Abstract
We calculated apparent stresses for 70 earthquakes (MW ≥ 5.0) occurring in the aftershock region of the 2010 MW8.8 Bio-Bío earthquake from January 1990 to September 2019. We identified that the average apparent stress was approximately 0.487 MPa between January 1990 and December 2005 and approximately 1.063 MPa within the period from January 2006 to January 2010. The latter one is 2.2-fold greater than the former, representing a significant difference as determined by a z test, with a 99% confidence level. Moreover, we analyzed the temporal evolution of the apparent stress and found that apparent stress rapidly increased from 0.43 to 1.2 MPa during the pre-event period from March 2006 to the occurrence of the Bio-Bío MW8.8 mainshock, and this increased apparent stress was found to be significant at the 98% confidence level. Furthermore, we calculated the spatial distribution of the apparent stress in the study region and observed two higher-apparent-stress regions, within one of which the epicenter of the MW8.8 event was located. On the basis of the inverse correlation between b value and stress, the temporal evolution and spatial distribution of b values were calculated and compared with those of the apparent stress. The comparison showed that the b values decreased approximately 4 years before the occurrence of the mainshock, while the apparent stress increased substantially; for the region of lower b, the apparent stress is higher, and vice versa. Therefore, the inverse correlation between b value and stress is supported by the results obtained in the present study and can be probably considered as one of the precursors to great earthquakes.
Highlights
On February 27, 2010, a strong earthquake struck central Chile
By analyzing earthquakes that occurred in the Bio-Bıo region, we assessed the variations in apparent stress as a function of time before the 2010 BioBıo MW8.8 mainshock and found a significant increase during the pre-mainshock period
We obtained the spatial distribution of the apparent stress and identified two distinct high-apparent-stress areas in the aftershock region, one of which was located near the epicenter of the MW8.8 mainshock
Summary
On February 27, 2010, a strong earthquake struck central Chile. The epicenter reported by the United States Geological Survey (USGS) was located at 36.122° S and 72.898° W, which is offshore to the west of Bio-Bıo, Chile. Some researchers have applied the apparent stress to study stress changes during the period prior to an earthquake. A recent study demonstrated an increase in the apparent stress before the mainshock followed by decreasing values (Picozzi et al 2019). A decreases in the b value in the Gutenberg–Richter relation, log N = a - bM, is interpreted to reflect a stress increase before an impending seismic mainshock (Scholz 1968b; Wyss 1973; Main et al 1989; Urbancic et al 1992; Hainzl et al 1999). There exists no previous research related to regional stress changes during the years before the 2010 Bio-Bıo MW8.8 earthquake. We will focus on regional stress changes before this earthquake through apparent stress and b value to expose how tectonic stress changed during the preevent period in and around the source region
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