GEOPHYSICAL UNREST
Pinatubo was not seismically monitored before April 1991. Distant stations detected a few earthquakes in the vicinity of Mount Pinatubo following the magnitude 7.8 Luzon earthquake of July 1990. During April 1991, following steam explosions, several analog and digital seismometers were installed in a small network on the northwest side of Mount Pinatubo (Sabit and others). In late April and early May, the U.S. Geological Survey's Volcano Crisis Assistance Team (VCAT) helped the Philippine Institute of Volcanology and Seismology (PHIVOLCS) to install a network of seven radio-telemetered stations (Lockhart and others). Murray and others describe how data from these stations were processed on IBM-compatible PC's using software from the International Association of Seismology and Physics of the Earth's Interior (IASPEI). Earthquakes were located and examined for spectral characteristics, and the overall seismicity was examined in three dimensions and in plots of depth and magnitude versus time. High-frequency volcano-tectonic earthquakes 5 km northwest of Pinatubo and 5 km deep were succeeded in the first days of June by progressively shallower high-frequency and then low-frequency earthquakes beneath the north flank of Pinatubo. Escalating, shoaling high- and low-frequency events beneath the north flank became the main basis for successful forecasts of when magma might reach the surface; distinctive "long-period earthquakes," interpreted as qualitative pressure gauges for the volcanic system, provided continued warning after initial explosive events, until the climactic eruption on June 15 (Harlow and others).
Several refinements to the IASPEI software provided additional insights. Real-time Seismic Amplitude Measurement (RSAM, described by Endo and others), allowed tracking of the overall level of seismic energy release by one simple parameter. Seismic Spectral Amplitude Measurement (SSAM, described by Power and others) allowed tracking of changes in the spectral character of seismicity. SSAM was not used for forecasting during the crisis, owing principally to some cumbersome aspects of the software that have since been resolved, but SSAM did capture a distinct shift toward low-frequency events after June 1, 1991. Plotting of RSAM and SSAM data as inverse rates could have led to forecasts that were more precise at an earlier date than those based on non-inverted data (Cornelius and Voight).
In hindsight, R.A. White discovered deep long-period earthquakes (35 km deep) that are thought to record initial ascent of basaltic magma that eventually triggered the eruption. Although correlation of specific deep swarms with specific events near the surface might still be debated, a general correlation seems clear and represents the first seismic documentation of basalt intrusion and subsequent triggering of eruption of silicic magma.
Limited funds and logistical support in the preeruption period precluded detailed monitoring of ground deformation. The overall picture is unknown, but tiltmeters showed deformation precursors immediately before early dome growth (Ewert and others). In contrast, a quadrilateral array across the April 2 fissure showed no change above measurement error (about 1 cm in 20 m) between May 1 and May 28, even though this quadrilateral was just meters outside what would become the new caldera rim.
Posteruption seismic monitoring, by a reestablished network, showed exponentially decaying seismicity spread over wide area (Mori, White, and others). Episodic seismicity in late June and July 1991 probably correlated with small phreatomagmatic ash eruptions. Composite focal plane determinations by B. Bautista and others suggest that most preeruption, syneruption, and posteruption earthquakes were driven by regional east-west compression around the expanding and, later, partly vacated magma reservoir beneath Pinatubo.
An interesting outgrowth of the study of posteruption focal mechanisms was reexamination of changes in the regional stress field caused by the magnitude 7.8 1990 Luzon earthquake. Bautista and others, with coauthor R. Stein, conclude that compressive stress on the magma reservoir was about 1 bar and possibly enough to have triggered ascent of magma; alternatively, strong ground shaking associated with the earthquake might have triggered a chain of fault slippage that in turn allowed magma ascent.
Posteruption earthquakes also formed the basis for seismic tomography and a reevaluation of the three-dimensional velocity structure beneath Mount Pinatubo (Mori, Eberhart-Phillips, and Harlow). A low-velocity zone between 6 and 11 km beneath Pinatubo has an estimated volume of 40 to 90 km3; an apparent extension beneath neighboring Mount Negron raises the total volume of low-velocity material to between 60 and 125 km3. The improved three-dimensional velocity structure was also used to improve hypocenter locations. Two new programs by Hoblitt, Mori, and Power (PINAPLOT and VOLQUAKE) help visualize the seismicity of Pinatubo as it evolved, as well as seismicity during future volcanic crises.
Renewed seismicity and magma intrusion occurred in July through October 1992 (Ramos and others). As many as 1,400 low-frequency earthquakes occurred in a single day and included many multiplets (identical earthquakes), which are suggestive of repeated stress and failure at the same location. Were these earthquakes precursors to additional explosions, or were they the seismicity of a sluggish intrusion? They were from a sluggish intrusion, but they looked remarkably like events that in 1991 had preceded explosive eruptions. An interesting variety of tremor, possibly related to long-period seismic events, is also described by Ramos.
Ed Laguerta (PHIVOLCS), Andy Lockhart (USGS), Joey Marcial (PHIVOLCS), and John Power (USGS) installing a seismometer before the eruptions. Photograph by Val Gempis, USAF.
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Last updated 09.24.98