Earthquake swarms and slow slip on a sliver fault in the Mexican subduction zone Shannon L. Fasolaa,1, Michael R. Brudzinskia, Stephen G. Holtkampb, Shannon E. Grahamc, and Enrique Cabral-Canod aDepartment of Geology and Environmental Earth Science, Miami University, Oxford, OH 45056; bGeophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775; cDepartment of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA 02467; and dInstituto de Geofísica, Universidad Nacional Autónoma de México, 04510 Ciudad de México, México Edited by John Vidale, University of Southern California, and approved February 25, 2019 (received for review August 24, 2018) The Mexican subduction zone is an ideal location for studying a large amount of inland seismicity, including a band of intense subduction processes due to the short trench-to-coast distances seismicity occurring ∼50 km inland from the trench (Fig. 1A) that bring broad portions of the seismogenic and transition zones (19). SSEs and tectonic tremor have been well documented of the plate interface inland. Using a recently generated seismicity further inland from this seismicity band (Fig. 1A) (19, 24–28), catalog from a local network in Oaxaca, we identified 20 swarms suggesting this band marks the frictional transition on the plate of earthquakes (M < 5) from 2006 to 2012. Swarms outline what interface from velocity weakening to velocity strengthening (6). appears to be a steeply dipping structure in the overriding plate, Other studies have used shallow-thrust earthquakes to define the indicative of an origin other than the plate interface. This steeply downdip limit of the seismogenic zone in southern Mexico (29– dipping structure corresponds to the northern boundary of the 31), and this corresponds with where the seismicity band occurs Xolapa terrane. In addition, we observed an interesting character- (Fig. 1A). However, these studies pointed out that the depth of istic of slow slip events (SSEs) where they showed a shift from ∼25 km is nearly half of the typical depth for most subduction trenchward motion toward an along-strike direction at coastal zones (45 ± 5 km and up to 55 km; refs. 32 and 33). GPS sites. A majority of the swarms were found to correspond in The narrow seismogenic zone is thought to be a result of the time to the along-strike shift. We propose that swarms and SSEs geometry of the subducting plate and the fact that the overriding are occurring on a sliver fault that allows the oblique convergence plate is oceanic in origin and only ∼45 km thick, half the litho- to be partitioned into trench-perpendicular motion on the subduc- spheric thickness in South America, where the seismogenic zone EARTH, ATMOSPHERIC, tion interface and trench-parallel motion on the sliver fault. The AND PLANETARY SCIENCES is almost twice as deep (30, 34). The overriding plate in this resistivity structure surrounding the sliver fault suggests that SSEs region consists of different lithologic terranes, mainly the Mixteco, and swarms of earthquakes occur due to high fluid content in the Oaxaca, and Xolapa terranes (Fig. 1B). In fact, the Xolapa terrane fault zone. We propose that the sliver fault provides a natural – pathway for buoyant fluids attempting to migrate upward after overlies the band of intense seismicity, and the Xolapa Mixteco being released from the downgoing plate. Thus, sliver faults could boundary is consistent with its inland extent. The Xolapa terrane be responsible for the downdip end of the seismogenic zone by consists of undeformed plutonic intrusions and the highly deformed creating drier conditions on the subduction interface trenchward of Xolapa Metamorphic Complex, which by some has been proposed the sliver fault, promoting fast-slip seismogenic rupture behavior. to be of accretionary origin (35, 36). Some studies have proposed that faults and shear zones occur along portions of the northern – swarm seismicity | slow slip | sliver faults | subduction zone | Mexico boundary of the Xolapa terrane (22, 35, 37 39). In fact, an in- teresting characteristic of slip has been observed in the Mexican subduction zone, where coastal GPS sites have shown a small any of the world’s most devastating earthquakes have been Mruptures of the seismogenic zone along the subduction but significant along-strike motion during the trenchward interface (1), highlighting the importance of studying fault slip in subduction zones. Within the last couple decades, slow slip Significance events (SSEs) have been observed to occur regularly downdip of the seismogenic zone, where there is a transition in frictional We provide an interpretation for the interaction of crustal faults, behavior from stick slip to stable sliding, as a way of releasing clusters of earthquakes (swarms), and slow slip (a slower form of accumulated strain (2–4). SSEs typically last weeks to months fault rupture) in southern Mexico. Our observations