Paul G. Okubo

The use of earthquake rate changes as a stress meter at Kilauea volcano.

Stress changes in the Earths crust are generally estimated from model calculations that use near-surface deformation as an observational constraint. But the widespread correlation of changes of earthquake activity with stress has led to suggestions that stress changes might be calculated from earthquake occurrence rates obtained from seismicity catalogues. Although this possibility has considerable appeal, because seismicity data are routinely collected and have good spatial and temporal resolution, the method has not yet proven successful, owing to the nonlinearity of earthquake rate changes with respect to both stress and time. Here, however, we present two methods for inverting earthquake rate data to infer stress changes, using a formulation for the stress- and time-dependence of earthquake rates. Application of these methods at Kilauea volcano, in Hawaii, yields good agreement with independent estimates, indicating that earthquake rates can provide a practical remote-sensing stress meter.

Imaging the crustal magma sources beneath Mauna Loa and Kilauea volcanoes, Hawaii

Three-dimensional seismic P-wave traveltime tomography is used to image the magma sources beneath Mauna Loa and Kilauea volcanoes, Hawaii. High-velocity bodies (>6.4 km/s) in the upper 9 km of the crust beneath the summits and rift zones of the volcanoes correlate with zones of high magnetic intensities and are interpreted as solidified gabbro-ultramafic cumulates from which the surface volcanism is derived. The proximity of these high-velocity features to the rift zones is consistent with a ridge-spreading model of the volcanic flank. Southeast of the Hilina fault zone, along the south flank of Kilauea, low-velocity material (

Structure of the mobile south flank of Kilauea Volcano, Hawaii

How do huge landslides occur in Hawaii? We can begin to address this question by examining an incipient landslide structure on the mobile south flank of Kilauea volcano. Between 1970 and 1989 this part of the south flank moved in a southeast direction (seaward by 4 m on its eastern end and by 10 m on its western end), parallel to the western boundary of a large oifshore topographic bench. We show that abrupt decreases in cumulative flank movement correspond to abrupt decreases in hypocenter concentration. Most of the earthquakes occur in a 7- to 10-km-deep band beneath the south flank, and relative relocation of a subset of these events shows that they are derived from a single horizontal plane which appears to be a decollement. Focal mechanisms for this deep seismicity are dominated by thrust fault mechanisms, with the overlying block moving seaward. A comparison of south flank bathymetry with decollement structures on other volcanoes suggests that the distal end of this decollement coincides with the distal end of the offshore topographic bench (30–50 km from shore and 5 km below sea level). The fault system forming the Hilina pali is connected with this decollement, so that slip on the Hilina palis normal faults coincides with slip of the decollement and with reverse faulting of the distal end of the bench. (This interpretation is supported by studies of the events associated with the M7.2 Kalapana earthquake in 1975.) When the Hilina pali is connected to the outer bench with a decollement inferred from seismicity, a wedge 8-km thick adjacent to Kilaueas magma system and 1–2 km thick along the edge of the outer bench (an area over 3000 km2) is defined. The thickness adjacent to the magma system decreases eastward along Kilaueas East Rift Zone and southwestward from Kilauea caldera, whereas the thickness of the lip of the outer bench decreases only eastward. Geologic observations and geophysical studies of the subaerial flank reveal an extensive magma system beneath the Koae fault system as well as beneath Kilauea caldera and its East Rift Zone. We infer that the boundary of the proximal end of the mobile flank is the southern boundary of this magma system and that this boundary coincides with the southern boundaries of both the Koae fault system and Kilaueas East Rift Zone.

Volcanic spreading at Kilauea, 1976–1996

The rift system traversing about 80 km of the subaerial surface of Kilauea volcano has extended continuously since the M 7.2 flank earthquake of November 1975. Widening across the summit has amounted to more than 250 cm, decelerating after 1975 from about 25 to 4 cm yr−1 since 1983. Concurrently, the summit has subsided more than 200 cm, even as the adjacent south flank has risen more than 50 cm. The axes of the upper zones, about 10 km from the summit, subsided before 1983 at average rates of 9 and 4 cm yr−1, respectively, and at rates of 4 and 3 cm yr−1 since. The middle southwest rift zone is also subsiding and, at the other end of Kilaueas subaerial rift system, subsidence along the lower east rift zone has averaged 1–2 cm yr−1. Deformation of Kilaueas south flank has been continuous, although subject as well to displacements caused by major rift zone seismic swarms. Whereas horizontal strains across the subaerial south flank seem to have been generally compressive after 1975, they have been extensional since about 1980 or 1981, interrupted only by the east rift zone dike intrusion of 1983. Because the magnitudes of these contractions and extensions are much less than the extension across the rift system, the subaerial south flank is apparently sliding seaward on its basal decollement more than it is accumulating horizontal strains within the overlying volcanic pile. Kilauea suffers from gravitational spreading made even more unstable by accumulation of magma along the rift system at depths in excess of about 4–5 km in the presence hot rock incapable of withstanding deviatoric stresses. This seismicly quiescent zone decouples the south flank from the rest of Hawaiis volcanic edifice; the rift zones at lesser depths exhibit a more brittle and, therefore, sporadic extensional behavior. Judging from the modern extension record of the summit, which both predates the M 7.2 earthquake of 1975 and has outlived its 10-year period of aftershocks, Kilauea will continue to spread along its rift system as its south flank slips seaward to accommodate the accretion of magma and its relatively dense olivine-rich differentiate.

