The earthquake cycle of stress accumulation and release is associated with the elastic rebound hypothesis proposed by H.F. Reid following the M7.9 San Francisco earthquake of 1906. However, observing details of the actual values of time- and space-dependent tectonic stress is not possible at the present time. In previous research, we have proposed two methods to image the earthquake cycle in California by means of proxy variables. These variables are based on correlations in patterns of small earthquakes that occur nearly continuously in time. One of these is based on the construction of a time series by the unsupervised detection of small earthquake clusters. The other is based on expanding earthquake seismicity in PCA-derived patterns, to construct a weighted correlation time series. The purpose of the present research is to compare these two methods by evaluating their information content using decision thresholds and Receiver Operating Characteristic methods together with Shannon information entropy. Using seismic data from 1940 to present in California, we find that both methods provide nearly equivalent information on the rise and fall of earthquake correlations associated with major earthquakes in the region. We conclude that the resulting time series can be viewed as proxies for the cycle of stress accumulation and release associated with major tectonic activity. The figure shows the PCA patterns of small earthquakes associated with 5 major M>7 earthquakes in California since 1950.
Equatorial Guinea’s Bioko Island is located in the Atlantic Ocean off the west coast of Cameroon. Bioko is a volcanic island and the first off-shore expression of the Cameroon Volcanic Line. It is home to three shield volcanoes: Pico de Basile, Pico Biao, and San Carlos. Eruptive histories are not known for Pico Biao or San Carlos. Pico de Basile erupted within the past 100 years, and steam vents were observed as recently as 2012. Malabo, the capital city of Equatorial Guinea, sits in the shadow of Pico de Basile. There is no permanent seismic monitoring; the closest seismic stations are in Cameroon and have not reported data since 2015. In November 2017 Drexel University researchers, supported by the Bioko Biodiversity Protection Program (BBPP) and the Universidad Nacional de Guinea Ecuatorial (UNGE), installed 4 broadband seismometers. In February 2018, the data were retrieved, and stations serviced. Preliminary earthquake detection and location was completed using an automated STA/LTA algorithm. S wave arrivals were added manually. The initial locations use the global IASP91 model and events were relocated using a local model. The events detected cluster into two areas: those near Bioko Island and those near Cameroon. Between 12-Dec-2017 and 17-Feb-2018, 77 events were recorded. Local magnitudes range between 0.16 and 2.61. Of these events, 49 are located near Cameroon and 28 are near Bioko. Most of the depths are crustal, mostly upper to mid crust. Our preliminary results show there is seismicity associated with Bioko Island as well as Cameroon. The locations match well with events recorded by a local network installed in Cameroon in 2007. The four stations were serviced again in November 2018. One station failed due to water infiltration and one was vandalized within a week of the previous service. One station was still operational at service with only a few days down and the last station was operational until the height of the rainy season when power failed.