by Esteban J. Chaves (PhD candidate at U.C. Santa Cruz, OVSICORI-UNA) and
Susan Y. Schwartz, Ph.D. (U.C. Santa Cruz)
A moderate magnitude (MW = 6.5) but powerful earthquake struck the Pacific coast of Costa Rica on Sunday night (02:28:22 UTC time) generating damage to buildings and houses near to the mainshock epicenter (Figure 1). Unfortunately, 2 people are reported dead due to heart attacks during the strong ground shaking generated by this earthquake. High Mercalli intensities (VII-VIII) were also reported by locals close to the epicenter, where the estimated maximum acceleration was close to 18% g. Near the capital of San José, Mercalli intensities were between IV and V and ground accelerations reached up to 6% g. Furthermore, a M=5.1 aftershock hit the coast 4 minutes after the mainshock, increasing the damage to the structures affected by the main event and alarming the population. Since then, hundreds of aftershocks have been recorded by the seismic network of the Volcanological and Seismological Observatory of Costa Rica at Universidad Nacional (OVSICORI-UNA).
A very active seismic region
The subduction of the Cocos plate underneath the Caribbean plate (plate rate is estimated to be ~85 mm/yr) along the Pacific coast of Costa Rica is one of the principal mechanisms responsible for generating large and destructive earthquakes in the country. An example of this is the recent September 5, 2012, Mw=7.6 Nicoya Peninsula Earthquake (Figure 1), with an epicenter located about 50 km to the northwest of yesterday’s temblor. The central Pacific subduction zone is one of the most seismically active regions in the country, producing tens of small magnitude events daily.
Ancient seamounts (typically extinct seafloor volcanoes) that stud the Cocos plate are subducted (shoved under) Costa Rica. This appears to be one of the main characteristics of the area, which small high-friction patches getting stuck and then slipping suddenly in moderate magnitude earthquakes like the event that hit Esterillos on Sunday night.
In a fascinating discovery, seismologists from OVSICORI-UNA, among them, Dr. Marino Protti, have recognized that moderate-magnitude events in the area tend to occur as doublets; temporally and spatially close earthquakes with similar magnitudes (Table 1), with each of the main events presenting a characteristic mainshock-aftershock sequence. Although the reason for these earthquake ‘twinning’ is unknown, it would not be a surprise to observe an event with similar magnitude in the subsequent days.
The magnitude, depth and style of faulting
The moment magnitude (MW) is the most accurate measurement of an earthquake size, it is determined by modeling or analyzing the long period seismic waves generated by an earthquake, which are least affected by heterogeneities of the earth structure. The earthquake focal mechanism, which describes the style of faulting of the earthquake is also computed using the long period seismic waves. Figure 2 shows the modeling of the long-period seismic wavefield using the permanent local and regional seismic stations provided by the OVSICORI-UNA seismic network. The moment magnitude estimated is MW= 6.5, with the hypocenter located at 25 km depth. The focal mechanism is consistent with thrust faulting, a compression system that is generated by the diving of the oceanic Cocos plate underneath the continental Caribbean plate.