawaii because the sequences of volcanic earthquakes driven by changes in the movement of magma or volcanic eruptions are much more complex than can be described by the characteristics of the simple aftershocks described above. After earthquakes of magnitude 5 or greater in Hawaii, the USGS earthquake forecasting team manually releases forecasts if USGS Hawaiian Volcano Observatory scientists conclude that the sequence is likely to be a simple sequence of aftershocks . For the October 14 earthquake, this process meant that the first prediction was not issued until 3 hours after a magnitude 5 earthquake. But this delay has given us confidence in the predictions, and so far, they have been accurate.

To learn more about the USGS aftershock forecast, visit https://earthquake.usgs.gov/data/oaf/.

Volcano activity updates

Kīlauea volcano erupts. USGS Volcano Alert Level in WATCH (https://www.usgs.gov/natural-hazards/volcano-hazards/about-alert-levels). Kīlauea updates are released daily.

Over the past week, lava has continued to erupt from the western vent inside Halemaʻumaʻu crater in Hawai’i Volcanoes National Park. Sulfur dioxide emission rates remain high and were last measured at around 700 tons per day (tons/day) on October 22. Seismicity is high but stable, with few continuous earthquakes and volcanic tremors. Over the past week, slope metrics have recorded several deflation and inflation (DI) events. For more information on the current eruption of Kīlauea volcano, see https://www.usgs.gov/volcanoes/kilauea/recent-eruption.

Mauna Loa is not erupting and is still at the volcano alert level. This alert level does not mean that a volcano eruption is imminent or that progression to an eruption from the current level of disturbance is certain. Mauna Loa updates are released daily.

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In the past week, nearly 130 earthquakes on the small Richter scale have been recorded below the summit and upper elevation sides of Mauna Loa – and most of these have occurred at shallow depths less than 15 kilometers (9 miles) below sea level. GPS measurements show a continuous deformation of the Earth consistent with the magma chamber swelling below the summit. Gas concentrations and fumarole temperatures at both the summit and in the sulfur cone of the Southwest Rift Zone have remained stable over the past week. Webcams do not show any changes to the landscape. For more information on current monitoring of Mauna Loa, see https://www.usgs.gov/volcanoes/mauna-loa/monitoring.

There have been 2 events with 3 or more poetic reports in the Hawaiian Islands over the past week: The M3.3 earthquake occurred 9 km (5 mi) east of Pahala at a depth of 31 km (19 mi) on October 23 at 10:44 PM HST and M2 earthquake occurred .5 3 km (1 mi) northwest of Pa’auilo at a depth of 0 km (0 mi) on Oct. 20 at 3:26 PM HST.

HVO continues to monitor the ongoing eruption of Kīlauea and Mauna Loa closely for any signs of increased activity.

Visit the HVO website for past Volcano Watch articles, Kīlauea and Mauna Loa updates, volcano images, maps, recent earthquake information, and more. Email questions to [email protected].

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