Mapping the Rivera and Cocos subduction zone

textThe crust and upper mantle seismic structure beneath southwestern Mexico was investigated using several techniques including teleseismic tomography using 3D raytracing, a joint tomographic inversion of teleseismic and regional data that included relocation of regional seismicity, and a P to S converted wave study. The data used in these studies came from a broadband seismic deployment called MARS. The seismic deployment lasted 1.5 years from January 2006 to June 2007 and the stations covered much of Jalisco and Colima states as well as the western part of Michoacan states. At depth less than 50 km, P-wave receiver function images show a clear dipping slow velocity anomaly above a fast velocity layer. The slow anomaly convertor seen in receiver functions is directly above a fast dipping seismic anomaly seen in regional tomography results. The slow velocity with high Vp/Vs ratio is interpreted as a high pore fluid pressure zone within the upper layer of subducting oceanic crust. Regional seismicity was located using the double difference technique and then relocated in a tomography inversion. The seismicity is located very close to the slow dipping boundary to depths of 30-35 km and thus along the plate interface between the subducted and overlying plate. Deeper events are below the slow layer and thus are intraplate. Receiver function results also show a weaker continental Moho signal above the dipping slab that I interpret as a region of mantle serpentinization in the mantle wedge. Inland of the subduction zone, a clear Moho is observed with a maximum thickness of near 42 km although it thins to near 36 km depth towards the north approaching the Tepic-Zacoalco Rift. Using H-K analysis to examine Vp/Vs ratios in the crust, I find a band of very high Vp/Vs along the Jalisco Volcanic lineament as well as beneath the Michoacan-Guanajuato volcanic field. These observations suggest the continental crust is warm and possibly partially molten over broad areas associated with these two magmatic regions and not just locally beneath the volcanoes. I also found seismicity associated with the Jalisco Volcanic Lineament but it was trenchward of the volcanoes. This may indicate extension in this region is part of the explanation for this magmatic activity. At depths below 100 km, the tomography results show clear fast anomalies, about 0.3 km/s faster than the reference model, dipping to the northeast that I interpret as the subducting Rivera and Cocos plates. Tomography models show that the Rivera slab is dipping much steeper than the Cocos plate at depth. Below 150 km depth, the Rivera plate shows an almost vertical dip supporting the interpretation that the slab has steepened through time beneath Jalisco leading to a coastward migration of young volcanism with mixed geochemical signatures. The location of the young volcanism of the Jalisco Volcanic Lineament is just at the edge of the steeply dipping slab seen in the tomography. The magmatism is thus likely a nascent arc. The models also display evidence of a gap between the Rivera and Cocos plates that increases in width with depth marking the boundary between the two plates. The gap lies just to the west of Colima graben and allows asthenosphere to rise above the plates feeding Colima volcano. Another interesting finding from this study is a possibility of a slab tear along the western edge of the Cocos plate at a depth of about 50 km extending 60 km horizontally. The tear is coincident with a lack of seismicity in this region although there are events below and above the tear.Geological Science

SEIZMIČKO MODELIRANJE NA PODRUČJU OTOKA SUMATRE I NJEGOVE OKOLICE, INDONEZIJA, POMOĆU P-VALNE SEIZMIČKE TOMOGRAFIJE LOKALNIH I REGIONALNIH POTRESA

Sumatra Island and its surroundings, Indonesia, are one of the most active tectonics in the world. The Aceh-Andaman earthquake, one of the most destructive earthquakes in the world, occurred there. It has attracted many earth scientists to apply various methods, including seismic tomography, to understand the island’s subsurface structure and tectonic system. This study is the first to delineate subsurface imaging beneath the island and its surroundings using a local-regional earthquake catalogue from the Indonesian Agency for Meteorology, Climatology, and Geophysics (BMKG) seismicnetwork. The tomographic imaging of P-wave (Vp) conducted in this study has successfully delineated subduction slabs (high Vp), partial melting zones (low Vp), volcanic arcs (low Vp), and Sumatran Fault zones (low Vp). The relationship between the subduction zone and the volcanic arc on the island can be seen on several vertical sections where a partial melting zone occurs at a depth of about 100 km, which functions as magma feeding for some volcanoes on the island. The oceanic slab model also exhibits a more pronounced and steeper slope towards the southern regions of Sumatra Island, possibly attributed to the slab’s aging process in that direction. The results highlight the importance of the BMKG seismic network in imaging local-regional subsurface structures beneath Indonesia’s archipelago, especially for the main islands such as Sumatra.Otok Sumatra i njegova okolica, Indonezija, jedno su od najaktivnijih tektonskih područja na svijetu. Tamo se dogodio potres Aceh-Andaman, jedan od najrazornijih potresa na svijetu. Privukao je mnoge znanstvenike koji su u svojim istraživanjima primijenili različite metode, uključujući seizmičku tomografiju, kako bi razumjeli podzemnu strukturu i tektonski sustav otoka. Ova studija prva je koja prikazuje model podzemlja ispod otoka i njegove okolice koristeći se lokalno-regionalnim katalogom potresa iz seizmičke mreže Indonezijske agencije za meteorologiju, klimatologiju i geofiziku (BMKG). Tomografski model brzine P-valova (Vp), uspješno je razgraničio subduciranu ploču (velika brzina P-valova), zonu djelomičnoga taljenja (mali Vp), vulkanski luk (mali Vp) i rasjedne zone Sumatre (mali Vp). Odnos između subdukcijske zone i vulkanskoga luka na otoku može se vidjeti na nekoliko vertikalnih presjeka gdje se na dubini od oko 100 km javlja zona djelomičnoga taljenja koja služi kao izvor magme za neke vulkane na otoku. Model oceanske subducirane ploče također pokazuje izraženiji i strmiji nagib prema južnim regijama otoka Sumatre, što se vjerojatno može pripisati procesu starenja ploče u tome smjeru. Rezultati naglašavaju važnost BMKG seizmičke mreže u identifikaciji lokalno-regionalnih podzemnih struktura ispod indonezijskoga arhipelaga, posebno za glavne otoke kao što je Sumatra

Preliminary study results of crustal structure beneath Mount Merapi, Central Java, Indonesia

International audienceIn this study, we put an effort to estimate crustal depth and image crustal structure beneath Merapi volcano by employing multicomponent analysis popularly known as Receiver Function technique. We collected a series of waveforms from teleseismic events recorded from October 2013 to mid-April 2015 at 53 stations as a part of DOMERAPI project. We processed selected seismograms by simple deconvolution process between radial and vertical components to estimate the depth of Moho discontinuity beneath the volcano. Current results show complex structure beneath the volcano and a relatively potential Moho depth at about 30 km, which becomes shallower to the North at about 23 km. Stations located at Southern and Northern area show potential low velocity zone though a velocity modelling is necessary to confirm its depth and how low the velocity is