Seismic imaging of the shallow crust beneath the Krafla central volcano, NE Iceland

Abstract

We studied the seismic velocity structure beneath the Krafla central volcano, NE Iceland, by performing 3‐D tomographic inversions of 1453 earthquakes recorded by a temporary local seismic network between 2009 and 2012. The seismicity is concentrated primarily around the Leirhnjúkur geothermal field near the center of the Krafla caldera. To obtain robust velocity models, we incorporated active seismic data from previous surveys. The Krafla central volcano has a relatively complex velocity structure with higher P wave velocities (Vp) underneath regions of higher topographic relief and two distinct low‐Vp anomalies beneath the Leirhnjúkur geothermal field. The latter match well with two attenuating bodies inferred from S wave shadows during the Krafla rifting episode of 1974–1985. Within the Leirhnjúkur geothermalreservoir, we resolved a shallow (−0.5 to 0.5 km below sea level; bsl) region with low‐Vp/Vs values and a deeper (0.5–1.5 km bsl) high‐Vp/Vs zone. We interpret the difference in the velocity ratios of the two zones to be caused by higher rock porosities and crack densities in the shallow region and lower porosities and crack densities in the deeper region. A strong low‐Vp/Vs anomaly underlies these zones, where a superheated steam zone within felsic rock overlies rhyolitic melt.

Focal mechanisms and size distribution of earthquakes beneath the Krafla central volcano, NE Iceland

“…Thus, we believe that this event is also located close to an underlying melt zone. This is supported by tomographic images [Schuler et al, 2015]. A source dominated by near-vertical single force or a vertical-CLVD mechanism might produce only dilatational first motions as well at the stations, but this presumes that there is a small region at the surface where we could have recorded compressional first motions.…”

“…Arrival-time picks of events with high signal-to-noise ratio (SNR) were manually refined. Hypocenter locations were taken from Schuler et al [2015], who determined them by a 3D tomographic inversion. Improved relative locations ( Figure 1) are achieved by double-difference calculations [Waldhauser and Ellsworth, 2000] using the 3D velocity model.…”

“…Whether the drilled melt in IDDP-1 is part of a large magma chamber has not been fully determined. Seismic studies [e.g., Einarsson, 1978;Brandsdóttir and Menke, 1992;Schuler et al, 2015] as well as joint magnetotelluric and transient electromagnetic soundings [e.g., Árnason et al, 2009] point towards the presence of a larger heat source emanating from multiple shallow dikes, a larger melt pocket cooling at shallow depth, and/or heat being supplied from a depth further below.…”

“…Seismic data were acquired initially to image the shallow magma chamber [Schuler et al, 2015]. Here, we investigate the earthquake seismicity and source mechanisms close to the melt-rock interface and in the overlying geothermal field to better understand the c 2016 American Geophysical Union.…”

“…Seismicity ceased after the rifting episode and has been mostly confined to two high-temperature geothermal systems [e.g., Arnott and Foulger , 1994a;Schuler et al, 2015], where faults and fissures facilitate the transfer of hot geothermal fluids to the surface. The Bjarnarflag-Námafjall field is located outside whereas the Krafla-Leirhnjúkur field is located inside the caldera.…”