Figure 1. The large inset plots the westward migration of M ³ 6 earthquakes in SW Iceland, as inspired by Einarsson . The hypocenters of the two M 6.5 earthquakes in 2000 on June 17 (J17) and June 21 (J21) are separated by 81 hours (~4 days) and 16 km for an apparent stress transfer speed of 5 km/day or 5 cm/s. ÒMap showing the main tectonic features of the study area. The Reykjanes Peninsula (RP), the Hengill triple junction (He), the Western Volcanic Zone (WVZ), the South Iceland Seismic Zone (SISZ), and the Eastern Volcanic Zone (EVZ) are indicated. The light shaded areas are individual fissure swarms with associated central volcanoes. Mapped surface faults of Holocene age are shown with black lines[Einarsson, 1987]. The epicenter locations of earthquakes on June 2000 are shown with black stars. The 17 June main shock is labeled J17. The Reykjanes Peninsula events are on the Hvalhnœkur fault (H), near Lake Kleifarvatn (K), and Nœpshlõ’darh‡ls (N). Location of the 21 June 2000 main shock is shown with a white star labeled J21. Available focal mechanisms for the main events in June 2000 are shown with a lower hemisphere projection (USGS and K. Vogfjord personal communication, 2003). The location of Reykjavik, the capital of Iceland, is noted. The geothermal area at Svartsengi (Sv) is noted by a hexagon. [Small] inset shows a simplified map of the plate boundary across Iceland, with the location of main part of figure indicated by the black rectangle. The black arrows show the direction of the NUVEL-1A plate motion of North America and Eurasian plates.Ó [çrnad—ttir, et al., 2004].
Figure 2. Pairs of triggered earthquakes showing the distance and time between events from the literature (large symbols). Dashed lines show apparent propagation speeds of P-waves (7 km/s), surface waves (3 km/s), Òseismicity frontsÓ observed in Iceland (5 km/day), and a ÔglaciallyÕ slow process (10 km/year). Pairs with asterisks denote pairs with a second, ÒreceiverÓ event in a volcanic setting. Data from [Gomberg, et al., 1998; Gomberg, et al., 2001; Hill, et al., 1993; Hjaltadottir, et al., 2005; Kilb, et al., 2002; Toda and Stein, 2003; Vogfjord, 2003; Vogfjord and Slunga, 2003; Vogfjord and Slunga, 2004].
Considerable evidence indicates that one earthquake can trigger another by stress transfer. The textbook example is a pair of magnitude 6 earthquakes that shook the Superstition Hills, California in 1987. First, the left-lateral Elmore Ranch fault ruptured. Some 12 hours later, the right-lateral Superstition Hills fault ruptured in a second earthquake. The distance between the first (source) epicenter and second (receiver) epicenter is about 10 km. Similar sequences of large (magnitude 6 at least) earthquakes occurred in 1784, 1896, and 2000 in the South Iceland Seismic Zone (SISZ). The research seeks to explain the separation in both time and in space between such triggered earthquakes, focusing in particular on the 2000 SISZ earthquake sequence. The triggering appears to be a two-step process. In the first step, the seismic waves transmit a dynamic stress over the distance from the triggering ÒsourceÓ event to the (future) location of the triggered ÒreceiverÓ event. In the second, slower step, another local process is involved that leads to the second event. In this heuristic model, the first step produces the large separation in space (distance) between the source and receiver events, while the second step produces the separation in time (delay). This is one of the hypotheses that the proposed research is testing. The other hypotheses include: (1) dynamic friction that depends on the faultÕs slip rate and a physical state variable; (2) bubbles produced as the seismic wave releases gas from volatile magma; (3) aseismic fault slip; and much more speculatively, (4) some kind of stress pulse that propagates with the observed apparent velocity.
