An elevated number of earthquakes and an increase in the rate of ground swelling since mid-summer 1997 at Long Valley caldera has intensified the volcano-monitoring efforts of USGS and University scientists. The more than 10,000 small earthquakes beneath Long Valley caldera since July 1997 represents the second most energetic earthquake swarm since unrest at the caldera intensified in May 1980 and the rate of deformation in the caldera is the highest so far measured. Although the activity has not changed the "condition" of the caldera (and the potential for an eruption) according to the USGS response plan developed in the early 1990's, scientists are carefully monitoring the day to day and hour to hour changes in activity and making preparations to expand their efforts if the activity escalates even further.
What does all this mean? According to Dave Hill, USGS volcanologist in charge of the Long Valley monitoring studies, "The recent and continuing level of activity is the most energetic since 1983, when magma apparently was intruded into the caldera system but did not produce any eruptive activity." To date, however, the level of unrest at Long Valley has not matched that in 1983. Until scientists have opportunity to fully analyze and interpret the wealth of monitoring data being collected, it is difficult to determine whether the current heightened activity reflects possible magma intrusion, adjustments to changes in the hydrothermal envelope surrounding the magmatic system, or both.
Long Valley caldera and the nearby Mono-Inyo volcanic chain in east-central California form one of several geologically youthful volcanic systems in California that USGS scientists consider to pose a potential hazard to people living nearby from future volcanic eruptions. A strong earthquake swarm in May 1980 consisted of four magnitude 6 earthquakes near the southern margin of the caldera, and it marks the onset of unrest in the caldera that continues today. Between 1980 and the current prolonged earthquake activity, the unrest has been characterized by as many as 10 to 20 earthquakes daily with magnitudes less than M=2 and intermittent small to moderate earthquake swarms accompanied by steady uplift of the resurgent dome in the middle part of the caldera at a rate of about 1 inch (2 to 3 cm) per year. In 1983, a series of M=5 earthquakes struck beneath the caldera in a swarm that surpasses the current activity at this time. Such unrest strongly indicates that the magmatic system beneath the caldera and the Mono-Inyo volcanic chain are still active and capable of renewed volcanic eruptions. The USGS monitors the area intensively for earthquake activity, ground deformation, and gas and has developed a response plan in the event of increased unrest.
Here are a few excerpts from the USGS Long Valley website describing the recent earthquake activity that has been punctuated by several M=4 earthquakes.
January 5, 1998
A M=4.1 event at 6:11 AM on January 5 was located 6 miles ESE of Mammoth Lakes about 0.5 miles west of the Hot Creek/Airport junction along Highway 395. This earthquake was preceded by a pair of M=2.9 events at 4:21 AM and 5:16 AM on the 5th located 5 and 7 miles east of Mammoth Lakes, respectively.
December 31, 1997
The onset of an earthquake swarm in the south moat of the caldera (centered in the vicinity of the airport, 6 to 7 miles ESE of Mammoth Lakes) was marked by a M=4.8 earthquake at 12:37 PM on December 31. This swarm has included over 1,800 events through 9:45 AM on January 3rd as detected and located by the realtime computer system. Some 60 of these earthquakes have magnitudes of M=2.5 or greater and 16 have magnitudes of M=3.0 or greater. Swarm earthquakes with magnitudes of M=3.5 or greater include M=3.6 and M=3.7 events at 12:40 PM, a pair of M=3.5 events at 4:55 PM and 9:32 PM all on the 31st, and a M=3.8 event at 4:19 AM on the 1st.
December 29, 1997
Earthquake activity from the morning of December 29 through the morning of the 30th was dominated by a M=4.3 earthquake at 12:02 PM (PST) on the 29th. This earthquake was located in the Sierra Nevada block less than a mile south of the caldera boundary (in the vicinity of Sherwin Lakes and 5 miles SE of Mammoth Lakes). This earthquake was preceded by half a dozen small foreshock beginning at 1:46 AM. The M=4.3 earthquake itself was a complex event composed of two M>4 events in quick succession. Based on recordings from their broad-band seismic stations, U.C. Berkeley estimates that the largest of the "sub-events" had a moment magnitude of Mw=4.1. Seismic activity within the caldera increased following this M=4.3 earthquake with most of the activity concentrated near the airport and in a cluster just 2 miles E of Mammoth Lakes. The realtime computer system detected an located some 278 earthquakes during this 24-hour (through 10:00 AM on the 30th) including "aftershocks" to the M=4.3 earthquake.
December 4, 1997
A M=4.0 earthquake at 1:38 PM on the 4th was located just 1 mile SE of Mammoth Lakes (about 0.5 mile SE of the Minaret Mall) at a depth of 7.1 km (4.3 miles). The only other M>2.5 earthquake during the 24-period from the morning of the 4th through 6:00 AM on the 5th was a M=2.7 earthquake at 12:38 PM on the 4th located 2 miles SE of Mammoth Lakes. Seismic activity in the previous 24 hours also included just two earthquakes of M=2.5 or greater. The largest was a M=3.0 event at 11:54 AM on the 3rd located 6 miles ESE of Mammoth Lakes. The other was a M=2.8 event at 11:58 PM located 1 mile ESE of Mammoth Lakes. The number of smaller earthquakes detected and located by the realtime computer system had gradually diminished over the previous few days averaging just under 200 events per day detected and located by the realtime computer system during this 24-hour period.
November 30, 1997
A M=4.8 earthquake that occurred at 1:17 PM on Sunday afternoon (Nov 30th) indicates that it was centered at a depth of roughly 7.3 km (4.4 miles) beneath the eastern end of Meridian Blvd (near the Water District Offices about 1.5 miles ESE of the Mammoth Lakes post office). Data from the U.C. Berkeley "broad-band" seismic stations indicate that the magnitude of this earthquake was in fact closer to M=4.9 than the 4.8 value initially reported. This may explain why many people reported that this earthquake felt considerably stronger than the M=4.8 event that occurred at 9:21 AM on Saturday October 22nd. The magnitude scale is logarithmic, and a M=4.9 earthquake is roughly 40% more energetic than a M=4.8 earthquake. Sunday's earthquake was preceded by several small foreshocks, the largest of which were M=2.2 events at 1:14 PM and 1:15 PM. It was followed by more than 30 events with magnitudes of M=2.5 or greater, four of which had magnitudes of M=3.0 or greater. The latter include three M=3.0 events at 1:26 PM, 2:10 PM, and 3:05 PM, respectively, and a M=3.1 event at 3:16 PM all on Sunday afternoon (Nov 30th). The continuously monitored deformation data show minor changes in trend following this earthquake. The condition remains GREEN at this time, although Sunday's earthquake came close to one of the criteria for a YELLOW condition (one or more M=5.0 or greater earthquakes within the caldera).
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