Unusual seismic events in Mendocino area

The Mendocino Triple Junction is a structurally and seismically complex region where the North American, the Pacific and the Juan de Fuca-Gorda plates intersect. The triple junction at Mendocino is the junction of the northern part of the San Andreas fault, the Cascadia subduction zone and the Mendocino Transform fault. Several unusual seismic events have been recorded in this area, including non-volcanic tremors, repeating earthquakes and slow/low-stress-drop earthquakes (Figure 2.31).

Thus far we have concentrated our efforts on the slow earthquakes or low-stress-drop earthquakes. This class of events is identified by a large discrepancy ($\geq$0.5) between the moment magnitude $M_{w}$ and the local magnitude $M_{L}$, where $M_{w}$ is larger. Based on the Berkeley Moment Tensor catalog, at least 18 slow earthquakes have occurred and have been recorded between 1992 to the present with a $M_{w}$ range between 4.0 and 6.8. Smaller slow earthquakes may have occurred, but they have not been detected. Analyses show that some of the slow earthquakes have stress drops of 0.4 bar when the stress drop for a normal earthquake is between 10 and 100 bars. Whereas the corner frequency of a $M_{L}$$\approx$$M_{w}$$\approx$4 earthquake is about 2Hz, for a slow event ($M_{L}$=4 and $M_{w}$=5) the corner frequency is on the order of 0.1Hz, which is more appropriate for a $M_{w}$6.5+ event.

Figure 2.31: Map of the unusual seismic activity in Mendocino region. The green square is a BDSN station (JCC), the triangles are NCSN stations. Seismicity (gray points) has been relocated with hypoDD using the NCSN stations. Red circles are few slow earthquakes and small black circles show repeating earthquake seismicity.
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A catalog of relocated events shows a sequence of four low-stress-drop earthquakes which occurred in 1997 and 1999. All have a very similar location in latitude and longitude, but their depth varies in a range of 4 km. Because they are located at the same place on the Mendocino transform fault and also because they have about the same moment magnitude, these four events may appear as potential repeating slow-earthquakes.

Using a cross-correlation analysis with two of these four events as references, we have scanned about 6 years, between 2000 and the end of 2006, of continuous broadband data recorded at four stations of the Berkeley network (ORV, MOD, WDC and YBH) and filtered between 0.02 and 0.05 Hz. We run the cross-correlation analysis for the three components at each station and we study all the hits giving at least 60% of similarities with the references and seen at, at least, 3 stations.

We obtained a large number of hits due to large teleseisms. Because of the large S-P arrival time of the teleseisms, a positive result in the cross-correlation occurred for either the P-waves or S-waves. With this method, we have not detected any additional slow earthquakes in the Mendocino transform fault that we did not know about. This result may also be due to the choice made in the references.

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