Joint Hypocenter Determination Method Research Articles

Determination of Earthquake Hypocenter Distribution using the Modified Joint Hypocenter Determination (MJHD) Method throughout the Palu-Koro Fault (Case Study: August-October, 2018)

The area of Sulawesi, especially along the Palu Koro Fault, is an area that is largely influenced by the confluence and movement of plates as well as regional fault activity pathways with high levels of seismicity. Determining the location of the hypocenter accurately through relocation is required in identifying the detailed tectonic structures in the area. Relocation of the hypocenter using the Modified Joint Hypocenter Determination (MJHD) method using the IASP91 velocity model in the period August to October 2018 with the arrival time data from BMKG catalog. The results of hypocenter relocation using the MJHD method show that from 132 earthquake distribution points to 63 earthquake hypocenter points after the relocation. The change in the location of the hypocenter was much denser along the Palu Koro Fault route than before the relocation as evidenced by the mean value of rms (root mean square) before relocation was 1.31 and after relocation it became smaller (0.61). Changes in parameter values after relocation using the MJHD method caused the distribution of the earthquake hypocenter to be tighter towards the Palu Koro fault than before the relocation, where the distribution had a random and scattered pattern.

Hypocenter relocation using 2006 aftershock data and the current seismic activity in the Yogyakarta region, Indonesia

On May 27, 2006, a significant earthquake Mw 6.4 struck Yogyakarta Area, Indonesia. In this study, we analyzed aftershocks waveform data from 16 temporary network stations of Geo Forschungs Zentrum (GFZ) located around the Yogyakarta Area. We determined the P-and S-wave arrival times manually, with the criteria recorded at least five stations starting from June 3 to July 5, 2006. We successfully relocated 1228 events consisting of 8122 P-and S-wave phases, respectively, and update the 1-D velocity model simultaneously using the Joint Hypocenter Determination Method. To confirm the aftershock distribution, we also analyze the seismic activity around the Yogyakarta Area starting from 2006 until 2019. Our results show that the aftershocks are divided into 2 clusters; cluster located in the southern part of Opak Fault and 10-20 km east of the Opak Fault. We proposed that the main fault rupture of the 2006 Yogyakarta earthquake located in the east of Opak Fault, where this zone is still seismically active until 2015. We conclude that the main fault rupture zone and the Opak Fault are joined in the same fault complex. However, further studies of this complex fault zone are still needed; hence, we will use the results of this study as input in conducting three-dimensional velocity and attenuation inversion.

Hypocenter relocation and velocity model for major earthquakes in northwest Himalaya

The proper understanding of earthquake geometry and location is very important to understand the seismicity of an area. In the present study, 185 earthquakes that occurred in the northwest Himalaya of magnitude Mw > 0.6 between 2010 and 2013 were relocated using the Modified Joint Hypocenter Determination method. After examining seven velocity models, average VP/VS ~ 1.73 computed by Wadati diagram is used to improve the hypocenter locations of the earthquakes. Based on hypocentral distributions, these earthquakes were classified into four groups. Group 1 is along the Punjal Thrust in Hazara Kashmir Syntaxes. Group 2 belongs to the Indus-Kohistan Seismic Zone whereas group 3 is along the Besham dome and the fourth group is along the Main Mantle Thrust. Shallow crustal earthquakes were relocated to 0–50 km, deep earthquakes were relocated to 50–100 km, and much deep earthquakes were relocated to 250–350 km depth. This analysis will be helpful to understand the seismicity of the northwest Himalaya of Pakistan.