indicate that and are often accompanied by tectonic (nonvolcanic) tremor that swarms and slow slip are occurring on a sliver fault in the over- appears to represent a swarm of small, low-frequency earth- riding plate that allows the oblique plate convergence to be quakes (2, 5–8). There have also been multiple cases where SSEs separated into a trench-perpendicular and -parallel motion on the have been suggested to promote swarms of traditional earth- subduction interface and sliver fault, respectively. We propose quakes (9–14). Earthquake swarms are empirically defined as an the sliver fault provides a natural pathway for buoyant fluids attempting to migrate upward after being released from the increase in seismicity rate above the background rate without a downgoing plate. Thus, sliver faults could be responsible for the clear triggering mainshock, having an abrupt beginning and end, downdip end of the seismogenic zone by creating drier condi- and having several earthquakes of magnitudes similar to that of tions on the subduction interface trenchward of the sliver fault. the largest-magnitude event (15–17). When considered as a single event, swarms have durations and cumulative moment Author contributions: M.R.B. and S.G.H. designed research; E.C.-C. and M.R.B. led data similar to SSEs, as well as similar along-strike propagation ve- collection; S.L.F., M.R.B., S.G.H., S.E.G., and E.C.-C. performed research; S.L.F., M.R.B., locities (∼10 km/d; ref. 18). S.G.H., and S.E.G. analyzed data; and S.L.F. and M.R.B. wrote the paper. The Mexican subduction zone is a natural laboratory for The authors declare no conflict of interest. studying these slip processes due to the relatively short (∼50 km) This article is a PNAS Direct Submission. trench-to-coast distance that brings broad portions of the seis- Published under the PNAS license. mogenic and transition zones ∼250 km inland, providing a 1To whom correspondence should be addressed. Email: [email protected]. chance to pursue detailed studies of the plate interface. A re- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. cently developed catalog of seismicity in the Oaxaca region, re- 1073/pnas.1814205116/-/DCSupplemental. ferred to as the Oaxaca Network (OXNET) catalog, has revealed www.pnas.org/cgi/doi/10.1073/pnas.1814205116 PNAS Latest Articles | 1of9 Downloaded by guest on October 1, 2021 A promoting episodic slow slip (43, 44) and dynamic triggering of -100˚ -99˚ -98˚ -97˚ -96˚ tectonic tremor (45). The fate of fluids trenchward of the USL remains debated. Zones of high conductivity near the coast (Fig. Guerrero 1B) suggest that fluids are released from the slab updip of the 18˚ USL (21, 22) which could impact the occurrence of seismicity. In Oaxaca, Colella et al. (46) suggest that trapped fluids in the low- permeability seal along the plate interface (47–49) may be re- leased by SSEs which then flow upward along the plate interface Oaxaca generating seismicity, as a swarm of earthquakes was found to correspond to an SSE. The migrating fluids would raise pore- 17˚ fluid pressures and lower the effective stresses over a period of time to promote swarm seismicity. While earthquake swarms are not as well-studied in Mexico, the band of intense seis- micity below the Xolapa terrane appears in clusters near the downdip edge of the seismogenic zone, indicating swarms may 16˚ be prevalent (19). In this study, we aim to better understand the shallow portion of the megathrust as it relates to various slip processes (i.e., seismicity, swarms, and slow slip), upper crustal structures, and fluids. We focus on the Oaxaca region (Fig. 1), where we can 15˚ 50 km utilize the OXNET catalog of local seismicity and continuous GPS (cGPS) network in place (24–26). We seek to identify swarms of earthquakes, their relationship to SSEs, and whether B they occur along the plate interface as in most subduction zones -100˚ -99˚ -98˚ -97˚ -96˚ (9–14) or along forearc crustal faults (50–52). GMP Methods 18˚ OXAC Earthquake swarms are generated from the OXNET catalog of local seismicity, which spans from June 2006 to March 2012 and utilizes the local and per- CUICA manent networks of broadband seismometers (19). Swarms are identified LVF using the swarm-characterization method of Holtkamp and Brudzinski (18). MIX OXNC Every burst of seismicity, the apparent line of vertical dots in a magnitude vs. time plot, was inspected for the three empirical traits of swarm sequences: 17˚ no clear triggering mainshock, many events near the maximum size, and a OMTP OAX2 XOL relatively constant seismicity rate throughout the sequence (SI Appendix, OXEC Fig. S2). For slow slip comparison with the swarms, GPS data were obtained OXMA OXGU OAX from 11 cGPS stations in Oaxaca spanning the duration of the OXNET cat- PINO alog. GPS data were processed with Release 6.1 of the GIPSY software suite OXLP from the Jet Propulsion Laboratory.