To test these competing hypotheses, the research is employing
state-of-the-art techniques for measuring and modeling stress transfer
in Iceland. The measurements include: (1) geodetic time series from
continuously-recording Global Positioning System receivers (CGPS), (2)
interferometry using satellite radar images (INSAR), (3) hydrological
recordings of water level in boreholes, (4) meteorological recordings
of barometric pressure, rainfall and temperature, and (5) precise
estimations of earthquake locations and focal mechanisms. The numerical
models employ 3-dimensional finite element modeling (FEM) to account
for poro-elastic and visco-elastic processes in a fault zone. The model
calculates three quantities: (1) the displacement field, which can be
compared to INSAR and CGPS measurements; (2) the fluid pressure field,
which can be compared to the water level measured in boreholes; and (3)
the stress tensor, which can be resolved onto known fault planes to
evaluate failure by the Coulomb criterion.
Intellectual merit of the activity: The research seeks to delineate the processes causing the triggering over distances longer than several fault lengths and time scales longer than the travel time of seismic waves. It involves both measurements and models in the traditionally separate disciplines of geodesy, seismology, and hydrology. This interdisciplinary research seems poised to add to fundamental understanding of earthquake processes.
Broader impact of the activity: The investigators, including
colleagues in Iceland, posses a detailed familiarity with the strengths
and weaknesses of the observations that places their team in an ideal
position to undertake these modeling studies. They are fostering the
career of a junior scientist at the intersection of three disciplines:
seismology, geodesy and hydrology. By the end of the two-year project,
the junior scientist will have mastered three techniques of modern
geophysics (CGPS, INSAR and FEM) with applications beyond the
university setting. The new information coming from this study is
likely to be directly relevant to understanding time-dependent
earthquake hazards in Iceland. Improved understanding of earthquake
triggering also contributes to the objectives of the National
Earthquake Hazard Reduction Program.
çrnad—ttir, T., et al. (2004), Coseismic stress changes and crustal deformation on the Reykjanes Peninsula due to triggered earthquakes on 17 June 2000, Journal of Geophysical Research (Solid Earth), 109, 09307.
Einarsson, P., and K. S¾mundsson (1987), Earthquake epicenters 1982Ð1985 and volcanic systems in Iceland, in Festschrift for Th. Sigurgeirsson, edited by T. I. Sigfusson, Menningarsjodur, Reykjavõk.
Einarsson, P., et al. (1981), Seismicity pattern in the south Iceland seismic zone, in Earthquake Prediction, an International Review. M. Ewing Ser. 4, edited by D. Simpson and P. Richards, pp. 141-152, American Geophysical Union, Washington, D.C.
Gomberg, J., et al. (1998), Earthquake triggering by transient and static deformations, Journal of Geophysical Research, 103, 24411-24426.
Gomberg, J., et al. (2001), Earthquake triggering by seismic waves following the Landers and Hector Mine earthquakes, Nature, 411, 462-466.
Hill, D. P., et al. (1993), Seismicity remotely triggered by the magnitude 7.3 Landers, California earthquake, Science, 260, 1617-1623.
Hjaltadottir, S., et al. (2005), Mapping subsurface faults in southwest Iceland using relatively located microearthquakes, paper presented at EGU General assembly, European Geoscience Union, Vienna.
Kilb, D., et al. (2002), Aftershock triggering by complete Coulomb stress changes, Journal of Geophysical Research (Solid Earth), 107d.
Toda, S., and R. Stein (2003), Toggling of seismicity by the 1997 Kagoshima earthquake couplet: A demonstration of time-dependent stress transfer, J. Geophys. Res.,, 108, 2567 doi 2510.1029/2003JB002527.
Vogfjord, K. (2003), Triggered Seismicity after the June 17, Mw=6.5 Earthquake in the South Iceland Seismic Zone: The first five minutes, EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #11251, 11251.
Vogfjord, K., and R. Slunga (2003), Rupture in the South Iceland Seismic Zone forced by magmatic intrusion in the Hengill area, EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #9685, 9685.
Vogfjord, K. S., and R. Slunga (2004), Fault Mapping in the Hengill Region, SW Iceland by Joint Interpretation of Microearthquake Distribution and Collective Focal Mechanisms, AGU Fall Meeting Abstracts, 51, 05.