Three-dimensional velocity structure of the Kilauea caldera, Hawaii

High-resolution velocity models (0.5 km resolution) of the Kilauea caldera region are obtained by the tomographic inversion of both P- and S-wave arrival times. Data are from the permanent Hawaiian Volcano Observatory (HVO) seismic network, a broadband seismic network, and a temporary array of stations centered on the southern boundary of the caldera. A low-velocity P-wave anomaly is imaged centered on the southeastern edge of the caldera, with a velocity contrast of about 10% and a volume of 27 km³. The VP/VS model mimics the spatial extent of the P-wave anomaly, but is partitioned into two discrete anomalous volumes centered on the southern boundary of the caldera and on the upper east rift of the volcano. The corresponding Poissons ratio in these zones is high (ν=0.25–0.32) which is consistent with a densely-cracked, hot volume which may contain partial melt. The large-scale features of the models are consistent with results obtained from an earlier, larger-scale (2 km resolution) tomographic image of Kilauea Volcano based on HVO network data.

Magmatically Triggered Slow Slip at Kilauea Volcano, Hawaii

We demonstrate that a recent dike intrusion probably triggered a slow fault-slip event (SSE) on Kilauea volcanos mobile south flank. Our analysis combined models of Advanced Land Observing Satellite interferometric dike-intrusion displacement maps with continuous Global Positioning System (GPS) displacement vectors to show that deformation nearly identical to four previous SSEs at Kilauea occurred at far-field sites shortly after the intrusion. We model stress changes because of both secular deformation and the intrusion and find that both would increase the Coulomb failure stress on possible SSE slip surfaces by roughly the same amount. These results, in concert with the observation that none of the previous SSEs at Kilauea was directly preceded by intrusions but rather occurred during times of normal background deformation, suggest that both extrinsic (intrusion-triggering) and intrinsic (secular fault creep) fault processes can lead to SSEs.

Seismicity and deformation induced by magma accumulation at three basaltic volcanoes

We analyzed the evolution of volcano-tectonic (VT) seismicity and deformation at three basaltic volcanoes (Kilauea, Mauna Loa, Piton de la Fournaise) during phases of magma accumulation. We observed that the VT earthquake activity displays an accelerating evolution at the three studied volcanoes during the time of magma accumulation. At the same times, deformation rates recorded at the summit of Kilauea and Mauna Loa volcanoes were not accelerating but rather tend to decay. To interpret these observations, we propose a physical model describing the evolution of pressure produced by the accumulation of magma into a reservoir. This variation of pressure is then used to force a simple model of damage, where damage episodes are equivalent to earthquakes. This model leads to an exponential increase of the VT activity and to an exponential decay of the deformation rate during accumulation phases. Seismicity and deformation data are well fitted by such an exponential model. The time constant, deduced from the exponential increase of the seismicity, is in agreement with the time constant predicted by the model of magma accumulation. This VT activity can thus be a direct indication of the accumulation of magma at depth, and therefore can be seen as a long-term precursory phenomenon, at least for the three studied basaltic volcanoes. Unfortunately, it does not allow the prediction of the onset of future eruptions, as no diverging point (i.e., critical time) is present in the model.