KOREKSI LOKASI HIPOSENTRUM GEMPABUMI DI SULAWESI UTARA MENGGUNAKAN METODE MODIFIED JOINT HYPOCENTER DETERMINATION

North Sulawesi is one of the areas that has had a fairly high level of seismic activity that has occurred, due to the position of North Sulawesi Island which is located close to the earthquake source. One of the parameters of an earthquake that can be calculated is the earthquake hypocenter.Determining the hypocenter of an earthquake is very important in the world of seismology. The Modified Joint Hypocenter Determination (MJHD) method was carried out for relocation of the earthquake hypocenter. To find out how the results of the Hypocentrum Location Correction using MJHD and the tectonic pattern of the Sulawesi Sea Plate subduction zone. . The results of relocation using the MJHD (Moodifide Joint Hysopcentrum) method showed better and more accurate data quality seen from the residual value (RMS) before (≥1 s) and after relocation (≤1 S). Based on the seimicity results of relocation using the MJHD method, it was found that the earthquake clusters were concentrated at a shallow depth of the range of 20 - 80 km. At a depth of 250 - 300 and a distance of 400 km, there is a seimic gap so that earthquakes rarely occur in this zone.

Analysis of the mentawai region fault field based on earthquake relocation data using the modified joint hypocenter determination (MJHD) method

Tectonic conditions in an area are very important to know, one of them is the Mentawai region. Mentawai is an area prone to earthquake disasters, this is because the Mentawai is located between the subduction zone between the Eurasian Plate and the Indo-Australian Plate. One of the earthquake parameters that is needed in analyzing tectonic conditions is the hypocenter. Accurate hypocenter determination can be done by relocating earthquakes using the Modified Joint Hypocenter Determination (MJHD) method. The MJHD method is carried out by inversion of the main earthquake and aftershocks with correction of the station at the earthquake event. The earthquake data in this study began on September 1, 2017, until December 31, 2018, with 1 main earthquake and 104 aftershocks in the format of arrival time. The main earthquake before the relocation is at longitude coordinates 99.85o latitude -2.6o with a depth of 12 km. After being relocated it is at coordinates 99.5413o latitude 2.5779o with a depth of 29 km. The results after the relocation were identified were fault fields occurring in the B-B cross-section ‘nodal plane 2 with a strike of 141o 66o slip 92o. Earthquake relocation using the Modified Joint Hypocenter Determination (MJHD) shows a change in the Root Mean Square (RMS) value <1. This suggests that the earthquake hypocenter after relocation using the Modified Joint Hypocenter Determination (MJHD) method is more accurate.

RELOKASI HIPOSENTER GEMPABUMI MENGGUNAKAN METODE MODIFIED JOINT HYPOCENTER DETERMINATION (MJHD) UNTUK ANALISIS ZONA SUBDUKSI SUMATERA BAGIAN SELATAN

The part of south Sumatera is very vulnerable region in case of earthquake disaster caused by convergent boundary of two tectonic plates Indo-Australian Plates and Eurasian Plates. Precise hypocenter analysis is needed to understand about the accurate tectonic setting such as subduction zone in the area. Hypocenter relocation is used to recalculate earthquake hypocenter to become more accurate. To produce a more accurate hyposenter this hyposenter relocation is done by using the method of Modified Joint Hypocenter Determination (MJHD). Relocation using the Modified Joint Hypocenter Determination (MJHD) method uses IASP91 wave velocity which assumes that the inner structures are heterogeneous. In this study, used data P-wave and S-wave arrival time in the period January 2010 s.d December 2016 with coordinates -3.5º s.d -9º LS - 99º s.d 106.5º BT. The results of the relocation using MJHD showed a change of earthquake hypocenter shown by RMS (Root Mean Square) value ranging from 0.2 s.d 0.5. There are three subduction of the part in south sumatra. The subduction zone formed in Bengkulu is about 26.78º, the subduction zone of Lampung is around 30.225º and the subduction of the Sunda Strait is about 52.53º. Subduction zone of Bengkulu at depth of 250 km, Lampung and Sunda Strait at depth 400 km.

