"Gradual inflation began in November 1986 after almost two years of insignificant ground deformation. Extensive tilt measurements in late October 1986 and late April 1987, and several measurements in late February and early March 1987 show that the inflation affected the same area as it did in 1976-82. The observed tilt agrees reasonably with a single point source model. Tilt measurements in late February and early March 1987 indicated a maximum uplift of 17.8 cm since late October 1986 with a point source at 2.5 km depth. The observations in late April 1987 (figure 10) indicated a maximum uplift of 22.0 cm and a point source depth of 2.7 km.

Figure 10. Observed tilt at eleven optical level tilt stations near Krafla during the November 1986-March 1987 inflation. The open circle S of Leirhnjúkur shows the location of a single point source that can explain most of the observed ground deformation if source depth is 2.5-2.7 km. The dashed line marks the caldera boundary.

"Extensive distance measurements in the Krafla region in late February and early March 1987 were compared with measurements of March 1986. Lines inside the Krafla caldera had increased up to 70 ppm in length. Comparison with a point source model showed best agreement if the point source was at 2.7 km depth and the central uplift was 19.5 cm.

"Thus, both tilt measurements and distance measurements agree on the depth of a point source and the maximum uplift. Geographic coordinates of the indicated point source are about 65°42.8'N, 16°47.6'W if distance measurements are considered but about 0.6 km farther W if tilt measurements are considered. This discrepancy is apparently caused by systematic anomalies in the azimuth of tilt at several stations, making the coordinates obtained from distance measurements more reliable. The recording tiltmeters at the Krafla power station (figure 11) indicated a constant rate of inflation from early November 1986 until the end of March 1987, when inflation ceased. Seismic activity in the Krafla region increased slightly during this inflation period and reached its highest value in late February and March, 1987."

Figure 11. North component of tilt at the Krafla power station from mid-1983 to early May 1987. Up on the graph is uplift towards the N. The electronic tiltmeter (upper curve) shows high noise level during winter and minima in July or August, both caused by surface strain of thermal origin. The record of the water tube tiltmeter (lower curve) is less disturbed by thermal strain.

Further References. Tryggvason, E., 1986, Vertical ground movement in the Krafla region 1977-1986: Nordic Volcanological Institute, Univ of Iceland, no. 8602, 31 p.

Tryggvason, E., 1987, Myvatn lake level observations 1984-1986 and ground deformation during a Krafla eruption: JVGR, v. 31, p. 131-138.

Geological Summary. The Krafla central volcano, located NE of Myvatn lake, is a topographically indistinct 10-km-wide caldera that is cut by a N-S-trending fissure system. Eruption of a rhyolitic welded tuff about 100,000 years ago was associated with formation of the caldera. Krafla has been the source of many rifting and eruptive events during the Holocene, including two in historical time, during 1724-29 and 1975-84. The prominent Hverfjall and Ludent tuff rings east of Myvatn were erupted along the 100-km-long fissure system, which extends as far as the north coast of Iceland. Iceland's renowned Myvatn lake formed during the eruption of the older Laxarhraun lava flow from the Ketildyngja shield volcano of the Fremrinamur volcanic system about 3800 years before present (BP); its present shape is constrained by the roughly 2000 years BP younger Laxarhraun lava flow from the Krafla volcanic system. The abundant pseudocraters that form a prominent part of the Myvatn landscape were created when the younger Laxarhraun lava flow entered the lake.

Information Contacts: E. Tryggvason, NVI.