Crustal shear wave anisotropy in southern Hawaii: Spatial and temporal analysis

Split shear wave arrivals are analyzed in seismograms from local earthquakes in southern Hawaii recorded at five temporary arrays and one permanent network station. We identify split shear wave arrivals by their orthogonally polarized pulses, linear particle motions, and similar waveforms and estimate the delay time for the slow shear wave arrival (S 2 ) using a waveform cross-correlation method. Consistent leading shear wave polarizations were measured at the majority of our stations. Comparison of observed and predicted shear wave polarizations confirms that the former are due to anisotropy rather than earthquake source mechanism. Agreement between fast shear wave (S 1 ) polarizations and independently estimated directions of the maximum horizontal compressive stress (σ H ) for the Ainapo and Punaluu Gulch arrays leads us to conclude that the predominant source of the observed anisotropy for these two areas is stress-aligned cracks consistent with the extensive dilatancy anisotropy (EDA) hypothesis. Two distinct S 1 polarization directions were observed over distances less than 1 km for the Bird Park and South Flank arrays. S 1 polarizations parallel to the NE striking Kaoiki Pali fault system for the Bird Park array combined with a nonhorizontal maximum principal stress (σ 1 ) for the South Flank region suggest stress-induced cracks aligned by nearby faulting as a source for the observed anisotropy. Large station-to-station variations in S 1 polarization and the relationship between delay time and event depth for arrays in the Kaoiki and South Flank regions provide evidence for anisotropy that is predominantly shallow rather than pervasive. Average delay times for the five arrays vary from about 100 to 230 ms, with standard deviations of the order of 30 ms. Estimated anisotropic velocity variations and crack densities exceeding 10% indicate that the upper crust of southern Hawaii is highly fractured. A search for possible temporal changes in delay time associated with the 1983 Kaoiki main shock (M L = 6.6), at a station near the epicenter, finds no evidence for change.

Seismic Hazard in Hawaii: High Rate of Large Earthquakes and Probabilistic Ground-Motion Maps

The seismic hazard and earthquake occurrence rates in Hawaii are locally as high as that near the most hazardous faults elsewhere in the United States. We have generated maps of peak ground acceleration (PGA) and spectral acceleration (SA) (at 0.2, 0.3 and 1.0 sec, 5% critical damping) at 2% and 10% exceedance probabilities in 50 years. The highest hazard is on the south side of Hawaii Island, as indicated by the M I 7.0, M S 7.2, and M I 7.9 earthquakes, which occurred there since 1868. Probabilistic values of horizontal PGA (2% in 50 years) on Hawaiis south coast exceed 1.75 g . Because some large earthquake aftershock zones and the geometry of flank blocks slipping on subhorizontal decollement faults are known, we use a combination of spatially uniform sources in active flank blocks and smoothed seismicity in other areas to model seismicity. Rates of earthquakes are derived from magnitude distributions of the modern (1959–1997) catalog of the Hawaiian Volcano Observatorys seismic network supplemented by the historic (1868–1959) catalog. Modern magnitudes are M L measured on a Wood-Anderson seismograph or M S. Historic magnitudes may add M L measured on a Milne-Shaw or Bosch-Omori seismograph or M I derived from calibrated areas of MM intensities. Active flank areas, which by far account for the highest hazard, are characterized by distributions with b slopes of about 1.0 below M 5.0 and about 0.6 above M 5.0. The kinked distribution means that large earthquake rates would be grossly underestimated by extrapolating small earthquake rates, and that longer catalogs are essential for estimating or verifying the rates of large earthquakes. Flank earthquakes thus follow a semicharacteristic model, which is a combination of background seismicity and an excess number of large earthquakes. Flank earthquakes are geometrically confined to rupture zones on the volcano flanks by barriers such as rift zones and the seaward edge of the volcano, which may be expressed by a magnitude distribution similar to that including characteristic earthquakes. The island chain northwest of Hawaii Island is seismically and volcanically much less active. We model its seismic hazard with a combination of a linearly decaying ramp fit to the cataloged seismicity and spatially smoothed seismicity with a smoothing half-width of 10 km. We use a combination of up to four attenuation relations for each map because for either PGA or SA, there is no single relation that represents ground motion for all distance and magnitude ranges. Great slumps and landslides visible on the ocean floor correspond to catastrophes with effective energy magnitudes M E above 8.0. A crude estimate of their frequency suggests that the probabilistic earthquake hazard is at least an order of magnitude higher for flank earthquakes than that from submarine slumps.

Tomographic image of P-velocity structure beneath Kilauea's East Rift Zone and South Flank : Seismic evidence for a deep magma body

We present first results from the analysis of P-wave arrival time data recorded from November 11 to December 31, 1999, by a temporary 29-station network installed across Kilauea Volcanos East Rift Zone (ERZ) and South Flank (SF) on Hawaii, augmented by data from the permanent network of the Hawaiian Volcano Observatory. Starting with the inversion result for a minimum 1D velocity model, we use arrival time data from 135 local earthquakes to invert for the 3D P-velocity structure. The resulting tomographic image shows evidence for a deep magma body beneath the ERZ just east of its southward bend, and a smaller magma body at about 5 km depth beneath the WNW-ESE trending segment of the ERZ. We also observe a drastic change in velocities south of the Hilina fault system from velocities around 5.5 km/s in the west to 6.5 km/s to the east.