The First Study of 3-D Seismic Velocities Tomography Inversion to Delineate Reservoir Steam Zone at “FR” Geothermal Field West Java

Understanding reservoir conditions is one of the important aspects in fluid production monitoring activities in geothermal systems. The objective of this study to delineate the reservoir element especially the steam phase. It is the first study to requires mapping the existence and geometry of steam phase as an initial step to monitor reservoir conditions at “FR” Geothermal Field using the tomography travel time method. This study used seismogram data measured for 95 days and recorded by 11 seismometers. Arrivals time picking results get 213 microearthquake events with a minimum recorded by 3 recording seismometers. Hypocenter relocation is done by two methods in combination. The first use Joint Hypocenter Determination (JHD) and second use Double Difference relocation method. Then the travel time tomography inversion is using simulps12 software. The results of the hypocenter relocation distribution show one cluster around the main production well those are PPX well may be related to weak zones and reveal majority structure with NW-SE orientation. While in terms of hypocenter depth distributed clusters around the trajectory of production wells B and C from elevations +1 to 0 Km. The tomogram results show that the delineate steam zone at an elevation of +1 to 0 Km with strongly low perturbation VP, low VS and low VP/VS with arround 1.6 to 1.7 value. Integration with MT data show the steam zone has located at reservoir boundary zone with resistivity value of 30 to 60 Ohm meter. Integration with PPX temperature profiling shows the steam zone has convection heat transfers.

Reservoir Characterization around Geothermal Field, West Java, Indonesia Derived from 4-D Seismic Tomography

Observation of micro-seismic events induced by intensive geothermal exploitation in a particular geothermal field, located in West Java region, Indonesia was used to detect the fracture and permeability zone. Using local monitoring seismometer network, tomographic inversions were conducted for the three-dimensional Vp, Vs, and Vp/Vs structure of the reservoir for January - December 2007, January - December 2008, and January - December 2009. First, hypocenters location was relocated using joint hypocenter determination (JHD) method in purpose to estimate best location. Then, seismic tomographic inversions were conducted using delay time tomography for dataset of every year respectively. The travel times passing through the three-dimensional velocity model were calculated using ray tracing pseudo-bending method. Norm and gradient damping were added to constrain blocks without ray and to produce smooth solution model. The inversion algorithm was developed in Matlab environment. Our tomographic inversion results from 3-years of observations indicate the presence of low Vp, low Vs, and low Vp/Vs ratio at depths of about 1 - 3 km below sea level. These features were interpreted may be related to steam-saturated rock in the reservoir area of this geothermal field. The locations of the reservoir area were supported by the data of well- trajectory, where the zones of high Vp/Vs were observed around the injection wells and the zones of low Vp/Vs were observed around the production wells. The extensive low Vp/Vs anomaly that occupies the reservoir is getting stronger during the 3-years study period. This is probably attributed to depletion of pore liquid water in the reservoir and replacement with steam. Continuous monitoring of Vp, Vs, and Vp/Vs is an effective tool for geothermal reservoir characterization and depletion monitoring and can potentially provide information in parts of the reservoir which have not been drilled.

The Mw7.9 2014 intraplate intermediate-depth Rat Islands earthquake and its relation to regional tectonics

The 2014 Mw7.9 Rat Islands intermediate-depth (∼107 km) earthquake is studied using seismic observations at teleseismic distances. The nodal plane with a steep-dip angle (87°) and a strike of 308° inferred from the Global CMT solution is selected as the causative fault plane because of the consistency with the distribution of the aftershocks relocated using the Joint Hypocenter Determination method. Its slip history is constrained using broadband P and SH waveforms and long-period (3–6 mHz) seismic waves in vertical and transverse components. The rupture process associated with this plane shows a rupture propagating at ∼2 km/s. The rupture essentially breaks one large slip patch (peak slip amplitude of 3.5 m) located 30 km west of the hypocenter. The size of this slip patch extends from 80 to 140 km depth, which would be the largest depth extent of earthquake slip ever documented. We show that this earthquake breaks the entire cold core of the subducting slab defined by the depth of the 600–650 °C isotherm. In addition, the lateral extent of the rupture appears to be bounded by the heterogeneities of subducting slab. Our results further indicate that, along this segment of Aleutian Arc, the intermediate-depth earthquakes might contribute to the westward transporting motion of Rat block. Lastly, the slip distribution of this earthquake as well as that of two other well studied Mw>7.5 intermediate-depth earthquakes suggests that the coldest portion of the subducting slab, outlined by the double Wadati–Benioff zone, is seismic during large intermediate-depth earthquakes. Such a result is important for estimating the seismic hazard of future large intermediate-depth earthquakes.

