scholarly journals Repeating Earthquake Detection and Characterisation in New Zealand: A Catalogue for the Raukumara Peninsula, Northern Hikurangi Subduction Margin

2021 ◽  
Author(s):  
◽  
Laura Hughes
Keyword(s):  
New Zealand ◽  
Cross Correlation ◽  
Active Faults ◽  
P Wave ◽  
Slow Slip ◽  
Normal Faulting ◽  
Repeating Earthquakes ◽  
Long Duration ◽  
Recurrence Intervals ◽  
Repeating Earthquake

<p>Repeating earthquakes provide a novel way of monitoring how stresses load faults between large earthquakes. In this thesis, we develop a method and composite criterion for identifying repeating earthquakes in New Zealand and present New Zealand’s first long-duration repeating earthquake catalogue. This thesis addresses three primary objectives: (1) develop a method and composite criterion for identifying repeating earthquakes; (2) build a long-duration catalogue of repeating earthquakes for the Raukumara Peninsula; and (3) apply the method and composite criterion in different tectonic settings to investigate whether it can be applied more broadly elsewhere in New Zealand. The systematic identification of repeating earthquakes in New Zealand provides the first step in being able to monitor the state of stresses of New Zealand’s active faults in situ throughout the earthquake cycle.  Studies elsewhere, particularly in Japan and California, have developed case-specific criteria for identifying repeating earthquakes. Building on these studies, we develop a method and composite criterion for identifying repeating earthquakes in New Zealand, focusing on seismicity around the Raukumara Peninsula. Our composite criterion states that for events to be identified as repeating earthquakes, two or more events must have a normalised cross-correlation of at least 0.95 at two or more seismic stations, when calculated for 75% of the earthquake coda. Sensitivity to correlation window length, filtering frequency-band and correlation threshold were tested during the development of the composite criterion. These tests indicated that small perturbations to the parameter thresholds did not affect our ability to detect repeating earthquakes using the composite criterion.  By applying our composite criterion to seismicity around the Raukumara Peninsula, we identified 62 repeating earthquake families occurring between 2003 and 2018, consisting of 160 individual earthquakes. These families have a magnitude range of MW 1.5–4.5, and have recurrence intervals and family durations of < 1–12 years. High-precision absolute and relative locations were calculated using manual phase picks and cross-correlation re-picking. Focal mechanisms for 56 of the families were also determined, using P-wave first motions, revealing predominantly strike-slip and normal faulting at shallow depths, low-angle reverse faulting along the subduction interface, and normal faulting in the subducting plate. We compared the timing of the repeating earthquakes to slow-slip events previously identified using geodetic measurements around the Raukumara Peninsula and observed that repeating earthquakes occurred during 26 of the 31 identified periods of slow-slip. We also compared the seismic moment– recurrence interval relationship of the Raukumara Peninsula repeating earthquakes to that of earthquakes near Parkfield, California, identified by Nadeau and Johnson (1998), and observed a similar functional relationship. Slip-rates of the Raukumara Peninsula repeating earthquake families were also calculated using a slip-rate–moment relationship and were found to vary from < 10mm/yr to 80mm/yr.  We applied the method and composite criterion developed for the Raukumara Peninsula to two other locations to ensure it could be applied successfully in other New Zealand regions with different seismotectonic characteristics. Using our workflow, we successfully identified four families in Marlborough, and three families around Fiordland. These families differ from those identified around the Raukumara Peninsula in that they had relatively short recurrence intervals and family durations, of 2 minutes– 15 months. The ability of the composite criterion to identify these families confirms its suitability for further studies of repeating earthquakes throughout New Zealand.</p>

scholarly journals Repeating Earthquake Detection and Characterisation in New Zealand: A Catalogue for the Raukumara Peninsula, Northern Hikurangi Subduction Margin

2021 ◽  
Author(s):  
◽  
Laura Hughes
Keyword(s):  
New Zealand ◽  
Cross Correlation ◽  
Active Faults ◽  
P Wave ◽  
Slow Slip ◽  
Normal Faulting ◽  
Repeating Earthquakes ◽  
Long Duration ◽  
Recurrence Intervals ◽  
Repeating Earthquake

