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A new method to identify earthquake swarms applied to seismicity near the San Jacinto Fault, California

Map view of the detected swarm (solid circle) events in the San Jacinto Fault Zone.

Map view of the detected swarm (solid circle) events in the San Jacinto Fault Zone.

TitleA new method to identify earthquake swarms applied to seismicity near the San Jacinto Fault, California
Publication TypeJournal Article
Year of Publication2016
AuthorsZhang Q., Shearer PM
JournalGeophysical Journal International
Date Published2016/05
Type of ArticleArticle
ISBN Number0956-540X
Accession NumberWOS:000375092400023
Keywordsaftershocks; clusters; damage rheology; Earthquake interaction, forecasting, and prediction; heat-flow; models; occurrences; patterns; Seismicity and; sequence; southern california; statistical seismology; tectonics; tremor

Understanding earthquake clustering in space and time is important but also challenging because of complexities in earthquake patterns and the large and diverse nature of earthquake catalogues. Swarms are of particular interest because they likely result from physical changes in the crust, such as slow slip or fluid flow. Both swarms and clusters resulting from aftershock sequences can span a wide range of spatial and temporal scales. Here we test and implement a new method to identify seismicity clusters of varying sizes and discriminate them from randomly occurring background seismicity. Our method searches for the closest neighbouring earthquakes in space and time and compares the number of neighbours to the background events in larger space/time windows. Applying our method to California's San Jacinto Fault Zone (SJFZ), we find a total of 89 swarm-like groups. These groups range in size from 0.14 to 7.23 km and last from 15 min to 22 d. The most striking spatial pattern is the larger fraction of swarms at the northern and southern ends of the SJFZ than its central segment, which may be related to more normal-faulting events at the two ends. In order to explore possible driving mechanisms, we study the spatial migration of events in swarms containing at least 20 events by fitting with both linear and diffusion migration models. Our results suggest that SJFZ swarms are better explained by fluid flow because their estimated linear migration velocities are far smaller than those of typical creep events while large values of best-fitting hydraulic diffusivity are found.


We have developed a new method to search for swarms by comparing the number of neighbours to the number of background events in scalable spatiotemporal windows. Applying the method in the San Jacinto Fault Zone, we find swarms at a wide range of spatiotemporal scales. The SJFZ contains strong spatial and temporal variations of swarm events: the two ends of the SJFZ are dominated by more and larger swarms and the years between 1991 and 1998 host fewer swarms than the decades before and after. In general more swarms are found in normal faulting regions. While most of the estimated swarm linear migration speeds are lower than those of typical creep or slow slip events, large values of estimated hydraulic diffusivity are found near the southern end of the SJFZ (Salton Sea), which indicates that these swarms could be more correlated with the local fluid flow.

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