Earthquake History of the Sumatran Fault, Indonesia, since 1892, Derived from Relocation of Large Earthquakes

Abstract We map the fault planes of M ≥7.0 earthquakes along the Sumatran fault in Sumatra, western Indonesia, for the period since 1892. To obtain precise hypocenter locations of large earthquakes and identify fault planes of M ≥7.0 earthquakes, we relocated 27 M ≥6.0 earthquakes from 1921 to 2012 using the modified joint hypocenter determination method. We found that six M ≥7.0 events appear to have occurred on segments of the Sumatran fault as follows: Kumering for the 1933 ( M s 7.5) event; the northwestern and southeastern parts of Tripa for the 1935 ( M s 7.0) and 1936 ( M s 7.2) events, respectively; the southeastern half of Sunda for the April 1943 ( M s 7.1) event; Ketaun for the first ( M s 7.3) event of the June 1943 doublet; and Suliti and Sumani for the second ( M s 7.6) event of the June 1943 doublet. We also identified the faults of four M ≥7.0 earthquakes between 1892 and 1920 by using information on crustal deformation and damage distributions. From the results of earthquake locations between 1892 and 2012, and the corresponding fault lengths of M ≥7.0 earthquakes, we have interpreted the large ( M ≥7.0) earthquake history of the Sumatran fault for the past 120 years. Possible seismic gaps for M ≥7.0 earthquakes along the Sumatran fault are the northern half of the Sunda segment, and the Semangko, Dikit, Sianok, Barumun, Toru, Renun, Aceh, and Seulimeum segments.

Relocation of the 2011 off the Pacific coast of Tohoku earthquake sequence and fault planes of M ≥ 7 earthquakes

We relocated foreshocks, mainshock, and aftershocks of the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.1) using the modified joint hypocenter determination (MJHD) method in order to obtain their precise hypocenters and to identify fault planes of larger earthquakes. We used P-wave arrival times at stations worldwide reported by the U.S. Geological Survey (USGS). We confirmed by relocated hypocenters that the mainshock and aftershocks had occurred along the plate boundary between the North American and Pacific plates. We also confirmed that the Mw 7.5 foreshock, which occurred two days before the mainshock, and the largest aftershock (Mw 7.9), which occurred half an hour after the mainshock, were thrust earthquakes along the plate boundary. The second largest aftershock (Mw 7.6), which was a normal-faulting earthquake and was a bending-stress intra-plate event caused by the strain reduction on the subduction thrust, occurred at the outer rise of the Japan Trench and was well relocated with its aftershocks. We found that its fault plane dipped westward and it bounded the aftershock distribution on the seaward side. This implies that the western side of the fault plane had subsided, corresponding with the westward plate subduction.

Two seismic gaps on the Sagaing Fault, Myanmar, derived from relocation of historical earthquakes since 1918

[1] Relocation of six M (magnitude) ≥ 7.0 earthquakes near the Sagaing Fault in Myanmar since 1918 allows us to image earthquake history along the Sagaing Fault. All the earthquakes were relocated on the Sagaing Fault by using the modified joint hypocenter determination method. Combining the relocated epicenters with information on foreshocks, aftershocks, seismic intensities, and coseismic displacement, we estimated the location of the fault plane that ruptured during each earthquake. This analysis revealed two seismic gaps: one between 19.2°N and 21.5°N in central Myanmar, and another south of 16.6°N in the Andaman Sea. Considering the length of the first seismic gap (∼260 km), a future earthquake of up to M ∼7.9 is expected to occur in central Myanmar. Because Nay Pyi Taw, the recently established capital of Myanmar, is located on the expected fault, its large population is exposed to a significant earthquake hazard.