<p>Repeating earthquakes provide a novel way of monitoring how stresses load faults between large earthquakes. In this thesis, we develop a method and composite criterion for identifying repeating earthquakes in New Zealand and present New Zealand’s first long-duration repeating earthquake catalogue. This thesis addresses three primary objectives: (1) develop a method and composite criterion for identifying repeating earthquakes; (2) build a long-duration catalogue of repeating earthquakes for the Raukumara Peninsula; and (3) apply the method and composite criterion in different tectonic settings to investigate whether it can be applied more broadly elsewhere in New Zealand. The systematic identification of repeating earthquakes in New Zealand provides the first step in being able to monitor the state of stresses of New Zealand’s active faults in situ throughout the earthquake cycle.  Studies elsewhere, particularly in Japan and California, have developed case-specific criteria for identifying repeating earthquakes. Building on these studies, we develop a method and composite criterion for identifying repeating earthquakes in New Zealand, focusing on seismicity around the Raukumara Peninsula. Our composite criterion states that for events to be identified as repeating earthquakes, two or more events must have a normalised cross-correlation of at least 0.95 at two or more seismic stations, when calculated for 75% of the earthquake coda. Sensitivity to correlation window length, filtering frequency-band and correlation threshold were tested during the development of the composite criterion. These tests indicated that small perturbations to the parameter thresholds did not affect our ability to detect repeating earthquakes using the composite criterion.  By applying our composite criterion to seismicity around the Raukumara Peninsula, we identified 62 repeating earthquake families occurring between 2003 and 2018, consisting of 160 individual earthquakes. These families have a magnitude range of MW 1.5–4.5, and have recurrence intervals and family durations of < 1–12 years. High-precision absolute and relative locations were calculated using manual phase picks and cross-correlation re-picking. Focal mechanisms for 56 of the families were also determined, using P-wave first motions, revealing predominantly strike-slip and normal faulting at shallow depths, low-angle reverse faulting along the subduction interface, and normal faulting in the subducting plate. We compared the timing of the repeating earthquakes to slow-slip events previously identified using geodetic measurements around the Raukumara Peninsula and observed that repeating earthquakes occurred during 26 of the 31 identified periods of slow-slip. We also compared the seismic moment– recurrence interval relationship of the Raukumara Peninsula repeating earthquakes to that of earthquakes near Parkfield, California, identified by Nadeau and Johnson (1998), and observed a similar functional relationship. Slip-rates of the Raukumara Peninsula repeating earthquake families were also calculated using a slip-rate–moment relationship and were found to vary from < 10mm/yr to 80mm/yr.  We applied the method and composite criterion developed for the Raukumara Peninsula to two other locations to ensure it could be applied successfully in other New Zealand regions with different seismotectonic characteristics. Using our workflow, we successfully identified four families in Marlborough, and three families around Fiordland. These families differ from those identified around the Raukumara Peninsula in that they had relatively short recurrence intervals and family durations, of 2 minutes– 15 months. The ability of the composite criterion to identify these families confirms its suitability for further studies of repeating earthquakes throughout New Zealand.</p>

scholarly journals Spatiotemporal analysis of repeating earthquakes near Porangahau, Hikurangi Margin, New Zealand

2021 ◽  
Author(s):  
Olivia Pita Sllim
Keyword(s):  
New Zealand ◽  
Cross Correlation ◽  
Slip Rate ◽  
Pacific Plate ◽  
Focal Mechanisms ◽  
Slow Slip ◽  
Slow Slip Events ◽  
Repeating Earthquakes ◽  
The Pacific ◽  
Temporal And Spatial

