**3. Crustal faults activity triggered during and following Maule Earthquake**

#### **3.1 Pichilemu Fault**

On March 11, 2010, a series of earthquakes started at Pichilemu zone (34.3°S/72°W) with an Mw = 6.9 event, followed by several events of Mw = 5.5–6.5 in the next minutes. Then a crustal seismic activity continued in such zone to date. The focal mechanism of this earthquake was normal, with strike NW-SE, and the seismic activity is located in an alignment of such orientation with 50 km length [19, 20, 27]. Farias et al. [20] had fieldwork at Pichilemu after Maule earthquake but before March 11 event, indicating 0.2 m uplift. Quezada et al. [21] visited Pichilemu littoral zone in April 2010, and it exists at that date at a subsidence of around 50 cm. For this reason, the true dip of the NW-SE fault is toward SW. The littoral subsidence of the Pichilemu fault is bigger than the former uplift generated by Maule earthquake. The aftershocks of Pichilemu fault are separated from the events of the Wadati-Benioff plane [20, 27, 28]. This fault is explained because their strike is normal to the maximum extensional axis (T axis) of the rupture in Wadati-Benioff zone at this latitude [19].

#### **3.2 Santa María Fault**

A normal fault was activated close to the northern tip at Santa María Island (37°S) of NE-SW strike and visible open cracks and scarp until 30 cm height. The length of the trace is 600 m. Not significant seismicity or aftershocks are recognized near to this fault [27, 28]. One explanation of this fault is that this corresponds to a shallow extension due to a deeper splay fault of thrust mechanism [23]. This hypothesis was refused by Allmendinger et al. [22] that considers that this fault was reactivated due to the orientation of such fault normal to the maximum extensional axis (T axis) of Maule earthquake main rupture, at Wadati-Benioff zone in this latitude. In subduction earthquakes, the distribution of the T axis changes being normal to a curve trenchward concave between both tips of the main fault. It is the same explanation of the Pichilemu fault [19].

#### **3.3 Tirua Mocha Fault**

The vertical movements at the southern zone of Maule earthquake (38.3–38.4°S) are not typical trend with decreasing uplift from trench to arc. In fact, Mocha Island experienced an uplift of 0.25–0.3 m, whereas Tirua place, located at the continental margin in front of Mocha Island, experienced 0.9 m uplift. This anomaly was explained by the existence of a splay fault between Tirua and Mocha Island [5]. The activity of such fault began after 1960 earthquake due to the development of a strong asperity at the updip of Wadati-Benioff zone. This splay fault is synthetic, west vergent, also thrust movement. The main deformation due to the interplate convergence was in such fault explaining the permanence of the 1960 coseismic uplift at Mocha Island and the big interseismic uplift at Tirúa. The strong asperity remained even during Maule earthquake, and the main slip occurred along Tirua-Mocha splay fault explaining the abnormal uplift trend. Significant earthquakes of Mw = 6 magnitude occurred in such zone in the following months, evidencing the instability conditions, and finally, on January 2, 2011, an Mw = 7.1 earthquake occurred in such area that generated 0.5 m uplift at Mocha island and 0.2 m subsidence at Tirúa, indicating the final removal of the asperity at the updip of Wadati Benioff zone. Hicks and Rietbrock [29] indicated hybrid mechanism with thrust movement along Wadati-Benioff zone and a crustal normal fault. Quezada et al. [5] indicated that such normal fault was the Tirua-Mocha fault that experienced tectonic inversion because the movement is in a shallower position at the Wadati Benioff zone, being favorable conditions of such opposite movement.

#### **3.4 Seismicity linked to the crustal faults**

From the Centro Sismológico Nacional-U de Chile, we collected those seismic events occurred between January 2010 and November 2022 around Pichilemu (34°S), Santa María Island (37°S), and Tirúa-Mocha (38.3°S) areas. Around each of these locations, seismicity is exposed on a map view and along an ENE-WSW profile: in both cases, a subduction azimuth of 82° is considered (**Figures 2**–**7**). Distribution of seismicity is shown in circles according to its magnitude range: M < 3.5 cyan color, 3.5 < M < 4.5 yellow color, 4.5 < M < 5.5 red color, 5.5 < M < 6.5 green color, and 6.5 < M < 7.5 blue color. Distribution of seismicity and magnitudes in time are exposed in graphs. Here it can be seen that this data set is fully influenced by aftershocks linked

