Luciana Astiz

Temporal variation of large intraplate earthquakes in coupled subduction zones

The focal mechanisms of intraplate earthquakes within subducting lithosphere are frequently used to infer large-scale stress regimes induced by slab-pull, bending or unbending, and lateral segmentation and undulations of the slab. Numerous studies have further postulated that the intraplate activity is influenced by transitory regional stress regimes such as those associated with interplate thrust events. Temporal variations of the latter type may potentially play an important role in assessing regions of uncertain seismic potential, and possibly even in earthquake forecasting. A systematic analysis of 1130 focal mechanisms for intraplate earthquakes with m_b ≥ 5.0 in the depth range 0–300 km is conducted for nine circum-Pacific subduction zones, all of which are known to have large interplate thrust events. The spatial and temporal relationships of the earthquakes within the subducting slab to the large thrust events in each region are appraised. The earthquake catalog assembled contains all published focal mechanisms, and is probably complete for m_b ≥ 6.5 for the years 1963–1986. For many of the localized regions considered in detail the catalog is complete to lower thresholds of m_b ≥ 6.0 or m_b ≥ 5.5. This analysis provides compelling evidence for a temporal link between large interplate thrust activity and intraplate seismicity. For the seismically coupled regions considered here, outer rise compressional events have occurred prior to several large thrust events or are associated with seismic gaps, while outer rise tensional events generally only follow interplate ruptures. In the intermediate depth range, large down-dip tensional events generally precede interplate thrusts, and are often concentrated at the down-dip edge of the coupled zone. A transition to down-dip compressional stress or diminished tensional activity at intermediate depth is observed after several large thrust events (e.g., 1960 Chile, 1974 Peru, 1957 Aleutian, 1971 New Britain). These examples support the notion that the intraplate stress environment responds viscoelastically to the temporally varying interplate stress regime. Assuming that this concept is correct, the seismic potential of several seismic gaps is considered on the basis of both outer rise and intermediate depth earthquake activity.

Large intermediate-depth earthquakes and the subduction process

This study provides an overview of intermediate-depth earthquake phenomena, placing emphasis on the larger, tectonically significant events, and exploring the relation of intermediate-depth earthquakes to shallower seismicity. Especially, we examine whether intermediate-depth events reflect the state of interplate coupling at subduction zones, and whether this activity exhibits temporal changes associated with the occurrence of large underthrusting earthquakes. Historic record of large intraplate earthquakes (m_B ≥ 7.0) in this century shows that the New Hebrides and Tonga subduction zones have the largest number of large intraplate events. Regions associated with bends in the subducted lithosphere also have many large events (e.g. Altiplano and New Ireland). We compiled a catalog of focal mechanisms for events that occurred between 1960 and 1984 with M > 6 and depth between 40 and 200 km. The final catalog includes 335 events with 47 new focal mechanisms, and is probably complete for earthquakes with m_B ≥ 6.5. For events with M ≥ 6.5, nearly 48% of the events had no aftershocks and only 15% of the events had more than five aftershocks within one week of the mainshock. Events with more than ten aftershocks are located in regions associated with bends in the subducted slab. Focal mechanism solutions for intermediate-depth earthquakes with M > 6.8 can be grouped into four categories: (1) Normal-fault events (44%), and (2) reverse-fault events (33%), both with a strike nearly parallel to the trench axis. (3) Normal or reverse-fault events with a strike significantly oblique to the trench axis (10%), and (4) tear-faulting events (13%). The focal mechanisms of type 1 events occur mainly along strongly or moderately coupled subduction zones where a down-dip extensional stress prevails in a gently dipping plate. In contrast, along decoupled subduction zones great normal-fault earthquakes occur at shallow depths (e.g., the 1977 Sumbawa earthquake in Indonesia). Type 2 events, with strike subparallel to the subduction zone, and most of them with a near vertical tension axis, occur mainly in regions that have partially coupled or uncoupled subduction zones and the observed continuous seismicity is deeper than 300 km. The increased dip of the downgoing slab associated with weakly coupled subduction zones and the weight of the slab may be responsible for the near vertical tensional stress at intermediate depth and, consequently, the change in focal mechanism from type 1 to type 2 events. Events of type 3 occur where the trench axis bends sharply causing horizontal (parallel to the trench strike) extensional or compressional intraplate stress. Type 4 are hinge-faulting events. For strongly coupled zones we observed temporal changes of intermediate-depth earthquake activity associated with the occurrence of a large underthrusting event. After the occurrence of a large underthrusting event, the stress axis orientation of intermediate-depth earthquakes changes from down-dip tensional to down-dip compressional (e.g., 1960 Chile, 1974 Peru, 1982 Tonga and 1952 Kamchatka earthquakes), or the number of large intermediate events decreases for a few years (e.g., 1964 Alaska and 1985 Valparaiso earthquakes). We conclude that even though the stress changes induced by slab pull and slab distortion control the general pattern of intermediate-depth seismicity, spatial and temporal variations of the intraplate stress associated with interplate coupling are important in controlling the global occurrence of large intermediate-depth events.

