Additional Report for Northern Reykjanes Ridge

Northern Reykjanes Ridge

N Atlantic Ocean


Index of Monthly Reports

Reports are organized chronologically and indexed below by Month/Year (Publication Volume:Number), and include a one-line summary. Click on the index link or scroll down to read the reports.

05/1989 (SEAN 14:05) Possible submarine volcanism

08/1990 (BGVN 15:08) Strong seismicity SW of May 1989 swarm; possible new lava in 1989 swarm area

10/1990 (BGVN 15:10) Strong seismicity; turbid water and hydrothermal area but no new lava detected

12/1990 (BGVN 15:12) About 30 events near 61.7°N

08/1992 (BGVN 17:08) Young lava flow identified from submersible


Contents of Monthly Reports

All information contained in these reports is preliminary and subject to change.

All times are local (= UTC)

05/1989 (SEAN 14:05) Possible submarine volcanism

An earthquake swarm began shortly before midnight on 21 May, 500-550 km SW of the tip of Iceland's Reykjanes Peninsula. The most vigorous activity occurred between about 0200 and 0600 on [22] May, when 45 events were recorded on seismographs in SW Iceland (figure 1). Preliminary WWSSN data yielded locations for 22 events (M >= about 4.5) in the range 59.9-60.1°N, 29.6-29.9°W. The largest (4.9 mb/4.6 Ms) and best-located, on 22 May at 0202, was centered at 59.94°N, 29.90°W. The events were shallow, but no precise focal depths could be determined. The ridge crest in the area is at roughly 1 km depth. Seismologists noted that the long-period character of the seismicity suggested that it was related to magma movement, associated with an intrusive episode or an eruption onto the sea floor. No T-Phase signals were detected.

Figure 1. Number of earthquakes/day along the Reykjanes Ridge near 60°N, 21 May-15 June 1989. Courtesy of Páll Einarsson.

Low-level seismicity continued for the next 2.5 weeks, then a new swarm began on 11 June, when 29 earthquakes were detected on SW Iceland seismographs. On 13 June, a reconaissance aircraft dropped sonobuoys into the ocean and took thermal profiles (with XBT's) in the epicentral area. No thermal anomalies were detected. The sonobuoys recorded 140 discrete events during 4 hours of monitoring, and locations were refined to 59.73°N, 29.53°W ± 3 km, on the ridge axis. Although seismic stations in Iceland had recorded more than 100 shocks since activity resumed on 11 June, none of the events during the aerial monitoring were strong enough to be detected in Iceland. Further flights are anticipated that may better define the location and character of the activity.

Information Contacts: R. Stefánsson, Icelandic Meteorological Office; P. Einarsson, Univ of Iceland; R. Holcomb, Univ of Washington; B. Presgrave, NEIC; P. Vogt, Naval Research Laboratory.

08/1990 (BGVN 15:08) Strong seismicity SW of May 1989 swarm; possible new lava in 1989 swarm area

A 9 September earthquake swarm has been located on the Reykjanes Ridge, site of a large number of earlier swarms, including one in May-June 1989 (Nishimura and others, 1989, and 14:5). Bergman and Solomon (1990) made a detailed analysis of spatial, temporal, and other characteristics of Mid-Atlantic Ridge earthquake swarms detected by worldwide seismic networks. Their results indicated that the swarms are tectonic in origin, and not representative of volcanic activity, although not necessarily exclusive of it. Work by Shor et al. (1990) suggests that there has been recent volcanic activity at the site of the May 1989 swarm, potentially concurrent with the swarm activity.

A joint venture of the Hawaii Institute of Geophysics (Alex Shor and Clyde Nishimura) and the Naval Research Laboratory (Peter Vogt and Michael Czarnecki), aboard the RV Ewing (Lamont-Doherty Geological Observatory), visited the May 1989 swarm site during July (S,hor et al., 1990). Simultaneous SeaMARCII sidescan imagery and Hydrosweep multibathymetry was used to examine a 1,000 km2 area of the seafloor at depths ranging from 700 to 1,350 m. Gravity and magnetics studies were also conducted.