#### **Figure 2.**

*Seismicity at Pichilemu. It is noted in the profile the crustal seismicity in the littoral zone related to Pichilemu fault.*

*Crustal Faults Reactivated during 2010 Mw = 8.8 Maule Earthquake in South Chile DOI: http://dx.doi.org/10.5772/intechopen.109564*

#### **Figure 3.**

*Seismicity at Santa Maria Island. Shallow seismicity exists between the trench and littoral, but it is difficult to separate crustal from interplate events.*

**Figure 4.**

*Seismicity at Tirúa-Mocha Island. Shallow seismicity exists close to Tirúa that could be related to Tirúa-Mocha fault.*

#### **Figure 5.**

*Frequency of events and magnitude at Pichilemu.*

**Figure 6.** *Frequency of events and magnitude at Santa Maria Island.*

#### **Figure 7.**

*Frequency of events and magnitude at Tirúa-Mocha Island.*

with 2010 Maule Earthquake (Mw = 8.8). Error location for seismic events can be considered up to 10 km around determined hypocenter.

#### **3.5 Seismicity linked to the crustal faults**

As it can be seen in Pichilemu, Santa Maria Island, and Pichilemu areas, it exists as a continuous seismicity from trench to arc (**Figures 2**–**4**). However, the vertical profiles show separated clusters. The slab is clearly defined. Crustal seismicity occurs at Andean Cordillera. This seismicity must be related to the strike slip partitioning and compressive crustal faults linked to the growing of this relief. The magnitudes of these events are decreasing from north to south. At Pichilemu latitude (34°S, **Figure 2**), the biggest magnitude reached Mw = 5.5, at Santa Maria

#### *Crustal Faults Reactivated during 2010 Mw = 8.8 Maule Earthquake in South Chile DOI: http://dx.doi.org/10.5772/intechopen.109564*

Island latitude (37°S, **Figure 3**), the magnitude reached Mw = 4.5, and at Tirúa-Mocha Island latitude (38.3°S, **Figure 4**), the magnitude reached Mw = 3.5. Slab pull seismicity is visible in all profiles with depth hypocenters bigger than 50 km increasing depth eastward illuminating the slab. The number of events and bigger magnitude also occur at the northern profile at Pichilemu.

Between the trench and the littoral zone, important seismic activity being interplate and crustal exists. The Pichilemu profile shows a big amount of seismicity in the littoral zone linked to the Pichilemu fault with magnitudes reaching Mw = 5.5. West of this cluster until the trench, lesser shallow seismicity is noted. This light is better than the littoral seismic zone linked to the Pichilemu fault. The Santa María Island profile also displays shallow seismicity between the trench and littoral zone, but separate crustal seismicity to interplate one is more difficult, but crustal seismicity exists. Magnitudes are lesser than Mw = 5.5. At Tirúa-Mocha Island profile, crustal activity also exists between the trench and Tirúa, but with fewer events. Despite this, the hypocenters are located close to Tirúa and may illuminate the Tirúa-Mocha fault with some events of magnitude reaching Mw = 5.5.

About the frequency of the seismicity, it displayed the number of events every 7 days and the magnitude every 7 days between 2010 and 2022 (**Figures 5**–**7**). At Pichilemu (**Figure 5**), during all 2010 year occurred until 200 events every 7 days decreasing later stabilizing in 10 events since 2013, with some specific days with more activity. Average magnitude of these events is 3, but the maximum magnitudes decreased from Mw = 7 in 2010 to Mw = 5.5 since 2012 with some isolated events later.

The Santa María profile (**Figure 6**) shows a maximum of 40 events per 7 days in 2010 decreasing to 5 events, but since 2017, a small increase of the amount of events with some days with a big amount of seismicity exists. It is typically when occurred an event of magnitude between Mw = 5–6, then an aftershock sequence takes place related to the first event. Between 2010 and 2012, the average magnitude was between Mw = 3.5–4.5; then until 2021, it was Mw = 3, and since 2022, it increases.

The Tirúa-Mocha profile (**Figure 7**) shows important seismic activity between 2010 and 2012. Also occurred significant events of magnitudes of Mw = 6–7. The fourth peak corresponds to the Mw = 7.1 Araucania earthquake of January 2, 2011. Then the average magnitude of the events stabilized at Mw = 3.5, and since 2020, the amount of seismicity increased slowly.