An earthquake doublet in Ometepec, Guerrero, Mexico

On June 7, 1982 an earthquake doublet occurred in a gap near Ometepec, Guerrero, Mexico which had been given a high seismic potential. The two earthquakes (first event: M_s = 6.9, m_b = 6.0, 16.3°N, 98.4°W, d = 25 km; second event M_s = 7.0, m_b = 6.3, 16.4°N, 98.5°W, d = 8 km) of the doublet occurred within five hours of each other. We determine the source parameters of these events by inverting surface-wave data at a period of 256 s. The results are for the first event, strike = 116°, dip = 77°, slip = 88° and seismic moment of 2.8 × 10^(26) dyne • cm, and for the second event strike = 116°, dip = 78°, slip = 89° and seismic moment of 2.8 × 10^(26) dyne • cm. Modeling of long-period P waves suggests that the first event has a depth of 20 km and is represented by a single trapezoidal source time function, with an effective duration of 6 s. The second event is best modeled by two sources at depths of 15 and 10 km. The combined effective source duration time for the two sources is about 10 s. The ratio of the seismic moment, obtained from body waves to that from surface waves, is ∼ 0.5 for the first event and 1 for the second. Adding the seismic moment of the two events and considering the first week aftershock area, 3200 km^2, the stress drop is ∼4 bars. These results suggest that the first event, that involved a deeper asperity, caused an incremental stress change large enough to trigger the second event. If the two events of the doublet broke distinct areas of the subduction zone, the coseismic slip is 0.58 m, and accounts for about 25% of the total plate motion between the Cocos Plate and the North America Plate, accumulated since the last large earthquake in the region. Other doublets similar to the Ometepec doublet have occurred along the Middle America Trench during the past 70 years. A regional distribution of comparable-size asperities may be responsible for this relatively frequent occurrence of doublets and for the simplicity of earthquakes in the region. The high convergence rate, which produces rapid strain accumulation and short recurrence intervals for large earthquakes, and the smooth sea-floor subducted along the middle America Trench, may contribute to the homogeneous distribution of comparable size asperities. We found a relation, log T ≈ 1/3 log M_0 (T is the average recurrence time and M_0 is the average seismic moment) for large earthquakes along the subduction zone in the Guerrero-Oaxaca region, where the convergence rate and the properties of the subducted plate are considered relatively uniform. A simple asperity model predicts this relation.

GEOSCOPE Station Noise Levels

The noise level at GEOSCOPE seismograph stations operating in 1995 has been studied in order to quantify the quality of stations for periods ranging from 0.2 to 8000 sec. The power spectral density curves presented in this article are a useful tool for selecting stations as a function of signal-to-noise ratio in the frequency band of interest. Seismic-noise level is the lowest for continental stations in the entire frequency band. It is similarly low at most coastal stations (stations located less than 150 km away from the coast). Finally, the noise level is low for island stations at long periods but increases significantly for periods smaller than 20 seconds, and in particular in the period range of the microseismic peak. The noise level on horizontal components varies, in most stations, as a function of local time for periods greater than 20 sec, being higher during the day than during the night. Only stations located in cold areas with little daily temperature variations and stations installed in a long tunnel do not display these daily variations. There is no seasonal variations of short-period noise (periods less than 5 sec). For some continental stations, we observe variations in the amplitude of the 7-sec microseismic peak during the year. For all three components, the peak amplitude is higher and shifted toward longer periods in fall and winter than in spring and summer. This phenomenon can be explained by the increase of the number and the size of oceanic storms in fall and winter. Long-period seismic noise (periods greater than 30 sec) also varies for some stations as a function of the season; however, no systematic characteristics have been observed.