<p><b>The Hikurangi subduction zone beneath the eastern North Island, New Zealand exhibits a variety of fault-slip related phenomena including tsunami earthquakes, non-volcanic tremor, low-frequency earthquakes, episodic slow slip, and repeating earthquakes. The northern Hikurangi margin hosts shallow slow-slip and is weakly coupled to shallow depths. In contrast, the southern Hikurangi margin is strongly coupled, and only deep slow-slip has been observed. The transition in coupling occurs beneath the township of Porangahau, and is an exemplary focus region for studying how this change in locking is accommodated. </b></p><p>To examine slip processes beneath Porangahau, we have constructed and analysed a catalogue of repeating earthquakes that occurred between 2004 and 2018. Repeating earthquakes are thought to re-rupture the same fault patch at different times, and thus have nearly identical waveforms, locations and magnitudes. Because repeating earthquakes represent cyclic loading, they can be used to detect temporal and spatial changes of slip-rate at depth and hence monitor how stress is transferred to seismogenic zones. </p><p>To build a catalogue of repeating earthquakes we first clustered the GeoNet earthquake catalogue by distance and correlation to identify potential repeating events. We then used a stronger cross-correlation threshold of at least 0.95 normalised cross-correlation value at three or more stations to identify repeating earthquakes from the initial clusters. This threshold was determined by our group's previous work on the northern Hikurangi margin. We identified 225 families of repeating earthquakes, with each family having two or more earthquakes in the 14-year study period from 2004 to 2018. </p><p>We carried out manual phase picking and polarity identification for the most recent event in each family and computed absolute locations, local magnitudes calibrated with moment magnitude, and high-quality focal mechanisms. For the rest of the events in each family, we conducted cross-correlation re-picking to obtain precise relative locations and relative magnitudes. With precise locations and well-constrained focal mechanisms, we determined whether the repeating earthquake families originated within the Pacific Plate, Australian Plate or on the subduction interface. Most of the families are located within the Pacific Plate, and the majority of families that originate on the subduction interface are located near the township of Porangahau. At least 220 of the 532 identified repeating earthquakes locate at the transition from strong- to weak-coupling of the subduction interface near the township of Porangahau. </p><p>A variety of slow slip events have been detected near Porangahau in the last two decades. Even though some repeating earthquakes correlate spatially and temporally with slow slip events, temporal and spatial correlations between slow slip events and repeating earthquakes are scarce and sparse. The majority of repeating earthquakes are located up-dip or down-dip of modelled slow slip patches, with very few families having spatial correlation with slow slip events. We obtained a moment-recurrence interval relationship for the catalogue of repeating earthquakes near Porangahau and compared it to the relationship obtained by Nadeau and Johnson (1998) at Parkfield, California. Finally, we computed slip-rates using the families located on the subduction interface and obtained an average slip-rate of 13 mm/yr. The insights gained from this study lay the groundwork for future work constraining processes of strain accumulation at the creeping-to-locked transition zone near Porangahau.</p>

scholarly journals Spatiotemporal analysis of repeating earthquakes near Porangahau, Hikurangi Margin, New Zealand

2021 ◽  
Author(s):  
Olivia Pita Sllim
Keyword(s):  
New Zealand ◽  
Cross Correlation ◽  
Slip Rate ◽  
Pacific Plate ◽  
Focal Mechanisms ◽  
Slow Slip ◽  
Slow Slip Events ◽  
Repeating Earthquakes ◽  
The Pacific ◽  
Temporal And Spatial

<p><b>The Hikurangi subduction zone beneath the eastern North Island, New Zealand exhibits a variety of fault-slip related phenomena including tsunami earthquakes, non-volcanic tremor, low-frequency earthquakes, episodic slow slip, and repeating earthquakes. The northern Hikurangi margin hosts shallow slow-slip and is weakly coupled to shallow depths. In contrast, the southern Hikurangi margin is strongly coupled, and only deep slow-slip has been observed. The transition in coupling occurs beneath the township of Porangahau, and is an exemplary focus region for studying how this change in locking is accommodated. </b></p><p>To examine slip processes beneath Porangahau, we have constructed and analysed a catalogue of repeating earthquakes that occurred between 2004 and 2018. Repeating earthquakes are thought to re-rupture the same fault patch at different times, and thus have nearly identical waveforms, locations and magnitudes. Because repeating earthquakes represent cyclic loading, they can be used to detect temporal and spatial changes of slip-rate at depth and hence monitor how stress is transferred to seismogenic zones. </p><p>To build a catalogue of repeating earthquakes we first clustered the GeoNet earthquake catalogue by distance and correlation to identify potential repeating events. We then used a stronger cross-correlation threshold of at least 0.95 normalised cross-correlation value at three or more stations to identify repeating earthquakes from the initial clusters. This threshold was determined by our group's previous work on the northern Hikurangi margin. We identified 225 families of repeating earthquakes, with each family having two or more earthquakes in the 14-year study period from 2004 to 2018. </p><p>We carried out manual phase picking and polarity identification for the most recent event in each family and computed absolute locations, local magnitudes calibrated with moment magnitude, and high-quality focal mechanisms. For the rest of the events in each family, we conducted cross-correlation re-picking to obtain precise relative locations and relative magnitudes. With precise locations and well-constrained focal mechanisms, we determined whether the repeating earthquake families originated within the Pacific Plate, Australian Plate or on the subduction interface. Most of the families are located within the Pacific Plate, and the majority of families that originate on the subduction interface are located near the township of Porangahau. At least 220 of the 532 identified repeating earthquakes locate at the transition from strong- to weak-coupling of the subduction interface near the township of Porangahau. </p><p>A variety of slow slip events have been detected near Porangahau in the last two decades. Even though some repeating earthquakes correlate spatially and temporally with slow slip events, temporal and spatial correlations between slow slip events and repeating earthquakes are scarce and sparse. The majority of repeating earthquakes are located up-dip or down-dip of modelled slow slip patches, with very few families having spatial correlation with slow slip events. We obtained a moment-recurrence interval relationship for the catalogue of repeating earthquakes near Porangahau and compared it to the relationship obtained by Nadeau and Johnson (1998) at Parkfield, California. Finally, we computed slip-rates using the families located on the subduction interface and obtained an average slip-rate of 13 mm/yr. The insights gained from this study lay the groundwork for future work constraining processes of strain accumulation at the creeping-to-locked transition zone near Porangahau.</p>

