Temporal variations of non-volcanic tremor (NVT) locations in the Mexican subduction zone: finding the NVT sweet spot
Allen HuskerV. KostoglodovV. M. Cruz‐AtienzaD. LegrandН. М. ШапироJ. S. PayeroMichel CampilloEduardo Huesca–Pérez
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Epicentral locations of non-volcanic tremors (NVT) in the Mexican subduction zone are determined from the peak of the energy spatial distribution and examined over time. NVT is found to occur persistently at a distance of ∼215 km from the trench, which we term the Spot because this region probably has the proper conditions (i.e., temperature, pressure, and fluid content) for the NVT to occur with minimum shear slip. High-energy NVT episodes are also observed every few months, extending ∼190 km to ∼220 km from the trench with durations of a few weeks. During the 2006 slow slip event (SSE) the duration and the recurrence rate of the NVT episodes increased. Low-energy episodes were also observed, independent from the high-energy episodes, ∼150 km to ∼190 km from the trench during the 2006 SSE. Both the high and low energy episodes were made up of many individual NVT's that had a range of energy-release-rates. However, the highest energy-release-rates of the high-energy episodes were consistently double those of the low-energy episodes and the persistent activity at the Sweet Spot. We suggest that all of the high-energy episodes are evidence of small, short repeat interval SSE. Given this model, the increased recurrence rate of the high-energy NVT episodes during the 2006 long-term SSE implies that short-term SSE's also increase during the SSE and are therefore triggered by the SSE.Cite
We have investigated the characteristics of short‐term slow slip events (SSEs), recurrence interval and size distribution, and the slip rate at the transition zone on the plate interface beneath the Shikoku region, Japan, using nonvolcanic deep low frequency (DLF) tremors. On the basis of a proportional relationship between the seismic moment of SSE observed geodetically and the total size of DLF tremors of the corresponding episode, we estimated the seismic moment due to the slip on the plate interface from the DLF tremors and a temporal variation in the cumulative seismic moment. The recurrence interval of major short‐term SSEs is ∼6 months in the western area and 3 months in the central and the eastern areas. The size distribution of short‐term SSEs as well as DLF tremors is approximated by an exponential law rather than by a power law, showing a different scaling for regular earthquakes. The average slip rate at the transition zone estimated from the cumulative seismic moment with time of SSEs is 4.2 cm/yr, 3.3 cm/yr, and 4.9 cm/yr in the western, central, and eastern areas, respectively. These values compensate for the difference between the convergence rate at the trench and the slip deficit rate at the transition zone of the subducting Philippine Sea plate. In other words, the slip rate estimated from the DLF tremors provides a constraint of the slip deficit rate at the transition zone on the plate interface.
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A network of continuously recording GPS stations has operated in the Nicoya Peninsula of northern Costa Rica since 2002. We processed all available data from this network for the period of 2002–2011 to investigate the occurrence of Slow Slip Events (SSE) on the subduction interface between the Cocos and Caribbean plates. In order to overcome signal masking by high levels of tropospheric noise, we developed a new technique that facilitates detection of transient events in the presence of noise. We identified five significant SSEs during the 2002–2011 period, with event middle times in 2003, 2005, 2007, 2009 and 2011, with an average recurrence interval of 21 ± 6 months. Time series analysis shows that transient deformation imparts a signature similar to random walk. Removal of the SSEs and regional common mode errors from the time series reduced velocity uncertainty by nearly an order of magnitude. Limited available data for the 2003, 2005 and 2011 events preclude detailed characterization of these events. However, good spatiotemporal coverage of the 2007 and 2009 events suggest that both events had irregular duration and distribution. In the 2007 event, slow slip started in the northwest coastal area and migrated southeastward over a period of ∼1 month. The 2009 event had a significantly longer event duration and larger surface displacement. Stations in the northwest area observed two separate SSEs in 2008.6 and 2009.4, correlating well with the tremor episodes offshore, indicating a shallow SSE slip patch with shorter recurrence interval. Significant differences between the 2009 and 2007 events lead us to question the simple recurrence interval model for the SSE in Nicoya.
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Compilation and synthesis of neotectonic data from the Great Basin region (western U.S.), based on 173 published studies for 171 faults across the region, offer an unprecedented view into the spatiotemporal evolution of strain release in continental domains, at time scales of 1 k.y. to 1 m.y.
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Epicentral locations of non‐volcanic tremors (NVT) in the Mexican subduction zone are determined from the peak of the energy spatial distribution and examined over time. NVT is found to occur persistently at a distance of ∼215 km from the trench, which we term the “Sweet Spot” because this region probably has the proper conditions (i.e., temperature, pressure, and fluid content) for the NVT to occur with minimum shear slip. High‐energy NVT episodes are also observed every few months, extending ∼190 km to ∼220 km from the trench with durations of a few weeks. During the 2006 slow slip event (SSE) the duration and the recurrence rate of the NVT episodes increased. Low‐energy episodes were also observed, independent from the high‐energy episodes, ∼150 km to ∼190 km from the trench during the 2006 SSE. Both the high and low energy episodes were made up of many individual NVT's that had a range of energy‐release‐rates. However, the highest energy‐release‐rates of the high‐energy episodes were consistently double those of the low‐energy episodes and the persistent activity at the Sweet Spot. We suggest that all of the high‐energy episodes are evidence of small, short repeat interval SSE. Given this model, the increased recurrence rate of the high‐energy NVT episodes during the 2006 long‐term SSE implies that short‐term SSE's also increase during the SSE and are therefore triggered by the SSE.