Fine fracture structures in the geothermal region of Hakone volcano, revealed by well-resolved earthquake hypocenters and focal mechanisms

We investigated precise hypocentral distribution and the mechanisms of small earthquakes in the geothermal region of Hakone volcano, central Japan, where swarm activity has been frequently observed. Earthquake swarms were remarkably prevalent in 2001 and 2006, and were accompanied by crustal deformation. We first determined initial hypocenters, using station corrections and one-dimensional velocity structures as estimated by the joint hypocenter determination method. Then, we applied the double-difference method to relocate the initial hypocenters using the differential arrival time obtained by both manual picking and waveform cross-correlation analysis. Subsequently, we determined the focal mechanisms from the absolute P- and SH-wave amplitudes and P-wave polarities. From the relocated hypocenter distribution, we found that most swarm earthquakes are distributed on thin plane-like zones with width/length of 100 m to 1 km. We found that these plane-like hypocentral distributions range from the E–W to N–S strikes and are consistent with one of the nodal planes of the focal mechanism. It is likely that most swarm earthquakes occurred on pre-existing fracture planes, shaping the plane-like hypocenter distributions. Previous studies suggested that a highly permeable fracture system has developed in the fault interaction area, and these highly permeable fractures affect the occurrence of swarm earthquakes and surface geothermal activity. Since Hakone volcano is located in the interaction area of the active Tanna and Hirayama faults, it is suggested that the fracture planes revealed by the relocated hypocenter distribution have been developed by the activity of the two faults. The fracture planes in the caldera might be channels for the hydrothermal water of hot springs in the caldera of Hakone volcano which are rich in sodium chloride and considered to be composed of fluid heated by a deep-seated magma. The strike of the fracture plane on which the swarm earthquakes of the 2001 activity occurred is inconsistent with the orientation of open crack models estimated from geodetic data. It is therefore possible that the hydrothermal fluid aseismically intruded the open cracks during the 2001 activity.

Aysén seismic swarm (January 2007) in southern Chile: analysis using Joint Hypocenter Determination

A seismic swarm at the Aysen fjord started in January, 2007, reaching its highest activity between January and April, 2007. It was punctuated by the Mw5.3 23rd January event and the Mw6.2 21st April event which triggered a water-wave due to massive landslides; both with dextral strike slip focal mechanisms. More than 100 events were relocated with the Joint Hypocenter Determination method to study the depth distribution of the events of the swarm recorded by the Chile Ridge Subduction Project local network. The events are associated to a small area of approximately 7 × 7 km located in the Aysen fjord, on one of the main branches of the Liquine–Ofqui fault. Distribution in depth is located between 0 and 8 km and presents a west high dip with almost NS strike, consistent with the main trending of the Liquine–Ofqui fault and the published focal mechanisms. These characteristics, among others, suggest that the Liquine–Ofqui fault is active in this region and thus seismic risk has to be re-determined.