Crustal thickness of the Peninsular Ranges and Gulf Extensional Province in the Californias

We estimate crustal thickness along an east-west transect of the Baja California peninsula and Gulf of California, Mexico, and investigate its relationship to surface elevation and crustal extension. We derive Moho depth estimates from P-to-S converted phases identified on teleseismic recordings at 11 temporary broadband seismic stations deployed at ;318N latitude. Depth to the Moho is ;33 (63) km near the Pacific coast of Baja California and increases gradually toward the east, reaching a maximum depth of ;40 (64) km beneath the western part of the Peninsular Ranges batholith. The crust then thins rapidly under the topographically high eastern Peninsular Ranges and across the Main Gulf Escarpment. Crustal thickness is ;15-18 (62) km within and on the margins of the Gulf of California. The Moho shallowing beneath the eastern Peninsular Ranges represents an average apparent westward dip of ;258. This range of Moho depths within the Peninsula Ranges, as well as the sharp ;east-west gradient in depth in the eastern part of the range, is in agreement with earlier observations from north of the international border. The Moho depth variations do not correlate with topography of the eastern batholith. These findings suggest that a steeply dipping Moho is a regional feature beneath the eastern Peninsular Ranges and that a local Airy crustal root does not support the highest elevations. We suggest that Moho shallowing under the eastern Peninsular Ranges reflects extensional deformation of the lower crust in response to adjacent rifting of the Gulf Extensional Province that commenced in the late Cenozoic. Support of the eastern Peninsular Ranges topography may be achieved through a combination of flexural support and lateral density variations in the crust and/or upper mantle.

Earthquake Locations in the Inner Continental Borderland, Offshore Southern California

The inner Continental Borderland region, offshore southern California, is tectonically active and contains several faults that are potential seismic hazards to nearby cities. However, fault geometries in this complex region are often poorly constrained due to a lack of surface observations and uncertainties in earthquake locations and focal mechanisms. To improve the accuracy of event locations in this area, we apply new location methods to 4312 offshore seismic events that occurred between 1981 and 1997 in seven different regions within the Borderland. The regions are defined by either temporal or spatial clustering of seismic activity in the Southern California Seismic Network (SCSN) catalog. Obtaining accurate locations for these events is difficult, due to the lack of nearby stations, the limited azimuthal coverage, and uncertainties in the velocity structure for this area. Our location procedure is based on the L-1 norm, grid search, waveform cross-correlation method of Shearer (1997), except that we use a nearest neighbor approach (Astiz et al., 2000) to identify suitable event pairs for waveform cross-correlation and we explore the effect of different velocity models on the locations and associated station terms. In general, our relocated events have small estimated relative location errors and the events are more clustered than the SCSN catalog locations. A quarry on the south tip of Catalina Island provides a test of our location accuracy and suggests that, under ideal conditions, offshore events can be located to within 1 to 2 km of their true locations. Our final locations for most clusters are well correlated with known local tectonic features. We relate the 1981 Santa Barbara Island ( M L = 5.3) earthquake with the Santa Cruz fault, the 13 July 1986 Oceanside ( M L = 5.3) sequence with the San Diego Trough fault zone, and events near San Clemente Island with the known trace of the San Clemente fault zone. Over 3000 of the offshore events during this time period are associated with the 1986 Oceanside earthquake and its extended aftershock sequence. Our locations define a northeast-dipping fault plane for the Oceanside sequence, but in cross-section the events are scattered over a broad zone (about 4-km thick). This could either be an expression of fault complexity or location errors due to unaccounted for variations in the velocity structure. Events that occur near Coronado Bank in the SCSN catalog are relocated closer to the San Diego coast and suggest a shallow-angle, northeast-dipping fault plane at 10 to 15 km depth. Manuscript received 12 February 1999.

Precise relocations and stress change calculations for the Upland earthquake sequence in southern California

We relocate earthquakes that occurred near the 1988 (ML = 4.7) and the 1990 (ML = 5.5) Upland, California, earthquakes to map the fault geometry of the poorly defined San Jose fault and to test the static stress triggering hypothesis for this sequence. We adopt the L1 norm, waveform cross-correlation method of Shearer [1997] to obtain precise relocations for 1573 events between 1981 and 1997 in the Upland area. To limit computation time, we only perform waveform cross correlation on 60 of the nearest neighbors of each relocated event. Our final relocations show two linear features. The first is imaged by the locations of the initial month of aftershocks of the 1988 Upland earthquake, which delineate a fault with a dip angle of ∼45° between 7 and 9 km depth, consistent with the mainshock focal mechanism. The second linear feature is a plane dipping at about 74° from 2 to 9 km depth, which is illuminated by both the 1988 and 1990 Upland sequences, in agreement with the inferred location of the San Jose fault at depth. However, below 9 km the event locations become more diffuse, giving rise to two different interpretations of the fate of the San Jose fault at depth. One possibility is that the fault shallows at depth, consistent with our relocations but not with the focal mechanism of a ML = 4.7 deep aftershock. Alternatively, the fault may be offset at depth by the more shallow dipping fault strand broken during the 1988 earthquake. Using these inferred fault geometries, we compute stress changes resulting from slip during the mainshocks to test whether the relocated aftershocks are consistent with the hypothesis that more aftershocks occur where the change in static Coulomb failure stress is positive (on faults optimally oriented for failure). This requires an extension of previous models of changes in the failure stress to three dimensions and arbitrary fault orientation. We find that patterns of change in Coulomb failure stress differ little between the different fault geometries: all are nearly symmetric about the fault and so do not match the aftershock distribution, in which most of the off-fault events occur to one side of the fault plane.

