Characterizing the Temporal and Spatial Distribution

T41C-0636

Characterizing the Temporal and Spatial Distribution of Earthquake Swarms in the Puerto Rico - Virgin Island Block Francisco J. Hern´andez, Alberto Lopez Venegas, Elizabeth A. Vanacore ´

Introduction

Region 1 (Mona Canyon)

This research identifies event clusters from the Puerto Rico Seismic Network catalog between 2005 and 2016, with magnitudes > 2.3 in the northern Puerto Rico - Virgin Islands block using a novel application of declustering algorithms. Identified clusters were classified into either earthquake swarms or seismic sequences. These results have identified 128 clusters which have been categorized into 11 distinctive regions based on their spatio-temporal distribution. Statistical analysis of these clusters has permitted insight into regional intra- and interplate dynamics.

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Fig 2: Map indicates the events in Region 1; White stars are cluster centers. I

Most events occur to the west of an underwater canyon (Mona Canyon). Recurring period for sequences seems to be 4 years.

Events in a swarm in this region occur slower than events in a sequence (Figure 4).

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University of Puerto Rico - Mayaguez ¨ & Puerto Rico Seismic Network

Region 2A (Amphitheater West) I

I Fig 3: Timeline of events from Jan 2005 - Dec 2016 for region 1. Time interval for sequences seems to be 4 years. I Fig 12: Map indicates the events in Region 2A; White stars are cluster centers. I Fig 4: Progression of events (time vs event number) for different types of clusters. Colors indicate the different type of clusters.

Area contains one sequence triggered by a swarm. Located in the right flank the amphitheater. Most seismically active area of the PR Trench.

Events in this area are particularly close together forming an elliptical area of events

Region 2B (Amphitheater North) I

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Fig 13: Timeline of events from Jan 2005 - Dec 2016 for region 2A. No apparent pattern is observed. Fig 22: Map indicates the events in Region 2B; White stars are cluster centers. I

Fig 14: Progression of events (time vs event number) for different types of clusters. There appears to be no apparent time signature.

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Cluster contains the deepest epicenters are in the Puerto Rico trench. Events seem to be related to the plate boundary.

M5.5 event triggered several swarms in the area (Figure 26). Recurrence cannot be estimated with the current time window.

Region 2C (Amphitheater West) I

Fig 23: In this region, swarms and sequences happened simultaneously. Only 2 major periods of activity separated by 7 years. Fig 32: Map indicates the events in Region 2C; White stars are cluster centers. I Fig 24: Progression of events (time vs event number) for different types of clusters. One extraordinary sequence (shown below) exceeded 100 event.

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The January 13, 2010 Mw6.4 earthquake triggered 7 days of activity and 383 aftershocks. Located in the left flank of the amphitheatre.

Swarms cannot be distinguished from sequences from their time distribution.

Fig 33: Timeline of events from Jan 2005 - Dec 2016 for region 2C. No observable swarm or sequence recurrence.

Fig 34: Long sequence represents the highest recorded magnitude sequence aftershocks.

Fig 1: Identified regions with distinctive earthquake cluster activity. Table below shows the basic statistical characteristics.

Region # Sequences # Swarms # Unknowns Totals NS Trend NS-EW Trend No Trend

R 1 R 2A R 2B R 2C R 3 R 4A R 4B R 5A R 5B R 5C R 6 Totals 6 7 5 4 2 1 5 3 3 13 0 49 10 9 7 7 3 2 8 5 3 22 1 77 0 1 0 0 0 0 0 1 1 0 0 3 16 17 12 11 5 3 13 9 7 35 1 129 3 5 3 6 1 0 7 0 1 8 0 34 2 4 0 0 0 0 1 5 6 16 1 35 11 8 9 5 4 3 5 4 0 11 0 60

Method Three main stages for data processing are: 1. Declustering based on a probabilistic algorithm of Marsan and Lenglin´e (2008). 2. Spatio-temporal classification and reintegration of main shocks to associated clusters. 3. Classification of earthquake clusters as either sequences or swarms and statistical analysis. Some of the limitation of this methodology include small swarms detection ( < 10 events) and events are limited to magnitudes above catalog completion.

Conclusion I I I I

Fig 5: Typical swarm behavior for region one. Some NS trend is observed (Deepens towards south)

Fig 6: Typical sequence for Region 1. Very clear decay pattern, which is outstanding for a magnitude 4 main shock.

Region 4B (Culebra South) The third most active region in the PR Trench I No recurring pattern can be established Fig 7: Map indicates the I Area has strong NS events in Region 4B; White trends among its stars are cluster centers. clusters. I All the clusters in this region have less than 60 events. I Similar time distribution behavior between swarms and sequences.

