Magnitude-6.4 Indios, Puerto Rico Earthquake (January 7, 2020) and Associated Earthquake Sequence

January 13, 2020 Update [14:30 EST]

This summary includes updated information on the ongoing earthquake sequence along the southern coast of Puerto Rico.

Executive Summary • This earthquake sequence is consistent with expectations of seismicity in the region. Puerto Rico is tectonically active, and infrequent naturally occurring large earthquakes are expected. • It is possible that a larger earthquake will occur as part of this sequence. As of 3:40AM EST on January 13th, the U.S. Geological Survey (USGS) forecasts based on previously observed seismicity coupled with details of the ongoing sequence estimate a 4% chance of a magnitude greater than 6.4 earthquake occurring in the next week (ending 01/20). Potential aftershock scenarios for the next 30 days are available here. • USGS seismologists are continually analyzing the behavior of this ongoing sequence and the statistical approaches we are using for aftershock forecasts. The 01-13-2020 forecast reflects an updated approach informed by our observations as we continue to seek to deliver the best available science. • Earthquake locations and faulting mechanisms indicate that multiple faults have been active during this sequence. • Much of the building stock in the region is vulnerable to earthquake shaking. As such, economic losses resulting from the M 6.4 earthquake on Jan. 7th were modeled to exceed $100M. These were in agreement with more detailed FEMA HAZUS modeling estimates (losses > $800M). Subsequent aftershocks, like the Jan. 11th M 5.9 event, will likely increase these losses. • Earthquake preparedness is important for all citizens living in earthquake-prone regions. • The USGS partners with the Puerto Rico Seismic Network, operated by the University of Puerto Rico Mayaguez, to carefully monitor ongoing seismicity in real time. These groups have deployed an additional 5 stand-alone and 6 real-time seismic systems along the coast between Ponce and Bahia de Guanica. Observations from aftershock monitoring equipment will improve our ability to characterize and forecast earthquakes and better constrain seismic hazard.

Figure 1: The figure above shows the distribution of relocated seismicity southwest of Puerto Rico in December 2019 and January 2020. Earthquake symbols are sized by magnitude and colored according to the time of the earthquake relative to December 28, 2019.

Seismicity Overview Over the past several weeks, hundreds of small earthquakes have occurred offshore of southwest Puerto Rico, beginning in earnest with a M 4.7 earthquake late on December 28. Since the M 4.7 event, 66 earthquakes of magnitude 4 or larger have occurred (55 since current mainshock), nine of which were M 5+ (6 since mainshock). At the magnitude 3 level, at which earthquakes may be felt, our records are currently incomplete, though to date we have recorded over 200 such events as part of this sequence (160 since the current mainshock). The proximity of these events to Puerto Rico, and their shallow depth, mean that dozens of these events have likely been felt on land.

Figure 2: Timeline of recent earthquake sequence in Puerto Rico during December 2019 and early January 2020. Each symbol represents an earthquake sized by magnitude.

Tectonics Overview Tectonics in Puerto Rico (Figure 3, below) are dominated by the convergence between the North America and Caribbean plates, with the island being squeezed between the two. The convergence between these two plates occurs at a rate of about 20 mm/yr (about half the rate that fingernails grow) in a east-northeast to west-southwest direction. To the north of Puerto Rico, North America subducts beneath the Caribbean plate along the Puerto Rico trench. To the south of the island, and south of this sequence, Caribbean plate lithosphere subducts beneath Puerto Rico at the Muertos Trough. Several recent studies have documented upper plate extension surrounding Puerto Rico – both to the east, in the Anagada passage, and to the west in the Mona passage, such that Puerto Rico appears to be moving slowly westward with respect to the Caribbean, and southern Puerto Rico moves slowly to the southwest with respect to the north of the island.

Figure 3: 20th and 21st century seismicity and plate boundaries in the northeastern Caribbean. Historic earthquakes (circles) are colored by depth and sized by magnitude. The red star marks the location of the January 7, 2020, M 6.4 earthquake.

Earthquakes in this sequence are occurring in the offshore deformation zone bound by the Punta Montalva Fault on land and the Guayanilla Canyon offshore. Earthquake locations and focal mechanism solutions (Figure 4 below), which provide information about the type of faulting involved in an earthquake, indicate that multiple fault structures have been activated in this sequence – both strike-slip, and normal faults. This is an intraplate setting - that is, within the crust of the Caribbean plate, rather than on the nearby plate boundary.

Large intraplate earthquakes in this region are not unexpected, though they occur infrequently, particularly given the slow convergence between the major tectonic plates and the even slower documented upper plate extension surrounding Puerto Rico.

Figure 4: The figure above shows the distribution of relocated sequence seismicity overlain with focal mechanism solutions (beachballs), indicating the type of faulting in each event. Earthquake symbols are sized by magnitude and colored according to the time of the earthquake relative to December 28, 2019.