The 1983 Akita-Oki Earthquake (MW = 7.8) and Its Implications for Systematics of Subduction Earthquakes

The source parameters of the May 26, 1983, earthquake off the coast of Akita prefecture (02h59m59.6sUT, 40.462°N, 139.102°E, 24 km, m b = 6.8, M S = 7.7) determined from long-period surface waves are: 1st nodal plane; dip = 30° (E21°S), slip angle = 115°; 2nd nodal plane; dip = 63° (W7°S), slip angle = 76°; Seismic moment = 5.9 × 1027 dyne cm (M W = 7.8). This mechanism is consistent with a plate model, recently suggested by several investigators, that places the boundary between the Eurasian and North American plates along the Japan Sea Coast of Honshu Island, Japan. Within the framework of this model, Honshu, Hokkaido and Sakhalin reside on the North American plate, and the Akita-Oki event represents subduction of the Eurasian plate beneath the North American Plate. The convergence rate V between the North American and Eurasia plates is 1.1 cm/year along the Japan Sea Coast of Honshu and the age T of the plate being subducted is estimated to be about 20 M years. For these values of V and T, a previously determined empirical relation between earthquake magnitude, convergence rate and plate age for large earthquakes at subduction zones predicts a magnitude (M W) of 8.0 which agrees very well with that of the Akita-Oki earthquake. This good agreement between the observed and predicted values of M W suggests that the empirical relation is valid for subduction zones with very small T and V such as the Juan de Fuca subduction zone, Pacific Northwest, for which V = 3 to 4 cm/year, T = 10 to 15 M years and M W = 8.4 has been estimated.

The Long-Lasting Aftershock Series of the 3 May 1887 Mw 7.5 Sonora Earthquake in the Mexican Basin and Range Province

We study local and regional body-wave arrival times from several seis- mic networks to better define the active regional fault pattern in the epicentral region of the 3 May 1887 Mw 7.5 Sonora, Mexico (southern Basin and Range Province) earthquake. We determine hypocenter coordinates of earthquakes that originated between 2003 and 2007 from arrival times recorded by the local network RESNES (Red Sismica del Noreste de Sonora) and stations of the Network of Autonomously Recording Seismographs (NARS)-Baja array. For events between April and December 2007, we also incorporated arrival times from USArray stations located within 150 km of the United States-Mexico border. We first obtained preliminary earthquake loca- tions with the Hypoinverse program (Klein, 2002) and then relocated these initial hypocenter coordinates with the source-specific station term (SSST) method (Lin and Shearer, 2005). Most relocated epicenters cluster in the upper crust near the faults that ruptured during the 1887 earthquake and can be interpreted to be part of its long- lasting series of aftershocks. The region of aftershock activity extends, along the same fault zone, 40-50 km south of the documented southern tip of the 1887 rupture and includes faults in the epicentral region of the 17 May 1913 (Imax VIII, MI 5.0-0.4) and 18 December 1923 (Imax IX, MI 5.7-0.4) Granados-Huasabas, Sonora, earthquakes, which themselves are likely to be aftershocks of the 1887 event. The long aftershock duration can be explained by the unusually large magnitude of the mainshock and by the low slip rates and long mainshock recurrence times of the faults that ruptured in 1887.

Were the May 2012 Emilia-Romagna earthquakes induced? A coupled flow-geomechanics modeling assessment

Seismicity induced by fluid injection and withdrawal has emerged as a central element of the scientific discussion around subsurface technologies that tap into water and energy resources. Here we present the application of coupled flow-geomechanics simulation technology to the post mortem analysis of a sequence of damaging earthquakes (Mw = 6.0 and 5.8) in May 2012 near the Cavone oil field, in northern Italy. This sequence raised the question of whether these earthquakes might have been triggered by activities due to oil and gas production. Our analysis strongly suggests that the combined effects of fluid production and injection from the Cavone field were not a driver for the observed seismicity. More generally, our study illustrates that computational modeling of coupled flow and geomechanics permits the integration of geologic, seismotectonic, well log, fluid pressure and flow rate, and geodetic data and provides a promising approach for assessing and managing hazards associated with induced seismicity.