Fault healing inferred from time dependent variations in source properties of repeating earthquakes

We analyze two sets of repeating earthquakes on the Calaveras fault to estimate in-situ rates of fault strengthening (healing). Earthquake recurrence intervals t, range from 3 to 803 days. Variations in relative moment and duration are combined to study changes in stress drop, rupture dimension, rupture velocity, and particle velocity as a function of tr. Healing rates and source variations are compared with predictions of laboratory derived friction laws. Two interpretations of event duration τ are used: one in which τ: is given by the ratio of slip to particle velocity and one in which it scales as rupture dimension divided by rupture velocity. Our data indicate that faults strengthen during the interseismic period. We infer that source dimension decreases with tr due to aseismic creep within the region surrounding the repeatinge vents. Stress drop increases 1-3MPa per decade increase in tr, which represents an increase of a factor of 2-3 relative to events with tr between 10 and 100 days. This rate of fault healing is consistent with extrapolations of laboratory measurements of healing rates if fault strength is high, on order of 60MPa, ands tress drop is roughly 10% of this value

Toward Forecasting Volcanic Eruptions using Seismic Noise

During inter-eruption periods, magma pressurization yields subtle changes of the elastic properties of volcanic edifices. We use the reproducibility properties of the ambient seismic noise recorded on the Piton de la Fournaise volcano to measure relative seismic velocity variations of less than 0.1 % with a temporal resolution of one day. Our results show that five studied volcanic eruptions were preceded by clearly detectable seismic velocity decreases within the zone of magma injection. These precursors reflect the edifice dilatation induced by magma pressurization and can be useful indicators to improve the forecasting of volcanic eruptions.Comment: Supplementary information: http://www-lgit.obs.ujf-grenoble.fr/~fbrengui/brenguier_SI.pdf Supplementary video: http://www-lgit.obs.ujf-grenoble.fr/~fbrengui/brenguierMovieVolcano.av

High-resolution image of Calaveras Fault seismicity

By measuring relative earthquake arrival times using waveform cross correlation and locating earthquakes using the double difference technique, we are able to reduce hypocentral errors by 1 to 2 orders of magnitude over routine locations for nearly 8000 events along a 35-km section of the Calaveras Fault. This represents ∼92% of all seismicity since 1984 and includes the rupture zone of the M 6.2 1984 Morgan Hill, California, earthquake. The relocated seismicity forms highly organized structures that were previously obscured by location errors. There are abundant repeating earthquake sequences as well as linear clusters of earthquakes. Large voids in seismicity appear with dimensions of kilometers that have been aseismic over the 30-year time interval, suggesting that these portions of the fault are either locked or creeping. The area of greatest slip in the Morgan Hill main shock coincides with the most prominent of these voids, suggesting that this part of the fault may be locked between large earthquakes. We find that the Calaveras Fault at depth is extremely thin, with an average upper bound on fault zone width of 75 m. Given the location error, however, this width is not resolvably different from zero. The relocations reveal active secondary faults, which we use to solve for the stress field in the immediate vicinity of the Calaveras Fault. We find that the maximum compressive stress is at a high angle, only 13° from the fault normal, supporting previous interpretations that this fault is weak

New Insight on the Increasing Seismicity during Tenerife's 2004 Volcanic Reactivation

Starting in April 2004, unusual seismic activity was observed in the interior of the island of Tenerife (Canary Islands, Spain) with much evidence pointing to a reawakening of volcanic activity. This seismicity is now analyzed with techniques unprecedented in previous studies of this crisis. The 200 earthquakes located onshore during 2004 and 2005 have been classified by cross-correlation, resulting in a small number of significant families. The application of a relative location algorithm (hypoDD) revealed important features about the spatial distribution of the earthquakes. The seismic catalogue has been enhanced with more than 800 additional events, detected only by the closest seismic station. These events were assigned to families by correlation and as a consequence their hypocentral location and magnitude were estimated by comparing them to the earthquakes of each family. The new catalogue obtained by these methods identifies two major seismogenic zones, one to the northwest and the other to the southwest of the Teide-Pico Viejo complex and having a separation of at least 10 km between them. These regions alternate their activity starting in January 2004, i.e., three months earlier than previously thought. We propose a simple model based on the results of this work which will also concur with all previous geophysical and geochemical studies of the 2004 crisis. The model proposes a single magma intrusion affecting the central part of the island with lateral dikes driven by the rifts to the northwest and southwest.Comment: 20 pages, 15 figure