A probable young lava flow was identified, centered near 59.47°N, 29.43°W, within the error radius of sonobuoy-based epicenter locations from the May 1989 swarm. The apparent lava flow (roughly 4 x 2 km) extends from the E margin of the neo-volcanic zone (NVZ) S over sedimented and faulted terrain and is bounded on the E by an inward-facing normal fault with an 80 m throw. The region is heavily faulted (faults are W-facing, spaced <1 km apart, and have throws of 40-80 m), and 3 faults were visible within the flow. Geologists noted that these faults were either older and incompletely inundated, or younger, suggesting a pre-May 1989 age for the lava flow. The lava flow likely originated from the southernmost and smallest (0.5-1.5 km diameter, 50-150 m high) of a cluster of 10 volcanic cones. A second probable young lava flow was also found NE of the swarm location.

During the investigation of the Reykjanes Ridge, the Ewing followed and surveyed the ridge axis from 52.8°N to 63.0°N. More than 100 discrete volcanic cones were identified within one 65-km-long, 10-km-wide strip, with many additional cones on the flanks.

On 9 September, six earthquakes were recorded originating near 56.7°N and 34.4°W (table 1), about 500 km SW along the ridge from the May 1989 swarm (figure 2). Preliminary planning and requests have been made to conduct an airborne sonobuoy mission similar to that employed during the May 1989 swarm. Pre-swarm seafloor imaging of the site was obtained during the July ridge axis survey. A second Ewing mission to the site of the May 1989 swarm (Lindsay Parson, Institute of Oceanographic Sciences, U.K.), had already been planned for October and should complete "before and after" views of the September swarm site.

Table 1. Earthquake epicenters and magnitudes for Reykjanes Ridge earthquakes, 9 September 1990. Courtesy of the USGS National Earthquake Information Center.

    Time      Location        Magnitude (mb)

    0017   56.94°N, 34.29°W       5.0
    0030   56.1°N,  34.5°W        4.8
    0215   56.78°N, 34.37°W       5.5
    0625   56.60°N, 34.33°W       5.2
    0635   56.65°N, 34.59°W       5.1
    1235   56.75°N, 34.38°W       4.8
Figure 2. Locations of earthquake swarms on the Reykjanes Ridge, after Bergman and Solomon (1990). The approximate site of the September 1990 swarm has been added.

References. Bergman, E.A., and Solomon, S.C., 1990, Earthquake swarms on the Mid-Atlantic Ridge: products of magmatism or extensional tectonics?: JGR, v. 95, p. 4943-4965.

Nishimura, C.E., Vogt, P.R., Smith, L., and Boyd, J.D., 1989, Investigation of a possible underwater volcanic eruption on the Reykjanes Ridge by airborne sonobuoys and AXBT's (abs.): Eos, v. 70, p. 1301.

Shor, A.N., Nishimura, C.E., Czarnecki, M., and Vogt, P.R., 1990, Lava extrusion from the 1989 Reykjanes Ridge seismic swarm? probably yes (SeaMARCII) (abs.): Eos, v. 71 (Fall AGU Abstract Volume).

Information Contacts: R. Stefánsson, Icelandic Meteorological Office; P. Vogt, Naval Research Laboratory; B. Presgrave, NEIC.

10/1990 (BGVN 15:10) Strong seismicity; turbid water and hydrothermal area but no new lava detected

The following is a report from Páll Einarsson. "An intense earthquake swarm started on the N part of the Reykjanes Ridge on 30 October (figure 3). The first event recorded by seismographs in Iceland occurred at 1021 and had a magnitude of 3.5. Smaller events occurred at 1052 and 1152. The epicenters cannot be located accurately, but appear to be near 63°N, or ~180 km SW of Reykjavík. At 1229, activity in this area increased dramatically, and for the next 19 hours hundreds of earthquakes were recorded. The largest events approached M 5 and at least 14 were of M 4 or larger. For large parts of this time, the seismographs showed continuous motion due to the dense sequence of small and large earthquakes. However, motion resembling volcanic tremor could not be identified.