scholarly journals Repeating earthquakes and ground deformation reveal the structure and triggering mechanisms of the Pernicana fault, Mt. Etna

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Andrea Cannata ◽  
Adriana Iozzia ◽  
Salvatore Alparone ◽  
Alessandro Bonforte ◽  
Flavio Cannavò ◽  
...  
Keyword(s):  
Ground Deformation ◽  
Mount Etna ◽  
Fault System ◽  
Magma Intrusion ◽  
Repeating Earthquakes ◽  
Low Degree ◽  
Mt Etna ◽  
Recurrence Intervals ◽  
Triggering Mechanisms ◽  
Repeating Earthquake

AbstractStructure and dynamics of fault systems can be investigated using repeating earthquakes as repeatable seismic sources, alongside ground deformation measurements. Here we utilise a dataset of repeating earthquakes which occurred between 2000 and 2019 along the transtensive Pernicana fault system on the northeast flank of Mount Etna, Italy, to investigate the fault structure, as well as the triggering mechanisms of the seismicity. By grouping the repeating earthquakes into families and integrating the seismic data with GPS measurements of ground deformation, we identify four distinct portions of the fault. Each portion shows a different behaviour in terms of seismicity, repeating earthquakes and ground deformation, which we attribute to structural differences including a segmentation of the fault plane at depth. The recurrence intervals of repeating earthquake families display a low degree of regularity which suggests an episodic triggering mechanism, such as magma intrusion, rather than displacement under a constant stress.

Using P- to S- wave conversions from controlled sources to determine the shear-wave velocity structure along Hikurangi Margin Forearc, New Zealand

2020 ◽  
Author(s):  
Pasan Herath ◽  
Tim Stern ◽  
Martha Savage ◽  
Dan Bassett ◽  
Stuart Henrys ◽  
...  
Keyword(s):  
New Zealand ◽  
Pore Pressure ◽  
Shear Wave ◽  
P Wave ◽  
Unconsolidated Sediments ◽  
Eigenvalue Decomposition ◽  
Slow Slip ◽  
S Wave ◽  
Wave Velocities ◽  
Basement Rocks