Fig 15: Typical swarm in region 2A. There is a significant trend both in N-S and E-W

Region 5A (Sombrero East) I

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Fig 8: Timeline of events from Jan 2005 - Dec 2016 for region 4B Fig 17: Map indicates the events in Region 5A; White stars are cluster centers. I

Fig 9: Progression of events (time vs event number) for different types of clusters. No evident difference between swarm or sequence based on time between events.

Fig 16: Typical sequence in region 2A. Sequences also display a NS and EW trend.

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All events exhibit NS and EW coupling. Region exhibits 2 mayor traits: Deep events to the west; Shallower in the east.

Deep events seem to be related to downgoing slab Recurring period for swarms in this region are on average 2 years.

Fig 25: Typical swarm for region 2B. Shows a very weak NS trend that deepens toward south

Fig 26: An extraordinary sequence for a M5.5 event recorded in region 2B, where a total of 164 earthquakes were recorded over 5 days.

Region 5B (Sombrero North) Area contains swarms and sequences that have a strong NS - EW coupling. I 2nd most seismically active, however Fig 27: Map indicates the events in Region 5B; White only 7 clusters stars are cluster centers. detected. I An extraordinary sequence of sequences and swarms was recorded in this area (Fig. 31). I Events in these areas are related to the plate boundary.

Fig 35: Typical swarm for region 2C; with both a NS and an EW trend. Earthquakes seem to deepen toward the southwest.

Region 5C (Sombrero West)

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Fig 18: Timeline of events from Jan 2005 - Dec 2016 for region 5A

Fig 19: Progression of events (time vs event number) for different types of clusters. Two significant swarms in this time period (> 100 events). No clear distinction between swarms and sequences.

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Fig 28: Apparent separation of 1-3 years between clusters. The green symbol is a swarm that was triggered by a sequence. Fig 37: Map indicates the events in Region 5C; These events may be related to the Main Ridge in the PR Trench. I Fig 29: Progression of events for different types of clusters. A cluster marked as unknown is a sequence of sequences and swarms (shown below).

Fig 36: Sequence is the result of the highest magnitude earthquake that has been instrumentally recorded in the PRSN in modern history. It has a NS trend; deeper events in the south

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Most active region in terms of clusters, with 35 clusters in this area. At least 16 clusters contain a strong NS-EW trend that is very well defined in profile.

No observable recurrence pattern for either swarms or sequences. Events in these areas are related to intraplate and accommodation.

Fig 38: Timeline of events from Jan 2005 - Dec 2016 for region 5C. The most active region for earthquake clusters

Fig 39: Progression of events (time vs event number) for different types of clusters. Swarms are usually short lived (less than 50 events) and do not exhibit a pattern.

After data processing it is clear that most earthquake clusters occur in very specific areas, these have been classified into 11 distinct regions. Generally more swarms than sequences are classified in the study region. Most of these clusters exhibit a NS trend, in some cases coupled with a EW trend consistent with regional subduction. While some individual regions have demonstrate possible recurrence, generally the behavior lacks observable patterns and is chaotic.

Future Work & Acknowledgments Future work includes: (1) Extend study to events prior to 2005; (2) Package and publish the method; (3) Test this method in other tectonic settings. We would like to acknowledge the Puerto Rico Seismic Network for all the work they do to make the catalog used in this study. FJH would also like to thank the AGU Tectonophysics Section for sponsoring my AGU Student Travel Grant. References: Marsan, D. and Lenglin´e, O. Extending Earthquakes Reach Through Cascading Science, American Association for the Advancement of Science, 2008, 319, 1076-1079.

Fig 10: Typical swarm for Region 4B. Very clear trend in the N-S direction (depens towards south).

Fig 11: Typical sequence for Region 4B. Very clear trend in the N-S direction (deepens towards south, with some shallow events).

Fig 20: Typical swarm for region 5A. Large clustering of earthquakes around 60km depth. Trend in both NS and EW (with deeper events in the south and east).

Fig 21: Typical sequence for this area M4 mainshock. Earthquake exhibit same trend as swarms. Very clear decay rate.

Fig 30: A typical swarm for region 5B. There is a strong NSEW trend, with events deepening toward the southeast.

Fig 31: An extraordinary sequence of sequences and swarms for region 5B. Events strongly have a very strong trend. A total of 477 events were recorded over a period of 7 days.

Fig 40: Typical swarm in region 5C. Shows strong coupling in NS and EW. This trending is present in most of the swarms of region 5C.

Fig 41: Typical sequence for region 5C. Has a remarkable Omori-law decay, as well as strong coupling in NS and EW. This trend occurs over the majority of sequences in this area.