Earthquake distribution in southern Tibet and its tectonic implications

A temporary 37‐station seismic array was operated in southern Tibet from June 2004 to August 2005 close to Xigaze along a traverse from Tangra Yum Co rift in the west to Yadong‐Gulu rift in the east. This lateral array of the international Hi‐CLIMB project recorded a total of 885 local earthquakes during the 14‐month deployment. Hypocenter locations of these events were obtained using HYPOINVERSE2000, and about half of them were relocated by the joint hypocenter determination method. Lateral variations in crustal seismic P and S wave velocities beneath the array were obtained simultaneously as part of the station corrections during the relocation procedure. While earthquakes were scattered in the region, more than 250 earthquakes were clustered within a small area at about 50 km north of the Indus‐Yalu Suture and west of the Pumqu‐Xianza rift. Crustal and uppermost mantle earthquakes are concentrated beneath the Himalayan crest. Projection of all earthquakes to a north–south profile shows that the subducting Indian plate passes the Indus‐Yalu Suture below the Lhasa Terrane. While earthquakes are concentrated in the upper crust and upper mantle beneath the Himalayas, seismicity is mostly restricted to the upper crust to the north, which is consistent with a ductile middle/lower crust beneath southern Tibet. About 20 deep crustal and uppermost mantle events (at depths of 50–70 km) were observed to be associated with the subducting Indian lithosphere. Our relocation results of these local earthquakes in southern Tibet support the popular “jelly‐sandwich” model for the rheology of continental lithosphere in which the strong seismogenic layers of upper curst and uppermost mantle are separated by a weak lower crust.

Interpretation of the 1993, 2001, and 2002 Guam Earthquakes as Intraslab Events by a Simultaneous Relocation of the Mainshocks, Aftershocks, and Background Earthquakes

It has been unclear whether the 1993, 2001, and 2002 Guam earthquakes ( M w 7.8, 7.0, and 7.1, respectively) in the southern Mariana subduction zone were interplate events or intraplate ones within the Pacific slab. We relocated their mainshocks, aftershocks, and numerous long-term background earthquakes simultaneously with high relative accuracy using the modified joint hypocenter determination (MJHD) method. As a result, their mainshocks and three-day aftershocks are clearly distributed within the background slab seismicity. Therefore, we conclude that the 1993, 2001, and 2002 Guam earthquakes were not interplate events, which scarcely occur in the Mariana subduction zone, but intraslab earthquakes within the subducted Pacific plate. The 1993 fault plane is inferred to be the near-horizontal nodal plane of the mechanism solution, though the 2001 and 2002 fault planes remain unknown. The relocation of target events simultaneously with background earthquakes has turned out very effective for discriminating between large interplate and intraslab earthquakes in subduction zones.

Relocation of large intermediate-depth earthquakes in the Vrancea region, Romania, since 1934 and a seismic gap

AbstractWe relocatedM6 or larger intermediate-depth earthquakes in Vrancea, Romania that have occurred since 1934. To determine their absolute hypocenters accurately, we combined them with recent (from January 1996 to November 2003) smaller earthquakes recorded by the Romanian local network and then relocated all of them simultaneously using a modified joint hypocenter determination method. Using the data on recent small earthquakes we first obtained an estimate of the dimensions of the active seismic region of intermediate-depth earthquakes: length, 85 km; along a NE-SW direction; width, 20 km; depth 60-160 km. We then determined that the 1940 (M7.7), 1977 (M7.5) and 1990 (M6.9) major earthquakes were located near the NE edge of the seismic region, while the 1986 (M7.2) was located inside the seismic region. The focal depths of the 1940, 1977, 1986 and 1990 major earthquakes were calculated to be 124, 98, 135 and 84 km, respectively and their source areas to range from 60 to 140 km in depth. The only unbroken area is at a depth of 140–160 km, which we current consider to be a seismic gap. Based on the regularity of past large earthquakes, we propose that the nextM7 intermediate-depth earthquake can be expected to occur in Vrancea at a depth interval of 140–160 km sometime early in this century.

