Transient deformation following the January 30, 1997 Napau Crater eruption at Kilauea Volcano, Hawaii

Abstract

On January 30, 1997 an intrusion on Kilauea volcano opened a new fissure within the East Rift Zone (ERZ) at Napau Crater, 3 km uprift from the ongoing eruptions Pu'u O'o. The fissure eruption lasted for two days and opened a 5.15 km long, nearly vertical dike 1.96 m. It extended from the surface to a depth of 2.4 km \citep{OWEN2000b}. During the eruption, the lava pond at Pu'u O'o drained and eruptions ceased there until it eventually refilled in late February and eruptions resumed March 28, 1997.

Continuous GPS data show a large transient following the January 30, 1997 dike intrusion. After lengthening 40 cm during the initial eruption, the baseline between the two stations spanning the ERZ lengthened an additional 10 cm over the following six months. The coastal station KAEP also showed the transient as it continued to move southward (5 cm) over the same six months. The baseline between the two stations spanning Kilauea's summit caldera, UWEV and AHUP, contracted sharply during the eruption and gradually recovered to a little longer than it's previous length two months after the intrusion. We use the Extended Network Inversion Filter \citep{MCGUIRE2003} to invert continuous GPS data for volume change of a spherical pressure source under Kilauea's summit, opening distribution on a nearly vertical dike in the ERZ and potential slip distribution on a decollement 9 km beneath the south flank.

Following the January 30, 1997 intrusion under Napau Crater, rift extension continued below the initial intrusion dike for the duration of the transient. Decollement slip, regardless of its assumed depth, was not required by the data. The modeled transient summit reinflation and rift opening patterns under Napau Crater coincide with changes in observed behavior of Pu'u O'o's lava pond. Rift opening accelerated while Pu;u O'o eruptions paused and began to decelerate after the lava pond reappeared nearly a month after the Napau eruption. The transient deformation is interpreted as resulting from the mid-crustal accommodation of the new dike volume.

Figure 1. Volcanic landmarks and continuous GPS stations on Kilauea Volcano, Hawaii with cumulative post-intrusion transient displacements for the time period between February 1, 1997 and December 31, 1997. Error ellipses are 95\% confidence intervals. The initial January 30, 1997 dike is shown in green. The best-fitting cumulative transient dike, and predicted displacements are shown in red. The epicenter of the M5.7 earthquake that occurred on June 30, 1997 is shown by the yellow star.

Transient Signal in Continuous GPS Data

Figure 2. East, north and Vertical components of the Kilauea GPS network showing transient signals following the January 30, 1997 intrusion. The data are in blue and the model predicted positions are in red. The data have been differenced to Mauna Kea and were detrended using estimates of average velocity from three years of data following the transient. Error bars on the tightly constrained solutions are smaller than the size of the markers.

Movie of Transient Opening Rate (click picture for .wmv movie)

Figure 3. Space-time history of distrubuted dike opening rate following the January 30, 1997 dike intrusion.  Click picture for movie.

Figure 4. Time evolving rates of rift opening moment and magma chamber inflation. The red dotted lines show the time of the intrusion, the refilled lava pond at Pu'u O'o, and the time eruptions resumed there. Both rates sharply peak just after the lava pond returns to Pu'u O'o. Histogrammed below, are the number of earthquakes per day above 6 km in the middle frame and below 6 km in the lower frame.

Figure 5. Change in Coulomb failure stress on horizontal planes due to the initial January 30, 1997 dike intrusion. Warmer colors are encouraged to slip by the stress change and cooler colors discouraged. The white line shows the location of the January 30, 1997 dike. Relocated south flank earthquakes are also plotted with blacker circles indicating earthquakes that occurred early in the year and whiter circles occurred later in the year. The size of the circle indicates the magnitude of the event.

Conclusions

The continuous GPS data on Kilauea Volcano, Hawaii are capable of capturing not only
earthquakes, intrusions and eruptions, but also transient deformation events that develop
over longer timescales. The Extended Network Inversion Filter provides a framework
for estimating the time dependent source parameters causing the observed deformation.
Following the January 30, 1997 intrusion, the ENIF helps us draw these conclusions:

1. Additional transient deformation of Kilauea's ERZ occurred that resulted in up to 7 cm of additional displacement at sites close to the fissures.

2. Cumulative displacements from February 1, 1997 { December 31, 1997 are well modeled as opening of a dike-like body from 2-4.5 km deep and reinflation

3.  The opening distribution of the ENIF inversions show that the opening remains localized just below the coeruptive intrusion dike with a maximum opening of 1.5 m.

4. Temporal evolution of the model provides a time sequence of events following the intrusion:
       Jan. 30: Intrusion, Pu'u O'o drains, and summit de ates
       Jan. 30 { Mar. 3: Summit in ation and rift opening accelerate
       Feb. 24: Pu'u O'o's lava pond returns
       Mar. 3: Summit in ation and rift zone opening begin to decelerate
       Mar. 28: Pu'u O'o eruptions resume
       Apr. 15: Summit ceases transient in ation
       Aug. 01: Rift zone ceases transient opening

5. We interpret the transient deformation as resulting from the mid-crustal stress perturbation induced by the initial dike injection.

6. The initial dike did not cause a signi cant stress change at the level of the decollement to induce transient seaward slip beyond steady state rates.