Figure 3. Sketch map showing the approximate location of the earthquake swarms near 63°N and 63.7°N on the Reykjanes Ridge. After Perry and others (1980).

"The intense activity came to a rather abrupt halt at about 0800 on 31 October, but activity at a lower level continued, gradually diminishing. A temporary increase occurred 5-6 November (figure 4).

Figure 4. Number of earthquakes from the Reykjanes Ridge swarm area recorded daily at the Háhryggur seismic station in SW Iceland. Detection threshold of the station for this area is near magnitude 2.5. Courtesy of P. Einarsson.

"A second swarm started 3 November closer to Iceland, near 63.7°N. It began at 1426 with an event of M 3.8. Thirty events were recorded in the area that day, and five events the following day. This swarm was small and short-lived, and probably unrelated to the first one.

"The question of whether or not the swarm at 63°N is related to intrusive or extrusive activity at the sea floor cannot be answered from the available seismic data. Earthquake swarms are common on the Reykjanes Ridge and its landward continuation on the Reykjanes Peninsula. None of the recent swarms on the peninsula have been accompanied by eruptive activity, and they do not resemble the seismic swarms that accompany magmatic intrusions in the Krafla area along the rift zone in NE Iceland. Intrusion tremor, commonly observed at Krafla, has not been recorded during the swarms on the Reykjanes Peninsula despite a relatively dense seismograph network there.

"The current swarm at 63°N is unusual in both intensity and duration. The large distance to the nearest seismograph (roughly 150 km) means that intrusion and extrusion tremor could have occurred without being observed. Some characteristics of the swarm - for example the slow beginning, the high density of events at its culmination, and the abrupt end - in some respects resemble those of some of the Krafla eruptive events. If an analogy is drawn, one could speculate that the intense part of the swarm accompanied an intrusion of magma and that a dyke propagated for 19 hours. If an eruption occurred, it most likely began at about 0800 on 31 October when the seismic activity suddenly dropped to a lower level. Eruptive activity may have ended on 5 November, when there was a temporary increase in earthquakes."

A U.S. Navy P3 aircraft overflew the swarm area on 2 November between 1000 and 1400. Five sonobuoys were deployed; the central sonobuoy (at 63°15'40"N, 24°11'52") detected 50 Hz noise at 97 dB; sound intensity at four others (~ 9 km N, S, E, and W) was about 85 dB.

The following is from Jón Olafsson. "In response to the earthquake swarm on the Reykjanes Ridge, an international team assembled in Reykjavík on 2 November, sailing at midnight on the RV Bjarni Saemundsson of Iceland's Marine Research Institute.

"Investigations were concentrated on the area of the ridge crest between 62.9°N and 63.3°N, where the water depth ranged from 100 to 500 m. The ship is equipped with echosounders, sonar, and a CTD (Conductivity-Temperature-Depth) + light transmissometer with a rosette for water column sampling. On board were sonobuoys (provided by the U.S. Navy), equipment for analysis of dissolved silica, and a bottom dredge. Signs of possible eruptive activity were sought by deployment of sonobuoys, and water sampling on sections along and to the sides of the ridge crest. No signs could be detected of explosive activity of the type that created Surtsey in 1963, which would have given rise to extensive silica anomalies. However, the water above a segment of the ridge centered at 63.1°N showed some anomalous properties, particularly decreased light transmissivity and water column stability. A hydrothermal region was discovered near the summit of a seamount in this region, but has most likely been there beforehand, judging from previous information from fishermen. On the afternoon of 5 November, two nearby earthquake shocks were felt on the ship. Reports of earthquakes also came from deep-sea trawlers in this region, confirming that the research effort was in the region of seismic activity. Twelve dredge hauls brought up some fresh basalts but none were newly erupted.