Earthquake Relocations Single event earthquake locations are improved upon through a multiple event relocation technique called hypocentroidal decomposition. Multiple event relocation techniques improve the accuracy of both absolute and relative locations over standard methods. Uncertainty estimates for these events are estimated to be about 2-3 km laterally and in depth. Observations from rapidly deployed seismic stations can be tied into sequence relocations, further improving the accuracy of relocations. Earthquake depths range from ~10-20 km below mean sea-level.

Aftershock Forecast Earthquake forecasts are created using a statistical analysis based on past earthquakes and are presented in terms of probabilities of earthquakes of a given size occurring. The earthquake forecast continues to be updated based on observations of the rate and size of earthquakes in in this sequence. Details of the current aftershock forecast can be found here. It is important to note that it is not possible to predict earthquakes, and earthquake forecasts are presented as probabilities. Therefore, it is important to always remain prepared for more shaking.

Rapid Instrumentation Deployment In response to the M6.4 Indios, Puerto Rico earthquake, the USGS and PRSN rapidly deployed seismic instrumentation to record near-source aftershock ground motions from the evolving sequence. As of January 13, PRSN has installed 5 stand-alone stations and the USGS has installed 6 real-time stations (Figure 5 below). Seismic data from the USGS deployment will stream in real-time to the USGS ANSS and PRSN for improved monitoring of the evolving earthquake sequence and the threat and hazard posed to the people and property in Puerto Rico from earthquakes and tsunami.

Figure 5: The map above shoes seismicity (circles and stars) and seismic monitoring stations (triangles). Planned seismic stations are shown with transparency.

The portable seismic station deployment will supplement the permanent earthquake monitoring operations of the USGS Advanced National Seismic System (ANSS) and Puerto Rico Seismic Network (PRSN). In response to damage related to Hurricane Maria, the USGS and PRSN have upgraded and hardened 11 seismic stations across Puerto Rico. Within the coming months an additional 14 will also be upgraded. The data from these upgraded systems is shared in real- time between PRSN, USGS NEIC and the NOAA tsunami warning system.

Near-Source Ground Motions Near-source (<20 km) ground motion recordings (Figure 6 below) of the Puerto Rico earthquake sequence provide a rare opportunity to more accurately model the seismic hazard posed by the poorly understood Muertos trough subduction zone, and related faults in the region. The USGS rapid aftershock station deployed in SW Puerto Rico is the closest instrumental recording of the January 11, 2020 M 5.9 earthquake. The deployment is timely since recordings of near-source ground motion will be used to evaluate ground motion models that are input to planned updates to the Puerto Rico Seismic Hazard Model (PRSHM). The PRSHM will be used as input to future updates of the US building codes in this region.

Figure 6: Instrumental and Did You Feel It? ground motion observations of the January 11, 2020 M 5.9 earthquake. Rapidly deployed aftershock station (GS.PR01) is the closest station to the earthquake at a hypocentral distance of 22km.

Built Stock Vulnerability and Impact Unlike the continental United States, the building stock in Puerto Rico is composed of a high percentage (> 90%) of vulnerable construction such as reinforced concrete frames and unreinforced masonry buildings. Earthquake-resistant wood frame construction, which is most common in the continental US, represent only 5% of the total building stock. Given the extent of vulnerable construction of this region, the USGS PAGER system estimated an orange alert (direct shaking related losses exceeding 100 million USD, and a potential for some casualties) from this earthquake.

Ongoing USGS Response and Next Steps • In coordination with PRSN, the USGS have deployed six aftershock monitoring stations near the source zone of these earthquakes. Observations from these stations improve location estimates of these events. Ground-motion observations from these stations can be incorporated into ground-motion prediction equations, which are fundamental to improving regional seismic hazard assessment and subsequently building code. Strong ground motions from these rapidly deployed stations have already captured a M 5.9 aftershock. • USGS personnel are in the field to assess ground failure in the form of coseismic liquefaction and landslides. • USGS personnel are also on site with FEMA at the Joint Field Office to provide scientific and communications support during earthquake response efforts.

Useful Links to USGS Impact Products and Information USGS Top Story on the Indios, Puerto Rico Earthquake Earthquake Sequence https://www.usgs.gov/news/magnitude-64-earthquake-puerto-rico M 6.4 Event https://earthquake.usgs.gov/earthquakes/eventpage/us70006vll/executive Impact Assessment (PAGER) https://earthquake.usgs.gov/earthquakes/eventpage/us70006vll/pager Aftershock Forecast https://earthquake.usgs.gov/earthquakes/eventpage/us70006vll/oaf/commentary Ground Failure Modeling https://earthquake.usgs.gov/earthquakes/eventpage/us70006vll/ground- failure/summary Caribbean Tectonic Information and Summary https://earthquake.usgs.gov/earthquakes/eventpage/us70006vll/region-info