&lt;p&gt;The Hikurangi subduction margin offshore of the east coast of New Zealand displays along-strike variations in subduction-thrust slip behavior. Geodetic observations show that the subduction-thrust of the southern segment of the margin is locked on the 30-100 year scale and the northern segment displays periodic slow-slip on the 1-2 year scale. It is hypothesised that spatial variations in pore-pressure may play a role in this contrasting phenomenon. Higher pore-pressures would result in lower effective stresses, which promote slow-slip of the subduction-thrust. In addition, the presence of a sedimentary wedge with very low shear wave-speeds in the northern Hikurangi margin has been proposed to fit the ultra-long duration of ground motions observed following the 2016 Kaikoura earthquake. Compressional (P-) wave velocities (V&lt;sub&gt;p&lt;/sub&gt;) of the subsurface provide useful information about the lithological composition. Combined with shear (S-) wave velocities (V&lt;sub&gt;s&lt;/sub&gt;), the V&lt;sub&gt;p&lt;/sub&gt;/V&lt;sub&gt;s&lt;/sub&gt; ratio which is directly related to Poisson&amp;#8217;s ratio can be obtained. This is a diagnostic property of a rock&amp;#8217;s consolidation and porosity. Typical V&lt;sub&gt;p&lt;/sub&gt;/V&lt;sub&gt;s&lt;/sub&gt; ratio of consolidated and crystalline rocks range from 1.6 to 1.9 and that of unconsolidated sediments can range from 2.0 to 4.0.&lt;/p&gt;&lt;p&gt;We use the controlled sources of R/V Marcus G Langseth recorded by a profile of 49 multi-component ocean bottom seismometers (OBS) along the Hikurangi margin forearc for the Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE) to derive the V&lt;sub&gt;s&lt;/sub&gt; structure and estimate the V&lt;sub&gt;p&lt;/sub&gt;/V&lt;sub&gt;s &lt;/sub&gt;ratio. The orientations of the horizontal components of each OBS are found by a hodogram analysis and by an eigenvalue-decomposition of the covariance matrix. Using the orientations, the horizontal components of each OBS are rotated into radial and transverse components. P to S converted phases are identified on the radial and transverse components considering their linear moveout, polarisation angle, and ellipticity. We confirm incoming S-waves to OBSs by comparing them with their hydrophone components. We identify both PPS (up-going P-wave after reflection or refraction converts to an S-wave at an interface) and PSS (down-going P-wave from the controlled source converts to an S-wave at an interface) type conversions. The identified conversion interfaces are the sediment-basement interface and the top of the subducting crust. The travel-time delay of a PPS type conversion relative to its P-wave arrival is indicative of V&lt;sub&gt;s&lt;/sub&gt; above the converting interface. The linear-moveout of PSS type conversions are indicative of V&lt;sub&gt;s&lt;/sub&gt; along the raypath after the conversion. Preliminary results from the southern Hikurangi margin suggest V&lt;sub&gt;p&lt;/sub&gt;/V&lt;sub&gt;s&lt;/sub&gt; ratios of ~1.70 for the basement rocks above the subducting crust and ~1.90 for the sediments overlying the basement rocks. These values indicate that the basement rocks are consolidated and less porous than the overlying sediments.&lt;/p&gt;&lt;p&gt;We expect to estimate the V&lt;sub&gt;p&lt;/sub&gt;/V&lt;sub&gt;s&lt;/sub&gt; ratios in the northern Hikurangi margin to assess the role played by pore-pressure in the along-strike variation in subduction-thrust slip behavior. We also expect to ascertain the presence and estimate the thickness of the low-velocity sediment wedge in the northern Hikurangi margin.&lt;/p&gt;

The 2019 Ms 4.2 and 5.2 Beiliu Earthquake Sequence in South China: Complex Conjugate Strike-Slip Faulting Revealed by Rupture Directivity Analysis

2021 ◽  
Author(s):  
Xiaohui He ◽  
Hao Liang ◽  
Peizhen Zhang ◽  
Yue Wang
Keyword(s):  
South China ◽  
Source Parameters ◽  
Active Faults ◽  
Fault Zones ◽  
Strike Slip ◽  
P Wave ◽  
Complex Conjugate ◽  
South China Block ◽  
Rupture Directivity ◽  
Time Duration

Abstract The South China block has been one of the most seismically quiescent regions in China, and the geometries and activities of the Quaternary faults have remained less studied due to the limited outcrops. Thus, source parameters of small-to-moderate earthquakes are important to help reveal the location, geometry distribution, and mechanical properties of the subsurface faults and thus improve the seismic risk assessment. On 12 October 2019, two earthquakes (the Ms 4.2 foreshock and the Ms 5.2 mainshock) occurred within 2 s and are located in southern South China block, near the junction region of the large-scale northeast-trending fault zones and the less continuous northwest-trending fault zones. We determined the point-source parameters of the two events via P-wave polarity analysis and regional waveform modeling, and the resolved focal mechanisms are significantly different with the minimum 3D rotation angle of 52°. We then resolved the rupture directivity of the two events by analyzing the azimuth variation of the source time duration and found the Ms 4.2 foreshock ruptured toward north-northwest for ∼1.0 km, and the Ms 5.2 mainshock ruptured toward east-southeast (ESE) for ∼1.5 km, implying conjugate strike-slip faulting. The conjugate causative faults have not been mapped on the regional geological map, and we infer that the two faults may be associated with the northwest-trending Bama-Bobai fault zone (the Shiwo section). These active faults are optimally oriented in the present-day stress field (northwest-southeast) and thus may now be potentially accumulating elastic strain to be released in a future large earthquake.