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Abstract Two earthquakes of ML = 5.4 occurred within half an hour of each other within the Hilea area of Southern Hawaii on 21 January 1982. The aftershock distribution suggests that together they ruptured an area of approximately 5-km radius, and their joint equivalent magnitude was 5.6. The first motions indicate faulting on a near horizontal plane at 10 km depth, with the crust slipping to the southeast. The seismicity rate in the source area was studied using the earthquake catalog of the Hawaiian Volcano Observatory. This catalog contains some reporting rate changes that affect the count of smaller earthquakes strongly, with a period of low reporting during 1974 to 1977. Although we have used 2.5 as the minimum magnitude of homogeneous reporting after 1971, some of the artificial rate changes are still present in the data. The catalog was declustered using Reasenberg's algorithm, and a magnitude correction of −0.1 was applied to the data between 1974 and 1984. The seismicity rate for the period of November 1971 through 1985 was examined in four adjacent regions; one of these contained the aftershocks. The aftershock volume and the 3-km annulus around it showed a period of 46 weeks of low seismicity rate immediately before January 1982, during which the rate was decreased by 87 per cent. The seismicity rate in the other three volumes was normal during this time. We conclude that this low reporting rate was not likely due to artificial changes in the catalog. The fact that the quiescence anomaly coincided in space with the 1982 aftershock volume and that its termination coincided with the 1982 main shocks suggests that the quiescence was a precursor. Comparing the seismicity rate within all possible 46-week windows to the background rate, we found that the precursory rate decrease was more significant than any other rate decrease in all volumes studied except artificial low rate periods. Thus this quiescence precursor could be recognized without false alarms. However, the statistical significance estimated by the z- and β-tests was low, ≦75 per cent and ≦49 per cent, respectively. More case histories are needed to determine empirically the thresholds of these tests for accepting precursory anomalies without too many false alarms. The M = 6.6 Kaoiki earthquake that was located about 25-km north of the 1982 source area was preceded by 125 weeks of quiescence, while the M = 7.2 Kalapana earthquake quiescence precursor lasted about 200 weeks. These observations suggest that in Hawaii quiescence precursor times may be a function of magnitude.
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Abstract We determined m^b, the body-wave magnitude calculated from the maximum amplitude of short-period P waves, of 38 large earthquakes in various tectonic provinces. The data are divided into three groups: group 1 (subduction-zone thrust events); group 2 (nonsubduction-zone dip-slip and oblique-slip events); and group 3 (strike-slip events). Groups 2 and 3 include intraplate earthquakes. Comparison of m^b values for these three groups of events suggests that the source spectral amplitudes of intraplate events at a period of about 1.4 sec is 2 to 5 times larger than those of subduction-zone events with the same Mw. We also determined the source spectra of 28 large earthquakes (Mw = 6.5 to 7.7) directly from Global Digital Seismographic Network (GDSN) data, over a period range from 1 to 10 sec. At periods from 1 to 2 sec, the source spectral amplitudes of intraplate earthquakes are 2 to 4 times larger than those of subduction-zone events with the same Mw. The difference decreases as the period increases to 10 sec.
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Following the episodes of inflation of the resurgent dome associated with the May 1980 earthquake sequence (four M 6 earthquakes) and the January 1983 earthquake swarm (two M 5.2 events), 7 years of frequently repeated two‐color geodimeter measurements spanning the Long Valley caldera document gradually decreasing extensional strain rates from 5 ppm/yr in mid‐1983, when the measurements began, to near zero in mid‐1989. The corresponding seismic activity within the caldera persisted at a low rate of fewer than 10 M ≥ 1.2 earthquakes per week from 1985 through November 1989 with no events exceeding M 3.0. Early October 1989 marked a change in activity when measurements of the two‐color geodimeter network showed a significant increase in extensional strain rate (9 ppm/yr) across the caldera. The seismic activity began exceeding 10 M ≥ 1.2 per week in early December 1989 and rapidly increased to a sustained level of tens of M ≥ 1.2 per week with bursts having hundreds of events per day. Many events exceeded M 3.0 and the largest event was M ≈ 4. The 1989–1991 inflation episode is the first time that we have sufficient geodetic measurements in Long Valley to define the temporal relation between onset of an inflation episode and onset of brittle failure (earthquake swarm within the caldera). Here, the onset of deformation preceded the onset of increased earthquake activity by more than 2 months. The seismicity rate began to decrease in mid‐July 1990, consistent with a gradually slowing of extension across the caldera as measured by the two‐color geodimeter. The recent episode of inflation can be modeled by a single Mogi point source located about 7 km beneath the center of the resurgent dome. In contrast, the deformation pattern observed between mid‐1983 and mid‐1989 is best reproduced by fault slip in the south moat, inflation at 6.5 km depth near Casa Diablo Hot Springs and inflation beneath the resurgent dome. It appears that the 7‐km source beneath the resurgent dome that was active for the earlier episodes is the primary source for the more recent episode. The model used to satisfy the line length observations predicts 7.5 cm of uplift along leveling route along highway 395 from mid‐1983 to mid‐1989 and an additional 11 cm through the end of 1991. To comp are with the energy release from seismicity, the modeled inflation from late 1989 through the end of 1991 has a moment that is a factor of 40 more than the cumulative seismic moment from earthquakes located within the caldera from the same period. Thus the recent inflation episode represents a significant portion of the observed geodetic deformation with only little seismic energy release.
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