Validating Seismic Tomography Results for the 1992 Eruptions of Mount Spurr, Alaska

In previous studies of Mount Spurr, Alaska, seismic tomography has been used to locate hydrothermally altered zones and magmatic pathways during the 1992 eruptions. In this study we investigate the applicability of new tomography techniques to this problem. We use double-difference tomography (tomoDD) to determine P-wave velocity structure and improved earthquake locations using catalog travel time picks. The challenge for this and similar studies is the lack of dense station coverage in the mountainous terrain surrounding Mount Spurr. In 1992, Mount Spurr erupted three times from its Crater Peak vent. Seismicity was most active at shallow depths at the summit (-2 to 1 km depth) and at depths of 0-5 km and 10-20 km at the Crater Peak vent. Based on our relocated hypocenters, seismicity in these three regions form clusters reflecting structures within the volcano. At the summit, the hypocenters define structures that dip shallowly to the south. Focal mechanisms indicate that these are normal faults. The two clusters of seismicity beneath Crater Peak define two vertical columnar conduits associated with magma ascent during the eruptive sequence in 1992. Tomography reveals P-wave low-velocity anomalies as low as 4.5 km/s at depths of 1-4 km and 10-15 km beneath the Crater Peak vent. These anomalies are co-located with the two areas of seismicity. No velocity anomaly was observed in the summit region, which has not erupted for over 5,000 years. To investigate whether our tomography study is well resolved, we calculate travel times for a synthetic model using the 1992 station and hypocenter coverage and reinvert for the synthetic velocity model. TomoDD accurately recovers the 4.5 km/s P-wave velocity bodies modeled in our synthetic dataset beneath Crater Peak, indicating that this technique is robust for Mount Spurr despite its relatively sparse network.