scholarly journals Normal Faulting and Volcanism in the Kora 3D Seismic Volume

2021 ◽  
Author(s):  
◽  
Ian Hurst
Keyword(s):  
New Zealand ◽  
Causal Relationship ◽  
Petroleum Industry ◽  
Local Stress ◽  
3D Seismic ◽  
Normal Faulting ◽  
New Information ◽  
Basin Formation ◽  
Geologic Uncertainty ◽  
Local Tectonics

<p>The spatial and temporal relationship between normal faulting and volcanism in offshore Western North Island, New Zealand can be used to gain insight into basin formation, hydrocarbon resources, regional tectonics, and large subduction processes. It is hypothesised that there is a causal relationship between volcanic activity and faulting, however, within the Taranaki Kora 3D seismic volume (survey) this relationship has not yet been explored. The overall aim of this thesis was to map and identify whether there is a relationship between volcanism and normal faulting within the Kora 3D survey.  A causal relationship in location and timing between volcanic processes and fault activity was discovered in this study. Two novel models were created to explain the creation of the local stress leading to this causal relationship. The first model uses intrusive magma build up and the second extrusive cone building to explain the changes in local stress. These models not only support the causal relationship between volcanism and faulting activity but also provide a new understanding into how Kora volcanic cone activity may have influenced active faulting in the Kora 3D survey.  Application of this new information will allow innovative insights into basin formation, regional and local tectonics, and subducting plate geometry in the Taranaki Basin. This research could be utilized to increase knowledge for prospecting and reduce geologic uncertainty, which is of importance for the New Zealand petroleum industry at this northern end of the Taranaki Basin.</p>

scholarly journals Preliminary paleoearthquake investigations of active faults on Awaji Island, Japan,
 in relation to the 1995 Great Hanshin (Kobe) earthquake

1996 ◽  
Vol 29 (3) ◽  
pp. 172-177
Author(s):  
R. Van Dissen ◽  
J. Begg ◽  
Y. Awata
Keyword(s):  
New Zealand ◽  
Active Fault ◽  
Active Faults ◽  
Geological Survey ◽  
Historical Records ◽  
Surface Rupture ◽  
Kobe Earthquake ◽  
Nojima Fault ◽  
One Year ◽  
Hanshin Earthquake

Approximately one year after the Great Hanshin (Kobe) Earthquake, two New Zealand geologists were invited to help with the Geological Survey of Japan's paleoearthquake/active fault studies in the Kobe/Awaji area. Trenches excavated across the Nojima fault, which ruptured during the Great Hanshin Earthquake, showed evidence of past surface rupture earthquakes, with the age of the penultimate earthquake estimated at approximately 2000 years. A trench across the Higashiura fault, located 3-4 km southeast of the Nojima fault, revealed at least two past surface rupture earthquakes. The timing of the older earthquakes is not yet known, but pottery fragments found in the trench constrain the timing of the most recent earthquake at less than 500-600 years. Historical records for this part of Japan suggest that within the last 700 years there has been only one regionally felt earthquake prior to the 1995 Great Hanshin Earthquake, and this was the AD 1596 Keicho Earthquake. It thus seems reasonable to suggest that the Higashiura fault was, at least in part, the source of the AD 1596 Keicho Earthquake.

Induced versus natural earthquakes: Search for a seismic discriminant

1980 ◽  
Vol 70 (1) ◽  
pp. 269-281
Author(s):  
William A. Peppin ◽  
Charles G. Bufe
Keyword(s):  
Active Faults ◽  
P Wave ◽  
S Wave ◽  
Spectral Parameters ◽  
San Andreas ◽  
Time Period ◽  
Seismic Stress ◽  
Stress Drops ◽  
Natural Earthquakes ◽  
Steam Production

abstract A sizeable body (150 records) of three-component, wideband (0.2 to 50 Hz) digital seismic data has allowed a direct comparison between earthquakes at The Geysers geothermal area, California and along nearby active faults of the San Andreas system. An attempt has been made to find analog or spectral parameters which would permit discrimination between 12 events within the steam production field and 30 outside it. Results of the study for both classes of events are: (1) seismic moments vary with local magnitude ML as log M0 = (1.06 ± 0.11) ML + 16.9 ± 0.1; (2) the ratio of vertical P-wave to horizontal S-wave spectral corner frequencies is near unity; (3) seismic stress drops are low (1.0 to 10 bars); and (4) focal mechanisms are quite comparable during the time period of this study.

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