"The ship returned to Reykjavík on 6 November with water samples for analysis of helium isotopes, manganese, methane, and hydrogen. Processing of these samples and the instrument records will be conducted in the UK, Iceland, and USA."

Locations of four of the largest earthquakes in the swarm were determined at the U.S. National Earthquake Information Center on 4 November (table 2). Arrival time values were obtained from the NEIC database, and from two seismic stations in Iceland (~150 and 250 km from the epicentral area), reported by Páll Einarsson. The following is from Eric Bergman.

Table 2. Relocations of four large earthquakes from the Reykjanes Ridge swarm, 30-31 October 1990. Courtesy of Eric Bergman.

    Date    Time     Latitude       Longitude     Magnitude (mb)

    30 Oct  1307  62.95 ± 0.08°N  24.60 ± 0.07°W       4.7
    30 Oct  1403  63.06 ± 0.06°N  24.64 ± 0.07°W       5.0
    30 Oct  1915  63.11 ± 0.08°N  24.75 ± 0.10°W       4.7
    31 Oct  0658  63.17 ± 0.06°N  24.64 ± 0.07°W       4.6

"The swarm events were relocated as part of a multiple-event relocation analysis for earthquakes on the Reykjanes Ridge between 62.5°N and 63.5°N. In all, 30 well-recorded earthquakes were relocated, using the hypocentroidal decomposition technique. Locations were estimated using the 1968 Herrin tables for P-wave travel times, except for the two Icelandic stations. Because the Herrin tables assume a thick continental crust, the theoretical travel times are longer than the true travel times for these phases, which propagate predominantly as refracted waves along the oceanic Moho with a velocity of around 8 km/s. Theoretical travel times for the two Icelandic stations were calculated by dividing the epicentral distance by 8.0 km/s. This admittedly crude estimate is a substantial improvement over the standard tables and is in good agreement with other data. No station corrections were used in the relocation. All focal depths were fixed at 10 km, consistent with many studies of the depth distribution of mid-ocean ridge seismicity. Further work is needed to refine this type of analysis, and it should be recognized that the locations reported here are to some extent biased by these assumptions. The results of the analysis will also change as more arrival data accumulate."

Reference. Perry, R.K., Fleming, H.S., Cherkis, N.Z., Feden, R.H., and Vogt, P.R., 1980, Bathymetry of the Norwegian-Greenland and western Barents Seas: U.S. Naval Research Laboratory-Acoustics Division, map and chart series MC-21

Information Contacts: P. Einarsson, Univ of Iceland; J. Olafsson, Marine Research Institute; E. Bergman, NEIC; P. Vogt, Naval Research Laboratory; T. Stroh, Univ of Washington. Scientific team on the RV Bjarni Saemundsson: Jón Olafsson, Icelandic Marine Research Institute (leader); Johnson R. Cann, Univ of Leeds (deputy leader); Kjartan Thors, S. Kristmansson, and Jón Benjaminsson, Icelandic Marine Research Institute; David Francis, Univ of Leeds; Cherry Walker, Univ of Durham; and Marie de Angelis, State Univ of New York, Stony Brook. Sponsoring Institutions: Icelandic Marine Research Institute; Natural Environmental Research Council, UK; and RIDGE Office, National Science Foundation, USA.

12/1990 (BGVN 15:12) About 30 events near 61.7°N

Another Reykjanes Ridge earthquake swarm began on 2 January. About 30 events were detected from its start at 1947 to its end the next morning. S-waves were rather indistinct, so the distance from Iceland was difficult to determine accurately, but epicenters were probably at roughly 61.7°N. None of the shocks was larger than M 4.

Information Contacts: Páll Einarsson, Science Institute, Univ of Iceland, Dunhaga 5, 107 Reykjavík, Iceland.

