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It’s Swarmy Outside: Defining Swarms for the Purpose of Forecasting

Description: 

Compared to more common mainshock-aftershock sequences, earthquake swarms are difficult to define and are therefore often defined by what they lack in relation to aftershock sequences (e.g., a clear mainshock, Bath’s Law, Omori-like temporal rate decay). However, such definitions can be difficult to quantify, particularly in more complex sequences of elevated earthquake rate. They also do little to describe what a swarm actually is, particularly for the general public who may never hear the term until they are in the midst of one and receiving earthquake advisories that could cause confusion or alarm. Definitions that focus on seismogenic processes due to fluid migration, slow slip, or volcanic processes may be more informative, but it is difficult to identify these processes early in a swarm’s evolution. Because the U.S. Geological Survey (USGS) releases earthquake advisories and forecasts during some swarms, such as the 2016, 2020, and 2021 swarms near the Salton Sea in California (e.g., McBride et al., SRL, 2020), we propose a definition that helps us make better forecasts and reduces confusion.

Here we define a swarm simply as a sequence of earthquakes where changes in earthquake rates are driven by processes beyond typical mainshock-aftershock triggering. This definition allows a swarm to be described with a time-varying background rate term in the Epidemic-Type Aftershock Sequence model (Ogata, JASA, 1988), which is already used to produce aftershock forecasts in many regions. A swarm forecast can therefore be computed by adding a temporally varying background rate to the approach used for an aftershock forecast. Currently, the USGS forecasting method estimates this rate from the swarm so far observed and a swarm duration model (Llenos and van der Elst, BSSA, 2019). This procedure is applied using expert judgement when the inclusion of the extra rate improves the model. In general, we argue that the best definition of a swarm is one that helps us best forecast the future and is simple enough for people outside of our scientific sphere to understand.

Session: Advances in Operational and Research Analysis of Earthquake Swarms [Poster Session]

Type: Poster

Date: 5/3/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Andrea

Student Presenter: No

Invited Presentation: 

Authors