Jökull


Jökull - 01.06.2000, Síða 36

Jökull - 01.06.2000, Síða 36
Foulger and Field ons at the surface. The model contains no informati- on on near-surface areas between stations. This factor explains why the refraction proíiles of Brandsdóttir et al. (1997), which involved sensors deployed at 200 m intervals across the caldera floor, detected much lower surface velocities than are obtained by lateral extra- polation of the LET model at the surface. Other significant discrepancies between the LET model and the shallow refraction results, highlighted by Brandsdóttir et al. (1997), are at large distances from Víti along the profiles. Because rays from earth- quakes may travel for part of their paths outside the study volume (Figure 4) a velocity structure there must be defined for the LET inversion. However, this velocity structure is so poorly sampled by rays that it is held fixed in the inversion process. The perimeter nodes of ,the study volume, by their very nature, are also very sparsely sampled by rays and only at a few of them are the velocities resolved (Figure 4). For these reasons it is invalid to calculate by extrapolation velocities either outside the grid or in the outer rows of blocks in the study volume. This means that the parts of the refraction profi- les of Brandsdóttir et al. (1997) that are resolved by the LET are limited to distances from Víti of up to ~5.5 km to the west, up to ~1 km to the east, up to 3 km to the north and up to ~10 km to the south. With the exception of the lower velocities detected at very shallow depth by Brandsdóttir et al. (1997), explained above, the agreement between the LET model and the refraction results is remarkably good within these ranges. The LET study of the Krafla area was rather typical of this kind of study. The volume of int- erest was parameterised at 2-3 km intervals in the horizontal and 1 km intervals in the vertical. The inversion process determined the best fit velocities at nodes spaced at those intervals, assuming linear variations in velocity in between and a smooth gener- al model. Such a method returns a broad, average model of the velocity variations in the area. It is not designed to resolve small bodies nor velocity discont- inuities. This must be appreciated when comparing velocities determined by interpolation between grid nodes with methods that involve relatively localised and precise sampling such as small-scale seismic refraction experiments or drilling. Notwithstanding the imperfect earthquake distribution, a reasonable inversion result was obtained, with a data variance reduction of 84% from the starting model. The primary features imaged were high-velocity bodies beneath the caldera ring fault (Figures 4a-c). These were interpreted as gabbroic bodies intruded up the caldera fault. A smaller, high-velocity body was detected at shallow depth beneath Leirhnjúkur, and a low-velocity body to the SW of the caldera. GRAVITY DATA Hengill-Grensdalur Þorbergsson et al. (1984) measured gravity at 315 stations with average spacings of ~1.5 km covering an area ~450 km2 in the Hengill-Grensdalur area. The measured values of gravity at most stations have estimated uncertainties of <0.5 mGal. The data have been discussed in detail by Hersir et al. (1990). Krafla A gravity survey of 393 stations in the Krafla caldera is described by Karlsdóttir et al. (1978). The estimated accuracy is 0.5 mGal for most of the stations. Many of the stations are clustered around geothermal featur- es rather than being uniformly distributed throughout the area and the survey design was thus not ideal for comparison with the LET results. METHOD The approach adopted here is to convert the LET velocity field to density and then to calculate a “simulated” Bouguer anomaly field. This is compared with the “observed” Bouguer anomaly field calculated from the gravity data. The velocity-density relations- hip derived from measurements in the Reyðarfjörð- ur drillhole was used here (Christensen and Wilkins, 1982), which is p = 1530 + 230Vp (1) where Vp is the P-wave velocity in km/s and p is the rock density in kg/m3. The LET models were divided into cubes 0.25 km on a side and velocities interpola- ted linearly to determine an average velocity for each 34 JÖKULL No. 48
Síða 1
Síða 2
Síða 3
Síða 4
Síða 5
Síða 6
Síða 7
Síða 8
Síða 9
Síða 10
Síða 11
Síða 12
Síða 13
Síða 14
Síða 15
Síða 16
Síða 17
Síða 18
Síða 19
Síða 20
Síða 21
Síða 22
Síða 23
Síða 24
Síða 25
Síða 26
Síða 27
Síða 28
Síða 29
Síða 30
Síða 31
Síða 32
Síða 33
Síða 34
Síða 35
Síða 36
Síða 37
Síða 38
Síða 39
Síða 40
Síða 41
Síða 42
Síða 43
Síða 44
Síða 45
Síða 46
Síða 47
Síða 48
Síða 49
Síða 50
Síða 51
Síða 52
Síða 53
Síða 54
Síða 55
Síða 56
Síða 57
Síða 58
Síða 59
Síða 60
Síða 61
Síða 62
Síða 63
Síða 64
Síða 65
Síða 66
Síða 67
Síða 68
Síða 69
Síða 70
Síða 71
Síða 72
Síða 73
Síða 74
Síða 75
Síða 76
Síða 77
Síða 78
Síða 79
Síða 80
Síða 81
Síða 82
Síða 83
Síða 84
Síða 85
Síða 86
Síða 87
Síða 88
Síða 89
Síða 90
Síða 91
Síða 92

x

Jökull

Beinleiðis leinki

Hvis du vil linke til denne avis/magasin, skal du bruge disse links:

Link til denne avis/magasin: Jökull
https://timarit.is/publication/1155

Link til dette eksemplar:

Link til denne side:

Link til denne artikel:

Venligst ikke link direkte til billeder eller PDfs på Timarit.is, da sådanne webadresser kan ændres uden advarsel. Brug venligst de angivne webadresser for at linke til sitet.