08/1992 (BGVN 17:08) Young lava flow identified from submersible

The following is from a report by Lynn Johnson. "A young lava flow has been identified from deep-diving submersible observations on the northern Reykjanes Ridge at 59.87°N, 29.65°W. A complete lack of sediment cover or biological colonization, minimal alteration, and a distinct boundary between this flow and underlying sediments suggest that this flow is less than 10-20 years old. However, exceptional bottom conditions including unusually strong currents might act to prevent sediment accumulation and biological colonization, allowing this flow to be significantly older (a few hundred to a few thousand years).

"In June, 1992 a 5-member team from the United States joined 10 Russian scientists on Russian Mir deep-diving submersibles to study a possible young lava flow on the Reykjanes Ridge at 59.78°N 29.72°W. The cruise was a cooperative project organized by Kathy Crane (Lamont-Doherty Geological Observatory), Peter Vogt (Naval Research Lab), and Anatoly Sagalavitch (Laboratory of Deep Manned Submersibles, Shirshov Institute of Oceanology, Moscow). The site was chosen because of a microseismic swarm located there by the Worldwide Standardized Seismic Net and an airborne sonobuoy survey in 1989 (Nishimura and others, 1989, and 14:5). In 1990 a SeaMARC II sidescan sonar study showed a highly reflective patch of seafloor within the error radius of the sonobuoy-based epicentral region of the seismic swarm (Shor and others, 1990, and 15:8).

"Our Mir submersible observations reveal that the high backscatter region at 59.78°N is not a young flow, but a flat featureless plain covered with 20-40 cm of sediment. Subsequent dives identified a young flow farther north, but outside of the epicentral region calculated for the 1989 seismic swarm. Four dives in the region between 59.75°N and 59.85°N traversed the ridge axis and several of the ubiquitous small seamounts along the ridge crest (figure 5). Two of these dives also investigated the bounding scarps of the large double ridge segment at that location. Two dives traversed over the northern end of this large segment and the southern end of a smaller segment to the north (59.90°N). With the exception of the one young flow, all of the seafloor observed during over 50 hours of bottom time is comprised of relatively old, sediment-covered and biologically colonized pillow lavas. The young flow was observed and sampled on the flanks of a small seamount at the southern end of the smaller northern axial-ridge segment. Abundant fresh glass is preserved in rocks recovered from this location, and palagonite development is essentially non-existent. The flow lacks sediment cover and biological colonization, and appears to have advanced over an older, sedimented area. Although, except in a few special cases, radiometric dating of mid-ocean ridge basalts (MORB) younger than a few thousand years is not possible, we will try to have the sample dated by recent experimental methods to obtain a more accurate age for recent volcanism at this site.

Figure 5. Bathymetric map of a segment of the Reykjanes Ridge. Location is shown by a box on the inset map. The arrow marks the site of a young lava flow. Contour interval, 100 m; areas deeper than 1,000 m are shaded. Courtesy of L. Johnson.

"In addition to video and photographic observations we collected conductivity, temperature, and depth information from one of the submersibles and from the R/V Keldysh, the support ship. No temperature or salinity anomalies indicating hydrothermal activity were recorded. We also collected samples of volcanic rock, water, and sediment using both the submersibles and the surface ship, and conducted a magnetic survey over a 100 km2 area."

References. Nishimura, C.E., Vogt, P.R., Smith, L., and Boyd, J.D., 1989, Investigations of a possible underwater volcanic eruption on the Reykjanes Ridge by airborne sonobuoys and AXBT's: EOS, v. 70, p. 1301.

Shor, A.N., Nishimura, C.E., Czarnecki, M., and Vogt, P.R., 1990, Lava extrusion from the 1989 Reykjanes Ridge seismic swarm? Probably yes (SeaMARC II): EOS, v. 71, p. 1602.

Information Contacts: L. Johnson, Naval Research Lab.