Two parallel trench-normal fault planes within the Pacific slab associated with the 1994 and 2000 Kurile earthquakes as revealed by simultaneous relocation of their main shocks and aftershocks

Abstract The 1994 Hokkaido-Toho-Oki (M w 8.3) and the 2000 Nemuro-Hanto-Oki (M w 6.8) earthquakes have been regarded as great/large intraslab events within the subducted Pacific plate along the southern Kurile trench. Determining which nodal planes of their mechanism solutions are actual fault planes and whether or not the 2000 event occurred on the southwestern extension of the 1994 fault plane have been problematic. In order to clarify these points, we examine the relative location of their main-shock and aftershock distributions accurately by simultaneous relocation of their hypocenters using the Modified Joint Hypocenter Determination (MJHD) method. The result clearly shows that the two earthquakes occurred on trench-normal shallow-dipping fault planes within the Pacific slab, apart from and parallel to each other. Therefore, the 2000 earthquake was not a rupture on the deeper extension of the 1994 fault plane, but one on another fault plane. A large intraslab earthquake (M w 7.8; depth, about 100 km) occurred to the northwest of the 1994 event in 1978, and two trench-normal fracture zones exist outside the trench to the southeast of the 1994 and 2000 events. Therefore, these three intraslab great/large earthquakes are considered to have been ruptures along pre-existing trench-normal weak zones within the subducted Pacific plate.

Earthquake relocations, fault zone geometry and constraints on lateral velocity variations using the joint hypocenter determination method in the Taiwan area

AbstractFour subsets of earthquakes recorded by an island-wide seismic network from 1991 to 2002 in the Taiwan area are relocated using the joint hypocenter determination (JHD) technique. Relatively large horizontal and vertical shifts are observed during the relocation, which indicates there may be systematic earthquake miss-locations in the Taiwan earthquake catalog due to the over-simplification of complicated earth structures to a simple 1-D velocity model. Generally, earthquakes are more clustered after the JHD relocation. Subsurface structures defined by the relocated seismicity become clearer, while they are not as clear in the initial hypocenters. Significant subsurface structures defined by the relocated seismicity are fault systems beneath the Central Mountain Range, beneath the northern Longitudinal Valley and Coastal Range, beneath the southern Longitudinal Valley and Coastal Range, and beneath the Chao-Chou fault in southern Taiwan. Another set of results from the JHD analysis,P- andS-wave station corrections, provide valuable information on the lateral velocity variations. The JHD station corrections indicate that upper crustal materials in the Central Mountain Range are characterized by higher velocity than those in the Western Foothills and Coastal Plain. The patterns of the observed JHD station corrections are also thoroughly consistent with surface geology observations. Analysis of the four clusters of earthquakes resulted in a remarkable similarity in JHD station corrections, indicating that the overlapping subsurfaces where two or more clusters of rays had traveled through were relatively shallow. These subsurfaces are responsible for the observed patterns of positive and negative JHD station corrections.

Modern Seismic Observations in the Tatun Volcano Region of Northern Taiwan: Seismic/Volcanic Hazard Adjacent to the Taipei Metropolitan Area

The Tatun volcano group is located adjacent to the Taipei metropolitan area in northern Taiwan and was a result of episodic volcanisms between 2.8 and 0.2 Ma. Earthquake data collected over the last 30 years are analyzed to explore seismicity patterns and their associated mechanisms of faulting in the area. Using a Joint Hypocenter Determination (JHD) method, a few sequences of relocated earthquake hypocenters are tightly clustered; these seemed to be blurry in the original catalog locations. Numerous earthquakes, previously unnoticed and not reported in the CWB catalog, have been identified from careful examination of the continuous recordings of a nearby broadband seismic station. These newly identified earthquakes show similarities in waveforms and arrival time differences between direct P- and S-waves indicating that their hypocenter locations are very close to each other and their source mechanisms are similar. A relatively high b-value of 1.22 is obtained from the analysis of crustal earthquakes (depth < 30 km) in the region, which may suggest that clustered local seismicity in the Tatun volcanic region probably resulted from subsurface hydrothermal or volcano-related activities. Focal mechanism solutions determined in this study are dominated by normal faulting. Thus, these earthquake clusters are most probably associated with hydrothermal/magmatic activities in a back-arc extensional environment.