The low-frequency earthquake swarms at Mount Pinatubo, Philippines: implications for magma dynamics

Abstract

Mount Pinatubo's intense low-frequency (LF) earthquake swarms in 1992 provide insight into the volcano's shallow plumbing system and into the possible magmatic process that generate these unusual earthquakes. LF waveforms are remarkably uniform with spectral peaks at about 1 Hz. Recorded at Pinatubo 1 week prior to the June 1991 cataclysmic eruption, LF events reappeared 1 year later and a day before a new dome began to grow in the volcano's freshly-formed caldera. Seismicity with at least 23,000 LF events accompanied the 1992 dome growth, and both simultaneously ceased after 4 months. The magnitude range of the thousands of LF events is narrow at only 1.6 units, and show a more restricted range in size during any particular period within the crisis, defining a “group magnitude.” During two LF earthquake swarms, the group magnitude gradually varied, indicating a pervasive process affecting the mechanism of LF earthquake-generation. Hypocenters of Pinatubo's LF earthquakes define a vertical pipe extending from the crater floor to ∼4.2 km below sea level, coinciding with the column formed by high-frequency (HF) earthquakes prior to the 1991 cataclysmic eruption. A dike intrusion along the NW-trending Maraunot Fault is also suggested by the hypocenters. The temporal, amplitude and waveform features and hypocenter location of Pinatubo's LF earthquakes are distinctly different from those of HF events and indicate a close affinity of LF earthquakes to one another. LF events appear to be caused by interactions between more fluid magma intruding semi-ductile materials confined within the volcano's shallow plumbing system. This source mechanism can account for: the occurrence of LF earthquakes as swarms and multiplets; the limited magnitude range of the events; the temporally-changing group magnitudes; the obscurity of S-phase; the existence of hybrid LF–HF earthquakes; and the close relationship of LF earthquakes with dome extrusions. The 1992 LF seismicity at Pinatubo trails the 1991 paroxysmal eruption. Residual melt left in the shallow conduit following the 1991 eruption may have reached density and viscosity contrasts that allowed fluid fractions to intrude more viscous but still ductile part of the magma. This process may have produced the small-sized 1992 dome. By analogy, we suggest that the 1991 pre-eruptive dome and LF events may have involved viscous materials left by the ∼500 year BP eruption and intruded by the then ascending melt.